Reactions that warm you up and cool you down

Drop a magnesium ribbon into dilute hydrochloric acid and the test tube gets hot. Mix ammonium nitrate with water and the tube gets cold.

In both cases a chemical reaction has happened. In one, energy has been released to the surroundings; in the other, energy has been taken from them.

Chemistry calls this the enthalpy change of the reaction, written $\Delta H$.

Enthalpy ($H$) is a measure of the energy stored in a substance. $\Delta H$ compares the products with the reactants:

$\Delta H = H_\text{products} - H_\text{reactants}$

If the products hold less energy than the reactants, the difference is negative — the surplus went to the surroundings. That is exothermic.

If the products hold more energy, the difference is positive — energy came in from the surroundings. That is endothermic.

So the sign is not saying "the reaction has negative energy". It is saying which way the energy flowed. A minus sign means energy left the chemicals.

In the lab you cannot see $\Delta H$. You can only see the thermometer.

• Temperature of the mixture rises → exothermic → $\Delta H$ negative.
• Temperature falls → endothermic → $\Delta H$ positive.

The thermometer measures the surroundings (usually the water or the solution), not the chemicals themselves. That is exactly why it is a good test: energy leaving the reaction must arrive somewhere, and it arrives in the surroundings.