Question

ammonia and oxygen react to form nitrogen and water, like this:
4nh3(g)+3o2(g)→2n2(g)+6h2o(g)
imagine 154. mmol of n2 and 462. mmol of h2o are added to an empty flask, and then answer the following questions.
what is the rate of the reverse reaction before any n2 or h2o has been added to the flask?
zero.
greater than zero, but less than the rate of the forward reaction.
greater than zero, and equal to the rate of the forward reaction.
greater than zero, and greater than the rate of the forward reaction.
what is the rate of the reverse reaction just after the n2 and h2o has been added to the flask?
zero.
greater than zero, but less than the rate of the forward reaction.
greater than zero, and equal to the rate of the forward reaction.
greater than zero, and greater than the rate of the forward reaction.
what is the rate of the reverse reaction at equilibrium?
zero.
greater than zero, but less than the rate of the forward reaction.
greater than zero, and equal to the rate of the forward reaction.
greater than zero, and greater than the rate of the forward reaction.
how much n2 is in the flask at equilibrium?
none.
some, but less than 154. mmol.

1. mmol.

more than 154. mmol.

Explanation:

First question

Before any \(N_2\) or \(H_2O\) has been added to the flask, there are no products (\(N_2\) and \(H_2O\)) present. The reverse reaction (\(2N_2(g)+6H_2O(g)\to4NH_3(g) + 3O_2(g)\)) requires \(N_2\) and \(H_2O\) as reactants for the reverse - reaction molecules to collide and react. Since there are no \(N_2\) and \(H_2O\) (the reactants for the reverse reaction in this case), the rate of the reverse reaction is zero.

Second question

Just after \(N_2\) and \(H_2O\) are added to the flask, there are no \(NH_3\) and \(O_2\) (the products of the reverse reaction \(2N_2(g)+6H_2O(g)\to4NH_3(g)+3O_2(g)\)) present. The rate of the forward reaction (\(4NH_3(g)+3O_2(g)\to2N_2(g)+6H_2O(g)\)) is zero (because there are no \(NH_3\) and \(O_2\) for the forward - reaction to occur), and the reverse reaction rate is greater than zero (because \(N_2\) and \(H_2O\) are present and can start to react in the reverse direction).

Third question

At equilibrium, for a reversible reaction \(aA + bB
ightleftharpoons cC + dD\), the rate of the forward reaction \(r_f=k_f[A]^a[B]^b\) and the rate of the reverse reaction \(r_r = k_r[C]^c[D]^d\), and \(r_f=r_r\). So, the rate of the reverse reaction is greater than zero and equal to the rate of the forward reaction.

Fourth question

The reaction \(4NH_3(g)+3O_2(g)
ightleftharpoons2N_2(g)+6H_2O(g)\) is a reversible reaction. When \(N_2\) and \(H_2O\) are added to the flask, the reverse reaction \(2N_2(g)+6H_2O(g)\to4NH_3(g)+3O_2(g)\) will occur. Since the reaction is reversible, it will not go to completion (all \(N_2\) is not consumed). So, there will be some \(N_2\) left in the flask, but less than the initial amount (\(154\ mmol\)) because some of it will be consumed in the reverse reaction.

Answer:

1. Zero.
2. Greater than zero, and greater than the rate of the forward reaction.
3. Greater than zero, and equal to the rate of the forward reaction.
4. Some, but less than 154. mmol.