When the extent of reaction changes, how does the amount of A present change?

To understand why this choice is the appropriate answer, we need to consider the concept of extent of reaction (Δξ), which measures the progress of a chemical reaction. Specifically, it represents the change in the amount of reactants and products as the reaction proceeds.

In a chemical reaction, the stoichiometry tells us how much of each reactant is consumed and how much product is formed as the reaction progresses. For reactant A, when the extent of reaction changes by Δξ, this quantity indicates that some amount of A is being used up in the reaction.

If we start with an initial amount of A, denoted as n_A,0, as the reaction takes place and the extent of reaction increases, the amount of A will decrease. More formally, if A is a reactant, with every mole of A consumed, the extent of reaction corresponds to a decrease in the quantity of A by Δξ. Therefore, the updated quantity of A after the reaction can be expressed as:

n_A = n_A,0 - Δξ.

This relationship clearly shows that as the reaction progresses (indicated by an increase in Δξ), the amount of A present diminishes.

This understanding aligns with the noted choice, which accurately reflects that the