Rate equations (A-level only) common mistakes

Common mistakes

• Confusing rate constant with rate

  Students often confuse the rate constant (k) with the rate of reaction.

  Fix itRemember that the rate constant is a fixed value for a given reaction at a specific temperature, while the rate can change based on reactant concentrations.

• Incorrectly identifying the order of a reaction

  Students often confuse the coefficients in a balanced equation with the orders in the rate equation.

  Fix itRemember that the order must be determined experimentally and is not necessarily related to the stoichiometric coefficients.

• Incorrect unit for rate constant

  Confusing the units of k for different orders of reaction.

  Fix itAlways derive the units of k based on the overall order of the reaction to ensure accuracy.

• Confusing first-order reactions with second-order reactions

  Students often confuse the characteristics of first-order reactions with those of second-order reactions, particularly regarding half-life.

  Fix itRemember that for first-order reactions, the half-life is constant, while for second-order reactions, the half-life depends on the initial concentration.

• Confusing rate constant with rate

  Students often confuse the rate constant (k) with the rate of the reaction.

  Fix itRemember that the rate constant is a fixed value for a reaction at a specific temperature, while the rate can change based on the concentrations of reactants.

• Confusing fast and slow steps

  Students often mistakenly identify the fastest step as the rate-determining step.

  Fix itRemember that the rate-determining step is the slowest step, which controls the overall reaction rate.

• Confusing rate with rate constant

  Students often confuse the rate of a reaction with the rate constant (k).

  Fix itRemember that the rate is dependent on the concentrations of reactants, while the rate constant is a fixed value at a specific temperature.

• Confusing order of reaction with stoichiometry

  Students often assume that the order of reaction is the same as the stoichiometric coefficients in the balanced equation.

  Fix itRemember that the order of reaction is determined experimentally and may differ from the coefficients in the balanced equation.

• Confusing rate-determining step with the fastest step

  Students often think the rate-determining step is the fastest step in the mechanism.

  Fix itRemember that the rate-determining step is the slowest step that controls the overall reaction rate.