Rates of Reaction and Collision Theory
Rate of Reaction
The rate of reaction measures how quickly reactants are turned into products. It can be calculated by:
$$\text{rate of reaction} = \frac{\text{quantity of reactant used or product formed}}{\text{time taken}}$$
The rate of a chemical reaction can be changed by altering the: temperature, concentration of solutions (or pressure of gases), surface area of solid reactants, or by using a catalyst.
Rate of reaction can be increased by: raising temperature, increasing concentration/pressure, increasing surface area, or using a catalyst. The rate can be measured by collecting gas, measuring mass loss, or timing colour changes.
Collision Theory
Collision theory states that for a reaction to occur, particles must: (1) collide with each other, and (2) collide with sufficient energy. The minimum energy needed for a successful collision is called the activation energy.
Each factor affects the rate by changing the frequency or energy of collisions:
- Temperature — higher temperature gives particles more kinetic energy. They move faster, collide more frequently, and a greater proportion of collisions have energy \(\geq\) activation energy.
- Concentration / Pressure — more particles in the same volume means more frequent collisions.
- Surface area — smaller pieces expose more surface, giving more area for collisions to occur.
- Catalyst — provides an alternative reaction pathway with a lower activation energy, so more collisions are successful. The catalyst is not used up.
Collision theory: reactions only happen when particles collide with energy ≥ activation energy. Catalysts lower the activation energy — they are not used up and do not appear in the overall equation.
Common methods for measuring rate include: collecting gas in a gas syringe or over water, measuring the loss of mass on a balance (when gas escapes), or timing how long it takes for a solution to become opaque (e.g. sodium thiosulfate "disappearing cross" experiment). On a graph, a steeper curve means a faster rate.
On a graph of volume of gas vs time: the gradient (steepness) at any point represents the rate. A steeper line = faster rate. The line levels off when the reaction is complete (one reactant has been used up). Changing temperature or concentration affects the steepness but not the final volume (if the same amount of reactant is used). Increasing the amount of reactant increases the total gas produced.