Problem 18 - Chemistry An Atoms First Approach

Question

The enthalpy change for a reaction is a state function and it is an extensive property. Explain.

Step-by-Step Solution

Answer

The enthalpy change for a reaction, denoted by ΔH, is classified as a state function because it depends solely on the initial and final states of a system, and not how the system transitioned from one state to another. This means, irrespective of the path taken during the reaction, the enthalpy change will remain the same as long as the initial and final states are the same. The enthalpy change is also considered an extensive property, as its value is proportional to the quantity of substances involved in the reaction. In other words, if the amount of reactants increases or decreases, the enthalpy change will correspondingly increase or decrease. Thus, enthalpy change exhibits behavior of both a state function and an extensive property.

Step 1: Definition of a State Function

A state function is a property that depends only on the current state of a system and not on the path taken to reach that state. This means that any changes in its value depend solely on the initial and final states of the system, regardless of how the process occurred.

Step 2: Definition of an Extensive Property

An extensive property is a property that depends on the quantity or size of a substance within the system. Its value is proportional to the amount of substance in the system, which means if we change the amount of substance, the value of the property also changes linearly.

Step 3: Enthalpy Change as a State Function

The enthalpy change for a reaction, denoted by ΔH, is the difference in enthalpy between the products and reactants of a chemical reaction, calculated at constant pressure. Since enthalpy is a state function, its change depends only on the initial and final enthalpy values of the system. This means that ΔH is also a state function, as it merely demonstrates the difference in enthalpy between two states, without depending on the process or pathway by which the reaction took place.

Step 4: Enthalpy Change as an Extensive Property

As mentioned earlier, extensive properties depend on the quantity or size of the substance present in the system. Enthalpy change exhibits this behavior because it is proportional to the amount of reactants involved in the reaction. If we increase the amount of reactants, the enthalpy change will also increase proportionally. This linear relationship shows that enthalpy change is an extensive property. In conclusion, the enthalpy change for a reaction is considered a state function because it depends only on the initial and final states of a system, and not the path between them. It is also an extensive property since its value is proportional to the size or quantity of the substances involved in the reaction.