Question

Question: Consider an initial mixture of N₂ and H₂ gases that can be represented as follows.

The gases react to form ammonia gas (NH₃) as represented by the following concentration profile.

a. Label each plot on the graph as N₂, H₂, or NH₃ and explain your answers.

b. Explain the relative shapes of the plots.

c. When is equilibrium reached? How do you know?

Short answer

a. In the given graph, each plot is labelled as shown in Step 1.

b. The relative shapes of the plots are explained in Step 2.

c. When the reaction is reached to equilibrium, the rate of the forward reaction equals the rate of the reverse reaction.

Step-by-step solution

Step 1:

In the given graph, each plot is labelled as shown below:

The given initial mixture contains two nitrogen molecules and six hydrogen molecules. There are no ammonia present at initially. The ratio of hydrogen to nitrogen is 3:1 in the reaction mixture before the reaction starts. So, the line with the greatest initial concentration must be the H2 plot. In the graph, the green curve has the highest one. Hence, the green curve is labelled as H2.

The blue curve has the lowest initial concentration. So, it should be the N2 plot. Hence, the blue curve is labelled as N2.

The pink curve begins from the origin of the graph which indicates the initial concentration is zero before the reaction starts. So, it must be the NH3 plot. Hence, the pink curve is labelled as NH3.

Step 2:

The balanced chemical equation for the formation of ammonia from the gases of hydrogen and nitrogen as shown below:

$N_2\left(g\right)+3H_2\left(g\right)\to 2N{H}_3\left(g\right)$

According to the above balanced equation, 1 equivalent of nitrogen needs three equivalents of hydrogen to give two equivalents of ammonia. So, the concentration of hydrogen decreases three times faster than that of nitrogen as seen in the graph. This is due to the stoichiometric ratio of hydrogen to nitrogen that is 3:1 in the balanced equation. At the same time, the production of ammonia increases two times faster than nitrogen because the mole ratio of ammonia to nitrogen is 2:1 in the balanced equation.

Step 3: 

When the rate of the forward reaction equals the rate of the reverse reaction, equilibrium is established. Because no net change in any of the reactant or product concentrations has occurred at this point, the different graphs indicate that equilibrium has been attained when their concentrations no longer fluctuate with time.