Exothermic and endothermic reactions
In this topic you will
- learn about chemical reactions that give out energy
- plan and carry out an investigation
- learn about chemical reactions that absorb energy
- distinguish between exothermic and endothermic reactions and processes
- learn about the use of exothermic and endothermic reactions and processes.
Key words
- combustion
- dissipate
- exothermic reaction
- fuel
- oxidation reaction
- preliminary work
- endothermic process
- endothermic reaction
Burning
When something burns, a chemical reaction takes place. Burning is a chemical reaction in which a substance combines with oxygen. In a burning reaction, there are energy changes. The substance that reacts with oxygen is called a fuel.
Fuels have a store of chemical energy. Charcoal, wood, coal, natural gas and oil are examples of fuels.
When the fuel burns, the chemical energy is changed to thermal, light and sound energy. The thermal, light and sound energy dissipate (spread out) into the surroundings.
Combustion is another term for burning.
Look back at the equation in Getting started. You can see that, during the reaction, the atoms of carbon and oxygen join together in new ways. When this happens, chemical energy is changed to thermal energy and the temperature rises.
A chemical reaction in which thermal energy is given out is called an exothermic reaction.
Important Concept
Exothermic reactions are chemical reactions that release thermal energy to the surroundings. Burning is a classic example of an exothermic reaction.
Questions
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Combustion requires three things: a fuel, oxygen, and heat (thermal energy).
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An exothermic reaction is a chemical reaction that releases thermal energy to the surroundings.
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Burning releases thermal, light, and sound energy into the surroundings, showing that energy is being given out — a sign of an exothermic reaction.
Burning other substances
Hydrogen can be used as a fuel in a model rocket. The combustion of hydrogen is an exothermic reaction. The hydrogen and the oxygen combine to form water.
When the atoms of hydrogen and oxygen rearrange themselves and combine together, energy is given out. This chemical energy is changed into kinetic, thermal, sound and light energy.
In this experiment, a large plastic soda bottle filled with hydrogen and air is attached to a string across the room. The stopper in the bottle has wires that allow a spark to be generated. The hot spark provides the energy to start the reaction. The hydrogen and oxygen react together.
The reaction gives out a lot of energy and the stopper is pushed out. This energy makes the bottle shoot (move very quickly) along the string.
Example: Hydrogen Rocket Experiment
When hydrogen burns inside a closed bottle, it combines with oxygen and releases a large amount of energy. This energy pushes out the stopper and propels the bottle forward along the string — like a rocket.
The reactions of other substances burning in air are also exothermic reactions. An example is burning magnesium, which produces magnesium oxide. Energy is given out as heat and light as the magnesium and oxygen atoms rearrange themselves.
When a substance burns, it combines with oxygen and a new substance called an oxide is formed. Any reaction in which a substance combines with oxygen is an oxidation reaction.
With water
This is the equation for the reaction between potassium and water.
Water is made up of particles containing atoms of hydrogen and oxygen. In the potassium and water reaction, the bonds between the atoms of oxygen and hydrogen in the water break. The atoms rearrange to form the products potassium hydroxide and hydrogen. Stored chemical energy is changed to thermal energy, which dissipates into the environment.
With acid
If you add magnesium to dilute hydrochloric acid, the test tube gets hot. This reaction is an exothermic one.
Quick Fact
An oxidation reaction happens when a substance reacts with oxygen. Burning is a type of oxidation that often releases energy in the form of heat and light.
Measuring temperature rise during a reaction
Sofia and Marcus each measured 10 cm³ of dilute hydrochloric acid into a test tube and measured the temperature. Then they each added an identical piece of magnesium ribbon to their test tube of acid. When the reaction stopped, they each measured the temperature again.
| Sofia’s results | Start temperature in °C | End temperature in °C |
|---|---|---|
| 18 | 42 |
| Marcus’s results | Start temperature in °C | End temperature in °C |
|---|---|---|
| 21 | 45 |
Common Mistake
Students sometimes forget that a rise in temperature is evidence of an exothermic reaction. In this case, magnesium reacting with hydrochloric acid gives off heat, causing the temperature to increase.
Questions
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The products are magnesium chloride and hydrogen gas.
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They knew the reaction was finished when the temperature stopped rising and the magnesium had fully reacted.
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Sofia is likely correct. Both used the same amount of magnesium and acid, so the same amount of chemical energy was released. Marcus’s starting temperature was higher, so his final temperature was also higher, even though the energy released was the same.
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Safety glasses protect the eyes from splashes of acid or fragments if the reaction becomes vigorous.
Write each of these three ideas as a scientific question to be investigated.
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- Does adding more magnesium increase the temperature change?
- Which metal produces the highest temperature change when reacting with hydrochloric acid?
- How does using a different acid affect the temperature change in the reaction?
Think Like a Scientist
You will plan and carry out an investigation into the reaction between magnesium and an acid. Follow the steps below to complete your investigation independently.
Step 1: Choose a scientific question related to this reaction. Write a clear plan for investigating it.
Step 2: Carry out preliminary work: practise measuring temperature change during a trial run of the reaction.
Step 3: Make a list of equipment you will need for your experiment.
Step 4: Determine how much you need to change your variable (e.g. the length of magnesium ribbon) to see a measurable temperature change.
Step 5: Consider how energy dissipates. Ask yourself:
- Are you measuring the temperature change accurately?
- What can you do to reduce heat loss?
Step 7: Carry out a risk assessment and ensure safety throughout.
Step 8: Have your teacher check your plan before starting the experiment.
Step 9: Perform the investigation. If you make any changes, record and explain them clearly.
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You can conclude how the variable (e.g. magnesium length) affects temperature change, and whether your results support your prediction or hypothesis.
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If your results match others, this adds confidence to your conclusion. Differences may highlight errors or uncontrolled variables.
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You could repeat the experiment, improve accuracy in measuring temperature, insulate the container, or test more values of your variable.
Endothermic reactions
Some chemical reactions absorb thermal energy from their surroundings and change it to chemical energy stored in the chemical bonds. These are called endothermic reactions. When an endothermic reaction takes place, the temperature at the end of the reaction is lower than that at the start of the reaction.
Looking at endothermic reactions
This is the word equation for the reaction between sodium hydrogencarbonate and citric acid:
sodium hydrogencarbonate + citric acid → sodium citrate + water + carbon dioxide
During this reaction, thermal energy is absorbed from the surroundings and stored in the form of chemical bonds. So, if this reaction was carried out in a test tube, the surroundings will have a lower temperature and the test tube will feel cooler.
Important Concept
An endothermic reaction absorbs heat from its surroundings. This causes the temperature around the reaction to decrease, making it feel cooler.
Think Like a Scientist
You will investigate whether the reaction between citric acid (or lemon juice) and sodium hydrogencarbonate is endothermic by observing temperature changes.
Equipment: test tube, test tube rack, stirring rod, thermometer, spatula, lemon juice or citric acid, sodium hydrogencarbonate, safety glasses.
Step 1: Half-fill a test tube with citric acid or lemon juice.
Step 2: Measure and record the starting temperature using a thermometer.
Step 3: Add three spatulas of sodium hydrogencarbonate and stir with a stirring rod (not the thermometer).
Step 4: Measure and record the temperature again after stirring.
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The temperature decreased, indicating that the reaction absorbed thermal energy from its surroundings.
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Thermal energy is taken in from the surroundings, which is why the temperature drops—this is characteristic of an endothermic reaction.
Example: Sherbet sweets and endothermic reactions
If you eat sherbet sweets, this reaction takes place in your mouth. The sherbet is a mixture of dry citric acid and sodium hydrogencarbonate. When you eat the sweets, these substances dissolve in your saliva, and react together. This gives a cool, ‘fizzy’ feeling in your mouth (the surroundings), which is refreshing.
Questions
sodium hydrogencarbonate + citric acid → sodium citrate + water + carbon dioxide
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The reactants are sodium hydrogencarbonate and citric acid.
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The products are sodium citrate, water and carbon dioxide.
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An endothermic reaction absorbs thermal energy from its surroundings, causing the temperature to decrease.
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The citric acid and sodium hydrogencarbonate react and absorb heat from your mouth, making it feel cooler.
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The reaction produces carbon dioxide gas, which creates bubbles and gives the fizzy feeling.
Endothermic processes
If you place about 25 cm³ of water in a beaker and then stir in three spatulas of potassium chloride, you will find that the beaker gets cold. In this case, no chemical reaction has taken place. No new products are formed. The potassium chloride has just dissolved. A solution of potassium chloride has been formed. Potassium chloride is the solute and water is the solvent.
When potassium chloride dissolves in water, thermal energy is absorbed from the surroundings. This is why the beaker feels cold. This is an endothermic process.
Ice melting is another endothermic process. Thermal energy is absorbed from the surroundings as the solid ice changes to liquid water. Think about what happens to the particles when water changes state. The particles in the ice are lined up in rows and can only vibrate about fixed positions – they cannot move around inside the ice. The forces between the particles are strong.
As the particles absorb thermal energy from the surroundings, they vibrate more and more. The ice begins to melt. When the particles have enough energy, they can move and overcome the forces holding them in place. The particles can now slide past one another. The water is now in a liquid state.
Quick Fact
Not all endothermic processes are chemical reactions. Sometimes, like in dissolving or melting, heat is absorbed from the surroundings without forming new substances.
Questions
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Because no new substance is formed. Ice melting is a physical change, not a chemical reaction, so it's an endothermic process.
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Evaporation or boiling. Both require heat energy to change a liquid into a gas.
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The water on your skin evaporates and takes heat from your body to do so. This is an endothermic process, making you feel cold.
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Freezing is an exothermic process because heat is released into the surroundings as water turns into ice.
Endothermic or exothermic?
In exothermic reactions and processes, thermal energy is given out. In endothermic reactions and processes, thermal energy is taken in.
Think Like a Scientist
You will test a range of reactions and processes to determine whether they are endothermic (take in heat) or exothermic (release heat). Follow the procedure carefully and record your findings.
Equipment: insulated beakers or polystyrene cups, thermometer, stirring rod, safety glasses, chemicals listed below.
Reactions/processes to try:
- Sodium hydroxide and dilute hydrochloric acid
- Potassium chloride and water
- Melting ice cubes
- Copper sulfate solution and magnesium powder
- Ammonium nitrate and water
- Boiling water until steam appears
- Steam from a kettle directed at a cold surface
- Dilute hydrochloric acid and magnesium ribbon
- Sodium hydrogencarbonate and citric acid
Method:
Step 1: Construct a table to record temperature change and observations.
Step 2: Place one substance in a beaker or insulated cup.
Step 3: Measure and record the initial temperature.
Step 4: Add the second substance (if needed) and stir using a stirring rod.
Step 5: Allow the reaction or change to occur fully.
Step 6: Measure and record the final temperature.
Step 7: Clean your equipment before reusing.
Step 8: Decide whether each test was endothermic or exothermic and whether it involved a chemical reaction or physical change.
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Polystyrene is a better insulator than glass, so it helps to reduce heat loss or gain from the surroundings, making temperature changes easier to detect.
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The reaction between sodium hydroxide and hydrochloric acid is typically the most exothermic, producing a significant rise in temperature.
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The reaction between ammonium nitrate and water is highly endothermic and usually shows the greatest temperature drop.
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You could use your hand to feel for warmth or cooling on the outside of the container, or observe signs like steam (exothermic) or cooling to the touch (endothermic), though this is less accurate than using a thermometer.
Using exothermic reactions
Some exothermic reactions are used to produce self-heating cans of food or drink. For example, a can of self-heating coffee contains a small compartment at the bottom. The compartment is in two parts, separated by foil: one contains calcium oxide, the other contains water. Pressing a button breaks the foil seal, and the two parts are mixed.
When the water and calcium oxide are mixed together they react, and heat (thermal energy) is given off. The thermal energy is transferred to the coffee.
calcium oxide + water → calcium hydroxide
These cans can be useful if you are in a remote area, in an emergency when there is no power, or when you are camping.
The cans are expensive to produce because the compartments must be sealed from one another and from the food, so that it does not become contaminated. There have also been problems with the food not being heated evenly.
Common Mistake
Students sometimes confuse exothermic and endothermic reactions. In this case, heat is released to the surroundings, so it is clearly an exothermic reaction.
Using endothermic reactions
People sometimes use ice packs when they injure themselves. The ice packs are stored in a fridge or freezer until they are needed. When the ice pack is placed on the injured area, heat is transferred to the ice pack and the ice melts. (This is an endothermic process, not an endothermic reaction as no new substances are formed.)
The injured area is cooled which prevents it from swelling. After it has been used, the ice pack can go back into the freezer to be used again.
Some ‘ice’ packs are made from substances that undergo an endothermic process when they mix together.
These packs can be used even when you don’t have fridge or freezer. The pack has two compartments inside, each with a different substance. These are usually ammonium nitrate and water. When you push on the pack and break the compartment containing ammonium nitrate, the water mixes with it and the ammonium nitrate begins to dissolve. This is an endothermic process, so the temperature drops.
Common Mistake
It’s easy to confuse an endothermic process with an endothermic reaction. Melting ice is a physical change (a process), not a chemical reaction—no new substances are formed.
Questions
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Self-heating cans are expensive because they contain special chemicals and a heat-exchange mechanism, which increase manufacturing costs.
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The chemical reaction that produces heat occurs only once and cannot be reused, making the container single-use.
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Reusable icepacks:
Advantage – Can be used multiple times, making them cost-effective.
Disadvantage – Require freezing beforehand, limiting portability.
Instant icepacks:
Advantage – Can be activated without a freezer, useful in emergencies.
Disadvantage – Single-use only, creating more waste.