7.9 - Measuring temperature changes in reactions

Temperature Change Evidence

When a reaction or dissolving process happens in solution, the solution is part of the surroundings being measured. If heat energy is transferred to the solution, the measured temperature rises. If heat energy is taken from the solution, the measured temperature falls.

The temperature change is found from:

temperature change = final temperature - initial temperature

A positive value means the solution temperature increased. A negative value means the solution temperature decreased. The sign matters because +6.0 °C and -6.0 °C show opposite changes.

The temperature change does not tell you every detail about the energy transfer. Heat can be lost to the air, absorbed by the cup, or missed if the final reading is taken too late. It is still useful evidence because a larger measured rise or fall usually shows a larger heat change under the same conditions.

Four Solution Changes

Four types of change can be investigated by measuring temperature in solution.

For salts dissolving, do not assume every salt behaves the same way. Some dissolving changes make the solution colder, while others make it warmer. The measured temperature change is the evidence.

For a neutralisation example, dilute hydrochloric acid reacting with sodium hydroxide solution forms sodium chloride solution and water:

hydrochloric acid + sodium hydroxide -> sodium chloride + water
HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l)

For a displacement example, zinc can displace copper from copper sulfate solution:

zinc + copper sulfate -> zinc sulfate + copper
Zn(s) + CuSO4(aq) -> ZnSO4(aq) + Cu(s)

For a precipitation example, calcium chloride solution can react with sodium carbonate solution to form solid calcium carbonate:

calcium chloride + sodium carbonate -> calcium carbonate + sodium chloride
CaCl2(aq) + Na2CO3(aq) -> CaCO3(s) + 2NaCl(aq)

Measuring Temperature Change

A simple way to measure a temperature change in solution is to use an insulated cup. A polystyrene cup is often placed in a beaker to make it more stable. A lid helps reduce heat transfer to or from the air.

[DIAGRAM: asset_name: solution_temperature_change_apparatus_and_data: Measuring temperature changes in solution reactions - diagram 1; asset_slug: 171_7_9_temperature_changes_in_solution_reactions_diagram1; file: diagram_assets/171_7_9_temperature_changes_in_solution_reactions_diagram1.png; recommended_method: deterministic_drawn; description: Deterministic monochrome split-panel teaching visual. Left panel shows a polystyrene cup supported in a beaker with a lid, thermometer or temperature probe, stirring rod, measuring cylinder for a known volume of solution A, and solution B being added. Labels include record initial temperature and record highest or lowest temperature after mixing. Right panel shows a compact example data table for salts dissolving, neutralisation, displacement and precipitation with initial temperature, final temperature and temperature change values, including both positive and negative changes. The visual is exact practical/data support, so deterministic drawing is required; all values and labels are also explained in the lesson prose.]

A general method for a solution reaction is:

1. Measure a known volume of the first solution into the insulated cup.
2. Bring both reactants to the same starting temperature, then record the initial temperature.
3. Measure a known volume, or known mass, of the second reactant.
4. Add the second reactant, put the lid on if used, and stir gently.
5. Keep the probe bulb immersed without touching the cup. Record the highest temperature reached if the temperature rises, or the lowest temperature reached if the temperature falls.
6. Calculate the temperature change.