# 5.2.7 Rate-Concentration Graphs

### 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
• 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]
• 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

#### Exam 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.

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