At equilibrium, the concentrations of reactants and products can be predicted us
ID: 1004584 • Letter: A
Question
At equilibrium, the concentrations of reactants and products can be predicted using the equilibrium constant, Kc, which is a mathematical expression based on the chemical equation. For example, in the reaction
aA+bBcC+dD
where a, b, c, and d are the stoichiometric coefficients, the equilibrium constant is
Kc=[C]c[D]d[A]a[B]b
where [A], [B], [C], and [D] are the equilibrium concentrations. If the reaction is not at equilibrium, the quantity can still be calculated, but it is called the reaction quotient, Qc, instead of the equilibrium constant, Kc.
Qc=[C]tc[D]td[A]ta[B]tb
where each concentration is measured at some arbitrary time t.
A mixture initially contains A, B, and C in the following concentrations: [A] = 0.550 M , [B] = 0.900 M , and [C] = 0.650 M . The following reaction occurs and equilibrium is established:
A+2BC
At equilibrium, [A] = 0.370 M and [C] = 0.830 M . Calculate the value of the equilibrium constant, Kc.
Express your answer numerically.
Explanation / Answer
Here we are given initial concentrations of A, B and C. Using equilibrium concentrations of A and C (given) we can get concentration of B. Equilibrium concentrations are used in equilibrium expressions.
Set up ICE table.
A + 2 B == > C
I 0.550 0.900 0.650
C -x -2x +x
E 0.370 (0.900 -2x ) 0.830
Equilibrium constant expression
Keq = [C]/[A] [B]2
Here square bracket shows the concentrations.
Lets find the value of x and find out equilibrium concentration of B
Equilibrium concentration of A = 0.370 = 0.550 – x
x =0.18
Equilibrium concentration of B
0.900 – 2 x 0.18 = 0.54 M
Lets plug all the concentrations in equilibrium expression.
Keq = ( 0.830 ) / (0.370 ) x ( 0.54)2
=7.70
Equilibrium constant of this reaction would be 7.70
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