An industrial compromise means choosing conditions that give an acceptable yield at an acceptable rate and cost, because the conditions for maximum yield and those for maximum speed point in different directions. In your answer, you describe both effects and then say what is chosen and why.
This is the last lesson of reversible changes and equilibrium. It combines predicting shifts with separating rate from position.
Why is there a compromise at all?
Take an exothermic reaction where the gas count differs on each side, such as ammonia manufacture:
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) (forward reaction exothermic)
- Low temperature favours the exothermic forward reaction, so the equilibrium yield is higher. But the reaction is slow.
- High temperature makes it fast, but the position shifts towards the reactants, so the yield is lower.
- High pressure shifts the position towards fewer gas molecules and also speeds up the reaction. But strong, costly equipment and energy are needed.
- A catalyst gets equilibrium reached sooner at a moderate temperature without altering the yield.
The commonly quoted conditions for this process are a temperature of about 450 °C, a pressure of about 200 atmospheres and an iron catalyst. Treat these as course figures to check against your syllabus and use the numbers in any question.
How to write the compromise answer
- State the effect of the condition on yield (position).
- State the effect on rate.
- Say the two pull in different directions if they do, and mention cost or safety where the question gives a cue.
- Conclude with the chosen condition and the word “compromise”.
Worked example
The table shows invented data for practice only. It is not real plant data, and the numbers are chosen to make the pattern easy to see.
| Temperature (°C) | Relative rate (arbitrary units) | Equilibrium yield of product (%) |
|---|---|---|
| 200 | 1 | 85 |
| 350 | 30 | 50 |
| 450 | 150 | 25 |
| 550 | 600 | 10 |
The forward reaction is exothermic. A manufacturer is choosing a temperature. Explain why 200 °C and 550 °C are both poor choices and suggest a sensible range.
Step 1, 200 °C: the yield is the highest (85%), but the rate is only 1 unit. Equilibrium would be reached very slowly, so little product is made each hour.
Step 2, 550 °C: the rate is the highest (600 units), but the yield is only 10%. Most of the reactants would need to be recycled.
Step 3, the pattern: as temperature rises, rate rises and yield falls. This matches the exothermic forward reaction.
Step 4, conclusion: a temperature in the middle of the table, between 350 °C and 450 °C, gives a moderate yield at a useful rate. That is the compromise. A catalyst helps by letting a reasonable rate be reached without going even hotter.
The mistake to watch for
Mistaken answer: “The temperature is 450 °C because it gives the highest yield.”
For an exothermic reaction, a higher temperature lowers the yield. Stating that it gives the highest yield contradicts the rule.
The correction: “450 °C is a compromise. A lower temperature would give a higher equilibrium yield but the reaction would be too slow. A higher temperature would speed it up but reduce the yield. The catalyst allows an acceptable rate at this temperature.”
Check yourself
1. Give one reason why very high pressure is not always used, even when it increases the yield.
Show answer
Very high pressure needs thick-walled, expensive equipment and a lot of energy to compress the gases. The extra cost and safety demands can outweigh the extra yield.
2. In the invented table above, which temperature gives the highest rate, and what is the yield there?
Show answer
550 °C gives the highest relative rate (600 units). The yield there is 10%.
3. Explain why recycling unreacted gases is useful when the yield per pass is only moderate.
Show answer
After the product is separated, the unreacted reactants are fed back into the reactor. They get another chance to react, so less raw material is wasted and more of it is eventually converted to product.
Where this leads next
Test the whole module with the mixed practice set. The mole and equation-ratio tutor will help when a question adds a yield calculation, and the idea of percentage yield connects to concentration, gas and yield calculations.
If you can explain each effect but struggle to weave it into a balanced paragraph, our teachers in online one-to-one Chemistry tuition can work on structure with you.