Question

acetic acid and ethanol react to form ethyl acetate and water, like this:
hch₃co₂(aq)+c₂h₅oh(aq)→c₂h₅co₂ch₃(aq)+h₂o(l)
imagine 178. mmol of c₂h₅co₂ch₃ are removed from a flask containing a mixture of hch₃co₂, c₂h₅oh, c₂h₅co₂ch₃ and h₂o at equilibrium, and then answer the following questions.

Explanation:

Step1: Le Chatelier's Principle

When \(C_{2}H_{5}CO_{2}CH_{3}\) is removed, the equilibrium shifts to the right (forward direction) to produce more \(C_{2}H_{5}CO_{2}CH_{3}\). Before any \(C_{2}H_{5}CO_{2}CH_{3}\) is removed (initial equilibrium), the rate of the reverse reaction is equal to the rate of the forward reaction (\(v_{forward}=v_{reverse}\)).

Step2: Rate of Reaction After Removal (First Blank)

Just after \(C_{2}H_{5}CO_{2}CH_{3}\) is removed, the concentration of \(C_{2}H_{5}CO_{2}CH_{3}\) decreases. The rate of the reverse reaction (\(v_{reverse}\)) which depends on the concentration of \(C_{2}H_{5}CO_{2}CH_{3}\) (assuming an elementary reaction - rate of reverse reaction \(r_{reverse}=k_{reverse}[C_{2}H_{5}CO_{2}CH_{3}][H_{2}O]\)) becomes less than the rate of the forward reaction (\(r_{forward}=k_{forward}[HCH_{3}CO_{2}][C_{2}H_{5}OH]\)). So, the rate of the reverse reaction is greater than zero, but less than the rate of the forward reaction.

Step3: Rate of Reaction at New Equilibrium (Second Blank)

When the system has again reached equilibrium, \(v_{forward} = v_{reverse}\) (by the definition of equilibrium). The rates are greater than zero (because it is a dynamic equilibrium, reactants and products are constantly inter - converting).

Step4: Amount of \(C_{2}H_{5}CO_{2}CH_{3}\) (Third Blank)

Since the system shifts to the right to produce more \(C_{2}H_{5}CO_{2}CH_{3}\) to counteract the removal. But it cannot completely replenish the removed amount (Le - Chatelier's principle: the system can only partially counteract the change). So, some \(C_{2}H_{5}CO_{2}CH_{3}\) is produced, but less than the amount that was removed (\(178\) mmol).

Answer:

For the first blank (What is the rate of the reverse reaction just after the \(C_{2}H_{5}CO_{2}CH_{3}\) has been removed from the flask): Greater than zero, but less than the rate of the forward reaction.
For the second blank (What is the rate of the reverse reaction when the system has again reached equilibrium): Greater than zero, and equal to the rate of the forward reaction.
For the third blank (How much less \(C_{2}H_{5}CO_{2}CH_{3}\) is in the flask when the system has again reached equilibrium): Some, but less than \(178\) mmol.