A Level Chemistry OCR Practice Exam

What purpose does an enthalpy cycle serve?

1. To illustrate the reaction mechanism of a process

2. To determine the enthalpy change from other known changes using indirect routes

3. To compare the state of reactants with products directly

4. To represent the reaction rates of different substances

The correct answer is: To determine the enthalpy change from other known changes using indirect routes

An enthalpy cycle is a valuable tool in thermodynamics that allows chemists to calculate the enthalpy change of a reaction indirectly. This can be particularly useful when the direct measurement of the enthalpy change is difficult or impossible. By utilizing Hess's Law, which states that the total enthalpy change for a reaction is the same regardless of the pathway taken, an enthalpy cycle provides a way to relate the enthalpy changes of different reactions that lead to the same overall change. In practice, this involves using known enthalpy changes, such as formation or combustion enthalpies, to construct a cycle that connects these values to the sought-after enthalpy change. By tracing different routes through the cycle that represent individual steps of the reaction, you can effectively combine the enthalpy changes to find the overall change for the desired reaction. While the other choices describe important concepts in chemistry, they do not align with the primary function of an enthalpy cycle. For example, the reaction mechanism relates to the detailed path that reactants take to become products, while the comparison of reactants and products focuses more on their states rather than the energy changes involved. Similarly, reaction rates concern kinetics rather than thermodynamic changes, which