Chapter 6: Chemical Reactions — Part 2

Topic 6.2a · Rate of reaction: measurement and graphs

Defining and Factors of Reaction Rate

The rate of reaction is defined as the time taken for a reactant to be used up or a product to be formed.
Several factors affect the rate:
- Increasing the concentration of solutions increases the rate.
- Increasing the pressure of reacting gases increases the rate.
- Increasing the surface area of a solid (e.g., using a fine powder instead of a lump) increases the rate.
- Increasing the temperature increases the rate.
- Adding a catalyst increases the rate.

Rates are measured by tracking the change in mass of a reactant, mass of a product, or volume of gas formed over time.
Common apparatus includes:
- Digital balance: To measure the mass loss as a gas escapes or the mass of product formed.
- Gas syringe: To collect and measure the volume of gas produced in a closed system.
- Stopwatch: To measure the time in seconds.
- Precipitate (Cross) method: In reactions that form a precipitate, a beaker is placed over a cross; the time taken for the cross to disappear from view is recorded.

Exam Traps

Do not use an open flask on a balance for a gas-producing reaction without sealing — mass loss is inaccurate if gas escapes freely.

On a graph of product mass/volume against time, a steeper line represents a faster rate of reaction.
When the line becomes horizontal, the reaction is complete because all products have been formed.
To calculate the rate at a specific time, a tangent is drawn to the curve at that point.
The gradient of the tangent (Rate = ?y/?x) gives the rate of reaction at that moment.
The initial rate of reaction is found by drawing a tangent at t = 0 s and calculating its gradient.

Exam Traps

Do not use average rate (total change ÷ total time) when a tangent gradient is required.
Do not draw the tangent crossing the curve at two points far apart — it must touch the curve at the chosen time only.

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