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Science 7–10 · Year 8

Warmer or colder: measuring the energy change of four reactions

Change

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

A chemical reaction transfers energy to or from its surroundings, so the temperature of the mixture rises (exothermic) or falls (endothermic).

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

  • sodium hydroxide 0.4 mol/L and the acids are irritants
  • magnesium powder is highly flammable and hydrogen is produced
  • the magnesium powder and copper(II) sulfate mixture warms sharply: with no heat loss the 4.0 mmol of copper(II) ions in 10 mL of 0.4 mol/L solution would raise the liquid by about 50 °C, enough to scald, and fresh magnesium powder releases that heat fast
  • copper sulfate is harmful and toxic to aquatic life

Controls

  • eye protection
  • magnesium powder issued in single measures by the teacher; no flames
  • the magnesium powder goes into the copper(II) sulfate as one measure, never in repeated doses, and is stirred with the thermometer; learners keep their faces clear of the cup, leave it standing in its beaker and do not pick it up until it has cooled
  • copper residues to labelled waste

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson.

What you need

  • polystyrene cup standing in a 100 mL beaker, thermometer reading to 0.5 °C or a temperature probe
  • sodium hydroxide solution 0.4 mol/L, 10 mL, and hydrochloric acid 0.4 mol/L, 10 mL
  • sodium hydrogencarbonate solution 0.4 mol/L, 10 mL, and citric acid crystals, 4 small spatula measures
  • copper(II) sulfate solution 0.4 mol/L, 10 mL, and magnesium powder, 1 small spatula measure (issued by the teacher)
  • dilute sulfuric acid 0.4 mol/L, 10 mL, and one 3 cm piece of magnesium ribbon (about 0.03 g to 0.04 g; ribbon thickness varies)

How to do it

  1. Put the first solution in the cup and record its temperature to 0.5 °C.
  2. Add the second reagent, stir with the thermometer and record the highest (or lowest) temperature reached within 2 minutes.
  3. Rinse the cup and repeat for each of the four pairs. Record the temperature change with its sign.
  4. Classify each reaction as exothermic or endothermic and write its word equation.

What you should see

Acid with alkali warms by only a couple of degrees. Citric acid with sodium hydrogencarbonate cools by a few degrees while fizzing. Magnesium powder with copper sulfate warms by more than 10 °C and the blue fades as brown copper appears; with no heat loss the 4.0 mmol of copper(II) ions in the 10 mL would raise the liquid by about 50 °C, so the cup can become hot enough to scald. Magnesium ribbon in sulfuric acid warms by several degrees while hydrogen bubbles off. The learner knows it worked when the three exothermic mixtures (acid with alkali, magnesium with copper sulfate, magnesium with sulfuric acid) show a rise, the citric acid mixture shows a fall, and a repeat of each mixture gives the same sign and a temperature change within about 3 °C of the first. Closer agreement is not available at these quantities: the magnesium ribbon is issued as a 3 cm piece rather than weighed, and ribbon thickness moves its mass by about a quarter either way, which moves that run's rise by a few degrees.

What changes

What you change
reaction (four pairs of reagents)
What you measure
temperature change of the mixture
What you keep the same
  • volumes of solution
  • starting temperature
  • same cup and thermometer
  • stirring

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

  • Fizzing means a reaction is giving out heat.
  • A reaction that feels cold has no energy change.
  • The temperature rise comes from the stirring.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Sources

The pages the author read to write this activity.

  1. curriculum.nsw.edu.au/learning-areas/science/science-7-10-2023/outcomes
  2. edu.rsc.org/experiments/exothermic-or-endothermic-classifying-reactions/406.article
  3. edu.rsc.org/cpd/measuring-energy-changes/4010499.article
  4. education.nsw.gov.au/content/dam/main-education/asset-management/chemical-safety/5._Volume_2_Appendices.pdf

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