Interpreting Data
Section: 6. Chemical Reactions | Syllabus: Cambridge IGCSE Chemistry (0620)
Reading a Rate of Reaction Graph
A typical rate graph plots the amount of product formed (e.g. volume of gas, cm³) on the y-axis against time on the x-axis.
| Feature | What it means |
|---|---|
| Steep gradient at the start | the reaction is fastest at the start, when reactant concentration is highest |
| Gradient decreases over time | the reaction is slowing down as reactants are used up |
| Curve becomes horizontal (plateau) | the reaction has finished - at least one reactant is fully used up |
| Height of the plateau | the total amount of product - set by the amount of the limiting reactant |
rate of reaction = gradient of the graph = rise ÷ run
Comparing Curves for Different Conditions
Increasing temperature, concentration, pressure, or surface area, or adding a catalyst, all make the curve steeper and reach the plateau sooner.
| What changed | Effect on the curve | Effect on the final (plateau) height |
|---|---|---|
| Higher temperature | much steeper - reaches plateau much faster | same - the amount of reactant hasn't changed |
| Higher concentration (reactant in excess) | steeper - reaches plateau faster | same - limited by the other (unchanged) reactant |
| Smaller particle size (more surface area) | steeper - reaches plateau faster | same - same mass of solid used |
| Catalyst added | much steeper - reaches plateau much faster | same - a catalyst doesn't change the amount of product |
| More of the limiting reactant used | may also be steeper if concentration rises | higher - more product can be made |
A steeper curve means a faster reaction; curves reaching the SAME plateau height used the same amount of limiting reactant
Worked Example: Matching Concentrations to Reaction Times
- Question: small pieces of calcium carbonate react with excess dilute hydrochloric acid at three concentrations (0.75, 1.50, 3.00 mol/dm³). All other conditions are the same. The times recorded to finish reacting were 8 s, 16 s, and 32 s (in some order). Match each concentration to its time. [1]
- Answer: higher concentration reacts faster (shorter time): 0.75 mol/dm³ → 32 s (slowest), 1.50 mol/dm³ → 16 s, 3.00 mol/dm³ → 8 s (fastest)
Calculating Rate from Data
mean rate of reaction = amount of product formed ÷ time taken
Worked Example: Mean Rate of Reaction
- Question: in an experiment, 50 cm³ of gas was collected in 73 s. Calculate the mean rate of reaction, with units. [2]
- Answer: mean rate = 50 ÷ 73 = 0.68 cm³/s (2 s.f.)
An initial rate (the rate right at t = 0) is found by drawing a tangent to the curve at the start and calculating its gradient.
Worked Example: Reading Between Data Points
- Question: a graph plots time taken (y-axis) against volume of water added to dilute a reaction mixture (x-axis), using results from several experiments. Use the graph to estimate the time taken for a volume not directly tested.
- Answer: draw a vertical line up from the given volume on the x-axis to the line of best fit, then a horizontal line across to the y-axis, and read off the time - always show these construction lines on the graph as part of the working
Common Mistakes
- ❌ Thinking a steeper curve means MORE product is made
- ✓ a steeper curve means a FASTER rate, not necessarily more product - compare the plateau height for total amount
- ❌ Thinking the reaction is still happening once the curve is flat
- ✓ a horizontal line (gradient = 0) means the reaction has finished
- ❌ Giving a numerical answer without units
- ✓ always give rate with units, e.g. cm³/s, not just a bare number
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