Video: Describe the Rate of the Reversible Reactions

If a reversible reaction is allowed to reach equilibrium in a closed system, which of the following will be true? [A] The concentration of the products will gradually increase. [B] Increasing the pressure inside the closed system will not affect the equilibrium. [C] Increasing the temperature will not affect the equilibrium. [D] The concentration of the reactants and products will be the same. [E] The rate of the forward reaction will be the same as the rate of the backward reaction.

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Video Transcript

If a reversible reaction is allowed to reach equilibrium in a closed system, which of the following will be true? (A) The concentration of the products will gradually increase. (B) Increasing the pressure inside the closed system will not affect the equilibrium. (C) Increasing the temperature will not affect the equilibrium. (D) The concentration of the reactants and products will be the same. Or (E) the rate of the forward reaction will be the same as the rate of the backward reaction.

A reversible reaction is one in which reactants form products and at the same time products are forming reactants. When this occurs in a closed system, which is one in which energy can enter or leave but matter cannot, we say an equilibrium is reached. We can plot the progress of the reaction in terms of rate of reaction versus time from time equals zero. At time equals zero, reactants form products very fast. And at time equals zero, before any products are formed, the backward reaction hasn’t yet begun.

When some products are formed, some of it will react to reform the reactants. And the rate of the forward reaction begins to decrease. As more product forms, so the rate of the backward reaction increases. From a certain point in the reversible reaction, let’s call it time equals 𝑥, an equilibrium will occur. And the rate of the forward and backward reactions will be the same. We can see that from time equals 𝑥, both the black and green curves have the same 𝑦-value, in other words, the same rate of reaction. Option (E) is correct. The rate of the forward reaction will be the same as the rate of the backward reaction.

Let’s confirm this by having a look at the other answer options. We can do this by doing another sketch graph, this time of concentration versus time. At time equals zero, before the reaction has begun, when there is only a reactant in the vessel, the concentration of the reactants is very high and the concentration of the products is zero. As the reaction begins to proceed, so the concentration of the reactants decreases as they are converted into products. As more reactants are converted to products, so the concentration of the products increases.

However, from time equals 𝑥, the concentration of the reactants remains constant and the concentration of the products remains constant. An equilibrium is formed where the rate of conversion of reactants to products is the same as the rate of conversion of products to reactants. If we look carefully, however, we’ll see that the concentration of the reactants and products are not necessarily the same at equilibrium. So we can rule out option (D).

We can also rule out option (A) because the question asks about equilibrium. And at equilibrium, the concentration of the products will not increase but will remain constant. Now, concentration–time graphs can look different from the one drawn here. The reactant concentration can be higher than that of the products at equilibrium, as drawn in this graph. Or the reactants and products can have the same concentration at equilibrium. Or the products can have a higher concentration than the reactants.

Taking a ratio of the concentration of the products to reactants will give us a value either less than one, equal to one, or greater than one, depending on the graph. These values, which represent the equilibrium position, depend on the temperature, pressure, or concentration conditions under which the reaction occurs. In other words, temperature, pressure, and concentration do influence equilibrium. So we can rule out the remaining two answer options as they are incorrect.

Finally, if a reversible reaction is allowed to reach equilibrium in a closed system, the rate of the forward reaction will be the same as the rate of the backward reaction. And we call this dynamic equilibrium.

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