Calculate Equilibrium Constant for Polyprotic Acid Reaction

So the correct answer is (Kb)(Ka2) = (1.1*10^-14 / 4.3*10^-7) / (4.8*10^-11) = 2.5*10^-4. In summary, the equilibrium constant for the reaction between HCO3- and HCO3- to form H2CO3 and CO32- is 2.5*10^-4, calculated by dividing the product of the equilibrium constants for the two individual reactions by [H+][OH-].
  • #1
i_love_science
80
2
The principal equilibrium in a solution of NaHCO3 is
HCO3-(aq) + HCO3-(aq) <-> H2CO3(aq) + CO32-(aq)
Calculate the value of the equilibrium constant for this reaction.

My solution:

This overall reaction is the same as the sum of the following reactions:
HCO3-(aq) <-> H2CO3(aq) + OH-(aq)
HCO3-(aq) <-> CO32-(aq) + H+(aq)

The overall equilibrium constant is (Kb)(Ka2) = [10^(-14) / 4.3*10^(-7)]*[4.8*10^(-11)] = 1.1^10^(-18).

The correct answer is 1.1*10^(-4).

Can anyone explain where I went wrong? Thanks.
 
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  • #2
i_love_science said:
The overall equilibrium constant is (Kb)(Ka2)

Is it?
 
  • #3
If you multiply the expressions for the equilibrium constants of the two reactions, you will find you have [H+] and [OH-] in the numerator. To get the equilibrium constant for the original reaction, you need to divide by [H+][OH-].
 

Related to Calculate Equilibrium Constant for Polyprotic Acid Reaction

1. What is a polyprotic acid?

A polyprotic acid is an acid that has more than one ionizable hydrogen atom. This means that it can donate more than one proton in a chemical reaction.

2. How do you calculate the equilibrium constant for a polyprotic acid reaction?

The equilibrium constant for a polyprotic acid reaction can be calculated by multiplying the equilibrium constants for each individual step of the reaction. This is known as the overall equilibrium constant.

3. What factors can affect the equilibrium constant for a polyprotic acid reaction?

The equilibrium constant for a polyprotic acid reaction can be affected by temperature, pressure, and the concentrations of reactants and products. Changes in any of these factors can shift the equilibrium and therefore change the value of the equilibrium constant.

4. How can you determine the equilibrium constant experimentally for a polyprotic acid reaction?

The equilibrium constant for a polyprotic acid reaction can be determined experimentally by measuring the concentrations of reactants and products at equilibrium and plugging them into the equilibrium constant expression. This can be done using various analytical techniques such as titration or spectroscopy.

5. Can the equilibrium constant for a polyprotic acid reaction be greater than 1?

Yes, the equilibrium constant for a polyprotic acid reaction can be greater than 1. This indicates that the products are favored at equilibrium and the reaction proceeds in the forward direction. A value less than 1 indicates that the reactants are favored and the reaction proceeds in the reverse direction.

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