Tuesday, September 22, 2009 @ 7:22 AM Haber process Contact process and Reversible reaction.
Haber process - Ammonia is manufactured from nitrogen and hydrogen through a revisable process called Haber process. - The nitrogen and hydrogen must be pure. - 1 mole of nitrogen is reacted with 3 moles of hydrogen. - N2 (g) + 3 H2 (g) 2 NH3 (g) (ΔH = −92.4 kJ•mol−1) - This is done at 200 atmospheres and between 300 and 550 °C, passing the gases over four beds of catalyst, with cooling between each pass to maintain a reasonable equilibrium constant. On each pass only about 15% conversions occurs, but any unreacted gases are recycled, so that eventually an overall conversion of 98% can be achieved. Contact process - Is used to produced Sulphuric acid - The first step to is preparation and purification of sulfur dioxide - Catalytic oxidation (using vanadium pentoxide catalyst) of sulfur dioxide to sulfur trioxide - Conversion of sulfur trioxide to sulfuric acid - The average percentage yield of this reaction is around 30%. H2S2O7 (l) + H2O (l) → 2 H2SO4 (l) Reversible reaction - A reversible reaction is a chemical reaction that results in an equilibrium mixture of reactants and products. - The concentrations of reactants and products in an equilibrium mixture are determined by the analytical concentrations of the reagents and the equilibrium constant, K. - when the free energy change is large (more than about 30 kJ mol-1), then the equilibrium constant is large (log K > 3) and the concentrations of the reactants at equilibrium are very small. Such a reaction is sometimes considered to be an irreversible reaction, although in reality small amounts of the reactants are still expected to be present in the reacting system. - E.g. 2NaCl + CaCO3 → Na2CO3 + CaCl2 Na2CO3 + CaCl2→ 2NaCl + CaCO3
- This is an example of reversible reactions - H2(g) + F2(g) ------> 2 HF(g) forward - H2(g) + F2(g) <------ 2 HF(g) reverse - Reversible chemical reactions follow a similar pattern. The reactants are initially the only molecules around. They react to form products. The amount of reactants dwindles and the forward reaction slows down. The product amounts increase at the same time the reactants are disappearing. They products "decompose" to form reactants. The rate for this reverse reaction increases as the amount of product grows. Ultimately there comes a time when the forward reaction rate and the reverse reaction rates are equal.
- At equilibrium the numbers of reactant and product molecules stays constant. The identity of individual molecules keeps changing. by Poyu and Xavier
Monday, September 21, 2009 @ 7:27 AM .
BY JOY AND NATASHA
ATOMIC STRUCTURE
An atom is- the smallest particle
The name atom comes from the Greek word-ἄτομος/átomos, α-τεμνω, This means that it will not be able to be divided further.
Relative to everyday experience, atoms are minuscule objects with proportionately tiny masses. Atoms can only be observed individually using special instruments such as the scanning tunneling microscope.
An atom consist of
1)Protons
2)Neutrons
3)Electrons
Words we must know!
Atom- the unit particle of an element
Proton number- The number protons in one atom of the element
Nucleon number- The number of nucleons, that is, the number if protons and neutrons in one atom of the element.
Element- A substance that consists of atoms all with the same proton number. It cannot be broken down into anything simpler by chemical means.
Isotopes- Different atoms of the same element
Facts about an atom
-The size of the nucleus is <>
-Electrons of an atom are of negligible mass and are always revolving around the nucleusat high speed.
- The mass is concentrated in the nucleus
Student to professor: If the nucleus contains positively charged protons, and like charges repel each other, why don't the protons repel each other and blow the nucleus apart?
Professor to student: Although the positive charges on the protons do tend to push them apart, there is another force at work. It is called the Strong Nuclear Force and it only operates over very small distances - about 10-15 metres, which is about the size of the atomic nucleus. When protons are roughly this distance apart, the Strong Nuclear Force takes over and pulls the protons (and neutrons, if there are any) together and holds them tightly.
Myths
Facts
The electron shells are orbits, like rail track on which the electrons move.
The electron shells represent energy levels rather than physical orbits.
The mass of a proton and a neutron is 1g each.
The mass of a proton and a neutron are about 1.6X10-24, which means they are very light. The relative mass is 1. That is, when we compare the mass of proton and neutron, they are roughly the same.
An atom is a solid sphere.
The size of a neutron is extremely small compared to the entire size of the atom. In fact, most of the atom consists of empty space.
The isotopes have different chemical properties.
Isotopes are the atoms of the same element. They have similar chemical properties. Elements are defined by the number of protons and the number of electrons. Since isotopes only have a different number of neutrons, their chemical properties are the same.
Rutherford’s experiment.
Rutherford did an experiment to determine what atoms were like. He fired a beam of particles at a thin gold foil, tracking them using a detector. However, he discovered that most of the particles went straight through the foil, while some were deflected at an angle and some even bounced straight back. This showed that most of the particles passed through empty space but that some were being deflected by a small point charge.
@ 6:58 AM chemical structures by alex and clarisse.
STRUCTURE OF METAL
Arrangement of atoms in metals forms a closely packed regular giant structure.
·" Giant" implies that large but variable numbers of atoms are involved - depending on the size of the bit of metal.
· In metals, the atoms are ionized, loosing some electrons from the outer most shell.
·The electrostatic attraction between the positive metal ions and the negative delocalized electrons binds the ions together.
·
The delocalized electrons between the atoms enable metals to conduct heat and electricity, and give the metal malleability, as the atoms can slide over each other.
·Delocalized electrons means that they do not move in a fixed position.
·The electrons move freely in the empty space between the positive metal ions.
·Metals have high melting points, which suggests that there are strong chemical bonds between the atoms.
SIMPLE MOLECULAR SUBSTANCES
·The molecules of non-metals are often made up of two identical atoms covalently bonded together, for example: oxygen (O2), hydrogen (H2), and nitrogen (N2). There are also molecular compounds made up of different non-metal atoms bonded together, such as water (H2O), ammonia (NH3), and carbon dioxide (CO2)
·Simple molecular structures have distinctive properties. Although the covalent bonds within the molecules are very strong, the weak intermolecular forces are easily broken.
·As the molecules can be parted easily, simple molecular substances are not very hard or strong and have low melting and boiling points. At room temperature, most simple molecular substances are either a gas or liquid. They do not usually dissolve in water and do not conduct electricity because they have no ions.
There are two kinds of intermolecular forces,
·Van der Waals forces:
-Exist between simple covalent molecules
·Hydrogen bonding:
-Occurs in polar molecules containing hydrogen
-Stronger than Van der Waals’ forces but is still much weaker than covalent bond because it’s just an attraction.
IONIC COMPOUND STRUCTURE
·Ionic compounds consist of positive metallic ions and negative-metallic ions bonded together by electrostatic forces of attraction.
·They mostly form a giant crystal lattice pattern,this results in the compound forming crystals with definite shapes.
·These crystal lattice structures are very stable because of the strong ionic bonds
·between the two oppositely charged ions.
·These require large amount of energy to overcome.
An example is: Sodium Chloride
·Most ionic compounds are solids with hight melting and boiling point.
·They are also a non-conductor of electricity in solid state, but a conductor in
·molten or aqueous state.
·Chemical compounds are never strictly ionic. Even the most electronegative/electropositive pairs such as caesium fluoride exhibit a degree of covalency. Similarly, covalent compounds often exhibit charge separations. See also HASB theory.
MACROMOLECULAR STRUCTURE
·In a macromolecular structure, all the atoms in
·the crystal are held together by strong covalent
·bonds to give a giant three-dimensional lattice.
·This is the reason why it is so stable.
An example of such is diamond.
(tetrahedral structure)
·The carbon atoms are all bonded together by very strong covalent bonds to give a giant three-dimensional network.
·This results in a network of covalent bonds.
·Due to the large amount of energy required to overcome these bonds, the diamond is rigid and strong, and has a high melting point. They do not conduct electricity at all, and are generally insoluble in all solvents.
The term macromolecule was coined by Nobel Laureate Hermann Staudinger in the1920s although his first relevant publication on this field only mentions high molecular compounds (in excess of 1000 atoms). At that time the phrase polymer as introduced by Berzelius in 1833 had a different meaning from that of today: it simply was another form of isomerism for example with benzene and acetylene and had little to do with size.
Personal thoughts:
I honestly think that this topic isn't that very much interesting. This could because I have only learnt the basics of this topic. (Although the macromolecular one does seem interesting : D) Descriptions of the compound were pretty simple, whilst the macromolecular one was more complex, it showed how the (in a diamond) different carbon atoms are linked together by strong covalent bonds, this then goes on to show how it eventually ends up as a giant crystal lattice structure. The info from the internet about the structure of metals and simple molecular substances is explained in detail which is easy to understand and grasp the concept about howI hope through this project, it will help other students to understand more about these two. Credits to Cheryl Seah.
@ 6:29 AM .
CHEMICAL BONDING by: Jingxi and Slin History of Van der Waals forces: Van der Waals was born in Leiden,Netherlands to Jacobus van der Waals and Elisabeth van den Burg. He became a school teacher, and later was allowed to study at the university, in spite of his lack of education in the field of classical languages. He studied from 1862 to 1865, earning degrees in mathematics and physics. Van der Waals' doctoral thesis was entitled Over de Continuïteit van den Gas- en Vloeistoftoestand (On the continuity of the gas and liquid state). In this thesis he derived the equation of state bearing his name. This work gave a model in which the liquid and the gas phase of a substance merge into each other in a continuous manner. It shows that the two phases are of the same nature. In deriving his equation of state van der Waals assumed not only the existence of molecules (which in physics was disputed at the time), but also that they are of finite size and attract each other. Since he was one of the first to postulate an intermolecular force, however rudimentary, such a force is now sometimes called a van der waal's force.
Interesting Facts about Chemical Bonding: Pure ionic bonding is not known to exist. All ionic bonds have a degree of covalent bonding or metallic bonding. The larger the difference in electronegativity between two atoms the more ionic the bond. Ionic compounds conduct electricity when molten or in solution. They generally have a high melting point and tend to be soluble in water. In an ionic bond, the atoms are bound by attraction of opposite ions, whereas, in a covalent bond, atoms are bound by sharing electrons. In covalent bonding, the molecular geometry around each atom is determined by VSEPR rules, whereas, in ionic materials, the geometry follows maximum packing rules. Thus, a compound can be classified as ionic or covalent based on the geometry of the atoms. Reflection: From this, we learnt more facts about chemistry. Without Van der Waal, we probably would not be so advanced in chemistry and a lot of medical achievements that we made today would not have been discovered. The facts that we learnt helped us to better understand chemical bonding. And they are things that we cannot find
Sunday, September 20, 2009 @ 10:50 AM metals. like super duper wtf ftw late..
Metals What are metals? Metals are usually described as an arrangement of positive ions surrounded by a sea of delocalized electrons. They are distinguished on the periodic table by their ionization and bonding properties. Chemical Properties. Metals form cations when they lose electrons and they react with the oxygen in the air to form oxides over time. 4Na + O2 → 2Na2O (sodium oxide) 2Ca + O2 → 2CaO (calcium oxide) 4Al + 3O2 → 2Al2O3 (aluminum oxide) The oxides of metals are usually basic as compared to the oxides of non-metals which are acidic. Physical properties. Most metals in general have high electric and thermal conductivity. They also have high densities. There are several metals that have low densities and low melting points. They are called alkaline earth metals. Planes of atoms in a metal are able to slide over one another under stress, proving the ability of a crystal to form without shattering. Alloys An alloy is a mixture of two or more elements in solid solution in which the major component is a metal. Most pure metals are either too soft, brittle or chemically reactive for practical use. Combining different ratios of metals as alloys modifies the properties of pure metals to produce desirable characteristics. The aim of making alloys is generally to make them less brittle, harder, resistant to corrosion, or have a more desirable colour. Making the alloy • Weigh out 1 g each of lead and tin. Put the lead into the crucible, but keep the tin to one side. • If using casting sand, fill one of the sand trays with casting and push your finger into it to make an indent. This is your cast. • Put the crucible onto a pipe clay triangle. Make sure that it is stable on a tripod and mat. • Heat the crucible strongly with a Bunsen burner until the lead is molten. Add a spatula of carbon powder to the top of it to prevent a skin forming. • Add the tin and stir with a spatula until the metals are both molten and thoroughly mixed. • Move the Bunsen away from the tripod and put it onto a yellow flame. Wearing thermal protection gloves, pick up the crucible using the tongs, and pour the molten metal into the cast or onto a ceramic tile. Take great care as you do this to avoid splashing or dripping. • Let it cool down completely before you touch it. Testing the alloy • Hardness testing Try to scratch the alloy with the lead, and the lead with the alloy. The one which does not scratch is the hardest. • Density testing Hold the lead in one hand and the alloy in the other. Which seems to be the heaviest/most dense? • Melting-point testing Put the alloy, a piece of tin and a piece of lead onto a sand tray. They should all be the same distance from the middle of the dish. Heat the dish gently in the middle. When two of the metals have melted, stop heating.
Thursday, September 17, 2009 @ 7:19 AM Acid.
By: Lihui and Esther
Red Cabbage Litmus
Make some cabbage juice and find out it's unexpectedly colourful properties.
What you need:
A red cabbage
A grater
A bowl
A wooden spoon
A knife
A little water
Several cups
A sieve
Vinegar
Bicarbonate of soda
Fruit Juice
What to Do:
Chop the cabbage into managable lumps - say quarters
Grate some of it, - you won't need very much of it maybe a cm or so off the cabbage quarter.
Put it into the bowl, add a little water (not enough to cover it), and crush it with the wooden spoon, you are trying to get the coloured juice out.
You should now have some coloured liquid in amongst your bits of cabbage. sieve this into a cup to get rid of the cabbage.
You now have your test solution so share it out between several cups, and try adding different things to it.
Does anything interesting happen? Try and group things by what effect they have on the solution, is there anything similar about the groups?
What may Happen:
You should find that the solution starts off purple (even though it is called red-cabbage), if you add some things the solution will go a bright pink, and others will cause it to go a bright blue.
Try adding something that makes it go pink, and then something that makes it go blue or vice-versa.
What is going on?
In the red cabbage there are pigments that are very similar to those in litmus that you may have used in experiments at school.
You will probably have noticed that things with a sharp, sour taste caused it to go pink, these are called acids. Things with a chalky taste made it go blue and are called alkalis.
Acids and Alkalis are the opposite of each other and will cancel each other out, so if you add enough acid to something that is normally an alkali it will become acid and turn the cabbage pink, but if you add enough alkali to an acid solution the solution will become alkali and become blue.
Why does it fizz if you add an acid to bicarbonate of soda?
Bicarbonate of soda or Sodium bicarbonate is a special type of alkali, it is a type of carbonate. This means that it has carbon dioxide gas locked up inside it very tightly which is released as lots of bubbles when you add an acid.
Carbonate + Acid -> Carbon Dioxide + Water
HCO3- + H+ -> CO2 + H2O
Why is red cabbage called red cabbage not purple cabbage?
When you cook the red cabbage it will change colour from purple to red, because it becomes more acidic, causing it to change colour. This is why it is called red cabbage and not purple cabbage.
What is happening to the molecules?
Acids are things that have lots of free H+ ions (an ion is just a molecule that is positively or negatively charged) floating around, eg hydrochloric acid - HCl will break up in water (dissasociate) into H+ and Cl- ions.
Acidity is measured on the pH scale which is a measure of how many H+ ions there are in the solution, the lower the number the more H+ ions. Pure water has some H+ ions because water will split up occasionally too.
H2O -> H+ + OH-
Alkalis have lots of OH- ions which will react with the H+ ions forming water again,
H+ + OH- -> H2O
This reduces the number of H+ ions and makes the solution less acidic.
Why does red cabbage change colour?
It contains a type of anthocyanin pigment, these are very common in plants and are used to protect the plants from ultraviolet radiation. This has two forms; one with a hydrogen atom attached (which is more common in an acid because there are lots of H+ ions around), and one where it has fallen off which is more common in an alkali.
The pigment will look blue if it absorbs reddish long wavelength light, and look red if it absorbs blue and green shorter wavelength light.
In the Alkali form the electrons can move further than in the Acid form and so act like a large aerial and absorb long wavelength red light (so it looks blue), and in the Acid form it will act like a shorter aerial absorbing short wavelength blue light so it will look red.
@ 5:16 AM Why does salt melt ice?.
done by Winnie and Tiffany (3z)
Definition:
Salt: a salt is made of cation which comes from the base and an anion which comes from the acid.
NaOH + HCl -> NaCl + H2O
salt can form acidic, neutral or alkaline solution
i) form neutral solution: sodium chloride dissolves in water
NaCl -> Na+ + Cl-
ii) form alkaline solution: sodium ethanoate dissolve in water, its solution is slightly alkaline. derived from a weak acid CH3COOH and a strong base NaOH
CH3COONa -> CH3COO- + Na+
iii) form acidic solution: ammonium chloride dissolves in water, its slightly acidic. NH4Cl is derived from a weak base NH3 and a strong acid HCl.
NH4Cl -> NH4+ + Cl-
Some interesting facts about salt
1/ if you try the heat test to salt, there is nothing happens except you may find tiny dents in aluminium foil.
2/ Only 6% of the salt used in the U.S. is used in food; another 17% is used for de-icing streets and highways in the winter months.
3/ Sodium Chloride is formed when the unstable metal sodium reacts with chlorine gas. It is the only family of rocks regularly eaten by humans.
4/ After aviation fuel is purified, salt is mixed with it to remove all traces of water before it can be used.
Topic: experiment with salt NaCl and ice
Aim:
to investigate the effect of table salt ( sodium chloride , NaCl) on melting ice an freezing ice
Introduction:
salt has many advantages. in real life, such as ice melting, control of human body fluids, storage foods in long term,etc. in this experiment, we are going to investigate how salt NaCl is able to melt ice.
it is because when ice and water are placed in contact:
Molecules on the surface of the ice escape into the water (melting)
molecules of water are captured on the surface of the ice (freezing)
1/ When the rate of freezing is the same as the rate of melting, the amount of ice and the amount of water won't change on average. The ice and water are said to be in equilibrium with each other. The balance between freezing and melting can be maintained at 0°C (the melting point of water) unless conditions change in a way that favors one of the processes over the other.
the equilibrium will be change when adding NaCl crystals to the experiment. Consider replacing some of the water molecules with NaCl. it dissolves in the water, but do not pack easily with molecules in the solid.So fewer water molecules present in a system.it leads to total number of waters captured by the ice goes down, so the rate of freezing decreases. The rate of melting is unchanged by the presence of NaCl, so melting occurs faster than freezing.
( temperature is constant at 0°C)
It is a reason why salt melts ice.
( can refer to the video on http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-melts-ice.shtml )
2/ However, as more water melts, the NaCl is diluted, so it does not prevent the liquid water joining to the ice as effectively. Eventually the NaCl is diluted enough that some of the water can be captured by the ice again. Moreover, if the condition of temperature falls down below 0°C, water will change from a liquid to a solid. This change in composition makes it harder for water molecules to join together and form crystals. As a result, liquid water will be freeze.
It is why salt freezes ice.
Experiment:
apparatus and materials:
2 ice cube ~ 6 x 8 x 2 cm3
beaker x2
1 teaspoon of NaCl salt x 2
stop watch x 2
thread
ice container
thermometer
Procedure of ice melting by using salt
1/ putting 6 x 8 x 2 cm3 ice cube into a beaker in a ice container and using a thermometer to measure the temperature of the ice container at0°C
2/ putting a teaspoon of NaCl into the beaker
3/ using a stop watch to measure time until the ice is completely melting.
Procedure of ice freezing by using salt
1/ putting 6 x 8 x 2 cm3 ice cube into a beaker in a ice container and using a thermometer to measure the temperature of the ice container at0°C
2/ lay the thread on top of the ice in the beaker
3/putting a teaspoon of NaCl into the beaker around the thread
4/ using a stop watch to measure time until the ice starts to melt.
5/ putting more ice into ice container to control the temperature drops down below 0°C.
6/ waiting for about 2 mins
7/ slowly pull the ends of the thread up. find out that the ice has frozen to the thread.
Applications:
1/ Since salt prevents ice crystals forming in water, adding salt to water effectively lowers its freezing point. This is why the oceans don't freeze, even when the temperature is well below 0 °C.
In colder parts of the world, salt is often used on footpaths and roads to melt ice and snow.
When ice melts, it absorbs heat energy from its surroundings. When you add salt to ice, the rapid melting of the ice can cool the ice and water to below 0 °C. Before refrigeration was invented, ice and salt was used to produce the rapid cooling needed to make ice cream.
2/ Salt is also used to make homemade ice cream by melt the ice and keep it from refreezing so we can freeze the ice cream.
Tuesday, September 22, 2009 @ 7:22 AM Haber process Contact process and Reversible reaction.
Haber process - Ammonia is manufactured from nitrogen and hydrogen through a revisable process called Haber process. - The nitrogen and hydrogen must be pure. - 1 mole of nitrogen is reacted with 3 moles of hydrogen. - N2 (g) + 3 H2 (g) 2 NH3 (g) (ΔH = −92.4 kJ•mol−1) - This is done at 200 atmospheres and between 300 and 550 °C, passing the gases over four beds of catalyst, with cooling between each pass to maintain a reasonable equilibrium constant. On each pass only about 15% conversions occurs, but any unreacted gases are recycled, so that eventually an overall conversion of 98% can be achieved. Contact process - Is used to produced Sulphuric acid - The first step to is preparation and purification of sulfur dioxide - Catalytic oxidation (using vanadium pentoxide catalyst) of sulfur dioxide to sulfur trioxide - Conversion of sulfur trioxide to sulfuric acid - The average percentage yield of this reaction is around 30%. H2S2O7 (l) + H2O (l) → 2 H2SO4 (l) Reversible reaction - A reversible reaction is a chemical reaction that results in an equilibrium mixture of reactants and products. - The concentrations of reactants and products in an equilibrium mixture are determined by the analytical concentrations of the reagents and the equilibrium constant, K. - when the free energy change is large (more than about 30 kJ mol-1), then the equilibrium constant is large (log K > 3) and the concentrations of the reactants at equilibrium are very small. Such a reaction is sometimes considered to be an irreversible reaction, although in reality small amounts of the reactants are still expected to be present in the reacting system. - E.g. 2NaCl + CaCO3 → Na2CO3 + CaCl2 Na2CO3 + CaCl2→ 2NaCl + CaCO3
- This is an example of reversible reactions - H2(g) + F2(g) ------> 2 HF(g) forward - H2(g) + F2(g) <------ 2 HF(g) reverse - Reversible chemical reactions follow a similar pattern. The reactants are initially the only molecules around. They react to form products. The amount of reactants dwindles and the forward reaction slows down. The product amounts increase at the same time the reactants are disappearing. They products "decompose" to form reactants. The rate for this reverse reaction increases as the amount of product grows. Ultimately there comes a time when the forward reaction rate and the reverse reaction rates are equal.
- At equilibrium the numbers of reactant and product molecules stays constant. The identity of individual molecules keeps changing. by Poyu and Xavier
Monday, September 21, 2009 @ 7:27 AM .
BY JOY AND NATASHA
ATOMIC STRUCTURE
An atom is- the smallest particle
The name atom comes from the Greek word-ἄτομος/átomos, α-τεμνω, This means that it will not be able to be divided further.
Relative to everyday experience, atoms are minuscule objects with proportionately tiny masses. Atoms can only be observed individually using special instruments such as the scanning tunneling microscope.
An atom consist of
1)Protons
2)Neutrons
3)Electrons
Words we must know!
Atom- the unit particle of an element
Proton number- The number protons in one atom of the element
Nucleon number- The number of nucleons, that is, the number if protons and neutrons in one atom of the element.
Element- A substance that consists of atoms all with the same proton number. It cannot be broken down into anything simpler by chemical means.
Isotopes- Different atoms of the same element
Facts about an atom
-The size of the nucleus is <>
-Electrons of an atom are of negligible mass and are always revolving around the nucleusat high speed.
- The mass is concentrated in the nucleus
Student to professor: If the nucleus contains positively charged protons, and like charges repel each other, why don't the protons repel each other and blow the nucleus apart?
Professor to student: Although the positive charges on the protons do tend to push them apart, there is another force at work. It is called the Strong Nuclear Force and it only operates over very small distances - about 10-15 metres, which is about the size of the atomic nucleus. When protons are roughly this distance apart, the Strong Nuclear Force takes over and pulls the protons (and neutrons, if there are any) together and holds them tightly.
Myths
Facts
The electron shells are orbits, like rail track on which the electrons move.
The electron shells represent energy levels rather than physical orbits.
The mass of a proton and a neutron is 1g each.
The mass of a proton and a neutron are about 1.6X10-24, which means they are very light. The relative mass is 1. That is, when we compare the mass of proton and neutron, they are roughly the same.
An atom is a solid sphere.
The size of a neutron is extremely small compared to the entire size of the atom. In fact, most of the atom consists of empty space.
The isotopes have different chemical properties.
Isotopes are the atoms of the same element. They have similar chemical properties. Elements are defined by the number of protons and the number of electrons. Since isotopes only have a different number of neutrons, their chemical properties are the same.
Rutherford’s experiment.
Rutherford did an experiment to determine what atoms were like. He fired a beam of particles at a thin gold foil, tracking them using a detector. However, he discovered that most of the particles went straight through the foil, while some were deflected at an angle and some even bounced straight back. This showed that most of the particles passed through empty space but that some were being deflected by a small point charge.
@ 6:58 AM chemical structures by alex and clarisse.
STRUCTURE OF METAL
Arrangement of atoms in metals forms a closely packed regular giant structure.
·" Giant" implies that large but variable numbers of atoms are involved - depending on the size of the bit of metal.
· In metals, the atoms are ionized, loosing some electrons from the outer most shell.
·The electrostatic attraction between the positive metal ions and the negative delocalized electrons binds the ions together.
·
The delocalized electrons between the atoms enable metals to conduct heat and electricity, and give the metal malleability, as the atoms can slide over each other.
·Delocalized electrons means that they do not move in a fixed position.
·The electrons move freely in the empty space between the positive metal ions.
·Metals have high melting points, which suggests that there are strong chemical bonds between the atoms.
SIMPLE MOLECULAR SUBSTANCES
·The molecules of non-metals are often made up of two identical atoms covalently bonded together, for example: oxygen (O2), hydrogen (H2), and nitrogen (N2). There are also molecular compounds made up of different non-metal atoms bonded together, such as water (H2O), ammonia (NH3), and carbon dioxide (CO2)
·Simple molecular structures have distinctive properties. Although the covalent bonds within the molecules are very strong, the weak intermolecular forces are easily broken.
·As the molecules can be parted easily, simple molecular substances are not very hard or strong and have low melting and boiling points. At room temperature, most simple molecular substances are either a gas or liquid. They do not usually dissolve in water and do not conduct electricity because they have no ions.
There are two kinds of intermolecular forces,
·Van der Waals forces:
-Exist between simple covalent molecules
·Hydrogen bonding:
-Occurs in polar molecules containing hydrogen
-Stronger than Van der Waals’ forces but is still much weaker than covalent bond because it’s just an attraction.
IONIC COMPOUND STRUCTURE
·Ionic compounds consist of positive metallic ions and negative-metallic ions bonded together by electrostatic forces of attraction.
·They mostly form a giant crystal lattice pattern,this results in the compound forming crystals with definite shapes.
·These crystal lattice structures are very stable because of the strong ionic bonds
·between the two oppositely charged ions.
·These require large amount of energy to overcome.
An example is: Sodium Chloride
·Most ionic compounds are solids with hight melting and boiling point.
·They are also a non-conductor of electricity in solid state, but a conductor in
·molten or aqueous state.
·Chemical compounds are never strictly ionic. Even the most electronegative/electropositive pairs such as caesium fluoride exhibit a degree of covalency. Similarly, covalent compounds often exhibit charge separations. See also HASB theory.
MACROMOLECULAR STRUCTURE
·In a macromolecular structure, all the atoms in
·the crystal are held together by strong covalent
·bonds to give a giant three-dimensional lattice.
·This is the reason why it is so stable.
An example of such is diamond.
(tetrahedral structure)
·The carbon atoms are all bonded together by very strong covalent bonds to give a giant three-dimensional network.
·This results in a network of covalent bonds.
·Due to the large amount of energy required to overcome these bonds, the diamond is rigid and strong, and has a high melting point. They do not conduct electricity at all, and are generally insoluble in all solvents.
The term macromolecule was coined by Nobel Laureate Hermann Staudinger in the1920s although his first relevant publication on this field only mentions high molecular compounds (in excess of 1000 atoms). At that time the phrase polymer as introduced by Berzelius in 1833 had a different meaning from that of today: it simply was another form of isomerism for example with benzene and acetylene and had little to do with size.
Personal thoughts:
I honestly think that this topic isn't that very much interesting. This could because I have only learnt the basics of this topic. (Although the macromolecular one does seem interesting : D) Descriptions of the compound were pretty simple, whilst the macromolecular one was more complex, it showed how the (in a diamond) different carbon atoms are linked together by strong covalent bonds, this then goes on to show how it eventually ends up as a giant crystal lattice structure. The info from the internet about the structure of metals and simple molecular substances is explained in detail which is easy to understand and grasp the concept about howI hope through this project, it will help other students to understand more about these two. Credits to Cheryl Seah.
@ 6:29 AM .
CHEMICAL BONDING by: Jingxi and Slin History of Van der Waals forces: Van der Waals was born in Leiden,Netherlands to Jacobus van der Waals and Elisabeth van den Burg. He became a school teacher, and later was allowed to study at the university, in spite of his lack of education in the field of classical languages. He studied from 1862 to 1865, earning degrees in mathematics and physics. Van der Waals' doctoral thesis was entitled Over de Continuïteit van den Gas- en Vloeistoftoestand (On the continuity of the gas and liquid state). In this thesis he derived the equation of state bearing his name. This work gave a model in which the liquid and the gas phase of a substance merge into each other in a continuous manner. It shows that the two phases are of the same nature. In deriving his equation of state van der Waals assumed not only the existence of molecules (which in physics was disputed at the time), but also that they are of finite size and attract each other. Since he was one of the first to postulate an intermolecular force, however rudimentary, such a force is now sometimes called a van der waal's force.
Interesting Facts about Chemical Bonding: Pure ionic bonding is not known to exist. All ionic bonds have a degree of covalent bonding or metallic bonding. The larger the difference in electronegativity between two atoms the more ionic the bond. Ionic compounds conduct electricity when molten or in solution. They generally have a high melting point and tend to be soluble in water. In an ionic bond, the atoms are bound by attraction of opposite ions, whereas, in a covalent bond, atoms are bound by sharing electrons. In covalent bonding, the molecular geometry around each atom is determined by VSEPR rules, whereas, in ionic materials, the geometry follows maximum packing rules. Thus, a compound can be classified as ionic or covalent based on the geometry of the atoms. Reflection: From this, we learnt more facts about chemistry. Without Van der Waal, we probably would not be so advanced in chemistry and a lot of medical achievements that we made today would not have been discovered. The facts that we learnt helped us to better understand chemical bonding. And they are things that we cannot find
Sunday, September 20, 2009 @ 10:50 AM metals. like super duper wtf ftw late..
Metals What are metals? Metals are usually described as an arrangement of positive ions surrounded by a sea of delocalized electrons. They are distinguished on the periodic table by their ionization and bonding properties. Chemical Properties. Metals form cations when they lose electrons and they react with the oxygen in the air to form oxides over time. 4Na + O2 → 2Na2O (sodium oxide) 2Ca + O2 → 2CaO (calcium oxide) 4Al + 3O2 → 2Al2O3 (aluminum oxide) The oxides of metals are usually basic as compared to the oxides of non-metals which are acidic. Physical properties. Most metals in general have high electric and thermal conductivity. They also have high densities. There are several metals that have low densities and low melting points. They are called alkaline earth metals. Planes of atoms in a metal are able to slide over one another under stress, proving the ability of a crystal to form without shattering. Alloys An alloy is a mixture of two or more elements in solid solution in which the major component is a metal. Most pure metals are either too soft, brittle or chemically reactive for practical use. Combining different ratios of metals as alloys modifies the properties of pure metals to produce desirable characteristics. The aim of making alloys is generally to make them less brittle, harder, resistant to corrosion, or have a more desirable colour. Making the alloy • Weigh out 1 g each of lead and tin. Put the lead into the crucible, but keep the tin to one side. • If using casting sand, fill one of the sand trays with casting and push your finger into it to make an indent. This is your cast. • Put the crucible onto a pipe clay triangle. Make sure that it is stable on a tripod and mat. • Heat the crucible strongly with a Bunsen burner until the lead is molten. Add a spatula of carbon powder to the top of it to prevent a skin forming. • Add the tin and stir with a spatula until the metals are both molten and thoroughly mixed. • Move the Bunsen away from the tripod and put it onto a yellow flame. Wearing thermal protection gloves, pick up the crucible using the tongs, and pour the molten metal into the cast or onto a ceramic tile. Take great care as you do this to avoid splashing or dripping. • Let it cool down completely before you touch it. Testing the alloy • Hardness testing Try to scratch the alloy with the lead, and the lead with the alloy. The one which does not scratch is the hardest. • Density testing Hold the lead in one hand and the alloy in the other. Which seems to be the heaviest/most dense? • Melting-point testing Put the alloy, a piece of tin and a piece of lead onto a sand tray. They should all be the same distance from the middle of the dish. Heat the dish gently in the middle. When two of the metals have melted, stop heating.
Thursday, September 17, 2009 @ 7:19 AM Acid.
By: Lihui and Esther
Red Cabbage Litmus
Make some cabbage juice and find out it's unexpectedly colourful properties.
What you need:
A red cabbage
A grater
A bowl
A wooden spoon
A knife
A little water
Several cups
A sieve
Vinegar
Bicarbonate of soda
Fruit Juice
What to Do:
Chop the cabbage into managable lumps - say quarters
Grate some of it, - you won't need very much of it maybe a cm or so off the cabbage quarter.
Put it into the bowl, add a little water (not enough to cover it), and crush it with the wooden spoon, you are trying to get the coloured juice out.
You should now have some coloured liquid in amongst your bits of cabbage. sieve this into a cup to get rid of the cabbage.
You now have your test solution so share it out between several cups, and try adding different things to it.
Does anything interesting happen? Try and group things by what effect they have on the solution, is there anything similar about the groups?
What may Happen:
You should find that the solution starts off purple (even though it is called red-cabbage), if you add some things the solution will go a bright pink, and others will cause it to go a bright blue.
Try adding something that makes it go pink, and then something that makes it go blue or vice-versa.
What is going on?
In the red cabbage there are pigments that are very similar to those in litmus that you may have used in experiments at school.
You will probably have noticed that things with a sharp, sour taste caused it to go pink, these are called acids. Things with a chalky taste made it go blue and are called alkalis.
Acids and Alkalis are the opposite of each other and will cancel each other out, so if you add enough acid to something that is normally an alkali it will become acid and turn the cabbage pink, but if you add enough alkali to an acid solution the solution will become alkali and become blue.
Why does it fizz if you add an acid to bicarbonate of soda?
Bicarbonate of soda or Sodium bicarbonate is a special type of alkali, it is a type of carbonate. This means that it has carbon dioxide gas locked up inside it very tightly which is released as lots of bubbles when you add an acid.
Carbonate + Acid -> Carbon Dioxide + Water
HCO3- + H+ -> CO2 + H2O
Why is red cabbage called red cabbage not purple cabbage?
When you cook the red cabbage it will change colour from purple to red, because it becomes more acidic, causing it to change colour. This is why it is called red cabbage and not purple cabbage.
What is happening to the molecules?
Acids are things that have lots of free H+ ions (an ion is just a molecule that is positively or negatively charged) floating around, eg hydrochloric acid - HCl will break up in water (dissasociate) into H+ and Cl- ions.
Acidity is measured on the pH scale which is a measure of how many H+ ions there are in the solution, the lower the number the more H+ ions. Pure water has some H+ ions because water will split up occasionally too.
H2O -> H+ + OH-
Alkalis have lots of OH- ions which will react with the H+ ions forming water again,
H+ + OH- -> H2O
This reduces the number of H+ ions and makes the solution less acidic.
Why does red cabbage change colour?
It contains a type of anthocyanin pigment, these are very common in plants and are used to protect the plants from ultraviolet radiation. This has two forms; one with a hydrogen atom attached (which is more common in an acid because there are lots of H+ ions around), and one where it has fallen off which is more common in an alkali.
The pigment will look blue if it absorbs reddish long wavelength light, and look red if it absorbs blue and green shorter wavelength light.
In the Alkali form the electrons can move further than in the Acid form and so act like a large aerial and absorb long wavelength red light (so it looks blue), and in the Acid form it will act like a shorter aerial absorbing short wavelength blue light so it will look red.
@ 5:16 AM Why does salt melt ice?.
done by Winnie and Tiffany (3z)
Definition:
Salt: a salt is made of cation which comes from the base and an anion which comes from the acid.
NaOH + HCl -> NaCl + H2O
salt can form acidic, neutral or alkaline solution
i) form neutral solution: sodium chloride dissolves in water
NaCl -> Na+ + Cl-
ii) form alkaline solution: sodium ethanoate dissolve in water, its solution is slightly alkaline. derived from a weak acid CH3COOH and a strong base NaOH
CH3COONa -> CH3COO- + Na+
iii) form acidic solution: ammonium chloride dissolves in water, its slightly acidic. NH4Cl is derived from a weak base NH3 and a strong acid HCl.
NH4Cl -> NH4+ + Cl-
Some interesting facts about salt
1/ if you try the heat test to salt, there is nothing happens except you may find tiny dents in aluminium foil.
2/ Only 6% of the salt used in the U.S. is used in food; another 17% is used for de-icing streets and highways in the winter months.
3/ Sodium Chloride is formed when the unstable metal sodium reacts with chlorine gas. It is the only family of rocks regularly eaten by humans.
4/ After aviation fuel is purified, salt is mixed with it to remove all traces of water before it can be used.
Topic: experiment with salt NaCl and ice
Aim:
to investigate the effect of table salt ( sodium chloride , NaCl) on melting ice an freezing ice
Introduction:
salt has many advantages. in real life, such as ice melting, control of human body fluids, storage foods in long term,etc. in this experiment, we are going to investigate how salt NaCl is able to melt ice.
it is because when ice and water are placed in contact:
Molecules on the surface of the ice escape into the water (melting)
molecules of water are captured on the surface of the ice (freezing)
1/ When the rate of freezing is the same as the rate of melting, the amount of ice and the amount of water won't change on average. The ice and water are said to be in equilibrium with each other. The balance between freezing and melting can be maintained at 0°C (the melting point of water) unless conditions change in a way that favors one of the processes over the other.
the equilibrium will be change when adding NaCl crystals to the experiment. Consider replacing some of the water molecules with NaCl. it dissolves in the water, but do not pack easily with molecules in the solid.So fewer water molecules present in a system.it leads to total number of waters captured by the ice goes down, so the rate of freezing decreases. The rate of melting is unchanged by the presence of NaCl, so melting occurs faster than freezing.
( temperature is constant at 0°C)
It is a reason why salt melts ice.
( can refer to the video on http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-melts-ice.shtml )
2/ However, as more water melts, the NaCl is diluted, so it does not prevent the liquid water joining to the ice as effectively. Eventually the NaCl is diluted enough that some of the water can be captured by the ice again. Moreover, if the condition of temperature falls down below 0°C, water will change from a liquid to a solid. This change in composition makes it harder for water molecules to join together and form crystals. As a result, liquid water will be freeze.
It is why salt freezes ice.
Experiment:
apparatus and materials:
2 ice cube ~ 6 x 8 x 2 cm3
beaker x2
1 teaspoon of NaCl salt x 2
stop watch x 2
thread
ice container
thermometer
Procedure of ice melting by using salt
1/ putting 6 x 8 x 2 cm3 ice cube into a beaker in a ice container and using a thermometer to measure the temperature of the ice container at0°C
2/ putting a teaspoon of NaCl into the beaker
3/ using a stop watch to measure time until the ice is completely melting.
Procedure of ice freezing by using salt
1/ putting 6 x 8 x 2 cm3 ice cube into a beaker in a ice container and using a thermometer to measure the temperature of the ice container at0°C
2/ lay the thread on top of the ice in the beaker
3/putting a teaspoon of NaCl into the beaker around the thread
4/ using a stop watch to measure time until the ice starts to melt.
5/ putting more ice into ice container to control the temperature drops down below 0°C.
6/ waiting for about 2 mins
7/ slowly pull the ends of the thread up. find out that the ice has frozen to the thread.
Applications:
1/ Since salt prevents ice crystals forming in water, adding salt to water effectively lowers its freezing point. This is why the oceans don't freeze, even when the temperature is well below 0 °C.
In colder parts of the world, salt is often used on footpaths and roads to melt ice and snow.
When ice melts, it absorbs heat energy from its surroundings. When you add salt to ice, the rapid melting of the ice can cool the ice and water to below 0 °C. Before refrigeration was invented, ice and salt was used to produce the rapid cooling needed to make ice cream.
2/ Salt is also used to make homemade ice cream by melt the ice and keep it from refreezing so we can freeze the ice cream.
