Concentration-Time Graphs (AQA A Level Chemistry): Revision Note
Exam code: 7405
Concentration-Time Graphs
Order of reaction from concentration-time graphs
In a zero-order the concentration of the reactant is inversely proportional to time
This means that the concentration of the reactant decreases with increasing time
The graph is a straight line going down
In a first-order reaction, the concentration of the reactant decreases with time
The graph is a curve going downwards and eventually plateaus
In a second-order reaction, the concentration of the reactant decreases more steeply with time
The concentration of the reactant decreases more with increasing time compared to a first-order reaction
The graph is a steeper curve going downwards
Initial Rates Method
The Initial Rate Method
The initial rate method is used to gather experimental data and to determine the order with respect to the reactants in the reaction
The initial rate of a reaction is the rate right at the start of the reaction
This is used because right at the start of the reaction, we know the exact concentration of the reactants used
This method refers to the method of initial rates, which involves carrying out a series of experiments to determine the rate equation
During the experiments, the temperature must be kept constant
In each experiment, the concentration of only one reactant is changed, while the concentrations of all other reactants are kept constant
The experiments are designed so that the results can be used to determine the order of reaction with respect to each reactant
For each experiment, a concentration–time graph is plotted
The initial rate is found by drawing a tangent to the curve at time t = 0 and calculating its gradient
The gradient of the tangent at t = 0 represents the initial rate of the reaction
General example of applying the initial rates method
Consider the general reaction:
2A + B + C → products
To determine how each reactant affects the initial rate, a series of experiments is carried out at different concentrations
First, carry out an experiment using fixed concentrations of A, B, and C
In a second experiment, change only the concentration of A, keeping the concentrations of B and C the same as in the first experiment
In a third experiment, change only the concentration of B, keeping the concentrations of A and C constant
This process is repeated so that each reactant is varied individually
For each experiment, plot a concentration–time graph and draw a tangent at t = 0 to calculate the gradient
This gradient represents the initial rate.
The results are then tabulated. By comparing how changes in concentration affect the initial rate, the order of reaction with respect to each reactant can be determined, allowing the rate equation to be written
Table of the results collected for the reaction
Experiment | Initial [A] / mol dm-3 | Initial [B] / mol dm-3 | Initial [C] / mol dm-3 | Initial Rate / mol dm-3s-1 |
|---|---|---|---|---|
1 | 1.5 x 10-3 | 1.5 x 10-3 | 1.5 x 10-3 | 2.1 x 10-3 |
2 | 3.0 x 10-3 | 1.5 x 10-3 | 1.5 x 10-3 | 2.1 x 10-3 |
3 | 1.5 x 10-3 | 3.0 x 10-3 | 1.5 x 10-3 | 4.2 x 10-3 |
4 | 1.5 x 10-3 | 3.0 x 10-3 | 3.0 x 10-3 | 1.7 x 10-2 |
5 | 4.5 x 10-3 | 4.5 x 10-3 | 1.5 x 10-3 | 6.3 x 10-3 |
6 | 6.0 x 10-3 | 6.0 x 10-3 | 4.5 x 10-3 | 3.4 x 10-2 |
Rate Constant & Zero Order Graphs
Finding the Rate Constant of a Zero-Order Reaction
As shown previously, a zero-order reaction will give the following concentration-time graph
In a zero-order reaction, the rate of reaction remains constant over time
If you calculate the gradient at different points on a concentration–time graph, the value will be the same
Since the reaction is zero order with respect to the reactant, changing its concentration does not affect the rate of the reaction
Therefore:
Rate = k
The rate of reaction is equal to the gradient of the concentration–time graph. Because the rate is constant, the rate constant, k, is equal to the gradient of the graph
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