Question

nitrogen and hydrogen react to form ammonia, like this:

n₂(g)+3h₂(g)→2nh₃(g)

imagine 192. mmol of n₂ and 576. mmol of h₂ are added to an empty flask, and then answer the following questions.

what is the rate of the forward reaction before any n₂ or h₂ has been added to the flask?

zero.

greater than zero, but less than the rate of the reverse reaction.

greater than zero, and equal to the rate of the reverse reaction.

greater than zero, and greater than the rate of the reverse reaction.

what is the rate of the forward reaction just after the n₂ and h₂ has been added to the flask?

zero.

greater than zero, but less than the rate of the reverse reaction.

greater than zero, and equal to the rate of the reverse reaction.

greater than zero, and greater than the rate of the reverse reaction.

what is the rate of the forward reaction at equilibrium?

zero.

greater than zero, but less than the rate of the reverse reaction.

greater than zero, and equal to the rate of the reverse reaction.

greater than zero, and greater than the rate of the reverse reaction.

how much n₂ is in the flask at equilibrium?

none.

some, but less than 192. mmol.

1. mmol.

more than 192. mmol.

Explanation:

• First question: Before adding \(N_2\) or \(H_2\), there are no reactants to undergo the forward reaction \(N_{2}(g)+3H_{2}(g)\to2NH_{3}(g)\). So the rate of the forward reaction is zero.
• Second question: Just after adding \(N_2\) and \(H_2\), the forward reaction starts (rate \(> 0\)) because reactants are present. At this moment, there is no \(NH_3\) formed yet, so the reverse reaction rate (which depends on the concentration of \(NH_3\)) is zero. Thus, the forward - reaction rate is greater than the reverse - reaction rate (which is zero).
• Third question: At equilibrium, for a reversible reaction, the rate of the forward reaction is equal to the rate of the reverse reaction (\(v_{forward}=v_{reverse}>0\)).
• Fourth question: The reaction \(N_{2}(g)+3H_{2}(g)

ightleftharpoons2NH_{3}(g)\) is a reversible reaction. Since the reaction proceeds forward to some extent (because it is a non - zero - sum reaction with reactants added), some \(N_2\) will be consumed. But it will not be completely consumed (because it is a reversible reaction), so the amount of \(N_2\) at equilibrium is some, but less than \(192\) mmol.

Answer:

• What is the rate of the forward reaction before any \(N_2\) or \(H_2\) has been added to the flask? → Zero.
• What is the rate of the forward reaction just after the \(N_2\) and \(H_2\) has been added to the flask? → Greater than zero, and greater than the rate of the reverse reaction.
• What is the rate of the forward reaction at equilibrium? → Greater than zero, and equal to the rate of the reverse reaction.
• How much \(N_2\) is in the flask at equilibrium? → Some, but less than \(192\) mmol.