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Endothermic and Exothermic Reactions

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  • 0:08 Chemical Reactions
  • 0:47 Exothermic and…
  • 2:41 Enthalpy
  • 4:26 Lesson Summary
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Lesson Transcript
Instructor: Amy Meyers

Amy holds a Master of Science. She has taught science at the high school and college levels.

Explore a chemical reaction at the molecular level. Learn about exothermic and endothermic reactions, what they look like and what happens when they occur. Understand enthalpy and how you can use it to predict whether a reaction will be exothermic or endothermic.

Chemical Reactions

A chemical reaction is when one or more substances are changed into one or more different substances. To know for certain that a reaction has happened, you need to have evidence that a substance has changed identity. Although you could analyze the products to make sure it has changed identity, you can also look for other clues to tell you.

One of the laws of science is the law of conservation of energy, which states that in any chemical or physical process, energy is neither created nor destroyed. What this means is that if energy of the system decreases, then the energy of the surroundings increases by the same amount.

Exothermic and Endothermic Reactions

If two substances combine and heat and light are produced, this is strong evidence that a reaction has taken place. If heat is given off, the reaction is exothermic. Exothermic means that during the reaction, molecules have moved from a higher state of energy to a lower state. An endothermic reaction is the opposite. This is when a reaction starts colder and ends up hotter, taking in energy from start to finish.

In an endothermic reaction, the system gains heat as the surroundings cool down. In an exothermic reaction, the system loses heat as the surroundings heat up.

That ice pack the coach is putting on the batter? It's an example of an endothermic reaction. An endothermic reaction is when heat is needed by the reaction, so it draws heat from its surroundings, making them feel cold. Just like that ice pack.

An example of an endothermic reaction
Endothermic Reaction

Oh, look, it was the boy's birthday, and they're celebrating at the ball park. His mother is lighting the candles. See that lit candle? That is an example of an exothermic reaction. Exothermic reactions give off heat.

An example of an exothermic reaction
lighting birthday cake candles

At the molecular level, an exothermic reaction is when a bond is formed. When two things come together, they're happier and take less energy to stay together. Since we have the conservation of energy law, we know that the energy they don't need has to go somewhere, so they give it off to their surroundings.

As we all know, it is easier to get together than to break up. It takes energy to break up. Just like when a chemical bond is broken, it needs energy to break. This is an endothermic reaction.

Another way to look at exothermic and endothermic reactions is to think of two magnets. If the magnets are stuck together, then it takes energy to pull them apart, just like it takes energy to break bonds. If the magnets are apart, they like to be together and actually pull each other together, releasing energy in the process. This is like the making of chemical bonds.

Enthalpy

Enthalpy change is the amount of energy as heat that is lost or made by a system. Every bond between molecules has a specific amount of energy in it. That means that you can figure out how much energy it took to form a compound and how much it will take to break that compound apart. You can get enthalpy numbers from a chart in a textbook. The numbers represent the strength of a chemical bond. They are usually measured in kcal/mole or kJ/mole.

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