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Learn about reactions that release energy and make their surroundings warmer.
Have you ever cracked a plastic pouch on a freezing day and felt it suddenly turn hot in your hands? How can something create heat instantly without a battery, a plug, or a flame?
In chemistry, we use the word exothermic to describe reactions that give off energy. To remember this, look at the word parts: 'Exo' means 'exit' (like an exit door), and 'thermic' means 'heat.' In an exothermic reaction, heat is 'exiting' the chemical setup and moving into the world around it. This is why the area around the reaction feels warmer to your touch. The chemicals are losing energy, but the surroundings—which includes your hands or a thermometer—are gaining that energy.
Quick Check
If the word 'thermic' means heat, what does the 'exo' part tell you about where the heat is going?
Answer
The heat is exiting or leaving the reaction and moving to the surroundings.
Inside a hand warmer, iron powder reacts with oxygen from the air. 1. You shake the bag to let oxygen in. 2. The iron and oxygen begin to form iron oxide (rust). 3. As the new bonds in the rust form, they release energy. 4. You feel your hands get warm because that energy is moving from the pouch into your skin.
Quick Check
If a thermometer in a beaker starts at and ends at , is the reaction exothermic?
Answer
Yes, because the temperature increased, meaning heat was released.
The most famous exothermic reaction is combustion, or burning. When you light a campfire, the wood reacts with oxygen. This reaction releases a massive amount of energy very quickly. We see this energy as bright light and feel it as intense heat. Even though you need a tiny spark to start the fire, the reaction produces so much of its own energy that it keeps itself going until the fuel (the wood) is all gone.
Imagine two different reactions in two separate beakers of water: 1. Beaker A: The temperature rises from to . 2. Beaker B: The temperature rises from to .
In Beaker B, the temperature change is , while in Beaker A it is only . This tells us that the chemicals in Beaker B released much more energy during their reaction.
Space shuttle engines use a reaction between liquid hydrogen and liquid oxygen. 1. The reaction creates water vapor (). 2. The energy released is so powerful it expands the gases rapidly. 3. This 'extra' energy provides the thrust to lift the heavy rocket into space. This is an extreme example of an exothermic reaction where heat is converted into motion!
Which of these is the best definition of an exothermic reaction?
Which of the following is an example of an exothermic process?
In an exothermic reaction, the temperature of the surroundings will decrease.
Review Tomorrow
Tomorrow morning, try to explain to a friend why a campfire feels hot using the word 'bonds' and the prefix 'exo-'.
Practice Activity
With an adult, mix a small amount of hydrogen peroxide and yeast in a cup. Touch the outside of the cup—does it feel warm? If so, you've just created an exothermic reaction!