Heat comes out

The idea

Reactions in which heat is released along with the formation of products are exothermic reactions.

The warm beaker

When water is poured on quicklime, the beaker becomes too hot to hold. Where did the heat come from? Nobody lit a flame.

Where does the heat come from?

• From the reaction itself: heat is released while the products form
• From the water, which was already hot
• From the beaker

Watch the thermometer

Pour water on quicklime. Then put Heat where it belongs in the equation.

Exo = exit

Reactions in which heat is released along with the formation of products are exothermic reactions.

Remember: exo sounds like exit. Heat exits the reaction.

• Exothermic reaction | Equation
• Quicklime + water | CaO + H₂O → Ca(OH)₂ + Heat
• Burning of natural gas | CH₄ + 2O₂ → CO₂ + 2H₂O
• Respiration (glucose + oxygen) | C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
• Compost forming from vegetable matter | it warms up

Notes

Reactions in which heat is released along with the formation of products are exothermic reactions.

Check yourself

What does “exothermic” mean?

Which of these is exothermic?

Respiration is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. What kind of reaction is it?

• Heat is taken in from outside. That is the opposite. Exothermic reactions give heat out.
• The reaction is very slow. Speed is a different thing. Exo is about heat released.
• Heat is released along with the products — correct. Yes! Heat exits.
• Burning of natural gas — correct. Yes! The flame and heat show it. CH₄ + 2O₂ → CO₂ + 2H₂O + heat.
• Quicklime lying dry in a jar. Nothing happens: there is no reaction.
• Ice cubes in a glass. Melting is not a chemical reaction.
• Endothermic. Here energy is given out (it is on the product side), so it is exothermic.
• Exothermic: energy is released — correct. Yes! The energy is how our body stays alive and warm.
• It is not a reaction. Glucose and oxygen are changed into new substances: it is a reaction.