Qualitative Effects on Rates of Reaction for the ESAT
Updated July 2026
The rate of reaction is a measure of how quickly reactants are converted into products. For the ESAT, you must understand how concentration, pressure, surface area, temperature, and catalysts qualitatively change this speed. These factors work by altering either the frequency or the energy of collisions between reacting particles.
According to collision theory, a reaction occurs only when particles collide with the correct orientation and with energy greater than or equal to the activation energy (). The rate is increased by factors that raise the collision frequency or the proportion of successful collisions.
The Collision Theory
To understand why various factors change the rate of a chemical reaction, we must first look at the collision theory. In any system, reactant particles are in constant motion and frequently collide. However, a collision only leads to a reaction if it is successful. A successful collision is defined by two criteria: the particles must collide with the correct orientation (they must hit each other in the right way to break and form bonds) and they must possess a minimum amount of combined kinetic energy. This minimum energy threshold is known as the activation energy ().
Concentration and Pressure
The concentration of a reactant in solution is a measure of how many particles are present in a given volume of solvent. When the concentration is increased, there are more reactant particles crowded into the same space. As a result, the frequency of collisions between these particles increases. Because more collisions occur every second, the overall number of successful collisions also increases, which speeds up the rate of reaction.
For reactions involving gases, increasing the pressure has a very similar effect to increasing concentration. Increasing the pressure of a gas involves squeezing the same number of gas particles into a smaller volume. This reduces the space between particles, which increases the frequency of collisions. Consequently, the rate of reaction for gaseous reactants increases as the pressure is raised.
Particle Size and Surface Area
When one of the reactants is a solid, the reaction can only occur at the surface of the solid where its particles are exposed to the other reactants. If a large lump of solid is broken down into smaller pieces or crushed into a fine powder, the total surface area increases significantly while the total mass remains the same.
By increasing the surface area, more reactant particles are exposed at the surface and are available to collide with the other reactant particles. This increases the frequency of collisions per unit of time, which leads to a faster rate of reaction. This is why many industrial processes use powdered reactants or catalysts to maximise efficiency.
Temperature
Temperature is perhaps the most significant factor affecting reaction rates. Increasing the temperature increases the average kinetic energy of the particles. This has two distinct effects on the rate. Firstly, because the particles are moving faster, they collide more frequently. Secondly, and more importantly, a much higher proportion of the particles now possess energy equal to or greater than the activation energy ().
This means that when a collision occurs at a higher temperature, it is much more likely to be successful. Even a small increase in temperature can lead to a large increase in the rate of reaction because of the exponential increase in the number of particles with sufficient energy to react. For most reactions, an increase of or can approximately double the reaction rate.
Catalysts
A catalyst is a substance that increases the rate of a chemical reaction without being used up or permanently changed itself. A catalyst works by providing an alternative reaction pathway that has a lower activation energy () than the original route.
Because the activation energy is lowered, a greater proportion of the collisions occurring at any given temperature will have enough energy to result in a reaction. It is important to note that a catalyst does not increase the frequency of collisions; rather, it increases the success rate of the collisions that are already happening. Catalysts are vital in both biological systems, where they are called enzymes, and in industrial chemistry for reducing energy costs.
Key takeaways
- A reaction requires particles to collide with sufficient energy () and correct orientation.
- Increasing concentration or gas pressure raises the collision frequency by increasing particle density.
- Decreasing particle size increases the surface area, exposing more particles to collisions.
- Higher temperatures increase both collision frequency and, more critically, the proportion of successful collisions.
- Catalysts speed up reactions by lowering the activation energy through an alternative pathway.
When explaining the effect of temperature on the ESAT, distinguish clearly between the frequency of collisions and the energy of the particles. You will gain more marks for noting that the increase in successful collisions due to energy is more significant than the increase in the frequency of collisions.
Avoid saying that a catalyst 'lowers the activation energy of the reaction'. Technically, it provides a different pathway with a lower activation energy. The original high energy pathway still exists but the particles take the easier, lower energy route provided by the catalyst.
The effect of temperature is non linear. A small increase in temperature can lead to a massive increase in rate because of the way energy is distributed among particles, which is described by the Maxwell Boltzmann distribution in advanced chemistry.
Frequently asked questions
Why does pressure not affect the rate of reactions between two solids?
Pressure only significantly affects the rate of reactions involving gases. Solids and liquids are virtually incompressible, so increasing the pressure does not significantly change the distance between particles or their concentration.
Does a catalyst change the final yield of a reaction?
No. A catalyst only changes the speed at which a reaction reaches its end point or equilibrium. It does not change the amount of product formed or the position of the equilibrium.
What is the most important effect of increasing temperature?
While higher temperatures increase the frequency of collisions because particles move faster, the most important effect is that a much larger fraction of the particles have energy equal to or greater than the activation energy ().
How do you qualitatively describe the effect of a powder compared to a ribbon of metal?
A powder has a much larger surface area to volume ratio than a ribbon. This means more reactant atoms are exposed on the surface, leading to a higher collision frequency and a faster rate of reaction.