5.1.4Physical Chemistry and Transition Elements
Investigating Reaction Rates
5 min read 5 parts Lesson 5 of 40
Section 1 of 5Continuous Monitoring Methods
A property that changes steadily as the reaction proceeds is recorded at regular time intervals.
Gas volume
- Collect the gas given off in a gas syringe and record volume against time;
- this suits reactions that release a gas such as hydrogen or carbon dioxide.
Mass loss
- Place the reaction on a balance and record the falling mass as gas escapes;
- it works best for dense gases like carbon dioxide.
Colorimetry
- Measure how much light a coloured species absorbs, using a filter of the complementary colour,
- because absorbance is proportional to concentration.
Titration
- Remove samples at set times, quench the reaction by dilution or cooling, then titrate to find the remaining concentration of a reactant.
Conductivity or pH
- If ions or hydrogen ions are used up or produced, the conductivity or pH of the mixture changes and can be logged by a probe.
Section 2 of 5Reading Rates from Graphs
Concentration–time data are converted to a rate by looking at the gradient.
1
Plot concentration of a reactant or product on the vertical axis against time on the horizontal axis, using the data recorded during continuous monitoring.
2
Draw a tangent to the curve at the chosen time so that it just touches the line without crossing it there.
3
Find the gradient of the tangent using a large triangle, ignoring the sign, to obtain the rate in mol dm⁻³ s⁻¹ at that time.
4
Repeat at several concentrations and plot rate against concentration: a straight line through the origin shows first order, a horizontal line shows zero order.
5
For a first-order reaction the half-life is constant, so measuring two successive half-lives on a concentration–time curve identifies the order.
Section 3 of 5The Initial Rates and Clock Methods
The Initial Rates and Clock Methods
Initial rate
- Run several experiments changing one concentration at a time and measure the gradient at time zero,
- when the concentrations are exactly known.
Clock reaction
- Time how long it takes for a fixed small amount of product to appear, such as the sudden blue-black colour in the iodine clock.
Approximation
- Rate is proportional to 1/t for a clock reaction, so doubling the rate halves the time taken, provided little reactant is used.
Controls
- Keep temperature, total volume and the other concentrations constant, otherwise the effect of one variable cannot be isolated.
| Method | Measured quantity | Suitable when |
|---|---|---|
| Gas syringe | Volume of gas | A gas is produced |
| Colorimeter | Absorbance | A reactant or product is coloured |
| Sampling and titration | Remaining concentration | No other property changes measurably |
Section 4 of 5Worked Example
Question
- A tangent drawn to a concentration–time curve passes through (0 s, 0.80 mol dm⁻³) and (50 s, 0.30 mol dm⁻³).
- In an iodine-clock experiment the time to the colour change was 80 s at one concentration of persulfate and 40 s when it was doubled.
- Find the initial rate and the order.
- Gradient of tangent = (0.30 − 0.80) ÷ (50 − 0) = −0.010 mol dm⁻³ s⁻¹.
- The rate is the size of the gradient, so the rate is 0.010 mol dm⁻³ s⁻¹.
- Rate is proportional to 1/t, so halving the time from 80 s to 40 s doubles the rate.
- Doubling the concentration doubles the rate, so the reaction is first order in persulfate.
Answer
- Rate = 0.010 mol dm⁻³ s⁻¹;
- first order with respect to persulfate.
Section 5 of 5Exam Checklist & Tips
5.1.4 – Key Points for the Exam
- Initial rate The rate at time zero, found from the gradient of the tangent to a concentration–time graph at the start.
- Clock reaction An experiment timing how long a fixed amount of product takes to form, so that 1/time is a measure of rate.
- Colorimetry A method using light absorbance to follow the concentration of a coloured species.
- Quenching Stopping a reaction in a sample, for example by dilution or cooling, so that it can be analysed later.
- Tangent A straight line touching a curve at one point, whose gradient equals the rate at that point.
Common Mistakes
- Taking a gradient between two points on the curve instead of drawing a true tangent at the chosen time.
- Quoting a negative rate;
- the gradient of a falling reactant curve is negative but the rate is always positive.
- Changing two variables at once in an initial-rates experiment, so the order cannot be deduced.
- Using 1/t as the rate in a clock reaction when a large fraction of reactant has already been used up.