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NDA Chemistry · Chemical Reactions

Endothermic and Exothermic Reactions

An exothermic reaction releases heat to the surroundings (gets warm); an endothermic reaction absorbs heat from the surroundings (needs heat in).

All Chemical Reactions notesNDA Chemistry strategy

Why this matters

A short subtopic (3 PYQs) that asks you to label a reaction by the direction the heat flows, or to pick the endothermic one from a list. The reliable rules: combustion and neutralisation are exothermic; most decompositions need heat in and are endothermic.

Concept 1 of 1

Which way does the heat flow?

Intuition

If the container gets hot, the reaction is releasing heat — exothermic. If it must be heated to keep going, it is absorbing heat — endothermic. Combustion and adding water to quicklime warm up; thermal decompositions need a constant flame.

Definition

The two categories and their markers:

  • Exothermic — releases heat (ΔH negative); surroundings warm up. Examples: combustion/burning (CH₄ + 2O₂ → CO₂ + 2H₂O), neutralisation, the Haber process (N₂ + 3H₂ → 2NH₃), the thermite reaction (Fe₂O₃ + 2Al → 2Fe + Al₂O₃), and CaO + H₂O → Ca(OH)₂ (slaking of lime).
  • Endothermic — absorbs heat (ΔH positive); needs heat supplied. Examples: most thermal decompositions (2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂), photosynthesis, and N₂ + O₂ → 2NO (needs very high temperature, which is why air's nitrogen and oxygen do not react at room temperature).
  • Quick test for the bank: combustion/neutralisation/slaking = exothermic; 'requires heating to decompose' = endothermic.
ReactionEndothermic or exothermic?
CaO + H₂O → Ca(OH)₂ (slaking lime)Exothermic — releases heat
Adding water to quicklime gets HOT — it is strongly exothermic.
Combustion of CH₄ or glucoseExothermic
Haber process N₂ + 3H₂ → 2NH₃Exothermic
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂Endothermic — needs heat in
Thermal decompositions absorb heat — they are endothermic.
N₂ + O₂ → 2NOEndothermic — needs very high temperature
Air's N₂ and O₂ do not react at ordinary temperatures because the reaction is endothermic and needs > 2000°C.
Practice this concept4 quick reps

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

  1. 1.
    Reaction of quicklime (CaO) with water is endothermic or exothermic?
  2. 2.
    Is the combustion of methane endothermic or exothermic?
  3. 3.
    Decomposition of lead nitrate on heating is which type?
  4. 4.
    Why don't N₂ and O₂ in air react to form NO at room temperature?

From the bank · past-year question

Example 1Chemical ReactionsMODERATE
Which one of the following is an endothermic reaction ?

[Q108 · Sep · 2025]

Combustion and slaking are exothermic; decomposition is endothermic

When asked 'which is endothermic?', rule out combustion (CH₄, glucose) and the Haber process — those RELEASE heat. The endothermic one is usually a thermal decomposition that needs heat in (e.g. lead nitrate).

N₂ + O₂ needs huge energy

Air's nitrogen and oxygen do not combine at room temperature because N₂ + O₂ → 2NO is ENDOTHERMIC and requires very high temperature (lightning or engine heat). It is not that the oxides are unstable or that a catalyst is missing.
Drill 2 more on which way does the heat flow?

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (1)

Which way does the heat flow?5 rows
ReactionEndothermic or exothermic?
CaO + H₂O → Ca(OH)₂ (slaking lime)Exothermic — releases heat
Adding water to quicklime gets HOT — it is strongly exothermic.
Combustion of CH₄ or glucoseExothermic
Haber process N₂ + 3H₂ → 2NH₃Exothermic
2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂Endothermic — needs heat in
Thermal decompositions absorb heat — they are endothermic.
N₂ + O₂ → 2NOEndothermic — needs very high temperature
Air's N₂ and O₂ do not react at ordinary temperatures because the reaction is endothermic and needs > 2000°C.

Watch out for (2)

Mastery check — 2 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Chemical ReactionsEASY
Reaction of quick lime (CaO\text{CaO}) with water to produce slaked lime (Ca(OH)2\text{Ca(OH)}_2) is an example of

[Q87 · Apr · 2021]

Example 2Chemical ReactionsMODERATE
Dinitrogen (N2\text{N}_2) and dioxygen (O2\text{O}_2) are the main constituents of air but they do not react with each other to form oxides of nitrogen because

[Q84 · Apr · 2019]

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