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Question Video: Identifying the Graph of a Dynamic Equilibrium Using the Initial and Equilibrium Concentrations Chemistry • 10th Grade

A dynamic equilibrium is established between two reactants A and B according to the equation shown: A (aq) ⇌ 2B (aq). Compound A has an initial concentration of 0.8 mol/dm³ that drops to 0.4 mol/dm³ once equilibrium is established. Which graph for this equilibrium is correct?

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Video Transcript

A dynamic equilibrium is established between two reactants A and B according to the equation shown: A, aqueous, is in equilibrium with two B, aqueous. Compound A has an initial concentration of 0.8 moles per cubic decimeter that drops to 0.4 moles per cubic decimeter once equilibrium is established. Which graph for this equilibrium is correct?

Dynamic equilibrium is established when the forward and reverse reactions occur at the same nonzero rate. When the forward and reverse reactions have the same rate, the reactant and product concentrations will remain constant even though both reactions are still occurring. Looking at the graphs, we can determine when equilibrium is established by identifying the point in time where the concentrations of A and B no longer change.

We are told in the question that compound A has an initial concentration of 0.8 moles per cubic decimeter that drops to 0.4 moles per cubic decimeter once equilibrium is established. Looking at the graphs, we can see that only graphs (C) and (D) show compound A as having an initial concentration of 0.8 moles per cubic decimeter. So, we can eliminate graphs (A) and (B). Graphs (C) and (D) also show that the concentration of compound A is 0.4 moles per cubic decimeter once equilibrium is established.

To determine which of these graphs is correct, we’ll need to consider how the concentration of B changes over time. Looking at the balanced chemical equation, we see that for every mole of A that reacts, two moles of B are produced. Over the course of this reaction, the concentration of A decreases by 0.4 moles per cubic decimeter. So, the concentration of B should increase by twice as much. We should therefore expect that compound B has an initial concentration of zero moles per cubic decimeter that increases to 0.8 moles per cubic decimeter once equilibrium is established. This is shown in graph (D).

In conclusion, the graph for the equilibrium between A and two B that is correct is the graph shown in answer choice (D).

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