Topic 16.1: Chemical kinetics – Rate expression and reaction mechanism
Rate expressions can only be determined empirically and these limit possible reaction mechanisms. In particular cases, such as a linear
chain of elementary reactions, no equilibria and only one significant activation barrier, the rate equation is equivalent to the slowest step of
the reaction.
• Understanding: Reactions may occur by more than one step and the slowest step determines the rate of reaction (rate determining
step/RDS).
▪ Reaction mechanism: sequence of reaction steps of a reaction pathway
from reactants to products
▪ Rate determining step: the slow step in a reaction mechanism that
determines the rate of reaction
• RDS has a higher activation energy
• Understanding: The molecularity of an elementary step is the number of
reactant particles taking part in that step.
▪ Molecularity: number of reactant particles taking part in an elementary
step of a reaction mechanism
• Unimolecular: single molecule involved in the reaction
• Bimolecular: two molecules involved in collision of reaction
• Termolecular: three molecules involved in collision of
reaction; unlikely to occur due to low probability of collision
• Understanding: The order of a reaction can be either integer or fractional in nature. The order of a reaction can describe, with
respect to a reactant, the number of particles taking part in the rate-determining step.
▪ Overall order of reaction: sum of the orders for each reactant
▪ Order of reaction: indicates how changing the concentration of a reactant affects the rate
• Relationship with rate determining step: determines the number of particles taking part in the rate determining step
• Understanding: Rate equations can only be determined experimentally.
▪ The rate of the overall reaction is equal to the rate of the slow step
▪ Has to be determined experimentally as a reaction may contain more than one pathway
• Understanding: The value of the rate constant (k) is affected by temperature and its units are determined from the overall order of
the reaction.
▪ Rate (mol dm-3 s-1) = k[A]m[B]n
• [A] = concentration of reactant A (in mol dm-3)
• [B] = concentration of reactant B (in mol dm-3)
• m = order with respect to reactant A
• n = order with respect to reactant B
• k = rate constant (unit is dependent on overall order)
▪ k = Ae-Ea/RT
• A = frequency factor
• Ea = activation energy
• Rate constant with increasing temperature: magnitude of the rate constant will increase as absolute temperature
increases; greater proportion of molecules reach the activation energy at higher temperature (topic 6.1)
• Understanding: Catalysts alter a reaction mechanism, introducing a step with lower activation energy.
▪ Catalysts: substance that increase the rate of a chemical reaction but are not consumed in the reaction itself
• Catalysts provide an alternative pathway for a reaction to lower the activation energy for the slow step
• Applications and skills: Deduction of the rate expression for an equation from experimental data and solving problems involving
the rate expression.
▪ Deduction of rate expression form experimental data
• When concentration is doubled
▪ Zero order: rate does not increases
▪ First order: rate ∝ (concentration)
▪ Second order: rate ∝ (concentration)2
▪ Solving problems involving the rate expression
• Rate (mol dm-3 s-1) = k[A]m[B]n
• Substitute given values and identified the unknown parameter
• Applications and skills: Evaluation of proposed reaction mechanisms to be consistent with kinetic and stoichiometric data.
▪ Process of evaluating proposed reaction mechanisms
• Deduce the overall reaction and identify any intermediates
Rate expressions can only be determined empirically and these limit possible reaction mechanisms. In particular cases, such as a linear
chain of elementary reactions, no equilibria and only one significant activation barrier, the rate equation is equivalent to the slowest step of
the reaction.
• Understanding: Reactions may occur by more than one step and the slowest step determines the rate of reaction (rate determining
step/RDS).
▪ Reaction mechanism: sequence of reaction steps of a reaction pathway
from reactants to products
▪ Rate determining step: the slow step in a reaction mechanism that
determines the rate of reaction
• RDS has a higher activation energy
• Understanding: The molecularity of an elementary step is the number of
reactant particles taking part in that step.
▪ Molecularity: number of reactant particles taking part in an elementary
step of a reaction mechanism
• Unimolecular: single molecule involved in the reaction
• Bimolecular: two molecules involved in collision of reaction
• Termolecular: three molecules involved in collision of
reaction; unlikely to occur due to low probability of collision
• Understanding: The order of a reaction can be either integer or fractional in nature. The order of a reaction can describe, with
respect to a reactant, the number of particles taking part in the rate-determining step.
▪ Overall order of reaction: sum of the orders for each reactant
▪ Order of reaction: indicates how changing the concentration of a reactant affects the rate
• Relationship with rate determining step: determines the number of particles taking part in the rate determining step
• Understanding: Rate equations can only be determined experimentally.
▪ The rate of the overall reaction is equal to the rate of the slow step
▪ Has to be determined experimentally as a reaction may contain more than one pathway
• Understanding: The value of the rate constant (k) is affected by temperature and its units are determined from the overall order of
the reaction.
▪ Rate (mol dm-3 s-1) = k[A]m[B]n
• [A] = concentration of reactant A (in mol dm-3)
• [B] = concentration of reactant B (in mol dm-3)
• m = order with respect to reactant A
• n = order with respect to reactant B
• k = rate constant (unit is dependent on overall order)
▪ k = Ae-Ea/RT
• A = frequency factor
• Ea = activation energy
• Rate constant with increasing temperature: magnitude of the rate constant will increase as absolute temperature
increases; greater proportion of molecules reach the activation energy at higher temperature (topic 6.1)
• Understanding: Catalysts alter a reaction mechanism, introducing a step with lower activation energy.
▪ Catalysts: substance that increase the rate of a chemical reaction but are not consumed in the reaction itself
• Catalysts provide an alternative pathway for a reaction to lower the activation energy for the slow step
• Applications and skills: Deduction of the rate expression for an equation from experimental data and solving problems involving
the rate expression.
▪ Deduction of rate expression form experimental data
• When concentration is doubled
▪ Zero order: rate does not increases
▪ First order: rate ∝ (concentration)
▪ Second order: rate ∝ (concentration)2
▪ Solving problems involving the rate expression
• Rate (mol dm-3 s-1) = k[A]m[B]n
• Substitute given values and identified the unknown parameter
• Applications and skills: Evaluation of proposed reaction mechanisms to be consistent with kinetic and stoichiometric data.
▪ Process of evaluating proposed reaction mechanisms
• Deduce the overall reaction and identify any intermediates
