Specifications

Core Practicals

Core Practical 6

Investigating reaction rates

Introducing the Practical

This investigation is in two parts. Both parts require the reaction to be observed with respect to time to obtain the rate. 

  

In the first part, marble chips must be added to hydrochloric acid, and the volume of gas collected and measured over time. This will lead to graphical analysis to calculate rate, as well as an appreciation for how the rate may change with varying concentration of acid/temperature/surface area of marble chips.

  

The second part involves sodium thiosulfate reacting with dilute hydrochloric acid to produce a precipitate using the idea of a ‘disappearing cross’ to observe the change in the appearance of the reaction mixture as a precipitate of sulfur is formed. This must be carried out at different temperatures by warming the thiosulfate solution. A graph must be drawn to show the time taken for the reaction to take place at different temperatures. 

Risk Assessment

As a general rule, eye protection (goggles) must be worn for all practicals.

  

This risk assessment is provided as an example only, and you must perform your own risk assessment before doing this experiment.

Apparatus

Each group will need:

  • marble chips (3 sizes)

  • hydrochloric acid (3 concentrations)

  • 250 ml conical flask

  • 100 ml gas syringe

  • electric waterbath

  • sodium thiosulfate

  • 50 ml measuring cylinder

  • stop clock or stopwatch

  • 10 ml measuring cylinder

  • thermometer

Experiment Set-up

Method

Part 1 - Investigate the rate of a reaction by measuring the production of a gas

  1. use a measuring cylinder to add 50 ml of hydrochloric acid to a conical flask

  2. add 0.5 g of marble chips (calcium carbonate) to the flask, and immediately connect the gas syringe and start a timer

  3. at every 20 seconds, record how much gas has been produced

  4. when the reaction is complete, clean the apparatus as instructed by your teacher

  5. repeat steps 1 to 5 with either:

  6. different sized marble chips

  7. different concentrations of hydrochloric acid

  8. different temperatures of the acid

Part 2 - Investigate the rate of a reaction by observing a colour change

  1. use a measuring cylinder to add 50 ml of sodium thiosulfate solution to a conical flask

  2. measure and record the temperature of the solution

  3. place the conical flask on a piece of paper with a black cross drawn on it

  4. use another measuring cylinder to add 10 ml of hydrochloric acid to the flask, and immediately start a timer

  5. when the cross is no longer visible record the time taken, and then clean the apparatus as instructed by your teacher

  6. repeat steps 1 to 6 with different starting temperatures of sodium thiosulfate solution

Results and Analysis

Part 1 - Investigate the rate of a reaction by measuring the production of a gas

For each concentration/chip size/temperature, plot a graph on the same set of axes to show:

  • volume of gas (ml) on the Y axis (vertical)

  • time (s) on the X axis (horizontal)

  • a curve of best fit

  

Calculate the mean rate of reaction for each concentration/ chip size/ temperature.

  

Describe the effect of increasing the concentration/increasing chip size/increasing the temperature on the mean rate of reaction.

Part 2 - Investigate the rate of a reaction by observing a colour change

Calculate 1000 ÷ time for each temperature. This value is proportional to the rate of reaction.

  

Plot a graph to show:

  • reaction rate (/s) on the Y axis (vertical)

  • temperature (°C) on the X axis (horizontal)

  • a curve of best fit

  

Describe the effect of increasing the temperature of the reaction mixture on the rate of reaction.

Exam Question and Model Answer

A chemical company makes calcium chloride by reacting calcium carbonate and hydrochloric acid. They think they can increase the rate of reaction by increasing the concentration of the acid. Describe an experiment they could do in a laboratory to be able to test this idea.

[6 marks}

Level 1 (1-2 marks)

Add calcium carbonate powder to a conical flask.
Pour hydrochloric acid into the flask, mix, and immediately attach a gas syringe.
Measure how much gas has been produced every 20 seconds, and record in a table.
Compare the results to find out which reaction has a faster rate.

Level 2 (3-4 marks)

Add calcium carbonate powder to a conical flask.
Pour hydrochloric acid into the flask, mix, and immediately attach a gas syringe.
Measure how much gas has been produced every 20 seconds, and record in a table.
Repeat these steps with other concentrations of hydrochloric acid as well (keep the volume the same).
Compare the results to find out which reaction has a faster rate.

Level 3 (5-6 marks)

Add 5 g of calcium carbonate powder to a conical flask.
Using a measuring cylinder, pour and mix 20 ml of 0.5 mol/dm3 hydrochloric acid into the flask, and immediately attach a gas syringe.
Measure how much gas has been produced every 20 seconds, and record in a table.
Repeat these steps with 1.0 mol/dm3 and 1.5 mol/dm3 hydrochloric acid as well (keeping the temperature, volume of acid the same, and the mass of the calcium carbonate the same).
Calculate the rate of each reaction (volume ÷ time), and compare the results to find out which reaction has a faster rate.

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