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).
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?
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.
| Reaction | Endothermic 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 glucose | Exothermic |
| Haber process N₂ + 3H₂ → 2NH₃ | Exothermic |
| 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ | Endothermic — needs heat in Thermal decompositions absorb heat — they are endothermic. |
| N₂ + O₂ → 2NO | Endothermic — 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
The same idea in a real exam question:
Example 1 · Chemical Reactions · Endothermic and Exothermic Reactions
Combustion and slaking are exothermic; decomposition is endothermic
N₂ + O₂ needs huge energy
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
| Reaction | Endothermic 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 glucose | Exothermic |
| Haber process N₂ + 3H₂ → 2NH₃ | Exothermic |
| 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ | Endothermic — needs heat in Thermal decompositions absorb heat — they are endothermic. |
| N₂ + O₂ → 2NO | Endothermic — 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)
- Combustion and slaking are exothermic; decomposition is endothermic→ Which way does the heat flow?
- N₂ + O₂ needs huge energy→ Which way does the heat flow?
Test yourself on Chemical Reactions
15 past NDA questions from this chapter, timed at 15 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.