Topic 6.1: Chemical Kinetics – Collision theory and rates of reaction
The greater the probability that molecules will collide with sufficient energy and proper orientation, the higher the rate of reaction.
• Understanding: Species react as a result of collisions of sufficient energy and proper orientation.
▪ Collision theory: model that helps understanding of why rate of reaction
depends on temperature
• Collision must occur – two particles must collide (physically be in
contact) for the reaction to occur
• Collision in correct orientation – colliding particles must have the
correct orientation (wrong orientation will not result in reaction)
• Collision with sufficient energy – colliding particles must have
sufficient kinetic energy to initiate reaction (activation energy)
• Understanding: The rate of reaction is expressed as the change in concentration of a particular reactant/product per unit time.
▪ Rate of reaction: change in concentration of reactant or product per unit time
• Unit: mol dm-3 s-1 (or other unit for time)
• Understanding: Concentration changes in a reaction can be followed indirectly by monitoring changes in mass, volume and colour.
▪ Means of monitoring concentration changes indirectly
• Change in mass: reaction involving solids (and gases)
▪ Graph y-axis: mass of flask + content (or mass loss)
▪ Apparatus: periodic measurement through electronic balance
▪ Example: CaCO3 (s) + HCl (aq) → CaCl2 (aq) + CO2 (g) + H2O (l)
• Change in volume: reaction involving gases
▪ Graph y-axis: volume of gas
▪ Apparatus: measurement through inverted measuring cylinder or data logger
▪ Example: CaCO3 (s) + HCl (aq) → CaCl2 (aq) + CO2 (g) + H2O (l)
• Change in conductivity: reaction involving electrolytes
▪ Graph y-axis: electrical conductivity
▪ Apparatus: measurement through conductivity probe and meter from net change in number of ions
▪ Example: IO3- (aq) + 5I- (aq) + 6H+ (aq) → 3I2 (aq) + 3H2O (l)
• Change in colour: reaction involving coloured complexes
▪ Graph y-axis: colour intensity (or absorbance)
▪ Apparatus: measurement through calorimeter through change in colour intensity
▪ Example: 2MnO4- (aq) + 5C2O42- (aq) + 2Mn2+ (aq) c 10CO2 (g) + 8H2O (l)
▪ Purple (from manganate (VII) ion) → pale pink (from manganese ion)
• Understanding: Activation energy (Ea) is the minimum energy that colliding molecules need in order to have successful collisions
leading to a reaction.
▪ Activation energy (Ea): minimum energy required in colliding molecules in order to have successful collisions for a reaction
• Understanding: By decreasing Ea, a catalyst increases the rate of a chemical reaction, without itself being permanently chemically
changed.
▪ Catalyst: substance that increases the rate of a chemical reaction but is not consumed in the reaction process
• Rate of reaction: increases as catalyst provides an alternative pathway with required a lower activation energy
▪ Homogenous catalyst: catalysts in the same physical state as the reactants
▪ Heterogeneous catalyst: catalysts in a different physical state as the reactants (typically solid with reactants liquid or gas)
• Applications and skills: Description of the kinetic theory in terms of the movement of particles whose average kinetic energy is
proportional to temperature in Kelvin.
▪ Kinetic theory of gases: model used to explain and predict the behaviour of gases in a microscopic level (note topic 1.3)
• Space: most of the volume of gaseous particles is occupied by empty space
• Direction: gaseous particles are constantly in straight lines of random direction
• Elastic collision: gaseous particles under collisions that gives no loss of kinetic energy
• Average kinetic energy: energy of particles is proportional to absolute temperature in kelvin
▪ At a given instant of time some particles will be travelling at greater velocities than other particle, and some lower
▪ The average will be proportional to the temperature: temperature increase will result in increased average kinetic energy
The greater the probability that molecules will collide with sufficient energy and proper orientation, the higher the rate of reaction.
• Understanding: Species react as a result of collisions of sufficient energy and proper orientation.
▪ Collision theory: model that helps understanding of why rate of reaction
depends on temperature
• Collision must occur – two particles must collide (physically be in
contact) for the reaction to occur
• Collision in correct orientation – colliding particles must have the
correct orientation (wrong orientation will not result in reaction)
• Collision with sufficient energy – colliding particles must have
sufficient kinetic energy to initiate reaction (activation energy)
• Understanding: The rate of reaction is expressed as the change in concentration of a particular reactant/product per unit time.
▪ Rate of reaction: change in concentration of reactant or product per unit time
• Unit: mol dm-3 s-1 (or other unit for time)
• Understanding: Concentration changes in a reaction can be followed indirectly by monitoring changes in mass, volume and colour.
▪ Means of monitoring concentration changes indirectly
• Change in mass: reaction involving solids (and gases)
▪ Graph y-axis: mass of flask + content (or mass loss)
▪ Apparatus: periodic measurement through electronic balance
▪ Example: CaCO3 (s) + HCl (aq) → CaCl2 (aq) + CO2 (g) + H2O (l)
• Change in volume: reaction involving gases
▪ Graph y-axis: volume of gas
▪ Apparatus: measurement through inverted measuring cylinder or data logger
▪ Example: CaCO3 (s) + HCl (aq) → CaCl2 (aq) + CO2 (g) + H2O (l)
• Change in conductivity: reaction involving electrolytes
▪ Graph y-axis: electrical conductivity
▪ Apparatus: measurement through conductivity probe and meter from net change in number of ions
▪ Example: IO3- (aq) + 5I- (aq) + 6H+ (aq) → 3I2 (aq) + 3H2O (l)
• Change in colour: reaction involving coloured complexes
▪ Graph y-axis: colour intensity (or absorbance)
▪ Apparatus: measurement through calorimeter through change in colour intensity
▪ Example: 2MnO4- (aq) + 5C2O42- (aq) + 2Mn2+ (aq) c 10CO2 (g) + 8H2O (l)
▪ Purple (from manganate (VII) ion) → pale pink (from manganese ion)
• Understanding: Activation energy (Ea) is the minimum energy that colliding molecules need in order to have successful collisions
leading to a reaction.
▪ Activation energy (Ea): minimum energy required in colliding molecules in order to have successful collisions for a reaction
• Understanding: By decreasing Ea, a catalyst increases the rate of a chemical reaction, without itself being permanently chemically
changed.
▪ Catalyst: substance that increases the rate of a chemical reaction but is not consumed in the reaction process
• Rate of reaction: increases as catalyst provides an alternative pathway with required a lower activation energy
▪ Homogenous catalyst: catalysts in the same physical state as the reactants
▪ Heterogeneous catalyst: catalysts in a different physical state as the reactants (typically solid with reactants liquid or gas)
• Applications and skills: Description of the kinetic theory in terms of the movement of particles whose average kinetic energy is
proportional to temperature in Kelvin.
▪ Kinetic theory of gases: model used to explain and predict the behaviour of gases in a microscopic level (note topic 1.3)
• Space: most of the volume of gaseous particles is occupied by empty space
• Direction: gaseous particles are constantly in straight lines of random direction
• Elastic collision: gaseous particles under collisions that gives no loss of kinetic energy
• Average kinetic energy: energy of particles is proportional to absolute temperature in kelvin
▪ At a given instant of time some particles will be travelling at greater velocities than other particle, and some lower
▪ The average will be proportional to the temperature: temperature increase will result in increased average kinetic energy
