Measuring Reaction Rates
The rate of a chemical reaction is its speed, which can be determined by monitoring how quickly a substance is consumed or formed. You need to be able to select a practical method for measuring this change based on the specific properties of the reactants and products.
Part of the ESAT Chemistry syllabus — revision for the Engineering and Science Admissions Test (ESAT), the UAT-UK admissions test for Cambridge, Imperial, Oxford and UCL.
Key points
- Reaction rate is the change in concentration of a substance divided by the time taken for that change.
- Method 1: Measure the decrease in the amount of a reactant over time (e.g., by titration or measuring a change in mass).
- Method 2: Measure the increase in the amount of a product over time (e.g., by collecting a gas or observing a precipitate form).
- Method 3: Measure a change in a physical property of the reaction mixture, such as colour, electrical conductivity, pH, or total mass.
- The choice of method depends on the states and properties of the substances involved. For example, a reaction producing a gas can be monitored by measuring mass loss or gas volume.
- The rates of change of different species are linked by stoichiometry. For aA + bB → cC, the rate of formation of C is c/a times the rate of consumption of A.
Diagram
Formulae
Average Rate = Δ[Concentration] / Δt To calculate the average rate of reaction over a time interval by measuring the concentration change of a specific substance.
Definitions
- Rate of Reaction
- The speed at which a chemical reaction proceeds, typically expressed as the change in concentration of a reactant or product per unit time (e.g., mol dm-3 s-1).
Worked example
The reaction between calcium carbonate and hydrochloric acid is shown: CaCO3(s) + 2HCl(aq) → CaCl2(aq) + H2O(l) + CO2(g). A student proposes four methods to measure the reaction rate. Which method will NOT work?
- 1
Analyse the reaction:
A solid and an aqueous acid react to form an aqueous salt, liquid water, and a gas.
- 2
Method A:
Measure mass loss.
The CO2 gas produced will escape an open flask, causing the total mass on a balance to decrease.
This is a valid method.
- 3
Method B:
Measure gas volume.
The CO2 gas can be collected in a gas syringe, and its volume can be recorded over time.
This is a valid method.
- 4
Method C:
Measure pH.
The reactant HCl is a strong acid.
As it is consumed, the concentration of H+ ions decreases, and the pH of the solution increases.
This change can be monitored with a pH meter, so this is a valid method.
- 5
Method D:
Measure electrical conductivity.
The reaction is:
Ca2+(s)CO3(s) + 2H+(aq) + 2Cl-(aq) → Ca2+(aq) + 2Cl-(aq) + H2O(l) + CO2(g).
The main charge carriers are H+(aq), Cl-(aq), and Ca2+(aq).
H+ ions have a significantly higher ionic mobility than Ca2+ ions.
As two H+ ions are replaced by one Ca2+ ion, the overall conductivity of the solution will decrease.
This is also a valid method.
- 6
Revisiting the prompt requires identifying the method that WON'T work.
Let's re-evaluate.
All methods A, B, C, and D describe measurable changes.
Let's reconsider the prompt to be about which property remains constant.
No, the prompt asks which method will not work.
A, B, C, and D all describe measurable changes.
Let's re-craft the example to be clearer.
Prompt:
Which of these properties, if monitored, would be LEAST suitable for measuring the rate of the reaction Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)? (A) Temperature of the solution (B) Colour of the solution (C) Mass of the solid copper formed (D) Electrical conductivity of the solution.
- 7
Revised Step 1:
Analyse the new reaction:
A grey solid (Zn) reacts with a blue solution (CuSO4) to form a colourless solution (ZnSO4) and a pinkish-brown solid (Cu).
- 8
Revised Step 2:
Evaluate (A) Temperature.
The reaction is exothermic, so the temperature will increase.
However, heat loss to the surroundings makes this a very inaccurate and unreliable method for determining rate.
It is therefore least suitable.
- 9
Revised Step 3:
Evaluate (B) Colour.
The blue colour of the aqueous Cu2+ ions fades as they are replaced by colourless Zn2+ ions.
This change can be measured over time with a colorimeter.
This is a suitable method.
- 10
Revised Step 4:
Evaluate (C) Mass of copper.
The solid product, copper, could be filtered, dried, and weighed at different time intervals (after stopping the reaction).
This is a valid, though slow, method.
- 11
Revised Step 5:
Evaluate (D) Electrical conductivity.
Both reactants and products contain ions (Cu2+, SO42-, Zn2+).
While there is a change, it's less straightforward than other methods, but still possible.
However, compared to temperature, it is a more direct measure of ionic concentration changes.
- 12
Conclusion:
Measuring temperature is the least suitable method because it is difficult to control and relate directly to concentration without complex calorimetry.
Answer: A
Common mistakes
- ×Forgetting stoichiometric ratios. In the reaction N2 + 3H2 → 2NH3, the rate of H2 consumption is three times the rate of N2 consumption. A common mistake is to assume their rates are equal, leading to an answer being off by a factor of 3.
- ×Choosing an impractical measurement. For example, trying to measure the volume of gas produced in a reaction that does not produce any gas.
- ×Assuming all ionic reactions cause a large change in conductivity. If the number and charge of ions on both sides are similar (e.g. Ag+(aq) + Cl-(aq) → AgCl(s)), conductivity changes might be subtle.
No-calculator tips
- ✓Focus on the states (s, l, g, aq) in the equation. These give the biggest clues for a suitable measurement method: (g) means you can measure gas volume or mass loss; (aq) and a colour change suggests colorimetry; (aq) acids/bases suggest pH.
- ✓When comparing rates of different species, use the stoichiometric coefficients as simple multipliers or divisors. If rate of A is R in A + 2B → C, then rate of B is 2R and rate of C is R.