A burning match gives off heat; an instant cold pack soaks it up. In Physical and Chemical Changes you saw that reactions make new substances; here you track the energy, the stored energy of Forms of Energy, as heat flows in or out.
Key idea
Split the world into the system (the reacting substances) and the surroundings (everything else). An exothermic process releases energy to the surroundings, which warm up: $\Delta H < 0$. An endothermic process takes energy from the surroundings, which cool down: $\Delta H > 0$.
Why it works
Chemical energy is stored in the arrangement of atoms and the bonds between them. If the products hold less chemical energy than the reactants, the difference leaves as heat (or light). If they hold more, the reaction must take that energy in. $\Delta H$ is the energy of the products minus that of the reactants, so its sign tells the direction of the flow.
| Process | Type | What you notice |
|---|---|---|
| methane burning | exothermic | heat and light |
| hand warmer (iron rusting) | exothermic | gets warm |
| water freezing | exothermic | heat leaves the water |
| instant cold pack | endothermic | gets cold |
| ice melting | endothermic | cools its surroundings |
| photosynthesis | endothermic | takes in light energy |
Worked example: A cold pack
Squeezing an instant cold pack dissolves ammonium nitrate in water, and the pack gets cold. Is the process exothermic or endothermic?
- The system is the dissolving salt; the surroundings include the water, the bag and your hand.
- The surroundings get colder, so they lost energy to the system.
Answer: Endothermic ($\Delta H > 0$).
Worked example: Reading a diagram
On an energy diagram, the reactants are at $250\ \text{kJ}$ and the products at $100\ \text{kJ}$. Find $\Delta H$.
- $$\Delta H = E_{\text{products}} - E_{\text{reactants}} = 100 - 250$$
- $\Delta H = -150\ \text{kJ}$: energy is released.
Answer: $-150\ \text{kJ}$, exothermic.
Common mistake
Mixing up the system and the surroundings. A cold pack feels cold because the reaction takes heat from your hand, not because it gives off "cold". Check: decide what is the system, then ask whether the surroundings warmed (exothermic) or cooled (endothermic).