Question

Acetic acid and water react to form hydronium cation and acetate anion, like this: (aq)(l)(aq)(aq) Imagine of are added to a flask containing a mixture of , , and at equilibrium, and then answer the following questions. What is the rate of the forward reaction before any HCH3CO2 has been added to the flask

Answer 1

The question is incomplete. Here is the complete question.

Acetic acid and water react to form hydronium cation and acetate anion, like this:

$HCH_{3}CO_{2}_{(aq)}+H_{2}O_{(l)} -> H_{3}O^{+}_{(aq)}+CH_{3}CO_{2}^{-}_{(aq)}$

Imagine 226 mmol of $CH_{3}CO_{2}^{-}$ are added to a flask containing a mixture of $HCH_{3}CO_{2}$, $H_{2}O$,$H_{3}O^{+}$ and $CH_{3}CO_{2}^{-}$ at equilibrium and then answer the following questions:

1) What's the rate of the reverse reaction before any $CH_{3}CO_{2}^{-}$ has been added to the flask?

a) 0

b) Greater than 0, but less than the rate of the forward reaction

c) Greater than 0, but equal to the rate of the forward reaction

d) Greater than 0, but  greater than the rate of the forward reaction

2) What is the rate of the reverse reaction just after the $CH_{3}CO_{2}^{-}$ has been added to the flask?

a) 0

b) Greater than 0, but less than the rate of the forward reaction

c) Greater than 0, but equal to the rate of the forward reaction

d) Greater than 0, but  greater than the rate of the forward reaction

3) What is the rate of the reverse reaction when the system has again reached equilibrium?

a) 0

b) Greater than 0, but less than the rate of the forward reaction

c) Greater than 0, but equal to the rate of the forward reaction

d) Greater than 0, but  greater than the rate of the forward reaction

4) How much more $CH_{3}CO_{2}^{-}$  is in the flask when the system has again reached equilibrium?

a) None

b) Some, but less than 226 mmol

c) 226 mmol

d) More than 226 mmol.

Answer: 1) c) Greater than 0, but equal to the rate of the forward reaction

2) d) Greater than 0, but  greater than the rate of the forward reaction

3) c) Greater than 0, but equal to the rate of the forward reaction

4) b) Some, but less than 226 mmol

Explanation: A reversible chemical reaction reaches its equilibrium when forward and reverse reaction are at the same rate. At that point, equilibrium has a constant called K.

Equilibrium constant depends on the concentration of its products and reagents.

For the reaction $HCH_{3}CO_{2}_{(aq)}+H_{2}O_{(l)} -> H_{3}O^{+}_{(aq)}+CH_{3}CO_{2}^{-}_{(aq)}$,

the forward is towards production of $CH_{3}CO_{2}^{-}$ and reverse is towards the production of $HCH_{3}CO_{2}$.

1) When equilibrium is reached, forward and reverse are at the same rate and are different from zero.

2) When adding a compound, the equilibrium is broken. So, to go back to the equilibrium, reaction tend to counteract the change. In the case of acetic acid and water above, when adding acetate anion, the reverse reaction will produce more acetic acid to restore equilibrium, so reverse reaction will be at a greater rate and different from 0.

3) After a while when the system is back to the equilibrium, the rate will be equal again.

4) After second equilibrium, acetate anion will have less mmol than when this new equilibrium state started.