AP Chemistry - Semester 2: Equilibrium: the extent of Chemical Reactions

Sections: Introduction   |  Section 1   |  Section 2   |  Section 3   |  Section 4   |  Section 5   |   Lab

17.2 The Reaction Quotient and the Equilibrium Constant As the concentrations of reactants and products are changing before equilibrium is reached, we refer to the reaction quotient Q. When the reaction reaches equilibrium and reactant and product concentrations no longer change, then Q=K. Q is the ratio of the concentration of the products multiplied together and divided by the concentration of reactants multiplied together with each term raised to the power of its stoichiometric coefficient in the balanced equation. Using the generic equation: Consider the following reaction: The form of the reaction quotient depends on the direction in which the balanced equation is written. If the reverse of the reaction above was written, the decomposition of SO3, the reaction quotient would be the reciprocal of Q. These equations work with reactions with homogeneous equilibria where all the reaction components are in the same phase. When the components are in different phases we have heterogeneous equilibria. A pure solid will always have the same concentration (moles per liter) at a given temperature throughout the reaction and since a solid does not change volume very much at different temperatures, we say that the volume of a pure solid is constant. As long as some of the solid reactant is present, the reaction quotient for the reaction equals the non solid concentration. The example below is taken from page 732. Basically the solids cancelled out leaving the concentration of the gas. Next page

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