Question about an equilibrium paper 1 SL question

[maturk]

Hello, I do not understand the answer to this question. The answer is A. 

 

Why does XWZ have to be the forward reaction, why not the backward reaction for example because the question states that it is reversible so it can be approached from either side. Would someone please help and explain?

 

My original answer was B. Why is B incorrect?

 

Thank you for your time

[Emmi]

B is incorrect because the time at point Y also corresponds to point X. Y is your reversible reaction rate, and X is your forward reaction rate. At this time, though Y is zero, X is not zero. This is why B is incorrect.

A is the correct answer. When we have a reversible reaction, initially we have a lot of reactants and no products. As time goes on, we will use up those reactants and begin to form product. The reverse reaction will generate our reactants back. The forward reaction begins with a lot of reactant and no product. The reverse reaction begins with no reactants and only product that is formed in the forward reaction, so we need at least some reactant to react first before we can start the reverse reaction. Initially we have a lot of reactants so our rate is large. Once we begin to form product our amount of reactants decreases, so our rate is a little slower. The only curve that fits this is curve XWZ for the forward reaction, and YWZ for the reverse reaction. Thus A is the correct answer.

As a bonus, C is incorrect because equilibrium does not mean that the concentrations are equal, just that their concentrations are no longer changing with time. D is incorrect because although we generate reactants in the reverse reaction, looking at the curves the rate of increase of the reactants in the reverse reaction is smaller than the rate of decrease in the reactants in the forward reaction, so our overall concentration of reactant decreases.

Edited May 4, 2015 by Emmi

[IbTrojan]

Well I would assume that because there are two starting points when t=0, one of them must be the fwd and the other the bwd so B cannot be correct because Y is not the point at which both the fwd and bwd reaction are occurring. (Just trying to look at it logically)

 

I can see where you may have gotten confused though. If the fwd and bwd are reversible reactions, then one would think that they're start and end point would be the same on the graph

 

Edit: Thanks Emmi!

Edited May 4, 2015 by IbTrojan

[maturk]

Thank you guys for the enlightenment! Upon reflection, I seem to have analysed the question way too much and hence gotten confused with the fundamental principles of equilibrium and reversible reactions. Thanks so much You guys are the best.