Why does the concentration of product stop changing

Because we only ever write K (rather than Q) at equilibrium, it is tautologous but very common to see K written as /C(eq) or Ke. 

The descriptor isotherm derives from the Greek iso meaning same and thermos meaning temperature.  

Jacobus van t Hoff was a Dutch scientist (1852-1911). Notice the peculiar arrangement of the apostrophe, and small and capital letters in his surname. 

It would be beneficial if we could increase the yield of a chemical reaction by just leaving it to react longer. Unfortunately, the concentrations of reactant and product remain constant at the end of a reaction. In other words, the reaction quotient has reached a constant value. 

At equilibrium, when the reaction stops, we give the reaction quotient the special name of equilibrium constant, and re-symbolize it with the letter K. The values of K and Q are exactly the same at equilibrium when the reaction stops. The value of Q is always smaller than K before equilibrium is reached, because some product has yet to form. In other words, before equilibrium, the top line of Equation (4.48) is artificially small and the bottom is artificially big. 

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