Reaction Orders (HL) (DP IB Chemistry): Revision Note

Reaction orders

How to determine reaction orders from graphs

• Reaction orders can be determined by using graphical representations of experimental data

• Two types of graphs are commonly used:

  1. Concentration-time graphs 

  2. Rate-concentration graphs

• Rate-concentration graphs show the distinction between zero, first and second order more clearly than concentration-time graphs

Reaction order using concentration-time graphs

• These graphs show how the concentration of a reactant changes over time

• The shape of the graph helps identify the order of reaction with respect to that reactant

Zero-order concentration-time graphs

• In a zero-order reaction, the concentration of the reactant decreases at a constant rate over time

  • The graph is a straight line sloping downwards

  • The gradient of the line is the rate of reaction

• The rate is independent of reactant concentration:

Rate = k

First-order concentration-time graphs

• In a first-order reaction, the concentration of the reactant decreases exponentially over time

  • The graph is a downward curve that gradually flattens

  • The rate of reaction decreases over time as concentration falls

• The rate is directly proportional to the reactant’s concentration:

Rate = k[A]

Second-order concentration-time graphs

• In a second-order reaction, the concentration decreases more steeply at first than in a first-order reaction

  • The graph is a steep downward curve

• The rate is proportional to the square of the concentration:

Rate = k[A]²

Reaction order using rate-concentration graphs

• These graphs show how the rate of reaction changes as the concentration of a reactant changes

• The shape of the graph helps identify the order of reaction with respect to that reactant

Zero-order rate-concentration graphs

• In a zero-order reaction, the rate is independent of the reactant’s concentration

  • The rate remains constant throughout

  • The graph is a horizontal line

• The rate is independent of reactant concentration:

Rate = k

First-order rate-concentration graphs

• In a first-order reaction, the rate is directly proportional to the reactant concentration

  • The graph is a straight line through the origin

• The rate is proportional to [A]:

Rate = k[A] 

Second-order rate-concentration graphs

• In a second-order reaction, the rate is proportional to the square of the concentration

  • As concentration increases, the rate increases more steeply

  • The graph is a curve starting from the origin

• The rate is proportional to [A]²:

Rate = k[A]²

Summary: Comparing reaction orders

Zero order

• Rate does not change when concentration changes

• Concentration–time graph: straight line

• Rate–concentration graph: horizontal line

• Rate = k

First order

• Rate doubles when concentration doubles

• Concentration–time graph: smooth curve that flattens

• Rate–concentration graph: straight line through origin

• Rate = k[A]

Second order

• Rate quadruples when concentration doubles

• Concentration–time graph: steep curve

• Rate–concentration graph: curved line from origin

• Rate = k[A]²