Equilibrium

REVERSIBLE REACTIONS AND EQUILIBRIUM

Some reactions are reversible. For example, when we heat blue copper sulfate crystals, they turn white. Then, we add a drop of water to the crystals and they go back to being a blue solid...and release a lot of energy!

When a reversible reaction is contained within a closed system (such as a boiling tube with a rubber/cork bung, or a bottle with a lid on it), the forward and reverse reactions "compete", until the rate of the forward reaction is equal to the rate of reaction of the reverse reaction. The concentrations of the reactants and products do not change, even though both reactions (forward and reverse) are happening. We call this dynamic equilibrium.

NOTE: The first reaction is not balanced - 5 moles of water would evaporate away.

LE CHATELIER'S PRINCIPLE

The position of a system in dynamic equilibrium can be described mathematically, using the equilibrium constant, KC.

We need to know how to:

Write a KC expression for an equilibrium
Calculate KC (when given the concentrations of reactants and products at equilibrium).
Calculate the concentration of a reactant/product (when given the KC value and concentrations of all other reactants and products).
Explain whether the equilibrium "favours" the forward or reverse reaction, based upon the KC value.

We can then use this knowledge to apply le Chatelier's Principle to changes in the equilibrium position due to outside influences (our next key concept).

Nitrogen Dioxide - Dinitrogen Tetroxide Equilibrium

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