Topic F: Chemical Kinetics 2
Topic A: Stoichiometry 8
Topic B: Atomic structure 14
Topic D: Models of bonding and structure 15
Topic E: R1.1 Chemical energetics 27
Topic E: R1.2 Energy cycles in reactions 30
Topic E: R1.4 Energy Cycles in Reactions 32
Topic G: Chemical equilibrium 34
, Topic F: Chemical Kinetics
Rate of reaction
- Rate of reaction is the decrease in concentration of a reactant per unit time or the
increase in concentration of a product per unit time
𝑑[𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡] 𝑑[𝑝𝑟𝑜𝑑𝑢𝑐𝑡]
𝑟𝑎𝑡𝑒 =− 𝑑𝑡
= 𝑑𝑡
- Units for rate of reaction: mol dm-3 s-1
Types of rate
- Initial rate – rate at the start of the reaction when an infinitesimally small amount
of reaction has been used up (gradient of tangent at t=0)
- Instantaneous rate – Rate of reaction at a particular instant in time (gradient of
tangent at t)
- Average rate – rate of reaction calculated from the change in concentration of a
reactant or product over a particular period of time
Experimental procedures
- Discontinuous measurement where the rate-concentration relationship is
established
- Continuous measurement where the concentration-time relationship is
established
- Common properties monitored during a reaction:
- Concentration of reactants or products
- pH of reacting mixture
- volume of gas evolved
- change in mass of reacting mixture
- colour of reacting mixture
- electrical conductivity of reacting mixture
Titration method
- Reactants of known concentrations and measured volumes are mixed; samples of
the reaction mixture are withdrawn at regular time intervals
- The withdrawn sample is quenched to significantly slow down or stop the reaction
- Adding a large volume of cold solvent – slows reaction by dilution and
reducing temperature
- Adding a quenching reagent – reacts immediately with one reactant
- Adding inhibitor or negative catalyst to slow down reaction
Measuring changed in gas volume
- Continuous measurement
- Determining volume of gas produced per unit time or loss in mass per unit time
(gas allowed to escape)
Measuring changes in colour intensity (colourimetry)
- Continuous measurement
- Beer-Lambert Law
, 𝐴𝑏𝑠 = ε𝑙𝑐
- Abs = absorbance of solution
- ε = molar absorptivity/extinction coefficient
- 𝑙 = path length
Stop clock method
- Discontinuous measurement – only takes 1 measurement for time
1
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑟𝑎𝑡𝑒 ∝ 𝑡𝑜𝑡𝑎𝑙 𝑡𝑖𝑚𝑒
- Assumptions: concentration of the product is constant when cross can non longer
be seen
- Product formation must be detected in a reasonably short time – rate can be
approximated as initial rate because concentrations of reactants remain close to
initial concentrations
Collision theory
Kinetic theory
- Reacting particles are moving at high velocities in random motion, frequently
colliding with each other and walls of the container.
- Attractive and repulsive forces are negligible
- Collisions are elastic – energy is transferred between molecules during collision,
but average kinetic energy is unchanged (temp must be constant)
- Proportional rise in temperature corresponds to increase in average kinetic energy
Activation energy
- The minimum amount of energy which the reactant particles must possess for a
collision to result in a reaction
Collision theory
- For a reaction between two reactant particles:
- Particles must collide with each other
- Particles must collide in the correct orientation
, - Particles must collide with sufficient kinetic energy that minimally meets
the requirement of the activation energy for the collisions
- Collisions that result in the formation of products are called effective collisions
Factors affecting ROR
Particle size
- Smaller the particle size, larger the surface area over which other reactants can
make contact, increasing the fraction of particles available for collision, increasing
frequency of effective collisions and rate of reaction
Concentration and pressure
- Increasing concentration will increase number of particles per unit volume,
increasing frequency of effective collisions and ROR
Temperature
- Maxwell-Boltzmann curve
- When temperature increases, reactant particles gain kinetic energy, the fraction of
particles possessing energy greater than or equal to the activation energy
increases, results in an increase in the frequency of effective collisions
- At higher temperatures, peak of the curve is shifted and distribution is ‘flattened
out’, fewer particles possess the new average value as distribution has widened
and number of particles with high kinetic energy has increased
- Shaded area indicates increase in number of particles possessing kinetic energy
more than or equal to activation energy
Catalyst
- Increases the rate of chemical reaction without being chemically changed at the
end of the reaction
Topic A: Stoichiometry 8
Topic B: Atomic structure 14
Topic D: Models of bonding and structure 15
Topic E: R1.1 Chemical energetics 27
Topic E: R1.2 Energy cycles in reactions 30
Topic E: R1.4 Energy Cycles in Reactions 32
Topic G: Chemical equilibrium 34
, Topic F: Chemical Kinetics
Rate of reaction
- Rate of reaction is the decrease in concentration of a reactant per unit time or the
increase in concentration of a product per unit time
𝑑[𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡] 𝑑[𝑝𝑟𝑜𝑑𝑢𝑐𝑡]
𝑟𝑎𝑡𝑒 =− 𝑑𝑡
= 𝑑𝑡
- Units for rate of reaction: mol dm-3 s-1
Types of rate
- Initial rate – rate at the start of the reaction when an infinitesimally small amount
of reaction has been used up (gradient of tangent at t=0)
- Instantaneous rate – Rate of reaction at a particular instant in time (gradient of
tangent at t)
- Average rate – rate of reaction calculated from the change in concentration of a
reactant or product over a particular period of time
Experimental procedures
- Discontinuous measurement where the rate-concentration relationship is
established
- Continuous measurement where the concentration-time relationship is
established
- Common properties monitored during a reaction:
- Concentration of reactants or products
- pH of reacting mixture
- volume of gas evolved
- change in mass of reacting mixture
- colour of reacting mixture
- electrical conductivity of reacting mixture
Titration method
- Reactants of known concentrations and measured volumes are mixed; samples of
the reaction mixture are withdrawn at regular time intervals
- The withdrawn sample is quenched to significantly slow down or stop the reaction
- Adding a large volume of cold solvent – slows reaction by dilution and
reducing temperature
- Adding a quenching reagent – reacts immediately with one reactant
- Adding inhibitor or negative catalyst to slow down reaction
Measuring changed in gas volume
- Continuous measurement
- Determining volume of gas produced per unit time or loss in mass per unit time
(gas allowed to escape)
Measuring changes in colour intensity (colourimetry)
- Continuous measurement
- Beer-Lambert Law
, 𝐴𝑏𝑠 = ε𝑙𝑐
- Abs = absorbance of solution
- ε = molar absorptivity/extinction coefficient
- 𝑙 = path length
Stop clock method
- Discontinuous measurement – only takes 1 measurement for time
1
𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝑟𝑎𝑡𝑒 ∝ 𝑡𝑜𝑡𝑎𝑙 𝑡𝑖𝑚𝑒
- Assumptions: concentration of the product is constant when cross can non longer
be seen
- Product formation must be detected in a reasonably short time – rate can be
approximated as initial rate because concentrations of reactants remain close to
initial concentrations
Collision theory
Kinetic theory
- Reacting particles are moving at high velocities in random motion, frequently
colliding with each other and walls of the container.
- Attractive and repulsive forces are negligible
- Collisions are elastic – energy is transferred between molecules during collision,
but average kinetic energy is unchanged (temp must be constant)
- Proportional rise in temperature corresponds to increase in average kinetic energy
Activation energy
- The minimum amount of energy which the reactant particles must possess for a
collision to result in a reaction
Collision theory
- For a reaction between two reactant particles:
- Particles must collide with each other
- Particles must collide in the correct orientation
, - Particles must collide with sufficient kinetic energy that minimally meets
the requirement of the activation energy for the collisions
- Collisions that result in the formation of products are called effective collisions
Factors affecting ROR
Particle size
- Smaller the particle size, larger the surface area over which other reactants can
make contact, increasing the fraction of particles available for collision, increasing
frequency of effective collisions and rate of reaction
Concentration and pressure
- Increasing concentration will increase number of particles per unit volume,
increasing frequency of effective collisions and ROR
Temperature
- Maxwell-Boltzmann curve
- When temperature increases, reactant particles gain kinetic energy, the fraction of
particles possessing energy greater than or equal to the activation energy
increases, results in an increase in the frequency of effective collisions
- At higher temperatures, peak of the curve is shifted and distribution is ‘flattened
out’, fewer particles possess the new average value as distribution has widened
and number of particles with high kinetic energy has increased
- Shaded area indicates increase in number of particles possessing kinetic energy
more than or equal to activation energy
Catalyst
- Increases the rate of chemical reaction without being chemically changed at the
end of the reaction
