Chemistry related question

Question

Given the reaction

2X(g) + Y(g) →← 2Z(g) ∆H = –335 kJ

Which combination of pressure and temperature gives the highest yield of Z at equilibrium?

(A) 1000 atm and 500 °C

(B) 500 atm and 500 °C

(C) 1000 atm and 100 °C

(D) 500 atm and 100 °C

(E) catalyst, 500 atm and 100 °C

Could you explain which answer/option is correct?

Answer 1

By looking at the answer choices you have here, you can see some similarities which could clue you in to what you have to do if you're stuck. Across all of the answer choices, there are three things you can change about the reaction to possibly increase the yield of Z: change pressure, change temperature, and add a catalyst. What does increasing the yield of Z at equilibrium mean? Well, simply we just want to produce the most Z, or want to make the reaction shift to the right. According to Le Chatelier's principle, when you stress (move away from equilibrium) a reaction, it will shift toward either producing more products or reactants to try to return to an equilibrium. So all we want to do is take a look at these 3 factors and see how each changes where the reaction shifts.

Let's start with Pressure:

Pressure affects reactions that have gases in them. From the ideal gas law, we know that P is directly proportional to n (more molecules create a higher pressure). If we stress the reaction by let's say increasing the pressure, the reaction will shift in such a way that it reduces this. The way that it can do this is by producing more of the side that contains fewer gas molecules. In this way is has a lower number of gas molecules, and therefore a lower pressure.

In our reaction, There are 3 gas molecules on the left side and 2 on the right side. We want our reaction to shift to the right, so we should increase the pressure so that the reaction shifts to the side with fewer moles of gas. So, our correct answer will have a high pressure.

Now, how about temperature:

We're given that the reaction has a negative ∆H, or that it is exothermic. This means that the products have less energy than the reactants, or that we release energy to the surroundings in the form of heat. One way that you can think of temperature as relating to Le Chatelier's principle is to write it as a reactant/product in your reaction and see what happens from there.

Here, heat is a product because our ∆H is negative. If we want the reaction to shift right, we should decrease this quantity. So temperature should be lower.

Now, if you look at your answer choices, the highest pressure and lowest temperature is choice C), so you can feel pretty confident in choosing that.

Curveball!

But E) talks about a catalyst, which may confuse you. Try and think about what a catalyst does. Catalysts provide alternate mechanisms for reactions that can speed them up, but can they make more product appear? They can adjust the rate of a reaction, but not the extent to which it happens because the change in energy between the products and reactants is still the same. Catalysts are relevant in reaction kinetics, whereas when we talk about equilibrium and spontaneity and things like that we are discussing thermodynamics. In short, whether there's a catalyst there or not doesn't really matter for the amount of Z produced in the long run, which is what we're looking for. So you can just say that E) is 500 atm and 100 ºC, which is the same as D) and also a worse choice than C).