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Grade 11-12 (age 16-18)

high school Chemical Equilibrium

Water in a capped bottle stops going down at some point, not because evaporation stops but because it condenses back as fast as it evaporates. A chemical reaction likewise reaches a dynamic equilibrium when the forward and reverse reactions keep running at the same rate and the concentrations stop changing; the ratio of product to reactant concentrations then is the equilibrium constant K. At a given temperature K comes out the same no matter what you started with, and comparing it with the reaction quotient Q, the same expression evaluated at any moment, tells you which way the reaction will go. In the diagrams, changing K changes the equilibrium concentrations and the two reaction rates, and changing Q changes the direction the reaction heads.

What Is Dynamic Equilibrium?

💡 Walking Down an Up Escalator
①Walk down an up escalator at the same speed it rises and you stay in place
②From outside you seem to be standing still, yet your legs and the steps never stop moving
③Chemical equilibrium is the same: the forward and reverse reactions do not stop; they run at the same rate, so the concentrations stay constant
④Water in a capped bottle that stops going down is also a dynamic equilibrium, with evaporation and condensation at equal rates

Reaching Equilibrium

2
🔍 Reading Equilibrium from the Size of K
①A large K means a high product concentration at equilibrium: with K = 10, [B] is 10 times [A], so the reaction has gone nearly to completion
②A small K leaves much reactant: with K = 0.1, [B] is 1/10 of [A]
③For a reaction like A ⇌ B with equal coefficients, K = 1 means the equilibrium concentrations are half and half
④In the second diagram the forward rate drops as [A] falls and the reverse rate grows as [B] rises; from the moment they are equal the concentrations no longer change
⑤K changes only with temperature; at a fixed temperature, changing concentration or pressure does not change it

Equilibrium Constant K and Reaction Quotient Q

Equilibrium Constant (Concentration)
Kc = [C]c[D]d[A]a[B]b
For aA + bB ⇌ cC + dD, the ratio of products to reactants calculated from equilibrium concentrations
0.5
💡 Predicting Direction with Q and K
①Q is the same expression as K, filled in with the concentrations at this moment
②If Q < K, there is less product than at equilibrium, so the forward reaction proceeds and Q rises toward K
③If Q > K, there is more product than at equilibrium, so the reverse reaction proceeds and Q falls toward K
④If Q = K, the system is already at equilibrium and shows no visible change
⑤The arrow in the diagram shows the way Q will move, which is the way the reaction proceeds

Working with the Equilibrium Constant

Pure Solids and Liquids
Pure solids and liquids are left out of the K expression
Their concentrations are treated as constant. Example: for CaCO₃(s) ⇌ CaO(s) + CO₂(g), K = [CO₂]
📐 Equilibrium Concentrations with an ICE Table
①I (initial): write the concentrations before reaction
②C (change): call the amount that reacts x and write each increase or decrease using the coefficient ratio
③E (equilibrium): add the change to the initial value to get each equilibrium concentration
④Put the equilibrium concentrations into the K expression and solve for x
⑤Most questions about equilibrium concentrations are solved in this order

Worked Examples

Example 1
For H₂ + I₂ ⇌ 2HI at equilibrium, [H₂] = 0.1 M, [I₂] = 0.1 M and [HI] = 0.8 M. What is Kc?
1
Raise each product concentration to the power of its coefficient and put it on top; put the reactants below.
Kc = [HI]2[H2][I2]
2
Substitute the equilibrium concentrations.
Kc = 0.820.1 × 0.1 = 0.640.01 = 64
▸ Kc = 64
The coefficient 2 of HI becomes an exponent, giving 0.8². All three substances are gases, so all three appear in the K expression.
Example 2
A reaction has K = 10, and right now its reaction quotient is Q = 2. Which way will the reaction proceed?
1
Compare Q and K. If Q is smaller than K, there is less product than at equilibrium.
Q = 2 < K = 10
2
The reaction moves toward making more product, that is, forward, and Q rises toward 10.
Q < K ⇒ forward
▸ Forward
Set K to 10 and Q to 2 in the diagram and the arrow points from Q toward K, to the right.

Summary

Equilibrium Constant
Kc = [products]coef[reactants]coef
Depends only on temperature; at a fixed temperature, changing concentration or pressure leaves it unchanged
CSAT-style problem, adapted
For A ⇌ 2B at equilibrium, [A] = 0.5 M and [B] = 1.0 M. What is Kc?
①1
②2
③0.5
④4
⑤0.25
▸ ② 2
1
The coefficient 2 of B becomes an exponent, so Kc = [B]² / [A].
Kc = [B]2[A]
2
Substitute the equilibrium concentrations.
Kc = 1.020.5 = 2
🎯 Exam Points
①At equilibrium, forward rate = reverse rate and the concentrations show no visible change
②K > 1 means the equilibrium lies toward products, K < 1 toward reactants
③Q < K: forward; Q > K: reverse; Q = K: at equilibrium
④Leave pure solids and liquids out of K
⑤Use an ICE table to find equilibrium concentrations
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Shift of Equilibrium

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