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Chemical Equilibrium

Chemical Equilibrium

Dynamic equilibrium looks unchanged from outside, yet forward and reverse reactions keep going at the same rate. A large K favors products and a small K favors reactants, and K depends only on temperature. Comparing Q with K predicts direction: Q < K drives forward, Q > K drives reverse. Here you change K and Q to watch the approach to equilibrium and the reaction direction.

What is Dynamic Equilibrium?
💡 Analogy: People on an Escalator
①Imagine someone walking down on an upward escalator
②If walking-down speed = upward speed → they stay in place!
③It looks stationary from outside, but motion never stops
④This is 'dynamic equilibrium' — no apparent change but forward/reverse reactions continue
⑤Same as water evaporating and condensing at equal rates
Visualizing the Approach to Equilibrium
2
🔍 Meaning of K
①Large K → high product concentration (reaction nearly complete)
②Small K → high reactant concentration (reaction barely proceeds)
③K = 1 → reactant and product ratios similar
④K depends only on temperature (concentration/pressure changes don't change K)
Equilibrium Constant K and Reaction Quotient Q
Equilibrium Constant (concentration)
Kc = [C]c[D]d[A]a[B]b
For aA + bB ⇌ cC + dD: ratio of products to reactants
0.5
💡 Predicting Reaction Direction with Q vs K
①Q = K → equilibrium (no change)
②Q < K → too few products → forward reaction proceeds
③Q > K → too many products → reverse reaction proceeds
④Q has same form as K but uses concentrations at any moment
Using the Equilibrium Constant
Pure Solids and Liquids
Pure solids and liquids are excluded from K
e.g.: CaCO₃(s) ⇌ CaO(s) + CO₂(g) → K = [CO₂]
📐 How to Use ICE Tables
①I (Initial): set initial concentrations
②C (Change): denote change by x
③E (Equilibrium): I + C = equilibrium concentrations
④Substitute into K → solve for x
⑤The most common solving pattern in exams!
Worked Examples
Example 1
At equilibrium for H₂ + I₂ ⇌ 2HI: [H₂] = 0.1, [I₂] = 0.1, [HI] = 0.8 M. What is Kc?
1
Kc = [products]coeff / [reactants]coeff.
Kc = [HI]2[H2][I2]
2
Substitute the equilibrium concentrations.
Kc = 0.820.1 × 0.1 = 0.640.01 = 64
▸ Kc = 64
Coefficients become exponents ([HI]²). All are gases, so all appear in K.
Example 2
A reaction has K = 10, but the current reaction quotient is Q = 2. Which way does the reaction proceed?
1
Compare Q and K. If Q < K, products are insufficient.
Q = 2 < K = 10
2
It proceeds toward making more products.
Q < K ⇒ forward
▸ Forward (toward products)
Q<K → forward, Q>K → reverse, Q=K → equilibrium. It proceeds to bring Q toward K.
Summary
Equilibrium Constant
Kc = [products]coef[reactants]coef
Depends only on T; concentration/pressure changes do not change K
CSAT-style
At equilibrium for A ⇌ 2B: [A] = 0.5 M, [B] = 1.0 M. What is Kc?
①1
②2
③0.5
④4
⑤0.25
▸ ② 2
1
Kc = [B]² / [A] (the coefficient 2 of B is the exponent).
Kc = [B]2[A]
2
Substitute the equilibrium concentrations.
Kc = 1.020.5 = 2
🎯 Exam Points
①Equilibrium: forward rate = reverse rate (no apparent change)
②K > 1: products favored; K < 1: reactants favored
③Q < K → forward, Q > K → reverse, Q = K → equilibrium
④Exclude pure solids/liquids from K
⑤ICE tables for equilibrium concentrations — frequent exam question!
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Reaction Enthalpy
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Equilibrium Shift
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