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5.1Equilibrium and Reaction Rates

Chemical equilibrium — EST 2 Chemistry Revision Notes & Topical Questions

5 min read 8 parts Lesson 1 of 3 1 quiz

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Lesson summary

Chemical equilibrium EST Chemistry notes: Write equilibrium expressions.Only temperature changes the value of K.A catalyst speeds up reaching equilibrium but does not change the position.Adding an inert gas at constant volume causes no shift.Reversing an equation gives Knew = 1/K…

  • Write K for N2(g) + 3H2(g) ⇌ 2NH3(g)
  • Chemical equilibrium EST Chemistry: what to do next
  • Reversible reaction
  • Dynamic equilibrium
  • Equilibrium constant (Keq)
  • Homogeneous equilibrium

Section 1 of 8Chemical equilibrium

The Haber process uses high pressure to favour NH3 (4 gas moles → 2).
Equilibrium can be approached from either side.
Concentrations are constant at equilibrium but not necessarily equal.
ICE
Initial, Change, Equilibrium.
Kp uses partial pressures for gases.

Section 2 of 8Process & sequence

1
Write the balanced equation.
2
K = [products]coefficients ÷ [reactants]coefficients.
3
Omit pure solids and liquids.
4
Substitute equilibrium concentrations (use an ICE table if needed).
5
Compare Q with K to predict direction.

Applying Le Châtelierwrite these steps in this order; each step is a marking point.

1
Identify stress
2
Which side relieves it?
3
Predict shift
4
Concentration changes
5
Is K changed? (only T)

Section 3 of 8Compare

Interpreting K
Value of KMeaning
K >> 1Products favoured
K ≈ 1Similar amounts
K << 1Reactants favoured
Le Châtelier stresses
StressShiftDoes K change?
Add reactantRight (to products)No
Remove productRightNo
Increase pressure (smaller volume)Toward fewer gas molesNo
Raise T (endothermic forward)RightYes, K increases
Raise T (exothermic forward)LeftYes, K decreases
Add catalystNo shiftNo
Comparing Q and K
ConditionDirection
Q < KForward (right)
Q = KAt equilibrium
Q > KReverse (left)

Section 4 of 8Worked examples

1Write K for N2(g) + 3H2(g) &rlhar; 2NH3(g).
  • K = [NH3]² ÷ ([N2][H2]³).
2
For H2 + I2 &rlhar; 2HI, equilibrium: [H2] = [I2] = 0.10 M, [HI] = 0.80 M. Find K.
  • K = 0.80² ÷ (0.10 × 0.10) = 64.

Section 5 of 8Keep going

Chemical equilibrium EST Chemistry: what to do next

These Chemical equilibrium EST Chemistry notes covered the key ideas, worked examples and exam technique for this topic. Check your Chemical equilibrium EST Chemistry understanding with the practice questions below, then reread any Chemical equilibrium EST Chemistry point you were unsure about before moving on.

1
Practice: Chemical equilibrium EST Chemistry practice questions
2
Previous lesson: 4.3 Reaction calculations
3
Next lesson: 5.2 Reaction rates
4
Course: EST 2 Chemistry — all units
5
Official syllabus: OpenStax free science textbooks

Chemical equilibrium EST Chemistry revision notes

Section 6 of 8Key terms

Section 7 of 8Command terms

Command terms
Identify / Describe (FRQ)
A short, specific statement; describing requires relevant characteristics, not a single word.
Explain / Justify (FRQ)
Give the cause-and-effect reasoning or evidence; a restated fact does not score.
Calculate
Show setup, substitution and the answer with units.
Predict
State the expected outcome; add a reason only if asked to justify.
MCQ technique
Read every option; eliminate distractors that are true statements but do not answer the question; watch for NOT/EXCEPT.

Section 8 of 8Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Reversible reaction · Dynamic equilibrium · Equilibrium constant (Keq) · Homogeneous equilibrium · Heterogeneous equilibrium · Reaction quotient (Q) · Le Châtelier's principle
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.

How to get full marks (examiner guidance)

1
Answer exactly what is asked; extra correct material does not earn credit, and contradictory statements can cancel credit (AP FRQ).
2
Use precise vocabulary (e.g. “active site”, “concentration gradient”, “allele”), never “it/stuff/things”.
3
Show units and working in calculations.
4
In MCQs, predict the answer before reading the options, then eliminate.

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