Question

Consider the following generic reaction:

$2A_{2}B\left(g\right)\to 2A_2\left(g\right)+B_2\left(g\right)$

Some molecules of A₂B are placed in a 1.0-L container. As time passes, several snapshots of the reaction mixture are taken as illustrated below.

Which illustration is the first to represent an equilibrium mixture? Explain. How many molecules of A₂B were initially placed in the container?

Short answer

The second illustration is the first to represent an equilibrium mixture. The number of A₂B molecules initially placed in the container is 6.

Step-by-step solution

Step 1:

Because the speeds of the forward and reverse reactions are similar, there is no net change in the number of reactants and products present when equilibrium is attained. In the first figure, there are four A₂B molecules, two A₂ molecules, and one B₂ molecule. Two A₂B molecules, four A2 molecules, and two B₂ molecules are shown in the second diagram. Because there was a measurable difference in reactants and products, the first figure cannot reflect equilibrium.

When moving from the second to the third diagram, there is a net change in reactants and products. Because the third figure has the same number and kind of molecules as the second figure which is the first figure of equilibrium.

Step 2:

Only A₂B was initially present in the reaction container. Two A₂ molecules and one B₂ molecule may be seen in the first diagram (along with 4 A₂B molecules). When two A₂B molecules disintegrated in a balanced reaction, two A₂ molecules and one B₂ molecule were created. As a result, the initial number of A₂B molecules is 6 A₂B molecules.

Hence, the second illustration is the first to represent an equilibrium mixture. The number of A₂B molecules initially placed in the container is 6.