The rate of a chemical reaction depends on the frequency and energy of collisions between reactant particles. For a reaction to occur, particles must collide with sufficient energy (≥ activation energy) and correct orientation.
Factors affecting rate:
1. Temperature — particles move faster, collide more often and with more energy
2. Concentration/Pressure — more particles in the same volume, more frequent collisions
3. Surface area — smaller pieces expose more particles to react
4. Catalyst — lowers activation energy, more collisions are successful
Measuring Rates of Reaction
Here are some common practical methods used to measure the rate of a reaction:
Measuring the volume of gas produced: If a reaction produces a gas, you can collect the gas in a gas syringe or over water and measure the volume at regular time intervals. The faster the volume increases, the faster the reaction.
Measuring change in mass: If a gas is produced and allowed to escape, the total mass of the reaction mixture will decrease. This can be measured using a balance over time. The faster the mass decreases, the faster the reaction.
Measuring turbidity (disappearing cross experiment): For reactions that produce a precipitate, the solution becomes cloudy (turbid). You can place a flask containing the reaction mixture over a cross mark on paper and time how long it takes for the cross to no longer be visible. A shorter time indicates a faster reaction.