A chemical company is testing a new catalysed reaction between two gaseous elements, X2 and Y2, which combine to form the industrially useful compound XY3: X2(g) + 3Y2(g) 2XY3(g) The table shows the percentage yield of XY3 at equilibrium, measured in laboratory-scale trials at d…

Answer: ((a)) Exothermic. At both pressures, the percentage yield of XY3 falls as temperature rises (82% to 55% at 50 atm; 91% to 68% at 200 atm), so a higher temperature favours the reverse reaction (the endothermic direction); the forward reaction, which is disfavoured by heat, must therefo… ((b)) There are 4 moles of gas molecules on the reactant side (1 + 3) but only 2 moles of gas molecules on the product side; increasing pressure shifts the position of equilibrium towards the side with fewer gas molecules, i.e. towards XY3, so the percentage yield increases, consisten… ((c)) (i) The rate at which equilibrium is reached increases, because the catalyst provides a reaction pathway with a lower activation energy; (ii) the percentage yield of XY3 is unchanged, because a catalyst speeds up the forward and reverse reactions equally and does not affect the… ((d)) At 200 atm, the percentage yield at 300 °C (91%) is much higher than at 500 °C (68%); operating at 300 °C therefore produces more XY3 from the same reactants, wasting less unreacted gas and giving a more profitable, more efficient use of the same high-pressure equipment. Part (a): Exothermic or endothermic? Compare the yield at the two temperatures, keeping pressure fixed: - At 50 atm: yield falls from 82% (300 °C) to 55% (500 °C). - At 200 atm: yield falls from 91% (300 °C) to 68% (500 °C). In both cases,…

IGCSE Chemistry 0620: Predicting Equilibrium Yield from Experimental Data

Pressure / atm	Temperature / °C	Percentage yield of XY₃
50	300	82
50	500	55
200	300	91
200	500	68

Pressure / atm

Temperature / °C

Percentage yield of XY₃

300

500

200

300

200

500

Part (a): Exothermic or endothermic?

Compare the yield at the two temperatures, keeping pressure fixed:

At 50 atm: yield falls from 82% (300 °C) to 55% (500 °C).
At 200 atm: yield falls from 91% (300 °C) to 68% (500 °C).

In both cases, raising the temperature lowers the percentage yield of $\text{XY}_3$ . By Le Chatelier’s principle, increasing temperature always shifts the position of equilibrium in the endothermic direction. Since raising the temperature here shifts the equilibrium away from $\text{XY}_3$ (towards $\text{X}_2$ and $\text{Y}_2$ ), the reverse reaction must be the endothermic one.

That means the forward reaction (formation of $\text{XY}_3$ ) is exothermic.

Part (b): Effect of pressure

Count the moles of gas molecules on each side of the equation:

$\underbrace{\text{X}_2(g)}_{1\text{ mol}} + \underbrace{3\text{Y}_2(g)}_{3\text{ mol}} \rightleftharpoons \underbrace{2\text{XY}_3(g)}_{2\text{ mol}}$

$1 + 3 = 4\text{ mol of gas (reactants)} \qquad \text{vs.} \qquad 2\text{ mol of gas (products)}$

There are fewer moles of gas molecules on the product side. Increasing the pressure shifts the position of equilibrium towards the side with fewer gas molecules, since that reduces the total number of particles and partially relieves the increased pressure. This favours the product, $\text{XY}_3$ , so the percentage yield increases. Matching the data, where yield rises from 82% to 91% at 300 °C, and from 55% to 68% at 500 °C, as pressure increases from 50 to 200 atm.

Part (c): Effect of a catalyst

(i) Rate: A catalyst provides an alternative reaction pathway with a lower activation energy, so a much greater proportion of collisions between particles are successful. This means equilibrium is reached faster.

(ii) Percentage yield: The percentage yield of $\text{XY}_3$ shown in the table is unaffected. A catalyst speeds up the forward and reverse reactions by the same factor, so it does not change the position of equilibrium. Only how quickly that position is reached.

Part (d): Why 300 °C rather than 500 °C

Since both options in this comparison use the same pressure (200 atm), the cost of the high-pressure equipment is the same either way. The relevant difference is yield:

$\text{yield at } 300\text{ °C} = 91\% \qquad \text{yield at } 500\text{ °C} = 68\%$

Operating at 300 °C converts a much greater fraction of the $\text{X}_2$ and $\text{Y}_2$ fed into the reactor into useful $\text{XY}_3$ . This means less unreacted gas needs to be separated and recycled (or is wasted), so the process is more economical at 300 °C even though the reaction is slower there (which is why a catalyst, as in part (c), is used to compensate).

Final answers

(a) Exothermic. Yield falls as temperature rises at both pressures.
(b) Fewer gas moles on the product side (2 vs 4) $\Rightarrow$ higher pressure favours $\text{XY}_3$ $\Rightarrow$ higher yield.
(c) (i) Rate increases; (ii) percentage yield unchanged.
(d) 300 °C gives a much higher yield (91% vs 68%) at the same pressure, so it is more economical.