Study energy changes in two chemical reactions using thermometer strips to measure temperature

Use this class practical to explore energy changes in neutralisation at dropscale. Students observe the reaction of hydrochloric acid with magnesium, before adding sodium hydroxide and measuring the temperature change during the neutralisation reaction.

The practical should take approximately 20 minutes.

Equipment

Apparatus

  • Eye protection
  • Student worksheet
  • One thermometer strip per group

Chemicals

Note

Solutions should be contained in plastic pipettes. See the accompanying guidance on apparatus and techniques for microscale chemistry, which includes instructions for preparing solutions.

  • Hydrochloric acid, 2 mol dm–3
  • Sodium hydroxide, 2 mol dm–3
  • Magnesium ribbon

Health, safety and technical notes

  • Read our standard health and safety guidance.
  • Wear eye protection throughout (splash-resistant goggles to BS EN166 3).
  • Hydrochloric acid, 2 mol dm–3 HCl(aq), is of low hazard.
  • Sodium hydroxide solution, 2 mol dm–3 NaOH(aq), is CORROSIVE.
  • Magnesium ribbon is FLAMMABLE.

Procedure

  1. Put two drops of dilute hydrochloric acid on each marked section of the temperature strip.
  2. Add a small piece of magnesium ribbon to each drop.
  3. Observe and explain your findings.
  4. Wipe the strip clean with tissue paper.
  5. Put one drop of hydrochloric acid on the strip in each of the places marked by the arrows.
  6. Add one drop of sodium hydroxide to each drop of acid.
  7. Observe and explain your findings.

Observations

In both cases – the reaction of the piece of magnesium with the hydrochloric acid and the neutralisation reaction – it should be apparent that energy has been given out as heat since the numbers under the drops should illuminate. The highest green number can be regarded as the highest temperature reached.

It might be possible to obtain a value for the enthalpy change of the neutralisation of a strong acid by a strong base. Students know the concentration of the reagents and will be able to observe the temperature rise. They will need to know the volumes involved – the volume of one drop is ca 0.02 cm3.

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Additional information

This resource is part of our Microscale chemistry collection, which brings together smaller-scale experiments to engage your students and explore key chemical ideas. The resources originally appeared in the book Microscale chemistry: experiments in miniature, published by the Royal Society of Chemistry in 1998.

© Royal Society of Chemistry

Health and safety checked, 2018