Q: What does H2 Chemistry Notes: CORE IDEA 3, 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 K_c, K_p, reaction quotient reasoning, and Le Chatelier justifications tailored for the 2026 exam style.
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
\(\ce{N2O4}\)
\(\ce{NO2}\)
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
\(\ce{SO2}\)
\(\ce{O2}\)
\(\ce{SO3}\)
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. Using either a graphing calculator, numerical solver, or iteration (as permitted in exams), we obtain x=0.0634.
Equilibrium concentrations:
[SOX2]=0.200−2x=0.073mol⋅L−1
[OX2]=0.200−x=0.137mol⋅L−1
[SOX3]=0.200+2x=0.453mol⋅L−1
Check:
(0.073)2(0.137)(0.453)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.
Mention compromise conditions: 450∘C, 200atm.
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).
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
Use these frameworks to articulate equilibrium reasoning with confidence.
