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Exothermic Reactions

Exothermic reactions: definition, ΔH is negative, and how to interpret energy profile diagrams and activation energy without common traps.

  • 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))

This lesson applies the energetics overview to exothermic evidence, profiles and familiar examples such as combustion and neutralisation.

1. Definition

A. Exothermic Reaction

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

B. Enthalpy Change (Δ H)

For an exothermic reaction, the enthalpy change is negative:

Δ H = energy of products - energy of reactants < 0

2. Key Ideas

  • Exothermic means the surroundings warm up because heat is released from the reacting chemicals.
  • Products have lower energy than reactants.
  • Δ H is negative.
  • Most reactions still require activation energy to start (even exothermic ones).
Must-write line

Exothermic reactions release heat to the surroundings, so Δ H is negative.

3. Detailed Explanations

A. Energy Profile Diagram (Exothermic)

In an exothermic energy profile:

  • reactants start at a higher energy level,
  • products end at a lower energy level,
  • Δ H is downward (negative),
  • there is usually an activation energy “hump”.
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. A dashed catalysed pathway has a lower peak but the same reactant and product energy levels.EnergyProgress of reactionReactantsProductsEₐΔH < 0catalysed pathwayuncatalysed pathway
Exothermic energy profile: products are lower than reactants, so ΔH is negative. A catalyst lowers activation energy without changing ΔH.
Catalyst trap

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

B. Examples You Can Write Safely

ExampleWhy exothermic?
Combustion (burning in oxygen)Releases heat/light
Neutralisation (acid + alkali)Releases heat (temperature rises)
Many oxidationsOften release heat

4. Common Mistakes

  • Writing “temperature increases” as the definition. Definition must mention heat released to surroundings.
  • Getting the sign wrong: exothermic is Δ H negative.
  • Thinking exothermic means “no activation energy”. Exothermic reactions can still need energy to start.
  • Confusing “heat released” with “rate of reaction”. Fast/slow is a different topic.

5. Exam Tips

Energy profile mark

If asked to sketch/label: label reactants, products, activation energy, and ΔH (downward arrow).

  • Use the word surroundings (container/solution) in explanations.

A. Phrase-level exam wording reminders

  • “Heat is released from the system to the surroundings, so Δ H is negative.”
  • “Products are at lower energy than reactants on the energy profile.”
  • “A catalyst lowers activation energy only; it does not change Δ H.”
  • “Temperature rise is evidence consistent with exothermic change, assuming limited heat loss.”

6. Worked Examples

Modelled example 1

Decide the Sign of ΔH

Core

Problem

Is Δ H positive or negative for an exothermic reaction? Explain using the relative energies of products and reactants.
Study the worked solution

  1. Compare the energy levels

    Method

    Place the products below the reactants.

    Reason

    An exothermic system transfers energy to the surroundings, so the products retain less energy.

    Working

    E_products < E_reactants.

  2. State the sign

    Method

    Use a negative Δ H.

    Reason

    Products below reactants show a net energy release.

    Working

    Δ H < 0; therefore Δ H is negative.

Guided practice 2

Read an Exothermic Energy Profile

About 5 min

Problem

An energy profile shows the products below the reactants. Choose the sign of Δ H and classify the reaction.

Compare the levels and classify

Product level
Reaction type

Hints

Hint 1: compare endpoints
Ignore the peak for now and compare only products with reactants.
Hint 2: interpret the direction
A lower product level means energy has left the reacting system.
View solution step by step

  1. Compare the endpoints

    Method

    Place the products below the reactants.

    Reason

    The reacting system has less energy after the reaction.

    Working

    Products lower → Δ H < 0.

  2. Interpret the result

    Method

    Classify the reaction as exothermic.

    Reason

    The negative sign shows that the system releases energy to the surroundings.

    Working

    Δ H < 0, so the reaction is exothermic.

Common misconception 3

Read an Energy Profile

Find and correct the mistake

Learner claim

On an energy profile, the curve rises above the reactants before ending with products below them. A student says, “Δ H is positive because the peak is higher than the reactants.” Explain the error and classify the reaction.

Identify what each vertical difference means

Reactants to peak
Products below reactants

View solution step by step

  1. Name the peak correctly

    Method

    Use the reactant-to-peak rise for activation energy.

    Reason

    It is the energy barrier to starting the reaction, not the net difference between initial and final states.

    Working

    The peak can be above the reactants even for an exothermic reaction.

  2. Compare initial and final levels

    Method

    Compare the product level with the reactant level for Δ H.

    Reason

    The products are lower, so Δ H is negative.

    Working

    Δ H < 0; the reaction is exothermic and releases heat to the surroundings.

Challenge 4

Use ΔH to Find Energy Released

Minimal support

Out-of-syllabus enrichment

The combustion of carbon has Δ H = -394 kJ mol⁻¹ for the reaction as written. How much heat is released when 0.25 mol of carbon burns completely?

Scale the energy per mole

Scaling operation
Requested energy

Hints

Hint 1: unit meaning
394 kJ mol⁻¹ means 394 kJ is released per mole of reaction as written.
View solution step by step

  1. Scale by the reacting amount

    Method

    Multiply the amount by the magnitude of the molar enthalpy.

    Reason

    Only one quarter of a mole reacts, so it releases one quarter of the per-mole energy.

    Working

    0.25 mol × 394 kJ mol⁻¹ = 98.5 kJ.

  2. Report the requested quantity

    Method

    State the positive amount of heat released.

    Reason

    The negative sign belongs to Δ H; “how much heat is released” asks for its magnitude.

    Working

    98.5 kJ released.

7. Mind Stretchers

Mind stretcher 1: Catalyst ConfusionExtension

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

Show Answer

Lowering activation energy changes the rate, not the energy difference between reactants and products. A catalyst does not change Δ H.

Mind stretcher 2: Temperature Observation TrapExtension

A student measures no temperature rise in a reaction and concludes “it is not exothermic”. Give one valid reason their conclusion could be wrong.

Show Answer

Heat could be lost to the surroundings (poor insulation), or the temperature rise could be too small to detect. Exothermic means heat is released; the observed temperature change depends on heat loss/gain.

8. Quiz

Quiz Time!

First sketch one exothermic energy profile from memory. Then continue with the topic practice.

K324 / 6092 Practice