AP Chemistry Flashcards: Hess’s Law
Written by AP Content Team, Verified for 2026 AP Exams, Last updated: July 2026
Review key ideas with interactive flashcards. This set includes 10 cards to help you master important concepts.
When combining multiple step-reactions to find an overall reaction, how do you calculate the overall ΔH?
To find the overall ΔH, you sum the ΔH values of the individual reactions that were combined to form the overall process.
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When combining multiple step-reactions to find an overall reaction, how do you calculate the overall ΔH?
To find the overall ΔH, you sum the ΔH values of the individual reactions that were combined to form the overall process.
How is the enthalpy change of an overall process related to the enthalpy changes of its individual steps?
The net enthalpy change of an overall process at constant pressure is equal to the sum of the enthalpy changes of its individual steps.
List the three main rules for manipulating reactions and their ΔH values according to Hess's Law.
1. Reversing a reaction reverses the sign of ΔH. 2. Multiplying a reaction by a factor 'c' multiplies ΔH by 'c'. 3. Summing reactions means summing their ΔH values.
If the coefficients of a balanced chemical equation are multiplied by a factor 'c', what happens to the reaction's ΔH?
If a reaction is multiplied by a factor 'c', its ΔH value is also multiplied by the same factor 'c'.
What is the fundamental principle behind Hess's Law?
Hess's Law is based on the principle of energy conservation, stating that the net enthalpy change of a process is the sum of the enthalpy changes of its individual steps.
What happens to the value of ΔH if a chemical reaction is reversed?
According to Hess's law principles, reversing a reaction reverses the sign of its enthalpy change (ΔH).
What is the core procedural idea behind using Hess's Law?
The core idea is to represent a chemical or physical process as a sequence of individual steps, each with a known energy change, and then sum those changes.
What fundamental law of physics provides the foundation for Hess's Law?
Hess's Law is a direct consequence of the law of conservation of energy.
Why is Hess's Law a useful tool for analyzing many chemical processes?
It is useful because many complex processes can be broken down into a series of simpler steps, each with its own measurable or known energy change.
Define Hess's Law in the context of breaking down a process.
Hess's Law explains that a chemical or physical process can be represented as a sequence of steps, and the overall enthalpy change is the sum of the enthalpies of those individual steps.