A) The equilibrium between carbon dioxide gas and carbonic acid is very importan

Question

A) The equilibrium between carbon dioxide gas and carbonic acid is very important in biology and environmental science.
CO2 (aq) + H2O (l) H2CO3 (aq)
Which one of the following is the correct equilibrium constant expression (Kc) for this reaction?
a) K =[H2CO3]/ [CO2]
   b) K=[CO2]/ [H2CO3]
   c) K=[H2CO3]/ [CO2][H2O]
   d) K=[CO2][H2O]/ [H2CO3]
   e) K=1/[H2CO3]

B) For the reaction PCl3(g) + Cl2(g) <-->PCl5(g) at a particular temperature, Kc = 24.3. Suppose a system at that temperature is prepared with [PCl3] = 0.10 M, [Cl2] = 0.15 M, and [PCl5] = 0.60 M. Which of the following is true based on the above?
a) Qc > Kc, the reaction proceeds from right to left to reach equilibrium
   b) Qc < Kc, the reaction proceeds from right to left to reach equilibrium
   c) Qc > Kc, the reaction proceeds from left to right to reach equilibrium
   d) Qc < Kc, the reaction proceeds from left to right to reach equilibrium
   e) Qc = Kc, the reaction is currently at equilibrium

Explanation / Answer

CO2 (aq) + H2O (l) ----> H2CO3 (aq)

Kc    = [H2CO3]/[CO2][H2O]

[H2O] =1 because of H2O is liquid

Kc =[H2CO3]/[CO2] >>> answer is A

2. PCl3(g) + Cl2(g) <-->PCl5(g)

Qc = [PCl5]/[PCl3][Cl2]

       = 0.6/0.1*0.15 = 40

Kc = 24.3

Qc> Kc

a) Qc > Kc, the reaction proceeds from right to left to reach equilibrium

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