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Summary OCR A Level Chemistry Notes Module 5
Covers the full OCR A level specification with notes from past paper mark schemes, textbooks and examples.
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Module 55.1.1
Rate of reaction: Rate of reaction is the change in concentration of a reactant or product in a given time
Rate = Change in concentration/change in time
Units: mol dm-3 s-1
Order: Order shows how rate is affected by concentration
- Zero Order: Rate is not affected by concentration
Note: Zero order may be due to a reactant being in excess because then the concentration of that reactant would
be constant
- First order: Rate changes by the same factor as a concentration change
- Second order: Rate changes by the same factor as a concentration change raised to the power of 2
- Note: Changing the pressure would affect the rate proportionally depending on the orders showing the same
relationship as changing the concentration does
Rate = k[A]m[B]n, where m and n are 0, 1 or 2
Overall order: Overall order is the sum of the individual orders of the reactants
Rate constant: The rate constant k is the proportionality constant in the rate equation
Half-life: The half-life is the time for the concentration of a reactant to fall to half its original value
Concentration-time graphs can be plotted from continuous measurements taken during the course of a reaction
(continuous monitoring). The graphs below show the concentration of the reactant
- Zero order: The straight line shows that the reaction rate does not change during the course of the
reaction because gradient is constant. Gradient = rate = k
- First order: The gradient decreases over time as the reaction gradually slows down. A first order has a
constant half-life. Gradient = rate
Rate-concentration data can be obtained from initial rates investigation of separate experiments using different
concentrations of one of the reactants
- Zero order: Rate does not change with concentration as it is a
straight horizontal line with zero gradient. As rate = k, the intercept on the y-axis gives the rate
constant
- First order: Rate is directly proportional to the concentration. Straight, diagonal line through origin.
Rate constant = gradient
- Second order: The rate constant cannot be determined directly from this graph
Initial Rates Method
The initial rate is the instantaneous rate when time= 0. The initial rate can be found by measuring the gradient of
the tangent drawn at t=0 on a concentration-time graph.
The reaction is repeated several times whilst varying concentrations of one reactant and keeping the concentration
of the other reactants the same and values of 1/t are calculated for each experimental run. The initial rate is
proportional to 1/t. The graphs of 1/t against concentration are plotted for each reactant and the shape of the rate-
concentration graph determines the order with respect to each reactant.
Clock reactions are a more convenient way of obtaining the initial rate and are an approximation of the initial
rates method, where the time is measured such that the reaction has not proceeded too far.
Continuous monitoring including the use of calorimetry
, During the reaction, the concentration of a reactant can be monitored continuously using a calorimeter. The
concentrations of the other reactants are kept constant. A calorimeter measures the intensity of light passing
through a sample. The filter is chosen to be the complementary colour to the colour being absorbed in the
reaction. The absorbance reading from the calorimeter is recorded and absorbance is directly linked to the
concentration of the solution because the absorbance decreases as the concentration of the solution decreases.
Continuous monitoring can also measure the volume of gas or mass loss over time
Rate-determining step: The rate-determining step is the slowest step in the reaction mechanism of a multi-step
reaction
- For a multi-step reaction, you should be able to predict a rate equation that is consistent with the rate-determining
step and any preceding steps prior to the RDS (once intermediates are cancelled).The stoichiometric values of the
reactants in the rate determining step and any preceding steps determine the orders in respect to each reactant as
the order is the number of molecules of each reactant in the rate-determining step
- Many reactions are multi-step reactions because it is unlikely that some reactions could take place in one step,
shown if the stoichiometry in the rate equation does not match the stoichiometry in the overall equation and also
collision is unlikely with more than two species
Increasing the temperature increases the rate constant. The rate constant is proportional to the rate of
reaction. An increase in the rate constant means an increase in the rate of reaction
The Arrhenius equation shows the exponential relationship between the rate constant, k and temperature, T
- Ea = Activation energy (ALWAYS use Joules in the Arrhenius equation calculations)
- A = Pre-existing factor
- R = Gas constant
- T = Temperature, in kelvins
5.1.2
A homogenous equilibrium is where species all have the same state
A heterogenous equilibrium is where species have different states
The concentrations of solids and liquids are constant therefore are omitted from the K c expressions
Mole fraction: The mole fraction of a gas is the same as its proportion by volume to the total volume of gases in a
gas mixture. The sum of the mole fractions of the gases in a gas mixture must equal one.
Mole fraction = Number of moles of reactant or product/ total number of moles in gas mixture
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