Chemical reactions are quite often accompanied by the changes in the temperature indicating whether they are exothermic or endothermic. In exothermic reactions heat energy is released which is absorbed by the surrounding causing their temperature to increase.
🔥 Exothermic example: When quick lime (CaO) reacts with water, slaked lime Ca(OH)2 is formed and a lot of heat energy is released.
The surrounding temperature increases noticeably — the container becomes hot.
In endothermic reactions heat energy is absorbed causing the surrounding to cool down.
❄️ Endothermic example: When ammonium chloride or ammonium nitrate is dissolved in water, heat energy is absorbed causing the container to cool down.
The beaker feels cold — temperature drops.
Effect of temperature on reaction rate
Increasing the temperature of the reaction increases its rate. For many reactions, the rate doubles with every 10°C rise in temperature. Cooking food uses high temperature to speed up chemical reactions that break down the components of food making it edible.
Reaction Kinetics (Chapter 17, Chemistry – X) — pages 58. Exothermic: heat released → surroundings hot. Endothermic: heat absorbed → surroundings cool. Rate roughly doubles per 10°C.
📌 17.3 Quick Check!
1. What type of reaction is CaO + H₂O → Ca(OH)₂? How does it affect temperature?
Answer: Exothermic — releases heat, so temperature increases.
2. Give an example of an endothermic reaction (from the text).
Answer: Dissolving ammonium chloride or ammonium nitrate in water — absorbs heat, cools the container.
3. By how much does the rate of many reactions increase per 10°C rise?
Answer: Approximately doubles.
4. Why do we cook food at high temperature?
Answer: High temperature speeds up chemical reactions that break down food components, making it edible.
🧠 Memorization tips & tricks for 17.3
✨ Exothermic = Exit heat — heat leaves the system, surroundings get hot.
✨ Endothermic = Enter heat — heat enters from surroundings, you feel cold.
✨ Quick lime + water → hot enough to cook an egg! (classic demo).
✨ NH₄Cl in water = cold pack (endothermic).
✨ 10°C rule: rate doubles — think “every 10 degrees, speed ×2”.
✨ Cooking: higher temp = faster breakdown of food (kinetic energy↑).
📚 Guidelines for students – 17.3
• Distinguish exothermic (heat released) vs endothermic (heat absorbed) using everyday examples: quick lime gets hot, ammonium chloride gets cold.
• Remember the 10°C doubling rule — it’s a handy estimate for rate increase.
• Connect cooking to kinetics: higher temperature → faster breakdown of food molecules.
• Use the MCQs to test yourself — click any option to see instant correct/red feedback.
• Adjust font size and theme for comfortable study sessions.