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Explore reactions that absorb energy and make their surroundings colder.
Have you ever wondered how a plastic bag filled with liquid can turn ice-cold in seconds—without ever touching a freezer?
In chemistry, every reaction involves energy. Some reactions release energy (like a fire), but others do the opposite: they soak it up! An endothermic reaction is a chemical change that absorbs energy from its surroundings. The word comes from Greek roots: endo means 'inside' and therm means 'heat.' Essentially, these reactions pull heat into themselves to make the reaction happen. Without a constant supply of energy, these reactions would simply stop.
Quick Check
Based on the Greek roots, what is the literal meaning of the word 'endothermic'?
Answer
It means 'heat inside' or absorbing heat into the reaction.
When you touch an instant cold pack, it feels freezing. This happens because the reaction inside is 'stealing' heat from your hand! In an endothermic process, the energy needed to break the chemical bonds in the reactants is greater than the energy released when new bonds form. This extra energy has to come from somewhere, so it is pulled from the nearby environment. When the air or your skin loses heat energy to the reaction, the temperature of those surroundings drops.
1. Inside the pack, there is a pouch of water and solid ammonium nitrate (). 2. When you squeeze the pack, the water pouch breaks. 3. The ammonium nitrate dissolves in the water, which is an endothermic process. 4. The reaction absorbs heat from the water and the plastic bag, making the whole pack feel icy cold to the touch.
Quick Check
If a reaction absorbs heat from the air around it, what happens to the temperature of that air?
Answer
The temperature of the air drops (it gets colder).
Think of chemical bonds like rubber bands. It takes energy to stretch and break them. In an endothermic reaction, the 'cost' of breaking the old bonds is very high. Even after new bonds are formed, the system hasn't 'paid back' all the energy it used. This results in a net absorption of energy. A famous example is photosynthesis. Plants take in energy from sunlight to turn carbon dioxide () and water () into sugar. Without that outside energy from the sun, the reaction couldn't happen!
You can see an endothermic reaction in your kitchen: 1. Mix baking soda () with vinegar (). 2. You will see bubbles of gas forming immediately. 3. If you hold the container or use a thermometer, you will notice the temperature drops by a few degrees. 4. This proves that the reaction is absorbing thermal energy to break the bonds of the ingredients.
Imagine a reaction where breaking bonds requires units of energy, but forming new bonds only releases units. 1. Total Energy Needed: 2. Total Energy Released: 3. Net Energy Change: units absorbed. Because the result is a positive number (), we know the reaction is endothermic and will cool down its surroundings.
What happens to the temperature of the surroundings during an endothermic reaction?
Which of the following is a biological example of an endothermic process?
In an endothermic reaction, more energy is released when making bonds than is used to break them.
Review Tomorrow
In 24 hours, try to explain to a friend why a cold pack feels cold using the words 'energy' and 'surroundings.'
Practice Activity
Try the 'Kitchen Cold' experiment: Mix a spoonful of baking soda with a splash of vinegar in a small cup. Touch the bottom of the cup before and after to feel the temperature change yourself!