What controls how fast a reaction goes, and how does a catalyst speed it up?
Collision theory, the effect of concentration, pressure, surface area and temperature on rate, the Boltzmann distribution and activation energy, and the action of catalysts including the difference between homogeneous and heterogeneous catalysis.
An OCR H432 module 3 answer on reaction rates: collision theory, the effects of concentration, temperature and surface area, the Boltzmann distribution, activation energy, and catalysis.
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What this topic is asking
OCR specification point 3.2.2 wants you to use collision theory to explain how concentration, pressure, surface area and temperature affect reaction rate, to interpret the Boltzmann distribution in terms of activation energy, and to describe how catalysts (homogeneous and heterogeneous) work. This qualitative treatment of rates is extended quantitatively in Module 5.
Collision theory
Anything that increases the frequency of successful collisions increases the rate:
- Concentration / pressure. More particles per unit volume means more frequent collisions.
- Surface area. Breaking a solid into smaller pieces exposes more particles for collision.
- Temperature. Faster particles collide more often, but the dominant effect is the energy of the collisions (see below).
The Boltzmann distribution
This is why a modest temperature rise produces a large rate increase: a small shift in the curve moves a disproportionate fraction of molecules past the activation energy.
Catalysis
Examples in context
Example 1. Catalytic converters. A heterogeneous catalyst of platinum and rhodium converts toxic exhaust gases into less harmful products; the gases adsorb onto the metal surface, react, and desorb, cutting pollution from cars.
Example 2. Why food spoils slower in a fridge. Lowering the temperature shifts the Boltzmann distribution left, so far fewer molecules exceed the activation energy for the reactions that spoil food, slowing them dramatically.
Try this
Q1. State two conditions that must be met for a collision to result in a reaction. [2 marks]
- Cue. The colliding particles must have energy greater than or equal to the activation energy, and they must collide with the correct orientation.
Q2. Explain, in terms of collision theory, why powdered marble reacts faster with acid than a single lump of the same mass. [2 marks]
- Cue. Powdering increases the surface area, so more acid particles can collide with the marble per second, increasing the frequency of successful collisions.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksUsing the Boltzmann distribution, explain why a small increase in temperature causes a large increase in the rate of reaction.Show worked answer →
The Boltzmann distribution shows the spread of molecular energies; only molecules with energy greater than or equal to the activation energy can react (1). Raising the temperature shifts the curve to the right and lowers the peak, so the area beyond the activation energy increases significantly (1).
A many-fold increase in the proportion of molecules with sufficient energy means many more successful collisions per second (1). The collision frequency also rises slightly, but the energy effect dominates, so even a rise can roughly double the rate (1).
Markers reward the link to activation energy, the rightward shift of the curve, the larger area beyond , and the conclusion that the energy effect outweighs collision frequency.
OCR 20203 marksExplain how a heterogeneous catalyst increases the rate of a reaction, using a labelled reference to its mode of action.Show worked answer →
A catalyst provides an alternative reaction pathway with a lower activation energy (1). A heterogeneous catalyst is in a different phase from the reactants, typically a solid surface with gaseous or liquid reactants (1).
Reactant molecules adsorb onto active sites on the surface, which weakens their bonds and holds them in the right orientation; products then desorb, freeing the site (1).
Markers reward the lower-activation-energy pathway, the different-phase definition, and the adsorption-reaction-desorption mechanism.
Related dot points
- Enthalpy and standard enthalpy changes, exothermic and endothermic reactions, calorimetry and the q = mcDeltaT equation, average bond enthalpies, and Hess's law including formation and combustion cycles.
An OCR H432 module 3 answer on enthalpy changes: standard enthalpy definitions, calorimetry with q = mcDeltaT, bond enthalpy calculations, and Hess's law cycles for formation and combustion.
- Dynamic equilibrium, Le Chatelier's principle and the effect of concentration, pressure and temperature, the role of a catalyst, the equilibrium constant Kc for homogeneous equilibria, and the compromise conditions used in industry.
An OCR H432 module 3 answer on chemical equilibrium: dynamic equilibrium, Le Chatelier's principle, the equilibrium constant Kc, and the compromise conditions used in industrial processes such as the Haber process.
- Orders of reaction and the rate equation, the rate constant and its units, concentration-time and rate-concentration graphs, half-life, the rate-determining step, and the Arrhenius equation.
An OCR H432 module 5 answer on rates: orders and the rate equation, the rate constant and its units, concentration-time and rate-concentration graphs, the rate-determining step, and the Arrhenius equation.
- The periodic table arranged by atomic number into periods and groups, the s, p and d blocks, and the periodic trends in atomic radius, first ionisation energy and melting point across Periods 2 and 3.
An OCR H432 module 3 answer on periodicity: the structure of the periodic table, the s, p and d blocks, and the trends in ionisation energy, atomic radius and melting point across Periods 2 and 3.
Sources & how we know this
- OCR A-Level Chemistry A (H432) specification — OCR (2015)