Learn to determine the enthalpy change of a reaction either directly or indirectly using Hess’s Law

The enthalpy change of a reaction is the energy change that takes place during a reaction per one mole of substance under standard temperature and pressure. Enthalpy changes are normally reported in kJ mol-1.

The enthalpy of neutralisation is fairly straightforward to measure as demonstrated in this video from Malmesbury Education. The enthalpy of neutralisation is defined as the enthalpy change when one mole of water forms from one mole of H+(aq) ions and one mole of OH- (aq) ions under standard temperature and pressure. Please note in the following video: the calculations and answers are correct, although the specific heat capacity of water is incorrectly written as 4.18 J/kg/°C rather than 4.18 J/g/°C.

Questions you can ask your students:

  • Why is the temperature change less than the enthalpy change might predict?
  • Why don’t you take a temperature measurement when the acid and base are mixed?

This experiment requires students to graphically represent their results and interpret them. To calculate the enthalpy change, students need to ‘extend’ the line of best fit beyond the time at which the reactants were added together and read off the change in temperature from the graph. When calculating the enthalpy change, students need to ensure that they are reporting the enthalpy change ‘per mole’ rather than in absolute terms.

It’s not always possible to measure the enthalpy change for a specific reaction, but Hess’s Law states that the enthalpy change for a chemical reaction is independent of the route taken. The following video by SpaceyScience highlights an experimental method to determine the enthalpy change for the reaction:

MgO (s) + CO2 (g) → MgCO3 (s)

Determining the enthalpy changes for the reaction of MgO and MgCO3 with HCl respectively enables students to determine the enthalpy change for ΔH1 using:

 ΔH1 = ΔH3 – ΔH2

There are a number of videos available online which do a good job of explaining the method, but are unreliable when it comes to the calculations. Many of the errors are very similar to the errors that will be made by students and you might want to use them as a teaching aid. Crucially, it’s common for exothermic and endothermic reactions to be reported incorrectly. Exothermic reactions should be reported as negative enthalpy changes and endothermic reactions should be reported as positive enthalpy changes. In addition, many common mistakes arise from the failure to convert enthalpy changes into kJ mol-1.

A final enthalpy change experiment involves determining the enthalpy change of combustion. In this experiment, a known quantity of fuel is burnt as it heats up a known quantity of water. The temperature change can be used to calculate the enthalpy change of combustion.

Questions you can ask your students:

  • Why will the enthalpy change of combustion be greater than your calculation?
  • Why must the spirit burner be left to cool before the mass is remeasured?
  • What is the black ‘soot’ on the bottom of the conical flask? How does this affect the reliability of the enthalpy change you have calculated? 

Students should consider sources of error in this experiment. Some water may be lost as water vapour to the atmosphere, and the heat from the spirit burner is being used to heat the conical flask as well as the surrounding air, so not all heat is directly transferred into the water. It’s important to measure the mass of the spirit burner as soon as it is safe to do so, as some vapour will be lost to the atmosphere as the burner cools.  

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