Heat Up or Cool Down?
Classify exothermic and endothermic reactions
What this lesson covers
Why it matters
A magnesium ribbon flares with blinding white light, while a lump of limestone (calcium carbonate) sits silent in a crucible until you crank up the heat.
The idea in plain words
Watch the energy bar drop as magnesium burns in oxygen (2Mg + O₂ → 2MgO): energy is released as heat and light, so this reaction is exothermic.
Predict first
Why does burning magnesium feel hot, but heating calcium carbonate requires constant energy input?
Chemical reactions involve breaking old bonds and making new ones. If the new bonds are stronger, energy is released. If you need to force bonds apart, energy is absorbed.
- Magnesium is a metal and calcium carbonate is a salt
- Some reactions release stored energy (exothermic) while others absorb energy to break bonds (endothermic) — correct
- Magnesium is lighter than calcium carbonate
What you do
First, ignite the magnesium ribbon. Feel the heat radiating. Next, heat the calcium carbonate. Notice you must keep the flame on to keep the reaction going.
Check yourself
Which observation proves that burning magnesium is an exothermic reaction?
Exothermic reactions release energy, often as heat and light. The dazzling white light and heat from 2Mg + O₂ → 2MgO are clear signs.
In the reaction CaCO₃ → CaO + CO₂, what happens if you remove the heat source?
Thermal decomposition is endothermic. It absorbs heat to break bonds. Without the heat source, the energy supply cuts off and the reaction stops.
Photosynthesis (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂) is an endothermic process. What provides the necessary energy?
Photosynthesis is photochemical. It requires light energy (absorbed by chlorophyll) to drive the endothermic conversion of CO₂ and water into glucose.
- It produces a white powder
- It emits dazzling white light and heat — correct
- It reacts with oxygen
- The reaction continues because it is exothermic
- The reaction stops because it needs continuous energy input — correct
- The reaction reverses immediately
- Heat from the soil
- Light energy captured by chlorophyll — correct
- Oxygen released by the plant