Video: Selecting the Graph That Matches the Concentration of an Equilibrium Product against Time

Which graph could represent the concentration of one product versus time as the reaction approaches and reaches equilibrium? [A] Graph A [B] Graph B [C] Graph C [D] Graph D [E] Graph E

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Video Transcript

Which graph could represent the concentration of one product versus time as the reaction approaches and reaches equilibrium?

In this question, we want to graph the concentration of a product over the course of a reaction. For the sake of this question, I’ll say we’re working with a reaction of the form A plus B forming C plus D. And I’ll choose to graph the concentration of C. Since we want to make a graph of the concentration of a product versus time, that means that the concentration should go on the 𝑦-axis and time should go on the 𝑥-axis.

The question wants us to consider this reaction as it approaches and reaches equilibrium. Equilibrium occurs when the rate of the forward reaction, which is where A plus B reacts to form C plus D, equals the rate of the reverse reaction, where C plus D react to form A plus B. Once our reaction reaches equilibrium, since the forward and reverse reactions are equal, there will be no net change in concentration for either the products or the reactants.

So now let’s think about what will happen over the course of our reaction. When we first add A and B together, we can imagine starting a timer. So this will be time equals zero. We haven’t had a chance to form any of our products yet. So the concentration of C at this time will be zero. Let’s go ahead and add that point to our graph. Once A and B start to react, they’ll begin to form some of our products. So the concentration of C will increase.

Now that we form some of our products, the reverse reaction can start happening. But since we overall have more of our reactants than we do our products, there will still be a net increase in the concentration of our products. This will continue until we reach equilibrium, where the rates of the forward and reverse reactions are the same. After equilibrium, the concentrations of both our products and reactants will remain constant. This will appear in our graph as a leveling off of the concentration of C over time. And it will remain constant after that.

Now that we’ve thought through our entire reaction as it approaches and reaches equilibrium, we can finish our graph by connecting our dots. The graph that we’ve constructed most closely resembles answer choice C, which is the correct graph to represent the concentration of a product over time as the reaction approaches and reaches equilibrium.

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