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A reversible reaction can proceed in both directions. In a closed system, equilibrium occurs when forward and reverse reaction rates are equal. Concentrations then stay constant, but are not necessarily equal. If products can continually escape, the same closed-system equilibrium model does not apply. A catalyst makes equilibrium establish sooner by accelerating both directions, without changing the equilibrium mixture at a fixed temperature.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Explain dynamic equilibrium in a closed system
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What is equal at equilibrium?
Forward and reverse rates. Concentrations are constant but need not match.
After watching 2: Does an iron catalyst change ammonia’s equilibrium yield at fixed temperature?
No. It speeds attainment of equilibrium.
What you'll be able to do
A reversible reaction can proceed in both directions. In a closed system, equilibrium occurs when forward and reverse reaction rates are equal. Concentrations then stay constant, but are not necessarily equal. If products can continually escape, the same closed-system equilibrium model does not apply. A catalyst makes equilibrium establish sooner by accelerating both directions, without changing the equilibrium mixture at a fixed temperature.
Increasing a reactant concentration favours its consumption. Increasing pressure favours the side with fewer gaseous molecules; if the gas counts are equal, that rule predicts no shift. Heating favours the endothermic direction. These predictions describe equilibrium position, while collision theory describes rate. A temperature increase can speed both reactions yet reduce the final yield of an exothermic product. Separate those two ideas in an explanation.
The Haber process combines nitrogen and hydrogen to make ammonia. The forward reaction is exothermic and reduces gaseous molecule count, so low temperature and high pressure favour equilibrium yield. Low temperature gives a slow rate and very high pressure raises cost and engineering demands. An iron catalyst and compromise temperature and pressure provide a useful production rate. Cooling separates ammonia and unreacted gases are recycled. Evaluate the supplied conditions rather than assuming maximum single-pass yield is the only objective.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Why does increasing pressure favour ammonia?
The ammonia proportion increases at the same temperature.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.