Kb: NH3+H2O<=>NH4++OH-: Why Omit H2O?

  • Thread starter alingy1
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    Equilibria
In summary, the conversation discusses the use of H2O in acid reactions and the dissociation constant for NH4+. It is mentioned that NH4+ is a weak acid and can be written in different ways. There is also a mention of the equilibrium constant and its relation to the dissociation constant for NH4+.
  • #1
alingy1
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Why is it that for the constant Kb, we always have to write H2O in the equation and we can omit H2O in acid reactions:

My textbook always writes:
NH3+H2O<=>NH4++OH-
NH4+<=>NH3+H+

Why can we not write NH3+H+<=>NH4+?
 
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  • #2
alingy1 said:
Why can we not write NH3+H+<=>NH4+?

We can. NH4+ is a weak acid, with a dissociation constant of 5.6×10-10, and above dissociation reaction is perfectly valid.

Your book my be sticking to one convention to make things easier to follow.
 
  • #3
Would the equilibrium constant be the same of that reaction? Or should we substitute OH- with Kw/H+?
Kw=10^-14
 
  • #5
I guess this means yes. :)
 

1. Why is H2O omitted in the Kb expression?

H2O is omitted in the Kb expression because it is present in both the reactant side (NH3) and product side (OH-) of the equilibrium equation. This means that its concentration remains constant and does not affect the equilibrium constant.

2. How do we calculate the Kb value without H2O?

The Kb value is calculated by taking the concentration of the products (NH4+ and OH-) and dividing it by the concentration of the reactant (NH3). This is because the concentration of H2O does not change and can be ignored in the calculation.

3. Can we include H2O in the Kb expression?

No, H2O cannot be included in the Kb expression because it does not affect the equilibrium constant. Including it would result in an incorrect value for the Kb constant.

4. What is the purpose of the Kb expression?

The Kb expression helps us determine the strength of a base in a solution. It allows us to calculate the concentration of hydroxide ions (OH-) based on the concentration of the base (NH3) and the Kb constant.

5. Is H2O omitted in all equilibrium constant expressions?

No, H2O is only omitted in equilibrium constant expressions when it is present in both the reactant and product side of the equation. In other cases, such as for acids, H2O may be included in the expression as it affects the equilibrium constant value.

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