Rates of Reaction and Collision Theory
What Is Rate of Reaction?
The rate of reaction measures how quickly a chemical reaction proceeds. It is calculated using:
$$\text{rate of reaction} = \frac{\text{quantity of reactant used or product formed}}{\text{time taken}}$$
The ‘quantity’ can be measured as mass (g), volume of gas (cm3), or by monitoring a colour change. The four main factors that affect rate are: temperature, concentration (or gas pressure), surface area of solids, and the use of a catalyst.
Rate of reaction increases with: higher temperature, greater concentration (or pressure for gases), greater surface area, and the presence of a catalyst. It can be measured by: collecting gas produced, monitoring mass loss, or timing a colour change.
Collision Theory
Collision theory explains why these factors affect rate. For a reaction to occur, particles must collide with each other and that collision must have enough energy to overcome the activation energy. Any change that increases the frequency of successful collisions increases the rate.
Temperature: higher temperature gives particles more kinetic energy. They move faster, collide more frequently, and a greater proportion of collisions have energy greater than or equal to the activation energy. Even a small temperature rise can dramatically increase rate.
Concentration / Pressure: a higher concentration means more particles in the same volume, so particles are closer together and collide more often. For gases, increasing pressure has the same effect — it compresses the gas and raises particle density.
Surface area: breaking a solid into smaller pieces exposes more surface for reactant particles to collide with. The total volume of solid is unchanged but more of it is accessible, so collisions are more frequent.
Catalyst: provides an alternative reaction pathway with a lower activation energy. At any given temperature, a larger fraction of collisions now have enough energy to react, so the rate increases. The catalyst is not consumed.
Core Practical — Measuring Rates of Reaction
Your course includes a core practical investigating how factors affect the rate of reaction. Two standard methods are used:
Marble chips (calcium carbonate) react with dilute hydrochloric acid to produce carbon dioxide gas:
$$CaCO_3 + 2HCl \rightarrow CaCl_2 + H_2O + CO_2$$
Rate is measured by collecting the gas in a gas syringe and recording the volume every 30 seconds. The independent variable (e.g. concentration of acid, or surface area of chips) is changed; the dependent variable is the volume of CO2 produced over time. Control variables include: mass of marble chips, temperature, total volume of solution.
The initial gradient of a volume-vs-time graph gives the initial rate. Steeper gradients indicate faster rates. Each experiment should be repeated to check reproducibility and calculate a mean.
When sodium thiosulfate solution reacts with dilute hydrochloric acid, a yellow precipitate of sulfur forms:
$$Na_2S_2O_3 + 2HCl \rightarrow 2NaCl + H_2O + SO_2 + S$$
Rate is measured by placing the flask over a cross drawn on white paper and timing how long it takes for the cross to disappear as the sulfur cloud builds up. A shorter time means a faster rate. The independent variable is concentration (or temperature); the dependent variable is time for cross to disappear. Control variables: volume of solutions, room temperature, the same cross viewed from the same height.
Rate can then be expressed as \(\frac{1}{\text{time}}\) — this makes comparison easier as a larger value corresponds to a faster rate.
Both core practical methods require clear identification of: independent variable (what you change), dependent variable (what you measure), and control variables (everything else kept constant). Include repeats and a method of calculating rate from collected data.
Interpreting Rate Graphs
A graph of product formed against time produces a curve that starts steep and levels off as reactants are used up. The steeper the initial gradient, the faster the initial rate. When the line is horizontal, the reaction has stopped. Comparing two experiments on the same axes, the one with the steeper initial gradient has the faster rate — caused by higher temperature, greater concentration, greater surface area, or the presence of a catalyst.
For calculation questions, if rate is defined as 1/time, a shorter time means a faster rate. When drawing graphs of volume of gas against time, the steeper the initial gradient, the faster the rate. Catalysts and higher temperatures both produce steeper gradients without changing the final total volume of product obtained at completion, because the same amount of reactant is consumed in both cases.