Rate of Reaction: AQA Disappearing Cross Practical

AQA GCSE Chemistry Rate of Reaction Practical using the ‘Disappearing Cross’ method, where we change the temperature each time, with step-by-step visuals. Combined & Triple Science.

A student investigates how the Rate of a Reaction changes with Temperature. He has the following solutions:

Sodium thiosulfate
Hydrochloric acid

Plan a valid method using the Disappearing Cross method.

[6 marks]

1. Preparation

- Draw a cross on a piece of paper
- Place an empty conical flask on it

Black cross drawn on white paper for the GCSE Chemistry disappearing-cross rates of reaction experiment.

Empty conical flask placed over a black cross on paper, ready for the sodium thiosulfate and hydrochloric acid reaction.

- Use a measuring cylinder to measure 50cm³ of sodium thiosulfate
- Use another measuring cylinder to measure 10cm³ of hydrochloric acid
- Pour the sodium thiosulfate into the conical flask first, then the acid

Two measuring cylinders containing 50 cm³ sodium thiosulfate solution and 10 cm³ hydrochloric acid.

Sodium thiosulfate solution added to a conical flask, followed by hydrochloric acid to begin the disappearing-cross reaction.

2. Measurement

- Start the stopwatch as soon as the hydrochloric acid is added, then immediately swirl the flask
- We can then measure temperature using a thermometer

Stopwatch started as hydrochloric acid is added and the conical flask is immediately swirled to mix the reactants.

Thermometer measuring the initial temperature of the sodium thiosulfate and hydrochloric acid reaction mixture.

- Stop the stopwatch, as soon as the cross disappears. Make sure you are looking down through the conical flask at the cross.
- We record the temperature again and calculate a mean from the two temperatures.

Stopwatch stopped when the cross beneath the flask is no longer visible because the reaction mixture has become cloudy.

Thermometer measuring the final temperature of the cloudy reaction mixture after the cross disappears.

3. Results

- Independent variable (what will change): Mean Temperature (°C) in degrees Celsius
- Dependent variable (what we are measuring): Time taken for cross to disappear (s) in seconds
Mean temperature is calculated from the two measured temperature values = (initial temperature + final temperature) ÷ 2.

Results table for recording mean reaction temperature in degrees Celsius and time taken for the cross to disappear in seconds.

- Relative rate of reaction (s⁻¹) = 1 ÷ time (s)
- Add that to your table

Formula for calculating relative rate of reaction: one divided by the time taken in seconds, giving units of s⁻¹.

Results table showing mean temperature, time taken for the cross to disappear and calculated relative rate of reaction.

- Warm sodium thiosulfate to different temperatures using a hot water bath
- Repeat all previous steps to find Rate of Reaction for different Temperatures

Sodium thiosulfate solution warmed in a water bath to investigate the effect of temperature on reaction rate.

Results table for comparing mean reaction temperature, time taken and relative reaction rate across different temperatures.

Variables

Thermometer representing the independent variable: temperature of the reaction mixture in the disappearing-cross experiment. Stopwatch representing the dependent variable: time taken for the cross to disappear during the reaction.

Challenge:

- Sulfur precipitate forms, making it cloudy enough

- Judgement of when the cross fully disappears can differ from person to person.
- Having the same person makes sure the endpoint is judged consistently, improving reliability.

- Instead warming the solution to obtain different temperatures and measuring with a thermometer, use different volumes of sodium thiosulfate solution and water, keeping their combined volume constant.
- Keep the hydrochloric acid volume & acid concentration, as well as the temperature unchanged.

Continue Revising Paper 2

GCSE Chemistry Visuals

Rate of Reaction: AQA Disappearing Cross Practical

6-marker on the AQA Rate of Reaction required practical, GCSE Chemistry, using the disappearing cross method. Temperature is the independent variable.

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