Monitoring the rate of reaction between two liquids: results

The results from a trial experiment using thiosulphate at room temperature and at bench concentrations are shown below:

Figure 3: graph from PicoLog

The y-axis is left scaled in mV although ideally this could be converted to percentage of light reaching the LED or perhaps more simply completeness of reaction. Although a little jagged, the trend can be seen to be exponential with the rate of reaction diminishing as the reaction progresses.

Answers to questions

Q1. Why is the gradient of the graph initially quite steep but becomes less steep as time progresses?

A1. As time progresses the reactants are converted to products and are therefore in less abundance to react with each other. The reaction slows down.

Q2. What shape does the graph take if the chemicals are warmer?

A2. The steepness of the curve is greater and tends to level out sooner as the reactant particles are moving more quickly and make more violent collisions with each other more frequently. The reaction takes less time to reach completion.

Q3. What shape does the graph take if the chemicals are diluted?

A3. The curve is shallower as there are less reactant particles available to collide with each other.

Teachers’ notes

The manufacture of the sensor is a lot simpler than it may first appear and can be assembled for about £2.00. Perhaps a friendly technologist may lend a hand. Our science department now has 10 for use with data-loggers and voltmeters! The 3k9 resistor in series with the LDR is to convert the output resistance into a measurable voltage. The data sheet from Maplins and a few calculations pointed towards the value used. As the DrDAQ will measure up to 5 volts the greatest range possible was desirable.

Once the hardware is made, it should take no longer than 10 minutes to set up ready to use. A trial experiment would be useful to ensure you have suitable concentrations of reactants to give suitable results before letting students loose as most students have less patience than teaching staff!

Please check your safety documentation before using thiosulphate as some LEAs are reluctant to use it in the lab. If you do use it please advise students to avoid putting their faces directly over the reacting chemicals to avoid inhaling sulfurous fumes.

Changes in concentration and temperature can produce marvellous results with the data-logger and only very small changes are needed to see a marked difference. Possibly the making of an assessed piece of coursework?

References

The idea of using a data-logger to monitor rates of reaction came from an article in the ASE magazine School Science Review submitted by Vincent Parkes of Nunthorpe Community School, Middlesbrough in September 1999. Vincent used a voltmeter attached to a LDR in circuit with a DC supply to monitor the amount of light getting through a reacting cocktail. Vincent deserves credit for the idea and the notes above form further development and extensions to his initial work.

Submitted by

Robert Tudor, Science Teacher at Minsthorpe Community College, West Yorkshire. You can e-mail me with comments or for advise at Rtudor2294@aol.com.

Finally, do please try the setup suggested, I think you will be quite impressed by its potential.