Where the Energy in a Reaction Comes From
- Chemistry
- Ages 14–18 · Grades 9–12
- Hands-on experiment
What you’ll find out
- Breaking bonds takes in energy; making bonds gives energy out.
- A reaction is exothermic when the bonds made release more energy than the bonds broken absorb.
- Bond energies and calorimetry both let chemists calculate the energy change of a reaction.
No sign-up and no real names. In the lesson you check your understanding as you read, run the experiment yourself, and practise until it sticks.
Breaking costs, making pays
Every reaction breaks some bonds and makes new ones. Breaking a bond always takes energy in, because you must pull the atoms apart. Making a bond always gives energy out. If the new bonds are stronger than the old ones, more energy comes out than went in. The extra energy warms the surroundings, and the reaction is exothermic. If the old bonds were stronger, the reaction takes energy in and is endothermic. Burning methane, the gas in many cookers, is exothermic. Breaking its bonds and the oxygen’s bonds costs about 2,650 kJ for each mole. Making the bonds in carbon dioxide and water gives out about 3,470 kJ. So about 820 kJ is released as heat. Many people think energy comes out when bonds break. In fact, it comes out when new, stronger bonds form.
Measuring the heat
You can also measure the energy of a reaction by using it to heat water. It takes 4.18 joules to warm one gram of water by one degree Celsius. So the heat the water gains is its mass × 4.18 × its temperature rise. In a school experiment, a small burner of ethanol heats 100 g of water in a copper can. Suppose burning 0.50 g of ethanol warms the water by 30 °C. Then the water gained 100 × 4.18 × 30, about 12,500 joules. Scaled up to a mole of ethanol, that is about 1,150 kJ. The true value is about 1,370 kJ. The result is too low because heat escapes into the air and the can, and some ethanol burns only partly and makes soot. Special sealed containers called bomb calorimeters lose almost no heat. They are used to measure the energy in foods.
Curriculum links: NGSS HS-PS1-4