Heat, light or electricity
Decomposition needs energy to get going.
Why does limestone not fall apart on its own?
Limestone has been lying on hillsides for millions of years and is still limestone. It breaks down only in a hot kiln.
What does breaking a compound apart need?
What this lesson covers
The idea
Decomposition reactions need energy in the form of heat, light or electricity to break down the reactants; a decomposition carried out by heating is called thermal decomposition.
Why does limestone not fall apart on its own?
Limestone has been lying on hillsides for millions of years and is still limestone. It breaks down only in a hot kiln.
What does breaking a compound apart need?
- Energy
- Nothing, it just takes long
- Cold
Pick the energy
Each substance needs one particular kind of energy. Try the buttons and see what happens.
Energy to break apart
Decomposition reactions need energy in the form of heat, light or electricity to break down the reactants. A decomposition carried out by heating is called thermal decomposition.
Silver chloride and silver bromide behave like this, and that is used in black-and-white photography.
- Energy | Example | Equation
- Heat | Limestone | CaCO₃ → CaO + CO₂
- Heat | Lead nitrate | 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂
- Light | Silver chloride | 2AgCl → 2Ag + Cl₂
- Electricity | Water | 2H₂O → 2H₂ + O₂
Notes
Decomposition reactions need energy in the form of heat, light or electricity to break down the reactants; a decomposition carried out by heating is called thermal decomposition.
Check yourself
White silver chloride turns grey in sunlight. Which energy is needed?
What is thermal decomposition?
Why must we supply energy for a decomposition reaction?
- Light — correct. Yes! The sunlight decomposes it into grey silver and chlorine.
- Heat. The change happens in sunlight, so light is the energy.
- Electricity. No electricity is used. Sunlight does the job.
- A decomposition carried out by light. That would be a photo-decomposition. Thermal is about heat.
- A decomposition carried out by heating — correct. Yes! “Thermal” means heat.
- A decomposition done by electricity. That is not called thermal. Thermal means by heating.
- Because the products are heavier. Mass stays the same. The energy is for breaking the reactants down.
- Because the reaction makes heat. It is the other way: a decomposition takes energy in.
- Energy is needed to break the reactants apart — correct. Yes! The energy breaks the compound down.