What controls how fast a reaction goes and whether it releases or absorbs energy?
Collision theory and the factors affecting the rate of reaction, the action of catalysts, exothermic and endothermic reactions, enthalpy changes and energy profile diagrams, and the calculation of enthalpy changes.
A CCEA Life and Health Sciences answer on rates and energetics: collision theory and the factors affecting rate, the action of catalysts, exothermic and endothermic reactions, energy profile diagrams, and calculating enthalpy changes.
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What this dot point is asking
CCEA wants you to use collision theory to explain the factors that affect the rate of a reaction, describe the action of a catalyst, distinguish exothermic from endothermic reactions, interpret energy profile diagrams, and calculate enthalpy changes from experimental data. This is the physical-chemistry foundation for the industrial processes (such as the Haber process) studied later in the unit, where rate and energetics decide the operating conditions.
Collision theory and rate
Factors that increase the rate do so by increasing the frequency of successful collisions. Increasing concentration (or, for gases, pressure) packs the particles closer, so they collide more often. Increasing surface area (using a powder rather than a lump) exposes more particles to collide. Increasing temperature has two effects: particles move faster and collide more often, and, more importantly, a much greater proportion of particles now have energy at or above the activation energy, so a far larger fraction of collisions are successful. This is why a modest temperature rise can roughly double the rate.
Catalysts
On an energy profile diagram, a catalyst lowers the peak (the activation energy) but leaves the energy of the reactants and products unchanged. Industrial catalysts (such as iron in the Haber process or vanadium(V) oxide in the Contact process) are vital because they allow reactions to proceed fast enough at lower, cheaper temperatures, saving energy and money. Catalysts are often specific to particular reactions.
Exothermic, endothermic and enthalpy
A reaction is exothermic if it releases heat to the surroundings, so the products have less energy than the reactants and the enthalpy change (the heat change at constant pressure, symbol delta H) is negative. A reaction is endothermic if it absorbs heat, so the products have more energy than the reactants and the enthalpy change is positive. Combustion and neutralisation are exothermic; thermal decomposition is usually endothermic. On an energy profile diagram, exothermic reactions show products below reactants, endothermic reactions show products above reactants, and the activation energy is the height of the peak above the reactants. The enthalpy change of a reaction can be measured experimentally by recording the temperature change it causes in a known mass of water and calculating the heat transferred.
Examples in context
Example 1. Catalytic converters. A car's catalytic converter uses platinum and rhodium catalysts to speed up the reaction of toxic carbon monoxide and unburned hydrocarbons with oxygen and nitrogen oxides, producing less harmful carbon dioxide, water and nitrogen. The catalyst provides a lower-activation-energy pathway so the reactions go fast enough at exhaust temperatures, showing the industrial importance of catalysis.
Example 2. Industrial energy costs. In the Haber process, an iron catalyst lets ammonia form at a useful rate at about 450 degrees Celsius rather than needing a much higher, more expensive temperature. Because the catalyst speeds the reaction without being consumed, it lowers the energy demand and running costs, linking rate and energetics directly to industrial economics.
Try this
Q1. Explain, using collision theory, why increasing the pressure of a gaseous reaction increases its rate. [2 marks]
- Cue. Higher pressure packs the gas particles closer, so they collide more frequently, giving more successful collisions per second.
Q2. State two things a catalyst does and one thing it does not do. [3 marks]
- Cue. Does: provides a lower-activation-energy pathway, speeds up the rate, is not used up. Does not: change the enthalpy change or the position of equilibrium.
Q3. A reaction has products at a lower energy than the reactants. State whether it is exothermic or endothermic and the sign of delta H. [2 marks]
- Cue. Exothermic; delta H is negative.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA AS 36 marksUsing collision theory, explain how increasing the temperature and adding a catalyst each increase the rate of a chemical reaction.Show worked answer →
The answer must use collision theory for both factors, distinguishing frequency from energy.
Temperature: raising the temperature gives the particles more kinetic energy, so they move faster and collide more frequently. More importantly, a greater proportion of the particles now have energy equal to or greater than the activation energy, so a larger fraction of collisions are successful. Both effects increase the rate, with the energy effect being the larger.
Catalyst: a catalyst provides an alternative reaction pathway with a lower activation energy. This means a greater proportion of collisions have enough energy to react, so more collisions are successful and the rate increases. The catalyst is not used up overall.
A successful collision requires the particles to collide with at least the activation energy and in the correct orientation.
Markers reward more frequent collisions and (crucially) a greater proportion exceeding the activation energy for temperature, and the lower activation energy via an alternative pathway for the catalyst.
CCEA AS 35 marksWhen 1.0 gram of methane is burned completely, 55.6 kilojoules of heat are released. Calculate the enthalpy of combustion of methane in kilojoules per mole, and state whether the reaction is exothermic or endothermic. (Molar mass of methane is 16.0 grams per mole.)Show worked answer →
Convert mass to moles, scale the energy to one mole, and assign the sign.
Moles of methane burned:
Energy released per mole:
The reaction releases heat to the surroundings, so it is exothermic and the enthalpy change is negative:
Markers reward the correct moles, the correct energy per mole, the negative sign and the word exothermic, with appropriate units.
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Sources & how we know this
- CCEA GCE Life and Health Sciences specification — CCEA (2016)