Question

The net heat evolved or absorbed in a chemical process is the same whether the reaction takes place in one or in several steps is known as:

• A. Kirchhoff's law
• B. Le Chatelier's principle
• C. Beer's law
• D. Hess's law

Hint:

Hess's law allows us to mathematically add and subtract chemical equations like regular algebraic equations to determine unknown reaction enthalpies.

Answer 1

The Correct Option is D

Concept: The question states a foundational definition in chemical thermodynamics related to reaction enthalpy. Enthalpy ($H$) is a state function. This means that its change ($\Delta H$) during a chemical process depends solely on the initial state of the reactants and the final state of the products, remaining independent of the specific mechanism or reaction pathway taken. Let us evaluate the choices:

• Kirchhoff's Law: Describes how the enthalpy of a chemical reaction changes with varying temperature using heat capacities ($\Delta C_p$).

• Le Chatelier's Principle: Predicts how a chemical system at equilibrium shifts when subject to a change in temperature, pressure, or concentration parameters.

• Beer's Law: Relates the absorption of light by a chemical species to its concentration and path length in spectroscopy.

• Hess's Law of Constant Heat Summation: States that the total enthalpy change for a chemical reaction is equal to the sum of the enthalpy changes for each individual step making up the total path.

Step 1: Relate the problem description to thermodynamic state variables.
Because enthalpy is a state function, if a process changes from state 1 to state 2 via a direct path, the net heat change is $\Delta H_{\text{direct}}$. If it proceeds through intermediate steps ($1 \to A \to B \to 2$): \[ \Delta H_{\text{overall}} = \Delta H_{1 \to A} + \Delta H_{A \to B} + \Delta H_{B \to 2} \] Hess's Law states that: \[ \Delta H_{\text{direct}} = \Delta H_{\text{overall}} \] This precisely matches the statement provided in the question.