Energy transfer during exothermic and endothermic reactions

Welcome to the topic of energy changes! In chemistry, reactions aren't just about making new substances; they are also about the flow of energy.

What you'll learn:

• The golden rule of energy conservation in chemical reactions.
• How to tell exothermic and endothermic reactions apart using a thermometer.
• Everyday examples and practical uses for both types of reaction.
• How to carry out an experiment to measure temperature changes accurately.

The Conservation of Energy

Before we dive into hot and cold reactions, we need to understand a fundamental rule of the universe: energy is conserved.

In any chemical reaction, the total amount of energy in the universe at the end of the reaction is exactly the same as it was before the reaction took place. Energy cannot be created out of thin air, and it cannot be destroyed. It can only be transferred from one place to another.

If a reaction transfers energy to its surroundings, the chemicals themselves must lose that exact amount of energy. This means the newly formed product molecules will have less energy than the original reactant molecules. Conversely, if a reaction takes in energy, the products will end up with more energy than the reactants had.

Exothermic Reactions (Giving out heat)

When a reaction transfers energy out to the environment, we call it an exothermic reaction.

If you were holding a beaker where an exothermic reaction was taking place, it would feel hot. The energy has left the chemical bonds and entered the surroundings (including your hands and the thermometer!).

Examples of Exothermic Reactions

Most chemical reactions you see day-to-day are exothermic:

• Combustion: Burning fuels like wood, coal, or methane gas releases a huge amount of heat.
• Many oxidation reactions: Adding oxygen to a substance often releases heat. For example, the slow oxidation of iron (rusting) is exothermic, though it usually happens too slowly for you to feel the warmth.
• Neutralisation: Mixing an acid and an alkali to make a salt and water will always cause the temperature of the mixture to rise.

Everyday Uses

Because they produce heat, we use exothermic reactions when we want to warm things up without a plug socket or a fire.

• Self-heating cans: These contain clever compartments with chemicals (usually calcium oxide and water). When you press a button, the compartments break, the chemicals mix, and the exothermic reaction heats your coffee or soup.
• Hand warmers: Disposable hand warmers use the exothermic oxidation of iron powder to keep your hands warm on a cold winter day.

Endothermic Reactions (Taking in heat)

Endothermic reactions do the exact opposite. They are much less common than exothermic reactions.

If you hold a beaker containing an endothermic reaction, it will feel cold. The chemicals are literally stealing heat energy from their surroundings (and your hand) to make the reaction happen.

Examples of Endothermic Reactions

• Thermal decomposition: Breaking down a compound by heating it (like turning calcium carbonate into calcium oxide and carbon dioxide) requires a continuous input of heat energy.
• Citric acid and sodium hydrogencarbonate: Mixing these two common household chemicals (found in bath bombs) pulls heat from the water, making it colder.

Everyday Uses

• Sports injury packs: Instant ice packs don't need to be kept in the freezer. They contain a pouch of water and a solid chemical (like ammonium nitrate). When you squeeze the pack, the pouch breaks, the chemical dissolves in the water, and the highly endothermic reaction instantly drops the temperature to soothe a sprained ankle.

Measuring Temperature Changes (Required Practical)

To investigate the variables that affect temperature changes, we need to mix two chemicals together and track the temperature using a thermometer. You can test reactions like acids mixing with metals, acids with carbonates, neutralisation, or metal displacement.

The biggest challenge in this experiment is heat loss. If the reaction is exothermic, heat will try to escape into the air before your thermometer can measure it. If you just use a normal glass beaker, your maximum temperature reading will be much lower than it should be.

To fix this, we use an insulated container—a simple calorimeter.

The Method

1. Place a polystyrene cup inside a glass beaker (this stops the lightweight cup from falling over). Polystyrene is an excellent thermal insulator, which traps the heat inside.
2. Measure a set volume of your first liquid (e.g., dilute hydrochloric acid) using a measuring cylinder and pour it into the cup.
3. Record the initial temperature of the liquid using a thermometer.
4. Add your second chemical (e.g., sodium hydroxide solution, or a metal powder) and immediately put a lid on the cup to reduce heat escaping to the air.
5. Stir gently using the thermometer through a hole in the lid.
6. Watch the thermometer closely. If it goes up, record the highest temperature reached. If it goes down, record the lowest temperature reached.
7. Calculate the temperature change.

---

Applying Your Knowledge

You need to be able to look at temperature data or product descriptions and evaluate what kind of reaction is happening, and whether it is fit for purpose.