Energy in reactions: exothermic and endothermic

Heat in reactions

Some reactions make the mixture hot. Others make it cold.

Exothermic reactions

In an exothermic reaction, heat is given out. The temperature of the surroundings rises. The products have less energy than the reactants, and the difference goes out as heat.

Examples:

• Burning of natural gas: $\text{CH}_4 + 2\,\text{O}_2 \rightarrow \text{CO}_2 + 2\,\text{H}_2\text{O} + \text{heat}$.
• Quicklime in water: $\text{CaO} + \text{H}_2\text{O} \rightarrow \text{Ca(OH)}_2 + \text{heat}$. A test tube becomes hot to the touch.
• Respiration: glucose and oxygen in our cells give carbon dioxide, water and energy. $\text{C}_6\text{H}_{12}\text{O}_6 + 6\,\text{O}_2 \rightarrow 6\,\text{CO}_2 + 6\,\text{H}_2\text{O} + \text{energy}$. This is why food gives us energy, and why our body is warm.
• The decay of vegetable matter into compost.

Endothermic reactions

In an endothermic reaction, heat is taken in. The temperature of the surroundings falls. The products have more energy than the reactants.

Examples:

• Heating of limestone: $\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$ needs heat all the time.
• Photosynthesis: plants take in energy from sunlight.
• Barium hydroxide with ammonium chloride: when they are mixed in a flask, the flask feels very cold, and water on a wooden board below it may freeze.

How much heat?

We can find the heat from a rise in temperature. If a reaction heats $m$ grams of water by $\Delta T$ degrees, the heat taken by the water is

where $c = 4.18\ \text{J/(g K)}$ is the specific heat of water.

Example. $2.0\ \text{g}$ of quicklime ($\text{CaO}$, $56\ \text{g/mol}$) react with $100\ \text{g}$ of water, and the reaction gives out about $64\ \text{kJ}$ for every mole.

• Moles: $\dfrac{2.0}{56} = 0.0357\ \text{mol}$.
• Heat: $0.0357 \times 64000 = 2286\ \text{J}$.
• Rise in temperature: $\Delta T = \dfrac{Q}{m c} = \dfrac{2286}{100 \times 4.18} = 5.5\ \text{K}$.

You should find that twice the water warmed by the same amount takes twice the heat, and that a negative change means heat was taken in.

Practise with new numbers

Try it in Python

This program does the calculation.

Remember these

• Exothermic: heat given out, the surroundings get warmer. Endothermic: heat taken in, the surroundings get cooler.
• Burning, respiration and slaking of lime are exothermic. Photosynthesis and heating of limestone are endothermic.
• $Q = m c \Delta T$.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. A compost heap of rotting leaves feels warm inside. Is the decay exothermic or endothermic?

2. Barium hydroxide and ammonium chloride are mixed in a flask standing on a wet wooden board. The water under the flask freezes. Is the reaction exothermic or endothermic?

3. How much heat warms $50\ \text{g}$ of water by $6\ \text{K}$? Use $c = 4.18\ \text{J/(g K)}$.

4. On the energy diagram of an exothermic reaction, are the products above or below the reactants?