AQA Chem2 Revision Notes

My revision notes for AQA AS Chemistry Unit 2, minus Haloalkanes, Alkenes, Alcohols & Analytical Techniques, mostly due to curly arrows and mechanisms being much easier to handwrite on paper!

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  • Created by: Bryony
  • Created on: 21-05-11 14:33
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CHEM2 REVISION
ENERGETICS
Enthalpy ­ energy per mole
ENDOTHERMIC
Endothermic describes a reaction in which heat energy is taken in as the reactants change to products; thus
the temperature drops
The products are less stable than the reactants; they have more energy
H = +ve
EXOTHERMIC
Exothermic describes a reaction in which heat energy is released as the reactants change to products; thus
the temperature rises
The products are more stable; the lower the chemical potential energy of a given chemical species, the more
stable it is
H = -ve
q = mcT
ENERGY REQUIRED = MASS HEATED X SPECIFIC HEAT CAPACITY X TEMPERATURE RISE
Specific heat capacity is the amount of heat required to heat 1g of a substance by 1K
H = q/mols
ENTHALPY CHANGE = ENERGY REQUIRED/MOLES
HESS'S LAW
Hess's law states that the enthalpy change for a chemical reaction is the same, whatever route is taken from
reactants to products
ENTHALPY OF FORMATION
The energy change when one mole of substance is formed from its elements in their standard state at 298K
and 100kPa
ENTHALPY OF COMBUSTION
The energy change when one mole of substance is completely burnt in a plentiful supply of oxygen, under
standard conditions of 298K and 100kPa

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Standard enthalpies of combustion almost always have negative values, as burning a substance in oxygen is
almost always exothermic
BOND DISSOCIATION ENTHALPY
The energy required to separate completely the atoms in one mole of covalent bonds
MEAN BOND ENTHALPIES
H = bonds broken ­ bonds made
Mean bond enthalpy means the energy that must be put in to break a particular bond.…read more

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KINETICS
COLLISION THEORY
COLLISION FREQUENCY
If a chemical reaction is to take place between two particles, they must collide first
The number of collisions between particles per unit time is known as collision frequency
COLLISION ENERGY
Unsuccessful collisions happen when the colliding species do not have enough energy to break the
necessary bonds
If they do not have sufficient energy, the collision will not result in a chemical reaction
The combined energy of the colliding particles is known as the collision energy
MAXWELL-BOLTZMANN DISTRIBUTION
The…read more

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PARTICLE SIZE
If a sample of a solid is crushed into smaller pieces, the surface area of the solid will increase
More of the solid particles are exposed to the other reactants, and there is more likely to be a collision;
collision frequency increases
TEMPERATURE
COLLISION ENERGY
The particles have greater mean kinetic energy, and so collision energy is increased
It follows that an increase in temperature will increase the number of colliding particles with an energy equal
to or greater than the activation energy…read more

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EQUILIBRIA
DYNAMIC
The forward and reverse reactions are both taking place simultaneously and at the same rate
EQUILIBRIUM
The reactions take place at the same speed; the concentration of all reactants and products remain
constant
POSITION OF EQUILIBRIUM
Equilibrium lying to the left means there are more reactants than products
Equilibrium lying to the right means there are more products than reactants
E.g.…read more

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WHEN THE TEMPERATURE IS INCREASED, THE SYSTEM WILL MOVE IN THE ENDOTHERMIC DIRECTION IN ORDER TO DECREASE THE
TEMPERATURE.…read more

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CATALYST
Catalysts increase rate of reaction without affecting position of equilibrium; the reaction therefore takes
place in the presence of a V2O5 catalyst, which ceases to be effective at very low temperatures, placing a
further constraint on the minimum temperature which can be used
Optimum conditions are 400° and a little above atmospheric pressure.…read more

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Chosen temperature is 300°C and chosen pressure is 65atm, and concentrated phosphoric acid catalyst
used
Yield achieved is only 5% but reactants can be recycled
PRODUCTION OF METHANOL
CO(g) + 2H2(g) CH3OH(g) H forward reaction = exothermic
Best yield obtained by using low temperature and high pressure
Compromise conditions of 230°C and 100atm used, and copper catalyst
Yield is only 5%, but reactants can be recycled
CARBON NEUTRALITY
A carbon neutral process is one which has no net carbon dioxide emissions into the atmosphere
Methanol…read more

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Oxidation Is Loss (of electrons)
Reduction Is Gain (of electrons)
OXIDATION NUMBERS
Oxidation numbers (or states) tell you how many electrons an atom has lost or gained
Oxidation occurs when the oxidation state of an element increases (becomes more positive)
Reduction occurs when the oxidation state of an element decreases (becomes more negative)
RULES FOR WORKING OUT OXIDATION STATES
0 = oxidation state of an uncombined element
0 = sum of the oxidation states of elements in an uncharged formula
Charge of formula = sum…read more

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THE RULES FOR WRITING HALF EQUATIONS
Only one element can gain or lose electrons
The half equation must balance for atoms
The equation must balance for charge (by adding e- or H+)
If your constructed half equation doesn't satisfy all these rules, rewrite it
We can add things to our equations to make everything balance:
All you are allowed to add is:
Electrons ­ to balance charge
Water ­ if you need a source of oxygen (will produce H+)
Hydrogen ions (H+) ­ if there…read more

Comments

Thivakar

Hey there, this is some really good notes. Thanks for uploading it :) 

fboco

hey 

Awesome notes thanx

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