Shake a hand warmer and it gets warm; press a cold pack and it gets cold. When a chemical reaction happens, energy moves in or out along with the change in substances.
A reaction that releases heat and warms its surroundings is exothermic; one that takes in heat and cools its surroundings is endothermic.
In the energy diagram above, the products of an exothermic reaction sit lower than the reactants and those of an endothermic reaction sit higher, and energy equal to that height difference passes to or from the surroundings.
The graph below shows how far the temperature of the surroundings rises or falls from its starting value as the same reactions take place.
Reactions That Give Off or Take In Heat
🔥 Reactions That Warm, Reactions That Cool
①Shaking a hand warmer lets iron powder react with oxygen in the air, giving off heat
②Pressing a cold pack lets ammonium nitrate dissolve in water, taking in heat
③In a chemical reaction energy changes along with the substances, and that energy moves to or from the surroundings as heat
Energy Diagrams
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Energy levels of reactants and products in exothermic and endothermic reactions
Energy Levels
Energy of reactants > energy of products → the difference is released
The extra energy leaves as heat, so the surroundings get warmer
How the Surrounding Temperature Changes
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How the surrounding temperature changes over time during exothermic and endothermic reactions
💡 Examples of Exothermic Reactions
①Combustion of wood or gas
②Magnesium reacting with dilute hydrochloric acid
③Neutralization of an acid by a base
④Oxidation of iron powder in a hand warmer
⑤Calcium oxide reacting with water in a self-heating meal pack
Exothermic and Endothermic in Daily Life
Exothermic reaction
Reactants → products + energy
Energy is given off and the surroundings warm up. Examples: combustion, neutralization, oxidation of iron
Endothermic reaction
Reactants + energy → products
Energy is taken in and the surroundings cool down. Examples: photosynthesis, dissolving ammonium nitrate, thermal decomposition
Energy is never created or destroyed; it only moves between the reacting substances and the surroundings
🎯 Common Test Points
①Exothermic: the surroundings warm up. Combustion, neutralization, hand warmers
②Endothermic: the surroundings cool down. Photosynthesis, cold packs
③If reactants are higher than products in an energy diagram, the reaction is exothermic
④If reactants are lower than products, it is endothermic
⑤The total energy before and after a reaction is conserved
Examples and Unit Review
Example 1
Shaking a hand warmer makes its surroundings warm. Is this reaction exothermic or endothermic, and does it release or absorb energy?
1
The surroundings warm up because the reaction gives off energy as heat.
2
The iron powder in the hand warmer oxidizes and heats its surroundings, so it is an exothermic reaction that releases energy.
▸ Exothermic; it releases energy
A hand warmer cools down after a while because the reaction has finished and no more heat is produced, just as the curve in the lower graph returns toward the starting temperature.
Example 2
In an endothermic reaction, which has more energy, the reactants or the products? Give one example of an endothermic reaction.
1
An endothermic reaction takes the energy it lacks from its surroundings.
2
The products end up with more energy than the reactants by the amount absorbed. Ammonium nitrate dissolving in water in a cold pack is an example.
▸ The products have more energy. Example: a cold pack (ammonium nitrate dissolving in water)
Choosing the endothermic reaction in the energy diagram above draws the products' line higher than the reactants' line.
Unit-test style
Which statement about exothermic and endothermic reactions is correct?
①An exothermic reaction lowers the surrounding temperature
②An exothermic reaction releases energy and raises the surrounding temperature
③An endothermic reaction releases energy
④Combustion is a typical endothermic reaction
⑤In an endothermic reaction the reactants have more energy than the products
▸ ② An exothermic reaction releases energy and raises the surrounding temperature
1
An exothermic reaction releases energy, so the surrounding temperature rises.
2
An exothermic reaction makes the surroundings hot (①), an endothermic reaction absorbs energy (③), combustion is exothermic (④), and in an endothermic reaction the products have more energy (⑤).