International General Certificate of Secondary Education (IGCSE) Chemistry Practice Exam

Why might experimental values of enthalpy change differ from theoretical values?

1. Heat energy is gained from the surroundings

2. Not all reactants are converted to products

3. Experimental methodology is never accurate

4. Only part of the energy is accounted

The correct answer is: Not all reactants are converted to products

The difference between experimental values of enthalpy change and theoretical values can often be attributed to the fact that not all reactants are converted to products during a reaction. In a perfect reaction, you would expect 100% conversion of reactants to products; however, in real-world scenarios, several factors prevent this from happening. For example, incomplete reactions may occur due to side reactions, equilibrium states, or the formation of by-products, which means that the amount of energy released or absorbed during the reaction may be less than what is calculated theoretically based on stoichiometric relationships. These incomplete reactions directly lead to discrepancies between measured and expected enthalpies because the theoretical calculations assume complete conversion. Experimental values, on the other hand, reflect the actual energy changes that occur under the specific conditions of the experiment, taking into account all these practical limitations and inefficiencies.