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Endothermic Reactions

Endothermic reactions: definition, ΔH is positive, and how to interpret energy profile diagrams and activation energy 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))

This lesson applies the energetics overview to endothermic evidence, profiles and reactions that need a continuing energy input, especially thermal decomposition.

1. Definition

A. Endothermic Reaction

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

B. Enthalpy Change (Δ H)

For an endothermic reaction, enthalpy change is positive:

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

2. Key Ideas

  • Endothermic means the surroundings cool down because heat is absorbed by the reacting chemicals.
  • Products have higher energy than reactants.
  • Δ H is positive.
  • Endothermic reactions still require activation energy to start.
Must-write line

Endothermic reactions absorb heat from the surroundings, so Δ H is positive.

3. Detailed Explanations

A. Energy Profile Diagram (Endothermic)

In an endothermic energy profile:

  • reactants start at a lower energy level,
  • products end at a higher energy level,
  • Δ H is upward (positive),
  • there is an activation energy “hump”.
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. A dashed catalysed pathway has a lower peak but the same reactant and product energy levels.EnergyProgress of reactionReactantsProductsEₐΔH > 0catalysed pathwayuncatalysed pathway
Endothermic energy profile: products are higher than reactants, so ΔH is positive. 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 endothermic?
Thermal decomposition of limestone (CaCO₃)Heat must be supplied for decomposition
Photosynthesis (overall)Energy absorbed from sunlight (overall process)
Some dissolving processesDissolving can absorb heat (temperature falls)

4. Common Mistakes

  • Writing “temperature decreases” as the definition. Definition must mention heat absorbed from surroundings.
  • Getting the sign wrong: endothermic is Δ H positive.
  • Forgetting to include a heat condition (e.g., “heat strongly”) for thermal decomposition equations when asked.
  • Confusing “endothermic” with “slow”. Rate is a different topic.

5. Exam Tips

Energy profile mark

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

  • Use the word surroundings in your explanation.

A. Phrase-level exam wording reminders

  • “Heat is absorbed by the system from the surroundings, so Δ H is positive.”
  • “Products are at higher energy than reactants on the energy profile.”
  • “A catalyst changes activation energy and rate, but not Δ H.”
  • “Temperature drop supports an endothermic interpretation when heat exchange is controlled.”

6. Worked Examples

Modelled example 1

Decide the Sign of ΔH

Core

Problem

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

  1. Compare the energy levels

    Method

    Place the products above the reactants.

    Reason

    An endothermic system absorbs energy from the surroundings, so the products retain more energy.

    Working

    E_products > E_reactants.

  2. State the sign

    Method

    Use a positive Δ H.

    Reason

    Products above reactants show a net energy absorption.

    Working

    Δ H > 0; therefore Δ H is positive.

Guided practice 2

Read an Endothermic Energy Profile

About 5 min

Problem

An energy profile shows the products above 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 higher product level means the reacting system has gained energy.
View solution step by step

  1. Compare the endpoints

    Method

    Place the products above the reactants.

    Reason

    The reacting system has more energy after the reaction.

    Working

    Products higher → Δ H > 0.

  2. Interpret the result

    Method

    Classify the reaction as endothermic.

    Reason

    The positive sign shows that the system absorbs energy from the surroundings.

    Working

    Δ H > 0, so the reaction is endothermic.

Common misconception 3

Thermal Decomposition Equation (Condition)

Find and correct the mistake

Learner response

A student writes “calcium carbonate → calcium oxide + carbon dioxide” and says no condition is needed because calling the change endothermic already shows that energy is absorbed. Explain the mistake and correct the response.

Separate energy classification from reaction condition

Word equation
Required condition

View solution step by step

  1. Retain the correct equation

    Method

    Keep the stated reactant and products.

    Reason

    Calcium carbonate thermally decomposes into calcium oxide and carbon dioxide.

    Working

    Calcium carbonate → calcium oxide + carbon dioxide.

  2. Supply the operational condition

    Method

    State “heat strongly”.

    Reason

    “Endothermic” classifies the net energy transfer; it does not tell the examiner that the necessary thermal-decomposition condition is supplied.

    Working

    Condition: heat strongly.

Challenge 4

Use ΔH to Find Energy Absorbed

Minimal support

Out-of-syllabus enrichment

The decomposition of calcium carbonate has Δ H = +178 kJ mol⁻¹ for the reaction as written. How much heat is absorbed when 0.40 mol of CaCO₃ decomposes?

Scale the energy per mole

Scaling operation
Energy statement

Hints

Hint 1: unit meaning
178 kJ mol⁻¹ means 178 kJ is absorbed per mole of reaction as written.
View solution step by step

  1. Scale by the decomposed amount

    Method

    Multiply the amount by the molar enthalpy.

    Reason

    Only 0.40 of the per-mole reaction occurs.

    Working

    0.40 mol × 178 kJ mol⁻¹ = 71.2 kJ.

  2. State the transfer direction

    Method

    Report the energy as absorbed.

    Reason

    The positive enthalpy change means energy enters the reacting system from the surroundings.

    Working

    71.2 kJ absorbed.

7. Mind Stretchers

Mind stretcher 1: Temperature Observation TrapExtension

A student measures no temperature drop and concludes “it is not endothermic”. Give one valid reason their conclusion could be wrong.

Show Answer

Heat could be absorbed from the surroundings slowly, or heat could be supplied by the surroundings (e.g., warm container/air), masking the temperature drop. Reaction type should be linked to heat absorbed/released, not only observed temperature.

Mind stretcher 2: Catalyst ConfusionExtension

A student says: “A catalyst makes a reaction more endothermic 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.

8. Quiz

Quiz Time!

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

K324 / 6092 Practice