Speed of Reaction

Imagine collecting gas during a reaction. Measuring 20 cm³ tells you how much gas has collected; you also need the time interval to say how quickly it formed. Reaction rate connects the change with the time taken.

1. Definition

A. Speed (Rate) of Reaction

The speed of reaction (rate of reaction) is the change in amount of reactant used up or product formed per unit time.

B. Reactants and Products

• Reactants: starting substances used up in a reaction.
• Products: new substances formed in a reaction.

2. Key Ideas

• In a simple reaction proceeding forwards, reactants are used up and products form. Measure a change that tracks this reaction.
• Rate is always “change per time”, so it must have units (e.g., cm³/s, g/min, mol dm⁻³/s).
• You can find rate by measuring an observable change (mass, gas volume, colour, pH, precipitate, etc.).
• A reaction can be fast but produce very little product if the amount of reactant is small. Do not confuse rate with amount.

3. Detailed Explanations

A. What “Per Unit Time” Means

An average rate describes a specified interval. Subtract the initial reading from the final reading and divide by the elapsed time. If the quantity decreases, such as mass when a gas escapes, use the positive amount lost.

Choose a quantity that tracks the reaction:

Same final gas volume, different initial rates

Gas volume against time for two illustrative reactions. Solid circles label the greater initial rate; dashed squares label the smaller initial rate. Both reach a horizontal plateau at 40 cubic centimetres.

Scroll across the graph to read all labels.

Open full-size graph

View figure data

Values for Same final gas volume, different initial rates

Time (s)	Greater initial rate	Smaller initial rate
0	0	0
10	14	7
20	25	13
30	33	19
40	37	25
50	39	30
60	40	34
70	40	37
80	40	39
90	40	40
120	40	40

Values for Same final gas volume, different initial rates

Time (s)	Greater initial rate	Smaller initial rate
0	0	0
10	14	7
20	25	13
30	33	19
40	37	25
50	39	30
60	40	34
70	40	37
80	40	39
90	40	40
120	40	40

The illustrative graph compares reactions with the same final gas volume. Over the first 10 s, the solid curve rises by 14 cm³ and the dashed curve by 7 cm³. Their average rates over that interval are 14/10 = 1.4 and 7/10 = 0.70 cm³ s⁻¹. Both eventually reach 40 cm³. A faster start has not increased the final amount.

B. Try this next

Next, choose a practical measurement method in Measuring Reaction Rate. For gradients and tangents, use Reading Reaction-Rate Graphs. The particle explanations follow in:

• Collision Theory

• Factors Affecting Rate of Reaction

4. Common Mistakes

• Writing “rate is how fast a reaction happens” without stating the measurable change per unit time.
• Giving only a time or amount when a rate is required. Include both the measurable change and “per unit time”.
• Confusing rate with time taken: shorter time usually means faster rate, but you must state the condition (same amount of reactant/product).
• Measuring the wrong thing (e.g., measuring “volume of air” instead of gas produced).
• Not stating units.

5. Exam Tips

• If given data in a table, rate is often “change in volume/mass/concentration ÷ time”.
• If asked for a method, write what you measure (e.g., “measure volume of gas every 10 s using a gas syringe”).
• For a fixed end-point comparison, use the same measurable change. When testing a factor, change that factor and control the other relevant conditions.

6. Worked Examples

7. Mind Stretchers

Practise and check