Rates experiments: methods, variables and data (Core Practical 5)

Ways to measure a rate

• Time for a fixed change: time for a precipitate to hide a cross, for a colour to appear, or for a precipitate to form. Rate is proportional to 1/time, unit s^-1.
• Amount changed over time: gas volume in a syringe or loss of mass on a balance, read at intervals; the gradient of the graph gives the rate.
• Start timing the moment the reactants are mixed.

Core Practical 5 skills

Halogenoalkanes react with water in ethanolic silver nitrate; the halide ion released forms a precipitate, and the time for it to appear measures the rate.

• Ethanol is the solvent because halogenoalkanes do not dissolve in water.
• Control variables: the same amount in moles of each halogenoalkane (not the same volume, because their densities and molar masses differ), the same volumes of ethanol and silver nitrate, and the same temperature (a water bath).
• Results: rate increases from chloro to bromo to iodo, because the C-I bond is weakest; tertiary react faster than primary.
• Precision: timing by hand has a reaction time of about ±0.5 s, and judging when a precipitate 'appears' is subjective. Record times consistently (for example to the nearest second).

Temperature experiments and data

• Warm each reactant separately in a water bath to the target temperature before mixing.
• A graph of rate against temperature is an upward curve (rate roughly doubles for each 10 °C rise in many reactions), not a straight line.
• Calculate 1/t to 2 significant figures, matching the data; include units (s^-1).
• Improvements: water bath for constant temperature, a light sensor or colorimeter instead of judging by eye, the same observer, more temperatures.

Worked example

A precipitate takes 25 s to appear. Calculate the rate as 1/t.

1. Rate = 1 / time
2. = 1 / 25 s

Answer: 0.040 s^-1

Check yourself

1. A cross disappears after 64 s. Calculate the rate as 1/t.
   Show answer

   0.016 s^-1. 1 / 64 = 0.0156 s^-1, so 0.016 s^-1 to 2 s.f.

2. Which halogenoalkane gives a precipitate fastest with ethanolic silver nitrate?
   • 1-bromobutane
   • 1-chlorobutane
   • 1-iodobutane
   • 1-fluorobutane
   Show answer

   1-iodobutane. The C-I bond is the weakest, so it breaks most easily.

3. Why are equal amounts in moles, rather than equal volumes, of the halogenoalkanes used?
   • Their densities and molar masses differ, so equal volumes contain different numbers of moles
   • Volumes cannot be measured accurately
   • Equal volumes would make the reaction too fast
   • The halogenoalkanes are gases
   Show answer

   Their densities and molar masses differ, so equal volumes contain different numbers of moles. A fair comparison needs the same number of molecules.

4. Why are the tubes placed in a water bath at a set temperature?
   • To make the precipitate dissolve
   • To speed up the timing
   • To stop the ethanol evaporating
   • Temperature affects rate, so it must be kept the same in each run
   Show answer

   Temperature affects rate, so it must be kept the same in each run. Temperature is a control variable.

5. Why is ethanol used as a solvent in Core Practical 5?
   • Ethanol is the nucleophile
   • Halogenoalkanes are insoluble in water but dissolve in ethanol
   • Ethanol reacts with silver nitrate
   • Ethanol keeps the precipitate white
   Show answer

   Halogenoalkanes are insoluble in water but dissolve in ethanol. Ethanol lets the halogenoalkane, water and silver nitrate mix in one phase.

6. Which improvement removes the judgement of when a precipitate appears?
   • Using a larger test tube
   • Timing to 0.01 s with a stopwatch
   • Repeating with a different observer each time
   • Using a light sensor or colorimeter to detect a set change in light passing through
   Show answer

   Using a light sensor or colorimeter to detect a set change in light passing through. A sensor detects the same amount of cloudiness every time.

7. At 30 °C a reaction takes 80 s; at 40 °C it takes 38 s. By what factor has the rate increased?
   Show answer

   2.1. Rate is proportional to 1/t, so factor = 80 / 38 = 2.1.

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Sources

• Pearson Edexcel IAL Chemistry (2018) specification
• Pearson IAL Chemistry teacher practical guide
• Pearson examiners' report, WCH13 October 2021

Last updated 8 November 2026. Spotted a mistake? Tell us.