Rate-Concentration Graphs
Order of reaction from initial rate
- The progress of the reaction can be followed by measuring the initial rates of the reaction using various initial concentrations of each reactant
- These rates can then be plotted against time in a rate-time graph
- In a zero-order reaction the rate doesn’t depend on the concentration of the reactant
- The rate of the reaction therefore remains constant throughout the reaction
- The graph is a horizontal line
- The rate equation is rate = k
Rate-concentration graph of a zero-order reaction
- In a first-order reaction the rate is directly proportional to the concentration of a reactant
- The rate of the reaction increases as the concentration of the reactant increases
- This means that the rate of the reaction decreases as the concentration of the reactant decreases when it gets used up during the reaction
- The graph is a straight line
- The rate equation is rate = k [A]
Rate-concentration graph of a first-order reaction
- In a second-order reaction, the rate is directly proportional to the square of concentration of a reactant
- The rate of the reaction increases more as the concentration of the reactant increases
- This means that the rate of the reaction decreases more as the concentration of the reactant decreases when it gets used up during the reaction
- The graph is a curved line
- The rate equation is rate = k [A]2
Rate-concentration graphs of a second-order reaction
Examiner Tip
Careful - sometimes when asked to complete calculations for k, the exam question will give you a graph which demonstrates the order of one of the reactants, as well as tabulated data to determine the order for the other reactants. Do not ignore the graph.