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Introduction to Energy Changes

Energy changes: exothermic vs endothermic, the sign of ΔH, and how to read basic energy profile diagrams in exam questions.

  • SEC G3 Pure Chemistry 2027
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Learning objectives

  • describe the meaning of enthalpy change in terms of exothermic (∆H negative) and endothermic (∆H positive) reactions
  • represent energy changes by energy profile diagrams, including reaction enthalpy changes and activation energies (see also 10(c), 10(d))

Energy changes are everywhere in chemistry, but exam questions usually test just three things: exothermic vs endothermic, the sign of Δ H, and how to read a basic energy profile diagram.

1. Definition

A. Exothermic Reaction

An exothermic reaction is a reaction that releases heat energy to the surroundings.

B. Endothermic Reaction

An endothermic reaction is a reaction that absorbs heat energy from the surroundings.

C. Enthalpy Change (ΔH)

The enthalpy change, Δ H, is the heat energy change during a reaction.

In G3 Pure Chemistry, compare the energy levels of reactants and products and use the sign of Δ H. Numerical enthalpy calculations are not required.

2. Key Ideas

  • Read the sign of Δ H from the relative energy levels of products and reactants.
  • Exothermic: products have lower energy than reactants, so Δ H is negative.
  • Endothermic: products have higher energy than reactants, so Δ H is positive.
  • Many reactions need activation energy to start, even if they are exothermic.
Sign of ΔH is not optional

Exothermic = Δ H negative. Endothermic = Δ H positive. Always include the sign because it states the direction of the energy change.

3. Detailed Explanations

Quick Recall (exo vs endo)
  • Compare product and reactant energy levels before choosing the sign of Δ H.
  • Exothermic: releases heat to surroundings; products lower energy; Δ H is negative.
  • Endothermic: absorbs heat from surroundings; products higher energy; Δ H is positive.
  • Energy profiles always include reactants level, products level, activation energy “hump”, and a Δ H arrow.

A. System vs Surroundings

  • System: the chemicals reacting.
  • Surroundings: everything else (solution, container, air).

If heat is released from the system, the surroundings warm up (exothermic). If heat is absorbed by the system, the surroundings cool down (endothermic).

B. Energy Profile Diagrams (What You Must Read)

An energy profile diagram shows:

  • reactants energy level
  • products energy level
  • activation energy (the “hump” to start the reaction)
  • Δ H (difference between products and reactants)
Exothermic energy profileExothermic energy profile. Reactants are at higher energy than products, so the enthalpy change is negative. The activation energy is measured from the reactant energy level to the peak.EnergyProgress of reactionReactantsProductsEₐΔH < 0
Exothermic profile: products are lower than reactants, so ΔH is negative. Activation energy is measured from the reactant level to the peak.
Endothermic energy profileEndothermic energy profile. Reactants are at lower energy than products, so the enthalpy change is positive. The activation energy is measured from the reactant energy level to the peak.EnergyProgress of reactionReactantsProductsEₐΔH > 0
Endothermic profile: products are higher than reactants, so ΔH is positive. Activation energy is still measured from the reactant level to the peak.
Catalyst trap

A catalyst lowers activation energy, but it does not change Δ H.

C. Common Uses (K324 / 6092)

Examples:

  • exothermic: combustion, neutralisation, many oxidation reactions
  • endothermic: thermal decomposition, photosynthesis (overall), some dissolving processes

4. Common Mistakes

  • Saying exothermic means “temperature always increases”. It releases heat, but the observed temperature change depends on the surroundings and heat loss.
  • Mixing up sign: writing Δ H positive for exothermic.
  • Thinking an exothermic reaction has no activation energy. It can still require energy to start.
  • Using the height of the activation-energy peak to decide the sign of Δ H.

5. Exam Tips

Keywords that score

Write “releases heat to surroundings” for exothermic and “absorbs heat from surroundings” for endothermic.

  • If asked for Δ H sign, write the sign and the reason using “products higher/lower than reactants”.
  • On a profile, compare only the reactant and product levels for Δ H; the peak is used for activation energy.

6. Worked Examples

Modelled example 1

Decide Exothermic or Endothermic From Energy Levels

Core

Problem

An energy profile shows the products below the reactants. State the sign of Δ H, classify the reaction and give the energy-transfer direction.
Study the worked solution

  1. Compare the energy levels

    Method

    Notice that the products are lower than the reactants.

    Reason

    The reacting system has less energy after the reaction.

    Working

    Products below reactants → Δ H < 0.

  2. Classify from the sign

    Method

    Identify the reaction as exothermic.

    Reason

    The products are lower than the reactants, so the system has lost energy.

    Working

    Δ H < 0: exothermic.

  3. State the transfer direction

    Method

    Describe energy moving from system to surroundings.

    Reason

    The lower-energy products account for energy released by the reacting system.

    Working

    Energy is released to the surroundings.

Common misconception 2

Decide the Sign of Δ H

Find and correct the mistake

Learner claim

A student says, “An endothermic reaction has negative Δ H because the surroundings lose energy.” Explain the reference-point error and give the correct one-sentence explanation.

Track the reacting system

Product energy
ΔH sign

View solution step by step

  1. Use the system as the reference

    Method

    Track energy gained by the reacting chemicals.

    Reason

    Δ H describes the system, even though the surroundings lose the transferred energy.

    Working

    The system absorbs energy from the surroundings.

  2. Connect levels and sign

    Method

    Place products above reactants and assign a positive sign.

    Reason

    Products above reactants show that the reacting system has gained energy overall.

    Working

    Δ H > 0 because products are higher in energy; the reaction is endothermic.

Challenge 3

Use Δ H to Find Energy Released

Minimal support

Out-of-syllabus enrichment

A reaction has Δ H = -120 kJ mol⁻¹. If 0.50 mol of reaction occurs, how much heat energy is released?

Scale and choose the reporting convention

Transfer wording

Hints

Hint 1: scale the per-mole value
Half a mole gives half of the 120 kJ mol⁻¹ magnitude.
View solution step by step

  1. Scale the magnitude

    Method

    Multiply amount by the per-mole energy.

    Reason

    The supplied value applies to one mole of reaction as written.

    Working

    0.50 mol × 120 kJ mol⁻¹ = 60 kJ.

  2. Report the direction

    Method

    State 60 kJ released.

    Reason

    The question asks for the amount released; the negative sign is already represented by the transfer direction.

    Working

    60 kJ released. If a signed system energy change is requested, write -60 kJ.

7. Mind Stretchers

Mind stretcher 1: Catalyst EffectExtension

A student says: “A catalyst makes a reaction more exothermic because it lowers activation energy.” Identify the mistake.

Show Answer

Lowering activation energy makes the reaction faster, but it does not change the energy difference between reactants and products. So Δ H does not change.

Mind stretcher 2: Temperature vs Reaction TypeExtension

A reaction mixture gets colder. Does that prove the reaction is endothermic? State a correct exam-safe answer.

Show Answer

It suggests heat is being absorbed, but temperature change alone does not prove the reaction type unless heat exchange is controlled. Exam-safe: endothermic reactions absorb heat from surroundings, so surroundings often cool down.

8. Quiz

Quiz Time!

Continue with the topic practice when you can classify both profile shapes and explain the sign of Δ H.

K324 / 6092 Practice