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Exothermic and Endothermic Reactions

Middle schoolChemistryFixed-enthalpy dissolution with an exponential dissolution rate, lumped solution and cup heat capacity, and Newton heat exchange with a fixed-temperature room

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About this experiment

Add a solid to a coffee-cup calorimeter and compare warming with cooling. The instrument couples dissolution progress to a measured temperature rather than treating temperature as heat itself. Vary stirring, cup heat capacity and heat exchange to see why a late thermometer reading can miss the adiabatic temperature change.

Learning goals

  • Distinguish exothermic from endothermic dissolution.
  • Calculate heat stored in both solution and calorimeter.
  • Account for heat exchange with the room during a temperature-time measurement.

How to use it

  1. 1Add anhydrous calcium chloride and record the initial warming curve.
  2. 2Repeat with ammonium chloride and compare the sign of the temperature change.
  3. 3Set room heat transfer to zero and compare the final temperature with the adiabatic prediction.
  4. 4Increase heat exchange, run longer and explain why the temperature returns toward the room even after dissolution is complete.

The model

Lumped calorimeter energy balance
Stored energy plus signed heat exchange

Assumptions

  • Rounded fixed dissolution enthalpies are -81.3 kJ/mol for anhydrous CaCl₂ and +14.8 kJ/mol for NH₄Cl, with molar masses 110.98 and 53.49 g/mol. Hydrates and concentration dependence are excluded.
  • The whole mixture has specific heat 4.18 J/(g·K), and the selected cup heat capacity is added. The liquid and cup share one uniform temperature and start at room temperature.
  • Dissolved fraction is 1 minus exp(-kt), where k = 0.025 times one plus stirring setting per second. Newton heat exchange uses the selected conductance. The coupled temperature solution is analytic and heat-transfer signs are retained.

Try this in class

  1. Q1Why is temperature change alone insufficient to compare heat between different water masses?
  2. Q2Why can an endothermic process have negative heat transferred to the room?

Frequently asked questions

Why does the same salt give different temperature changes with different water masses?

The released or absorbed heat is distributed over a different total heat capacity.

Why is room heat transfer sometimes negative?

When endothermic dissolution cools the cup below room temperature, the room supplies heat to the cup. The signed heat leaving the cup is therefore negative.