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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.
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: Result: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.
Because NHX3 is the desired product, process design emphasises efficient downstream removal to keep shifting the equilibrium mixture toward products.
6.2 Contact Process SOX2+21OX2⇋SOX3
Exothermic; uses 450∘C and VX2OX5 catalyst.
Excess oxygen drives forward reaction.
Remove SO3 as formed to shift equilibrium (Le Chatelier).
Sulfur dioxide: oxidised reactant in the Contact-process equilibrium.
Sulfur trioxide: product species removed to increase conversion.
Treat this as an equilibrium-engineering pattern: control temperature, use catalyst for rate, and remove product species to maximise yield.
7 Common Misconceptions
Assuming catalysts change K; they do not.
Forgetting temperature is the only condition affecting K.
Failing to square concentrations when stoichiometric coefficient > 1.
Using Kc with partial pressures without converting.
8 Quick Drills
For PClX5(g)⇋PClX3(g)+ClX2(g), given Kp=0.012 at 500K and initial PClX5 pressure 1.00bar, calculate equilibrium partial pressures.
Explain, using Le Chatelier's principle, how increasing temperature affects equilibrium yield of methanol in CO+2HX2⇋CHX3OH
A mixture contains 0.300molNX2 and 0.300molHX2
Common exam mistakes
Mistake: Writing "the equilibrium shifts to increase Kc" or "to decrease Kc" - Kc is fixed at a given temperature and never shifts; only the position of equilibrium shifts in response to concentration or pressure changes.
Mistake: Including pure solids or pure liquids in the equilibrium expression - their activities are defined as 1 and must be omitted from Kc and Kp.
Mistake: Forgetting to raise concentrations to the power of the stoichiometric coefficient - for 2NOX2, the term is [NOX2]2
Mistake: Assuming a catalyst shifts the equilibrium position - catalysts speed up attainment of equilibrium but do not change Kc, Kp, or the equilibrium concentrations.
Mistake: Mixing Kc (concentration) with Kp (partial pressure) in the same expression - they are distinct constants related by Kp=Kc(RT)Δn
Mistake: Stating "increasing pressure increases yield" without checking Δn - pressure only shifts equilibrium if there is a net change in moles of gas (Δn=0).
Mistake: Stopping an ICE table calculation without checking that equilibrium concentrations are all positive - a negative concentration signals an arithmetic error.
Frequently asked questions
What is the only condition that changes the value of Kc? Temperature is the only factor that alters Kc. Changing concentration, pressure, or adding a catalyst shifts the position of equilibrium but leaves Kc unchanged.
When should I use Kp instead of Kc? Use Kp when the question gives or asks for partial pressures of gases. If concentrations in mol⋅L−1 are given, use Kc. You can interconvert using Kp=Kc(RT)Δn with R=8.31J⋅K−1⋅mol−1 from the data booklet.
How do I decide which direction the reaction shifts? Calculate the reaction quotient Q using the current concentrations or pressures. If Q<K, the reaction shifts to the right (towards products); if Q>K, it shifts to the left (towards reactants).
How do compromise industrial conditions relate to equilibrium principles? Industrial processes like the Haber process use moderate temperatures (around 450∘C) and high pressures (around 200atm) because the optimal equilibrium conditions (low temperature for high yield) would be too slow without a catalyst. The operating conditions are a practical trade-off between yield, rate, and cost.
Struggling with Chemical Equilibria? Our H2 Chemistry tuition programme covers this topic with structured practice, Paper 4 practical drills, and worked exam solutions.
Use these frameworks to articulate equilibrium reasoning with confidence.
