Chemistry · Chemical equilibrium. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A reversible reaction can go in both directions. In a closed vessel it reaches chemical equilibrium: the forward and reverse reactions go on at equal rates, so the concentrations stop changing. The equilibrium is dynamic, because both reactions continue. The concentrations are constant, but they are usually not equal.
The law of mass action says the rate of a reaction is proportional to the product of the active masses (molar concentrations) of the reactants, each raised to the power of its coefficient in the balanced equation. Setting the two rates equal gives the equilibrium constant.
At a given temperature the constant has a fixed value. Its size shows how far the reaction goes, and it lets us predict the direction of change.
Students multiply a concentration by its coefficient instead of raising it to that power, or put reactants on top. Also, equilibrium does not mean equal concentrations; it means equal rates.
Answer: The reaction goes in the forward direction (more PCl5 decomposes) until Qc = Kc.
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