At equilibrium, the concentrations of reactants and products can be predicted us

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+bB?cC+dD

where a, b, c, and d are the stoichiometric coefficients, the equilibrium constant is

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.

Part A

A mixture initially contains A, B, and C in the following concentrations: [A] = 0.550M , [B] = 0.700M , and [C] = 0.650M . The following reaction occurs and equilibrium is established:

A+2B?C

At equilibrium, [A] = 0.350M and [C] = 0.850M . Calculate the value of the equilibrium constant, Kc

Explanation / Answer

initially

[A] = 0.55

[B] = 0.70

[C] = 0.65

in equilbirium

[A] = 0.55 - x

[B] = 0.70 -2x

[C] = 0.65 +x

And we know

[A] = 0.55 - x = 0.35

[B] = 0.70 -2x =

[C] = 0.65 +x = 0.85

then, get Kc

but we need [B] first so, get x

fro m[C]

0.65 +x = 0.85

x = 0.20

then

[B] = 0.70 -2x = 0.70 -2*0.20 = 0.30 ;

substitute in Kc

Kc = [C] /([A][B]^2)

Kc = (0.85)/(0.35*(0.3^2))

Kc = 26.9841

sig figs used --> 2

then

Kc = 27

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