As you read your chemistry textbook, you may notice that some reactions are written with arrows that point in both directions. This signifies that a reaction is reversible--that the reaction's products can re-react with one another and re-form the reactants. The point at which a reaction occurs at the same rate in both directions is known as equilibrium. When gases react at equilibrium, it's possible to calculate their pressures using a number known as the equilibrium constant, which is different for each reaction.
How to Calculate Equilibrium PressuresBy Robert Schrader
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