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6.2Thermochemistry

Energy cycles and spontaneity — EST 2 Chemistry Revision Notes & Topical Questions

5 min read 8 parts Lesson 2 of 2 1 quiz

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Lesson summary

Energy cycles and spontaneity EST Chemistry notes: Apply Hess's law.ΔH°f of an element in its standard state is zero.From bondsΔH = Σ(bonds broken) − Σ(bonds formed).Use kelvin in ΔG = ΔH − TΔS and match kJ with J.Spontaneous does not mean fast.The crossover temperature is T = ΔH…

  • Standard conditions
  • ΔH = +50 kJ, ΔS = +0.20 kJ/K. Above what T is it spontaneous?
  • Energy cycles and spontaneity EST Chemistry: what to do next
  • Standard enthalpy of formation (ΔH°f)
  • Bond enthalpy
  • Entropy (S)

Part 1 of 8Key ideas

ΔH°f of an element in its standard state is zero.
From bonds
ΔH = Σ(bonds broken) − Σ(bonds formed).
Use kelvin in ΔG = ΔH − TΔS and match kJ with J.
Spontaneous does not mean fast.
The crossover temperature is T = ΔH/ΔS.

Part 2 of 8Learning objectives

Learning objectives
Apply Hess's law.
Use enthalpies of formation.
Predict spontaneity using ΔG.

Part 3 of 8Essential knowledge

Diamond turning to graphite is spontaneous but extremely slow.
Standard conditions
298 K, 1 atm, 1 M.
Entropy of a perfect crystal at 0 K is zero.
ΔG° = −RT ln K links energy and equilibrium.
Born–Haber cycles apply Hess's law to ionic lattices.

Part 4 of 8Process & sequence

1
Write the target equation.
2
Rearrange given equations (reverse or multiply).
3
Apply the same changes to ΔH.
4
Add equations; cancel species on both sides.
5
Or use ΔH°rxn = ΣΔH°f(products) − ΣΔH°f(reactants).

Hess's law routewrite these steps in this order; each step is a marking point.

  1. Target equation
  2. Rearrange steps
  3. Adjust ΔH
  4. Add and cancel
  5. Total ΔH

Part 5 of 8Worked examples

ΔH°f: CO2 = −394, H2O(l) = −286, CH4 = −75 kJ/mol. Find ΔH for CH4 + 2O2 → CO2 + 2H2O.
  • (−394 + 2(−286)) − (−75) = −891 kJ.
ΔH
+50 kJ, ΔS = +0.20 kJ/K. Above what T is it spontaneous?
  • T = 50 ÷ 0.20 = 250 K.

Part 6 of 8Summary

Hess: ΔH is path-independent.
What the symbols mean:
ΔG = ΔH − TΔS.
What the symbols mean: ΔG = Gibbs free energy change (kJ mol−1; feasible when ≤ 0) · ΔH = enthalpy change (kJ mol−1) · T = temperature (K) · ΔS = entropy change (J K−1 mol−1, divide by 1000 to use kJ)
ΔG < 0 means spontaneous.

Part 7 of 8Keep going

Energy cycles and spontaneity EST Chemistry: what to do next

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

1
Practice: Energy cycles and spontaneity EST Chemistry practice questions
2
Previous lesson: 6.1 Energy changes
3
Next lesson: 7.1 Inorganic chemistry
4
Course: EST 2 Chemistry — all units
5
Official syllabus: OpenStax free science textbooks

Energy cycles and spontaneity EST Chemistry revision notes

Part 8 of 8Compare: Compare

Predicting spontaneity
ΔHΔSΔGSpontaneous?
−+Always −At all T
+−Always +Never
−−− at low TLow temperatures
++− at high THigh temperatures
Entropy increases when
ChangeExample
Solid → liquid → gasIce melting
More gas moles formed2H2O2 → 2H2O + O2
Solid dissolvesNaCl in water
Temperature risesFaster particle motion

Key terms

Command 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.

Exam technique

Keywords the mark scheme looks for in this lesson

Keywords the mark scheme looks for in this lesson
Hess's law · Standard enthalpy of formation (ΔH°f) · Bond enthalpy · Entropy (S) · Gibbs free energy (G) · Spontaneous process
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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