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Key Idea 6
Energy and Chemical Reactions
Concept Question: What is the role of energy in chemical reactions?

People who wish to slow climate change and protect the environment typically condemn the combustion of fossil fuels—and for good reason. The emission of carbon dioxide from the combustion of fossil fuels is the primary cause of climate change and will have drastic consequences for humans and the environment. However, there is also a good reason why humans burn fossil fuels: the chemical reaction between the fossil fuel molecules and oxygen to produce carbon dioxide and water vapor releases a massive amount of energy, which we can then use for our own purposes.

 Picture of an oil pump

Why does the combustion of fossil fuel generate such a large amount of energy? To answer this question, we can return to thermochemistry and a molecular level view of energy.


What do we know?

All chemical reactions involve energy changes; in some chemical reactions, energy is absorbed, while in other reactions, like the combustion of a fossil fuel, energy is released. Therefore, just like a phase change, a chemical reaction can be either endothermic or exothermic. However, as was emphasized earlier in the lesson, to accurately describe the energy transfer for a chemical reaction, we need to clearly define what the system and the surroundings are.


Your Turn!

Suppose you want to describe the energy transfer that occurs during the combustion of natural gas, which is often used for household heating:

CH_{4}(g)+2\ O_{2}(g)\rightarrow CO_{2}(g)+2\ H_{2}O(g)

For this situation, how would you define your system?

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In this situation, it makes sense to define the system to be all the particles that are involved in the chemical reaction. Then, the surroundings are everything in the universe that is not a part of the reaction.