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An illustration of a thermally agitated molecule.
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An illustration of a thermally agitated molecule.
Everyone knows what heat is. It's hot in July, the stove burner is hot and falling into lake water in winter could kill us, because we will lose our body heat.

But in chemistry heat is treated as a quanititative measure. Heat is "ENERGY" and heat can be stored in bodies which are warmer than their surroundings, but it can also be stored in, or released by, the formation of chemical bonds.

For instance when we subject water to electrolysis we separate the hydrogen atoms from the oxygen atoms and consume energy in the process. That energy is rereleased as heat when we recombine the hydrogen with the oxygen by burning.

H2 + 1/2 O2 --> H2O + heat

Chemical reactions which release heat are called exothermic reactions and reactions that absorb heat are referred to as endothermic. Chemical compounds that result from reactions that release heat are generally favoured over ones that absorb heat.

We can use this information to provide us with a method for ranking the stability of competing chemical compound products, simply by measuring the amount of heat released/absorbed in a reaction. Using this model we see that heat can be stored two ways, either stored in a chemical bond or stored as kinetic energy in vibrating molecules (see image above left).

In order to continue to discuss the movement of heat between these two forms we need one word that describes both forms. That word is enthalpy.

In this section we discuss the following:

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