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Q: What does H2 Chemistry Notes: Topic 9 - Chemical Equilibria cover? A: Master equilibrium constants, Le Chatelier shifts, and quantitative problem-solving for Core Idea 3 (Chemical Equilibria) in the 2026 H2 Chemistry syllabus.
Equilibrium mastery blends conceptual understanding with algebraic manipulation. This note covers Kc, Kp, reaction quotient reasoning, and Le Chatelier justifications tailored for the 2026 exam style.
Status: SEAB H2 Chemistry (9476, first exam 2026) syllabus and Chemistry Data Booklet last checked 2026-01-13. Core Idea 3 Topic 9 is assessed across Papers 1–3.
Quick revision box
What this topic tests:
Kc/Kp manipulation, reaction quotient, Le Chatelier, and equilibrium calculations.
Top mistakes to avoid: Wrong equilibrium expression terms; not checking state symbols; qualitative shifts without quantitative support.
20-minute sprint plan: 5 min Kc/Kp expression recall; 10 min equilibrium math; 5 min shift + Q vs K reasoning.
1 Defining Equilibrium Constants
For reaction aA+bB⇋cC+dD:
Kc=[A]a[B]b[C]c[D]d
Exclude pure solids and liquids activity=1. For gas-phase reactions:
Kp=PAaPBbPCcPDd,Pi=partial pressure
Relationship: Kp=Kc(RT)Δn, where Δn=(c+d)−(a+b).
Use R=8.31J⋅K−1⋅mol−1 from the SEAB Chemistry Data Booklet when converting between Kc and Kp, and keep units consistent (Pa or bar) as specified in the question.
2 Reaction Quotient Q
Computed using same expression as K but with initial concentrations/pressures. Compare with K:
If Q<K, reaction shifts right to reach equilibrium.
If Q>K, reaction shifts left.
If Q=K, system already at equilibrium.
State this logic explicitly in answers; include direction of shift and rationale.
3 Le Chatelier's Principle
When systems at equilibrium are disturbed, they shift to counteract the change. Provide particle-level reasoning:
Pressure/volume changes: for gases, stress number of moles Δn.
Temperature: consider exothermic/endothermic direction using ΔH.
Catalyst: no effect on position; only speeds reaching equilibrium.
Include quantitative context when data available (e.g. new equilibrium constant values).
4 ICE Table Method
Use Initial-Change-Equilibrium tables for calculations. Example for NX2OX4(g)⇋2NOX2(g):
Stage
NX2OX4
NOX2
Initial
C
0
Change
−x
+2x
Equilibrium
C−x
2x
Plug into Kc=C−x(2x)2. Solve for x (often via quadratic). Check reasonableness (concentration cannot be negative).
5 Worked Example
Question:2SOX2(g)+OX2(g)⇋2SOX3(g) has Kc=280 at 700K. If [SO2]=[O2]=[SO3]=0.200mol⋅L−1 initially, determine the equilibrium concentrations.
Solution:
Compute Q:Q=(0.200)2(0.200)(0.200)2=0.008000.0400=5.00
Since Q<Kc, position shifts to right.
ICE table with change x:
Stage
SOX2
OX2
SOX3
Initial
0.200
0.200
0.200
Change
−2x
−x
+2x
Equilibrium
0.200−2x
0.200−x
0.200+2x
Substitute into Kc:
280=(0.200−2x)2(0.200−x)(0.200+2x)2
Rearrange and solve for x. The cubic simplifies to 280(0.200−2x)2(0.200−x)−(0.200+2x)2=0. Solving (by iteration) gives x=0.0714 (3 s.f.).
Equilibrium concentrations:
[SOX2]=0.200−2x=0.0571mol⋅L−1
[OX2]=0.200−x=0.1286mol⋅L−1
[SOX3]=0.200+2x=0.3429mol⋅L−1
Check:
(0.0571)2(0.1286)(0.3429)2≈2.80×102
The large Kc value drives equilibrium heavily towards SOX3, leaving only modest amounts of SOX2.
(When calculators with equation solvers are unavailable, apply successive substitution or quadratic rearrangement; show the method used to secure reasoning marks.)
6 Industrial Applications
6.1 Haber Process NX2+3HX2⇋2NHX3
Exothermic ΔH<0; lower temperature favours NH3 but reduces rate.
High pressure favours fewer moles (forward reaction).
Use iron catalyst with promoters to increase rate.
