the composition of the final equilibrium mixture at a particular temperature depends upon the starting amount of reactants
Answers
Answer:
At a given temperature, the equilibrium composition is related to the equilibrium constant, Kc. For the general reaction aA + bB --> cC + dD Kc is the ratio of the product concentrations divided by the reactant concentrations, with each concentration raised to the power of its coefficient in the balanced chemical reaction.
Kc = [C]c[D]d
[A]a[B]b
Regardless of the initial concentrations, the final equilibrium concentrations must satisfy the equation specified by Kc. Usually Kc is written without units.
If an equilibrium involves reactants and products in a single phase that is called a homogeneous equilibrium, opposite to a heterogeneous equilibrium which involves reactants and products in more than one phase. For the latter case when phases are separated (for example solid presipitation) the phases with defined and thus not variable concentartions are excluded from the equilibrium constant expression. Example:
3Fe(s) + 4H2O(g) --> Fe3O4(s) + 4H2(g)
For the reaction of iron with steam, you would write
Kc = [H2]4
[H2O]4
Concentrations of Fe and Fe3O4 are omitted, because whereas the concentration of a gas can have various values, the concentration of a pure solid or a pure liquid is a constant at a given temperature. For the same reason, concentration of liquid solvent is usually omitted as well.
Qualitatively Interpreting the Equilibrium Constant
If Kc for a reaction, aA + bB -->cC + dD
is large, the equilibrium mixture is mostly products. If Kc is small, the equilibrium mixture is mostly reactants. When Kc is approximately 1, the equilibrium mixture contains appreciable amounts of both reactants and products.
Predicting the Direction of Reaction
The reaction quotient, Qc, is an expression that has the same form as the equilibrium constant expression but whose concentration values are not necessarily those at equilibrium. For the general reaction
aA + bB -->cC + dD.
the reaction quotient, Qc, is defined as
Qc = [C]ic[D]id
[A]ia[B]ib
where the subscript i on the concentrations means that the instantaneous concentration (not necessarily the equilibrium concentration) is to be used. To predict the direction of a reaction, substitute the concentrations of reactants and products at a particular time into the reaction quotient expression, and compare Qc with the equilibrium constant Kc:
If Qc > Kc, the reaction will go to the left.
If Qc < Kc, the reaction will go to the right.
If Qc = Kc, the reaction mixture is at equilibrium.
Calculating Equilibrium Concentrations
The equilibrium constant allows us to calculate final equilibrium composition of reactants and products for given initial concentrations. This can be accomplished in three steps:
Set up a table of concentrations (starting, change, and equilibrium expressions) using x for unknown quantities;
Substitute the values for equilibrium concentrations into the equilibrium-constant expression;
Solve the equilibrium-constant equation for the values of the equilibrium concentrations.
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