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6.1Thermochemistry

Energy changes — EST 2 Chemistry Revision Notes & Topical Questions

5 min read 8 parts Lesson 1 of 2 1 quiz

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Lesson summary

Energy changes EST Chemistry notes: Use q = mcΔT.1 calorie = 4.184 J; 1 food Calorie = 1 kcal.Water's high specific heat moderates climate.ΔH of a reverse reaction has the opposite sign.ΔH scales with the coefficients in the equation.Bond breaking absorbs energy; bond making rele…

  • How much heat raises 200 g of water from 20 °C to 45 °C?
  • 0.050 mol of acid neutralised raises 100 g water by 6.7 °C. Find ΔH
  • Energy changes EST Chemistry: what to do next
  • Specific heat (c)
  • Calorimeter
  • Enthalpy (H)

Part 1 of 8Key ideas

1
1 calorie = 4.184 J; 1 food Calorie = 1 kcal.
2
Water's high specific heat moderates climate.
3
ΔH of a reverse reaction has the opposite sign.
4
ΔH scales with the coefficients in the equation.
5
Bond breaking absorbs energy; bond making releases energy.

Part 2 of 8Learning objectives

Learning objectives
Use q = mcΔT.
Interpret calorimetry data.
Write thermochemical equations.

Part 3 of 8Essential knowledge

1
Coffee-cup calorimeters work at constant pressure.
2
Molar heat of fusion and vaporisation apply to phase changes.
3
Heat always flows from hot to cold.
4
System vs surroundings must be identified.
5
ΔHvap of water is 40.7 kJ/mol.

Part 4 of 8Process & sequence

1
Measure mass and temperature change in the calorimeter.
2
q = mcΔT for the water.
3
Heat lost by reaction = heat gained by water (qrxn = −qwater).
4
Divide by moles reacted to get ΔH per mole.
5
Assign the sign: temperature rise means exothermic.

Calorimetrywrite these steps in this order; each step is a marking point.

1
Measure ΔT
2What the symbols mean:
q = mcΔT
What the symbols mean: E, Q or q = energy transferred (J) · m = mass (kg; g in chemistry calorimetry) · c = specific heat capacity (J kg−1 °C−1; water 4.18 J g−1 °C−1) · ΔT or Δθ = temperature change (°C or K)
3
qrxn = −qwater
4
÷ moles
5
Sign of ΔH

Part 5 of 8Worked examples

1How much heat raises 200 g of water from 20 °C to 45 °C?
How much heat raises 200 g of water from 20 °C to 45 °C?
  • q = 200 × 4.18 × 25 = 20 900 J = 20.9 kJ.
20.050 mol of acid neutralised raises 100 g water by 6.7 °C. Find ΔH.
0.050 mol of acid neutralised raises 100 g water by 6.7 °C. Find ΔH.
  • q = 100 × 4.18 × 6.7 = 2.80 kJ; ΔH = −2.80 ÷ 0.050 = −56 kJ/mol.

Part 6 of 8Summary

  • q = mcΔT.
  • ExothermicΔH negative.
  • Breaking bonds absorbs energy.

Part 7 of 8Keep going

Energy changes EST Chemistry: what to do next

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

1
Practice: Energy changes EST Chemistry practice questions
2
Previous lesson: 5.3 Ionic equilibria
3
Next lesson: 6.2 Energy cycles and spontaneity
4
Course: EST 2 Chemistry — all units
5
Official syllabus: OpenStax free science textbooks

Energy changes EST Chemistry revision notes

Part 8 of 8Compare: Compare

Exothermic vs endothermic
FeatureExothermicEndothermic
ΔHNegativePositive
SurroundingsGet warmerGet cooler
Energy diagramProducts lowerProducts higher
ExamplesCombustion, neutralizationPhotosynthesis, melting ice
Specific heats
Substancec (J/g·°C)
Water (l)4.18
Ice2.03
Aluminium0.897
Iron0.449

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
Energy · Heat (q) · Specific heat (c) · Calorimeter · Enthalpy (H) · Exothermic · Endothermic · Thermochemical equation
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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