Thermodynamics may sound like a complicated word, but it can be broken down easily. "Thermo" means heat and "dynamics" means motion or change, so if we put them together, we get "thermodynamics" which means the motions of heat or the changes of heat. In chemistry, we talk about this in context of chemical reactions. Chemical reactions need energy to happen and they either give off or absorb energy as they happen. Most of the time this energy is in the form of heat. That means that some reactions get hot and some reactions get cold. As you might expect, we have special names for those kinds of reactions.

Activation Energy

Activation energy is the amount of energy needed to make a reaction happen. Reactions can't happen without a certain amount of energy, because in order for a reaction to happen, chemical bonds need to be broken. Breaking these bonds is like pulling apart a pair of magnets. It is a process that absorbs energy. When new chemical bonds are formed, energy is released, or given off. Before bonds can be formed, they have to be broken first, and that means that a certain amount of energy is needed for a reaction to happen. The symbol for activation energy is Ea. On the reaction energy diagram below, activation energy is the distance ebtween the energy of reactants and the energy of the activated complex, or label B.

Endo and Exothermic

Endothermic and exothermic are terms that describe how a chemical reaction relates to heat. If a chemical reaction absorbs heat, we say it is Endothermic. If a reaction releases heat, we say it is Exothermic. Even though all reactions absorb and release some energy (because they involve breaking and forming bonds), they don't absorb the same amount of energy as they release. Below are some ways to compare endo and exothermic reactions.
  • Exothermic
    • Releases heat energy
    • Has a positive net energy
    • The reaction feels hot
    • Exo = exit, external, exterior, outside
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  • Endothermic
    • Absorbs heat energy
    • Has a negative net energy
    • The reaction feels cold
    • Endo = enter, into, interior, internal, inside
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