Concentration-Time Graphs (College Board AP® Chemistry)

Concentration-Time Graphs

• The reaction order can be inferred from a graph of concentration of reactant vs time

• Zero-order, first-order and second-order reactions have characteristic concentration vs time graphs for each of them

• In a zero-order reaction, the concentration of a reactant is inversely proportional to time

  • The graph is a straight line that goes down as shown below

  • The slope of a zero order reaction is the rate constant

Concentration-time graph of a zero-order reaction

A concentration-time graph of a zero-order reaction shows that concentration is inversely proportional to time

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

  • The graph is a curve that goes downwards and eventually levels out, as shown below:

Concentration-time graph of a first-order reaction

A concentration-time graph of a first-order reaction curves downwards

• In a second-order reaction, the concentration of the reactant decreases more steeply with time

  • The graph is a curve that goes downwards, however the decrease in concentration is steeper and it eventually plateaus, as shown below:

Concentration-time graph of a second-order reaction

A concentration-time graph of a second-order reaction shows a downward curve with a steeper slope than the curve for a first-order reaction