Equilibria, Energetics and Elements

Comprehensive revision notes on all of OCR chemistry Unit 5

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  • Created on: 25-05-12 13:42
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A2 Unit F325: Equilibria, Energetics
and Elements
Module 1: Rates Equilibrium and pH
How Fast?
Rates graphs and Orders
Rate of Reaction:
o Measures the rate of formation of a product or the rate of removal of a product
o The time taken for the concentration of one of the reactants to fall by half
Rate determining step
o In a multi-step reaction this is the slowest step- the one with the highest
activation energy
Activation energy
o Minimum amount of energy that particles need to have if a collision between
them is to result in a reaction
The rate of reaction can be found by measuring the gradient of a concentration/time
graph (using a tangent if it's a curve).
If the gradient at several concentrations is calculated and a graph of rate against
concentration is plotted:
These graphs show how the rate of reaction varies with respect to a particular reactant
and is called its order.
The half-life of a first order reaction is independent of the concentration
[A] is the concentration of A
First order reactions
A reaction is first order with respect to a reactant if the rate of the reaction is proportional to the
concentration of that reactant. The concentration term for this reactant is raised to the power of 1 in
the rate equation.

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Page 2

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This means that doubling the concentration of a reactant doubles the rate of reaction. Rate is
proportional to [reactant]. This means a plot of rate vs. concentration produces a straight line
through the origin.
If you plot a concentration vs. time graph and then work out the half-life of the reaction then at
a constant temperature, the half-life of a first order reaction will be equal wherever it is measured
on the graph.…read more

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The rate vs. concentration graph for a second order reaction produces a curve.
Zero Order Reactions
A reaction that is zero order with respect to a reactant is unaffected by concentration. In a zero
order reaction, the concentration term for the reactant is raised to the power of zero.
The Initial Rate Method
This is based on finding the rate immediately after the start of the reaction; when all the
concentrations are known. Different mixtures are made up with differing initial concentrations.…read more

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Using experiments 1 and 2, a halving of [HCl] produces almost exactly a halving of the rate, so
the reaction is first order with respect to the acid catalyst [H+]. Using reactions 1 and 3, a halving of
[propanone] produces approximately a halving of the rate. The reaction is first order with respect to
[propanone]. Comparing reactions 1 and 4, a halving of [iodine] produces approximately no change
in the rate, so the reaction is zero order with respect to [iodine].…read more

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The shaded areas show the proportions of particles having at least the activation energy at 2
different temperatures. So at higher temperatures, there are more molecules with enough energy to
react when they collide, so the rate of reaction is higher.
Rate Determining Steps and Multi-Step Reactions
Most reactions do not take place in one step as suggested in a balanced chemical
equation, but occur in a number of steps.…read more

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This led to the suggestion of a one stage reaction in which both reactants are involved in
a single rate determining step:
In this substitution reaction, the nucleophile is the hydroxide ion. Chemists label this
mechanism SN2 ­ the two shows that there two species involved in the rate-determining
On the other hand, the hydrolysis of tertiary halogenoalkanes is 1st order overall (first
order with respect to the halogenoalkanes and zero order with respect to the OH- ions).…read more

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How Far
Reversibility and Equilibrium
In general, for a reversible reaction at equilibrium:
This is the form for the equilibrium constant KC, where the concentrations of the
reactants and products are measured in moldm-3. [A], [B] are the equilibrium
Equilibrium constants are constant at a fixed temperature.…read more

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Make sure that the KC expression always relates the balanced chemical equation.
Equilibrium constants and the direction of change
If the value of KC is large then the position of equilibrium lies to the right.
On the other hand if KC is small then the position of equilibrium lies to the left.
If KC equals 1 then the position of equilibrium is in the middle.
KC gives no indication of how long it will take for equilibrium to be reached.…read more

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Acids, Bases and Buffers
Brønsted-Lowry Acids and Bases
An acid is a proton donor
A base is a proton acceptor
Acid Metal/Ammonium Name/Formula of Salt Comment/Explanation
Ion formed
H2SO4 Mg2+ Magnesium sulphate, Both H+ ions in the acid are replaced.…read more

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A strong acid is one which dissociates fully in solution
A weak acid is one which only partially dissociates in solution
pH scales
( )
Ionic Product of Water
There are H+ and OH- ions in pure water because of the transfer of hydrogen ions
between water molecules.
Or more simply:
Therefore the equilibrium constant:
[ ( ) ][ ( )]
There is however a very large excess of water, so [H2O (l)] can be taken to be a constant.…read more


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