Investigating the Rate of Reaction

The rate of reaction tells us how fast reactants turn into products. It is like timing how quickly a queue moves. Faster reactions finish changes in a shorter time.

What does "rate" mean?

Rate is how much something changes in a certain time.

$$\text{rate} = \frac{\text{change in amount}}{\text{time}} \quad (\text{e.g. cm}^3\!/\text{s or g/s})$$

Collision theory: particles must collide with enough energy to react. Higher collision frequency and higher particle energy make reactions faster. Activation energy $E_a$ is the minimum energy needed.

Practical methods to measure rate

• Measuring gas made: Collect gas in a gas syringe or an upside-down measuring cylinder. Record volume every 10–20 s.
• Change in mass: If a gas escapes (e.g. CO₂), place the flask on a balance. Mass decreases as gas leaves. Record mass over time.
• Formation of a precipitate: For a cloudy solid forming (e.g. sodium thiosulfate + acid), time how long until a black cross under the flask is no longer visible.

Planning a fair test

• Independent variable (you change): concentration, temperature, surface area of a solid, pressure of a gas, or adding a catalyst.
• Dependent variable (you measure): volume of gas, change in mass, or time taken for a visible change.
• Control variables: total volumes, amounts of reactants, particle size (if not tested), acid strength, temperature (if not tested), same apparatus.
• Repeat trials and average your results.

From results to rate

Use average rate for a time period: $\text{rate} = \Delta \text{amount} / \Delta t$. On a graph, the steeper the line, the faster the rate. The initial slope shows the initial rate.

How conditions change rate (collision theory)

• Higher concentration/pressure: particles are closer, collisions happen more often → faster.
• Higher temperature: particles move faster and more have energy ≥ $E_a$ → faster.
• Greater surface area: more exposed particles on the solid → more collisions → faster.
• Catalyst: provides a route with lower $E_a$; it speeds up the reaction and is unchanged at the end.

Safety

• Wear eye protection. Handle acids carefully.
• Do not seal gas-producing reactions tightly; allow gas to escape safely into measuring apparatus.
• Warm water baths can be hot; use tongs and heatproof mats.