Rates of Reaction and Collision Theory
What Is Rate of Reaction?
The rate of reaction tells us how quickly a reaction happens. A fast reaction, such as an explosion, is over in a fraction of a second. A slow reaction, such as the rusting of iron or the weathering of stone, takes months or years. In industry, controlling the rate of a reaction is vital — too fast can be dangerous, too slow is uneconomical.
Rate can be measured in several ways: the volume of gas produced per unit time (using a gas syringe or measuring cylinder over water), the mass of a reaction mixture lost per unit time (as gas escapes, measured using a balance), or the time taken for a colour change or for a solution to become opaque. The choice depends on the particular reaction and what can be conveniently measured.
Collision Theory
Collision theory states that for a reaction to occur, the reacting particles must: (1) collide with each other, (2) collide with sufficient energy (at least the activation energy), and (3) collide with the correct orientation. Not every collision leads to a reaction — most are unsuccessful because the particles bounce off without reacting. The rate of reaction depends on the frequency of successful collisions: collisions that have both enough energy and the right orientation.
For a reaction to happen, particles must collide with enough energy (the activation energy) and the correct orientation. Anything that increases the frequency or energy of successful collisions increases the rate of reaction.
Factors Affecting Rate of Reaction
Four main factors affect the rate of a chemical reaction: temperature, concentration, surface area, and catalysts.
Temperature: Increasing the temperature gives particles more kinetic energy, so they move faster and collide more frequently. More importantly, a greater proportion of collisions have energy equal to or exceeding the activation energy, so a much larger fraction of collisions are successful. Even a small increase in temperature produces a significant increase in rate.
Concentration (for solutions) and pressure (for gases): Increasing concentration means more particles in the same volume, so they collide more frequently. For gases, increasing pressure pushes particles closer together, with the same effect. More collisions per unit time → more successful collisions → faster rate.
Surface area: Breaking a solid into smaller pieces increases the surface area to volume ratio. A greater surface area means more of the solid is exposed to the other reactant, so collisions occur more frequently. Powdered marble chips react faster with hydrochloric acid than large lumps, even with the same total mass. In extreme cases, a fine powder of combustible material in air can cause a dust explosion.
Catalysts
A catalyst increases the rate of a reaction without being used up. It provides an alternative reaction pathway with a lower activation energy, so a greater proportion of collisions are successful. On a reaction profile, the catalyst lowers the peak (activation energy) without changing the energy levels of the reactants or products. Each catalyst is specific to a particular reaction — iron catalyses the Haber process but would not catalyse the decomposition of hydrogen peroxide (which uses manganese dioxide).
Catalysts speed up reactions by lowering the activation energy. They are not used up and are not changed at the end of the reaction. Enzymes are biological catalysts found in all living organisms.
Interpreting Rate of Reaction Graphs
Rate graphs typically plot volume of gas (or mass lost) on the y-axis against time on the x-axis. The gradient (steepness) of the line at any point shows the rate at that moment — a steeper gradient means a faster rate. The line levels off when one reactant is used up (the reaction is complete). Two curves on the same axes that level off at the same point but with different steepnesses represent the same reaction with the same amount of reactant, but at different rates (e.g. different temperatures or concentrations).
In your practical work (PAG C8), you investigate the rate of reaction between a metal (such as magnesium) and dilute acid, or between marble chips and hydrochloric acid. Methods include:
- Gas syringe: collect gas in a syringe and record volume at regular intervals.
- Displacement of water: collect gas over water in an inverted measuring cylinder.
- Mass loss: place the flask on a balance; record mass at regular intervals as gas escapes through cotton wool.
- Disappearing cross: for the sodium thiosulfate and acid reaction, time how long it takes for a cross viewed through the solution to disappear as a precipitate of sulfur forms.
In all methods, change only one variable at a time and keep all other conditions constant to make it a fair test. Look for anomalous results in your data and exclude them when calculating means.