Exothermic & Endothermic Reactions

1. Heat Exchange in Reactions: System & Surroundings

Every chemical reaction involves a transfer of thermal energy. To understand this, we need two key definitions:

The System and the Surroundings

System: The chemicals that are actually reacting (the reactants and products).

Surroundings: Everything else—the beaker, the air, the table, the observer. Anything that isn't directly involved in the reaction.

Think of it like a boundary: draw an imaginary line around the chemicals. Inside the line = system. Outside = surroundings. The thermal energy flows across this boundary.

Chemical Bonds as Energy Stores

The energy in chemical reactions comes from chemical bonds themselves. Bonds store potential energy—think of them like compressed springs. When bonds break, that stored energy is released. When new bonds form, energy is released or absorbed depending on the strength of the new bonds.

2. Exothermic Reactions: Energy Released

What's Happening?

In an exothermic reaction, thermal energy is transferred from the system to the surroundings.

• The system loses energy (energy change is negative)
• The surroundings gain heat and get warmer
• More energy is released forming new bonds than absorbed breaking old bonds

In Practice: What You Feel

If you've held a hand warmer or seen a chemical reaction in class that got hot, that was exothermic. The beaker felt warm. The thermometer rose. The products have less energy than the reactants, so the "missing" energy was released as heat.

Real-World Examples

Practice Question

Key Terms & Definitions