Heat transfer in chemical reactions happens because energy is either absorbed or released when chemical bonds are broken and formed. The heat does not stay trapped in the reacting molecules — it flows between the system (the reacting chemicals) and the surroundings.
Ways Heat is Transferred in Chemical Reactions:
- Exothermic Reactions (Heat Released):
- Energy is released when new bonds in products are stronger than the bonds in reactants.
- Heat flows from the system to the surroundings.
- Example: Combustion of fuels (fire feels hot because heat is released).
- Endothermic Reactions (Heat Absorbed):
- Energy is absorbed to break bonds, and products have higher energy than reactants.
- Heat flows from the surroundings into the system.
- Example: Photosynthesis, or dissolving ammonium chloride in water (solution feels cold).
Modes of Heat Transfer (just like in physics):
- Conduction: Direct transfer through contact (e.g., hot beaker during a reaction).
- Convection: Movement of heated fluids or gases carrying energy away.
- Radiation: Heat emitted as infrared waves (e.g., burning magnesium produces bright light and heat).
In short:
During chemical reactions, heat flows between the system and surroundings depending on whether the reaction is exothermic (heat released) or endothermic (heat absorbed), and it is transferred by conduction, convection, or radiation.