Distinguish between reversible and irreversible reactions. What is chemical equilibrium? Give its characteristics.
How to lay out the answer. Follow the frame, then write it in your own words.
1 · Reversible reaction. Goes in both directions under the same conditions: N₂ + 3H₂ ⇌ 2NH₃.
2 · Irreversible reaction. Goes in one direction only, to completion: burning of paper, 2Mg + O₂ → 2MgO.
3 · Differences.
Both ways vs one way
⇌ vs →
Reaches equilibrium vs goes to completion
Closed vessel vs open vessel
4 · Define equilibrium. The state of a reversible reaction in which the forward and backward rates are equal and concentrations stay constant.
5 · Draw the rate graph. [Graph of rate against time: the forward rate curve falls, the backward rate curve rises, they meet and run flat together; mark 'equilibrium']
6 · Characteristics.
Reached only in a closed system
Dynamic: both reactions continue
Concentrations constant
Reached from either side
Catalyst does not change it
7 · Real-life link. A closed soda bottle holds CO₂ at equilibrium; cooking an egg is irreversible.
8 · Close in one line. Thus equilibrium is a balance of two reactions running at equal rates, not a stop.
Scoring tip: Use the ⇌ sign in every reversible example and say the word 'dynamic' with a reason.
