AQA Chemistry Unit 2

All you need to know for your Unit 2 Additional Science Chemistry exam. Hope these help & GOOD LUCK!

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Atomic Structure.

All atoms contain:

Protons (positively charged)

Neutrons (no charge/neutral)

& Electrons (negatively charged)

The protons and neutrons are found in the nucleus (centre) of the atom. Whereas the electrons are found in the outer shells of the atom.

The amount of protons in an atom can be found on your periodic table. (It's the atomic number!! Or the number below the symbol of the element). The amount of electrons is equal to the amount of protons. The amount of neutrons in an atom is the Ar (Relative Atomic Mass) of the substance minus the Atomic (proton) number.

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Giant (and not so giant) Structures

Examples of Giant Covalent Structures are Diamond and Graphite. Within these, there are weak forces of attraction in the layers but very strong bonds, meaning they have high melting points. In Graphite, there are delocalised electrons which allow it to conduct electricity.

Giant Ionic Structures are regular, latticed structures held together by strong forces of attraction (electrostatic forces) between oppisitely charged ions. Ionic compounds conduct electricity when in molten or solution because charged ions are free to move.

Contrastly, Nanoparticles. These are, quite simply, really, really, really small particles. In fact they're tiny. Less than tiny, they're one billionth of a metre. And they can combine to form nanostructures.

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Ionic Bonding

Ionic Bonding is the transfer of electrons from one atom to another and it occurs in a metal and non-metal. For example, Sodium and Chlorine or Magnesium and Oxygen etc. The atoms that lose electrons become positively charged and those that gain electrons become negatively charged. Ionic compounds are giant structures of ions that are held together by strong forces off attraction between oppositely charged ions. They have high melting and boiling points.

EXAMPLE Sodium and Chlorine. Sodium has 1 electron in it's outer shell, but Chlorine has 7. A complete outer shell of electrons has 8 electrons on it. Therefore, Sodium would give Chlorine it's electron. This would be written like this:

Sodium = Na atom - 2, 8, 1 ---> Na+ ion [2,8]+ Chlorine = Cl atom - 2, 8, 7 ---> Cl- ion [2, 8, 8]-

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Covalent Bonding

Covalent Bonding is the sharing of electrons and forms a very strong bond between two non-metal atoms. Some covalent bonds are simple whereas others have giant covalent structures called macromolecules. Covalently bonded substances generally have low melting and boiling points.


Water, H2O. Oxygen has 6 electrons in its outer shell and Hydrogen has 1. Therefore 2 Hydrogen atoms are needed to bond covalently with Hydrogen. Which is written like this: H-O-H

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Mass and Atomic Number

The Mass number (Ar) of an atom is the number above the element symbol and the Atomic (proton) Number is the number below it. The atomic number gives the number of protons witin the element, and consequently, the number of electrons.

Relative Formula Mass (Mr).

The Mr of a compound is easy to calculate. It's the Ar of all the elements added together.

Calculating % Mass of an element in a compound

% Mass = Ar / Mr X 100. That's The relative mass of the element in the compound divided by the relative formula mass of the compound, all multiplied by 100

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The Mole (mol)

A mole is a measurement of the number of particles in a substance. A mole is the Ar of the substance, but in grams.

The number of moles in a substance = Mass of the substance / Mass of one mole

Mass of the substance = Number of moles in a substance X Mass of one mole

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Yield & Atom Economy

Yield is the amount of product made. The % yield=Actual Yield/Theoretical Yield X100

Atom economy is the amount of product that ends up as useful after a reation.

Atom economy = Mr of useful products / Mr of reactants X 100

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Reversible Reactions

Some reactions are reversible. That means the products can react to produce the orignal reactants.

A + B [reversible reaction sign] C + D

A and B react to produce C and D, but C and D can also react to produce A and B.

If the forward reaction is equal to the backward reaction, it is said to be in equilibrium.

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The Haber Process

The Haber Process is:

N2 (g) + 3H2 (g) [reversible reaction sign] 2NH3 (g)

If the pressure is increased the eqm. (equilibrium) moves right because there's less moles and if the pressure is decreased the eqm. moves left because there's more moles of gas. Consequently, the Haber Process uses low temperatures and high pressures. This way it favours an exothermic reaction, moves forward and in the direction where there are less moles (so therefore, makes more product). However, the temperature can't be too low as the rate of reaction would be too low and the pressure can't be too high because it's dangerous and expensive. Therefore a temperature of 450C and a pressure of 200 atmospheres is used.

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Rates Of Reaction

The rates of rections can be changed by changing the:

temperature (higher temp. = more kinetic energy = more collisions with enough activation energy = faster rate of reaction)

concentration (higher concentration = more particles = more collisions = faster rate of reaction)

surface area (larger surface area = more surface exposed = more surface to react with)

or by using a catalyst (provides an alternative pathway to a reaction - has a lower activation energy)

Rate of reaction = (Amount of product used / product formed) / Time

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Endothermic & Exothermic reactions

An endothermic reaction takes in heat whereas an exothermic reaction gives off heat.

exothermic examples: combustion, neutralisation, oxidation

endothermic examples: dissolving ammonium nitrate crystals in water, thermal decmposition.

If a reaction is reversible, it is always endothermic one way and exothermic the other.

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Electrolysis is the breaking down of an ionic compound into into it's elements using an electrical current. During electrolysis ions gain or lose electrons forming electrically neutral atoms.

The anode is the positive electrode and the cathode the negative electrode. Positive ions move to the cathode and are reduced. Negative ions move to the anode and are oxidised. REMEMBER: Oxidation Is Lost Reduction Is Gained. (OILRIG) Electrolysis has to take place in solution or in molten as the electrons can then move freely. The products formed depend on the reactivity of the elements involved.

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reading through was good... im doing c3 and c2 ; <

I know more than i think i do.. if that make sense : )


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Thankyou !


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thankyou for this, was very helpful :):):):)


cheers man!!!!


Thanks this was really good, it covers almost everything i think :) thanks again, btw is this for the unit 2 exam coming up in 2012?


are you sure you've covered everythingg?? bcoz i'm relying on this!!!!!!!


this is kewl

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