Hess's+Law

Hess's Law is a way of calculating the enthalpy, entropy, or free energy change for a chemical equation.

It is unique in that it uses two or more chemical equations where the change in energy is known; these chemical equations are then manipulated to obtain the energy change for an overall chemical equation.

In other words, one chemical equation will be built using other chemical equations.

Any changes can be made to a chemical equation.
 * reversing: if a reactant needs to become a product in the overall reaction, you can flip a chemical equation around. When doing so, the sign on the change in energy is reversed.
 * multiplying by a coefficient: if you need more than one reagent, you can multiply the whole equation by a number or fraction; just do the same for the energy value.

Watch this video to learn how to use Hess's Law:

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