Comprehensive study guide for Chemistry A Level, made by an Oxford Biochemistry student with all 9s at GCSE and 3 A*s at A Level! Information arranged by spec point. Notes written using past papers, textbooks and more.
The rate of reaction looks at the speed at which reactants are converted into
products.
- It is a measure of how quickly a reactant is used up, or a product is formed,
for example by looking at changes in the concentration/mass/volume/etc. of a
reactant or product per unit time.
ii. rate equation
The rate equation for a chemical reaction is an equation that links the initial or
forward reaction rate with the concentrations or pressures of the reactants and
constant parameters.
- E.g. rate = k[NO]2[O2]
- N.B. a rate equation can only include the reactants and catalyst, not
intermediates.
iii. order with respect to a substance in a rate equation
Order with respect to a substance in a rate equation rate = k[A] n
- When n = 2, it is described as second order with respect to reactant A.
o If the concentration of A is doubled, the rate would increase by four
times.
o If the concentration of A is tripled, the rate would increase by nine
times.
- When n = 1, it is described as first order with respect to reactant A.
o If the concentration of A is doubled, the rate would double.
o If the concentration of A is tripled, the rate would triple.
- When n = 0, it is described as zero order with respect to reactant A.
o Rate = k[A]0 = k
o A change in the concentration of reactant A has no effect on the
reaction rate.
o The rate constant k can be calculated from the intercept with the y-
axis on a rate vs concentration graph.
iv. overall order of reaction
The overall order of reaction = the sum of the orders with respect to each
substance that appears in the rate equation.
- In the rate equation: rate = k[A]2[B], the overall order of reaction will be 2 + 1
= 3.
, v. rate constant
The rate constant for a reaction quantifies the rate of a chemical reaction.
- In a simple equation A products (e.g. thermal decomposition reaction), the
rate of reaction will be directly proportion to the concentration of A:
o Rate α [A]
o [A] = concentration of A
- Where all the reactants and products are gases or are aqueous, we can
replace the proportionality sign to write a rate equation in the form:
o Rate = k[A]
o The coefficient k is called the rate constant for the reaction. It is
independent of concentration but dependent on temperature and the
addition of a catalyst.
o k also changes depending on the reaction, where a reaction with a
greater activation energy will have a smaller k.
The units of the rate constant, k, depend on the overall order of the reaction.
- The definition of rate is the change in concentration per unit time.
- Therefore the units for rate are mol dm-3 s-1.
- We need to manipulate the units of k in order to make sure the overall units
are mol dm-3 s-1.
Order with Overall
Rate Order with Units of rate
respect to order of the
equation: respect to A constant, k
B reaction
Half-life = the time taken for the concentration of a given reactant to
decrease by a half.
- Successive half-lives of a reactant that is zero order (n = 0) get smaller.
- Successive half-lives of a reactant that is first order (n = 1) remain the same.
- Successive half-lives of a reactant that is second order (n = 2) get larger.
vii. rate-determining step
, Most reactions involve multiple elementary steps, each of which occur at
different rates.
- The step which determines the overall rate of reaction is aptly named the
rate-determining step.
- This will be the step with the slowest rate of reaction.
viii. activation energy
The activation energy is the energy required for a collision between reactant
particles to result in a successful reaction.
ix. heterogeneous and homogenous catalyst
Homogenous catalysis – the catalyst is in the same phase as the reactants.
Heterogenous catalysis – this involves the use of a catalyst in a different phase
from the reactants.
- This often involves a solid catalyst with the reactants as either liquids of
gases.
- A phase is different from the term ‘state.’
o State is either solid, liquid or gas.
o A phase of matter is uniform with respect to its physical and chemical
properties.
o For example, saltwater has a phase (aqueous) but not a state. The
salt is solid and the water is liquid.
o There is no ‘aqueous’ state, whereas there is an ‘aqueous’ phase.
2. be able to determine and use rate equations of the form: rate =
k[A]m[B]n, where m and n are 0, 1 or 2
Writing rate equations
- In a reaction mA + nB products, the rate equation would be written as:
o Rate = k[A]m[B]n
- The rate equation for C4H9Br (aq) + OH– (aq) C4H9OH (aq) + Br– (aq) would
be written as:
o Rate = k[C4H9Br (aq)][OH– (aq)]2
o N.B. this form on writing rate equations is only applicable if the
reaction occurs in one elementary step. In the reaction C 3H8 (g) + 5 O2
(g) 3 CO2 (g) + 4 H2O (g), there are multiple elementary steps.
Therefore we cannot simply write the rate equation as rate = k[C 3H8
(g)][O2 (g)]5
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