Exothermic reactions are a type of reaction that gives off heat.

When products form from the original materials, they release energy, mostly as heat.

This happens when it takes less energy to break the bonds in the starting materials than it gives off when new bonds form in the products.

Key Points:

• Energy Changes: In an exothermic reaction, energy is lost to the surroundings. We say that the change in energy ($\Delta H$) is negative.

• Examples:

  • Burning: When we burn fuels like methane ($\text{CH}_4$), it reacts with oxygen ($2\text{O}_2$) to produce carbon dioxide ($\text{CO}_2$) and water ($2\text{H}_2\text{O}$). This process releases heat.
  • Breathing: When our bodies break down sugar (glucose), it reacts with oxygen ($\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O}$) and releases energy as well.

Because of these reactions, exothermic reactions often feel warm when we touch them, showing how they give off heat!