Final answer:
Hess's law states that the enthalpy change of a reaction is the sum of the enthalpy changes of the steps involved. It allows for indirect calculation of reaction enthalpy, particularly in complex scenarios like the operation of an acetylene torch, by summing the enthalpy changes of individual reactions.
Step-by-step explanation:
Understanding Hess's Law in Calculating Enthalpy
Hess's law is a fundamental principle in chemistry that states the total enthalpy change in a chemical reaction is the same regardless of the number of steps the reaction takes. This means that if a process can be expressed as the sum of several steps, the enthalpy change for the overall process can be calculated by summing the enthalpy changes for each individual step.
Applying Hess's Law
To apply Hess's law, you can write out the chemical equations for each step and manipulate them to obtain the overall equation for the desired reaction. The enthalpy change of the reaction (ΔH°) can then be determined by summing the enthalpy changes of these individual steps, making sure to include any enthalpy changes associated with change of physical states, like the enthalpy of combustion (ΔH°comb) or the enthalpy of vaporization (ΔH°vap).
Additionally, standard heats of formation (ΔH°f) can be used in conjunction with Hess's law to indirectly calculate the heat of reaction for a reaction at standard conditions. This is particularly useful when direct measurement of a reaction is complex or impractical.
For example, in calculating the energy involved in using an acetylene torch, which has multiple reactions, Hess's law allows for the combination of the enthalpy changes of each chemical reaction to find the total enthalpy change.
Summarizing Hess's Law
Hess's law highlights that energy conservation applies to enthalpy in chemical reactions, and it enables chemists to calculate the energy change (ΔH) of reactions that might not be easily measured directly. The manipulation of equations to reflect the desired reaction's enthalpy change is a critical application of this principle.