Quick answer: Respiration is exothermic because glucose combines with oxygen in our cells to form carbon dioxide and water, releasing energy: + energy. Photosynthesis is endothermic because plants absorb energy from sunlight to make glucose from carbon dioxide and water. The decomposition of vegetable matter into compost is also exothermic: it gives out heat.
Exothermic and endothermic reactions
| Type | Energy | Example |
|---|---|---|
| Exothermic | Heat is given out to the surroundings | Burning of natural gas, respiration |
| Endothermic | Energy is absorbed from the surroundings | Photosynthesis, decomposition of calcium carbonate on heating |
In terms of equations:
- Exothermic: reactants → products + heat
- Endothermic: reactants + energy → products
Why respiration is exothermic
Step 1: Food gives glucose. During digestion, carbohydrates such as rice, potatoes and bread are broken down into glucose.
Step 2: Glucose reacts with oxygen in cells. Glucose combines with oxygen in the cells of our body:
Step 3: Energy is released. This reaction gives out energy. Part of it is stored as ATP and used for life processes; part is released as heat, which helps keep our body temperature at about 37 °C.
Exam answer: "During respiration, glucose combines with oxygen in the cells of our body and provides energy. Since energy is released, respiration is an exothermic reaction."
Why photosynthesis is endothermic
- Green plants make glucose from carbon dioxide and water.
- This needs energy, which the plant absorbs from sunlight using chlorophyll.
- So photosynthesis takes in energy and is endothermic.
Photosynthesis and respiration are roughly the reverse of each other: one stores energy in glucose, the other releases it.
Why composting is exothermic
- When vegetable peels, leaves and other plant waste are piled up, microbes (bacteria and fungi) break them down.
- This breakdown is a kind of slow oxidation, similar to respiration, and it releases heat.
- The inside of an active compost heap becomes warm; it can reach about 50–60 °C.
So the decomposition of vegetable matter into compost is an exothermic process.
Summary table
| Process | Energy change | Type |
|---|---|---|
| Respiration | Releases energy | Exothermic |
| Photosynthesis | Absorbs light energy | Endothermic |
| Composting of vegetable matter | Releases heat | Exothermic |
| Burning of natural gas | Releases heat and light | Exothermic |
| Heating calcium carbonate | Absorbs heat | Endothermic |
Exam tips
- For "why is respiration exothermic?", write the balanced equation and say that energy is released.
- Balance the respiration equation with 6s: .
- For photosynthesis, mention that sunlight energy is absorbed.
- Remember the NCERT example: decomposition of vegetable matter into compost is exothermic.
Common mistakes
- Confusing respiration with breathing. Breathing is moving air in and out; respiration is the chemical reaction in cells that releases energy.
- Calling photosynthesis exothermic because it produces oxygen. It absorbs energy, so it is endothermic.
- Writing an unbalanced respiration equation.
- Thinking all decomposition reactions are endothermic. Composting is a decomposition that releases heat.
Frequently Asked Questions
Why is respiration considered an exothermic reaction?
In respiration, glucose combines with oxygen in our cells to form carbon dioxide and water, and energy is released. A reaction that releases energy is exothermic.
Why is photosynthesis an endothermic reaction?
Plants absorb energy from sunlight to make glucose from carbon dioxide and water. Because energy is absorbed, photosynthesis is endothermic.
Is the decomposition of vegetable matter into compost exothermic?
Yes. Microbes break down the plant matter and release heat, so a compost heap becomes warm.
What is the balanced equation for respiration?
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.
What is the difference between exothermic and endothermic reactions?
Exothermic reactions give out heat to the surroundings, while endothermic reactions absorb energy from the surroundings.