Science 7–10 · Year 10
Zinc in copper sulfate: a displacement reaction you can measure by its heat
Reactions
School laboratory, not for home
In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
The idea
In a displacement reaction a more reactive metal takes the place of a less reactive one in its salt solution and releases energy, so the temperature rise in the same copper(II) sulfate solution compares how readily different metals displace copper.
Safety card
Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.
Hazards
- copper(II) sulfate 1.0 mol/L is harmful and causes eye damage
- zinc, iron and magnesium powders are flammable
- the zinc mixture can reach about 70 °C; magnesium powder releases far more energy with copper(II) sulfate (with no heat loss, enough to raise 20 mL of the solution by more than 100 °C), so fresh magnesium powder can make the mixture boil and spit
Controls
- eye protection
- powders issued in measured amounts by the teacher; no flames
- the magnesium run is a teacher demonstration behind a safety screen, with the powder added in small portions and learners at least 2 m back wearing eye protection
- metal residues to a bucket, never the sink
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. CSIS Appendix F lists zinc powder and iron powder under chemicals to be used with caution.
What you need
- copper(II) sulfate solution 1.0 mol/L, 20 mL per run
- zinc powder, two spatula measures (about 2 g, an excess), issued by the teacher; iron powder for a comparison run; magnesium powder for a comparison the teacher demonstrates
- polystyrene cup in a beaker, thermometer to 0.5 °C or a temperature probe with data logger, stop clock
How to do it
- Put 20 mL of 1.0 mol/L copper sulfate in the cup and record its temperature every 30 seconds for 2 minutes to get a steady start value.
- Add the zinc powder, stir with the thermometer and keep recording every 30 seconds until the temperature has peaked and started to fall.
- Plot temperature against time; extrapolate the cooling line back to the moment of mixing to estimate the true rise.
- Repeat with iron powder and rank the metals by temperature rise. The teacher then demonstrates the magnesium run behind a safety screen, adding the powder in small portions; compare the order of the rises with the order of the metals in the reactivity series.
What you should see
The blue solution fades toward colourless and a brown solid (copper) settles; the temperature climbs steeply and peaks within 2 minutes. The RSC lists typical measured rises of 49 °C for zinc, 39 °C for magnesium, 32 °C for iron and 22 °C for tin with 20 mL of 1.0 mol/L copper(II) sulfate, and notes that more extensive oxidation on the surface of the magnesium may explain why it comes out below zinc although it is the more reactive metal; the teacher's magnesium run, added in portions, is not a fair comparison either. The learner knows it worked when the measured rise for zinc is close to the RSC's 49 °C, the zinc rise is larger than the iron rise, and the blue colour has faded.
What changes
- What you change
- metal added (zinc and iron; magnesium in the teacher demonstration)
- What you measure
- maximum temperature rise
- What you keep the same
- volume and concentration of copper sulfate
- metal in excess
- same cup and thermometer
- stirring
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- The heat comes from the powder dissolving.
- Adding more zinc powder always gives a bigger temperature rise, even after all the copper ions have reacted.
- A reaction between a solid and a solution cannot release much energy.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Science 7–10 Syllabus (2023)SC5-RXN-01SC5-WS-05
- Australian Curriculum v9AC9S10U07AC9S10I04
Sources
The pages the author read to write this activity.