Both oxyhydrogen torches and fuel cells use the following reaction to produce energy: 2 H2(g) + O2(g) → 2 H2O(l). Both processes occur at constant pressure. In both cases, the change in state of the system is the same: the reactant is oxyhydrogen (“Knallgas”) and the product is water. Yet, with an oxyhydrogen torch, the heat evolved is large, and with a fuel cell, it is small. If heat at constant pressure is considered to be a state function, why does it depend on path?

Verified step by step guidance

1. Step 1: Understand the concept of a state function. A state function is a property whose value does not depend on the path taken to reach that specific value. Examples include enthalpy, internal energy, and entropy.
2. Step 2: Recognize that enthalpy (H) is a state function. In the given reaction, the change in enthalpy (ΔH) is the same regardless of the process (oxyhydrogen torch or fuel cell) because it depends only on the initial and final states of the system.
3. Step 3: Differentiate between the terms 'heat' and 'enthalpy'. While enthalpy is a state function, the heat (q) exchanged in a process can vary depending on the path, especially in terms of how the reaction is carried out (e.g., fast combustion in a torch vs. controlled reaction in a fuel cell).
4. Step 4: Consider the role of reaction kinetics and mechanism. In an oxyhydrogen torch, the reaction occurs rapidly, releasing a large amount of heat quickly, whereas in a fuel cell, the reaction is controlled and slower, resulting in a smaller, more gradual release of heat.
5. Step 5: Conclude that while the total enthalpy change (ΔH) is the same for both processes, the rate and manner in which heat is released (the path) can affect the perceived amount of heat evolved, even though the overall energy change is consistent.