# Acid-base reaction equilibrium

Huzaifa
TL;DR Summary
Equilibrium constants in acid-base reactions equations
$$\begin{array}{c} \mathrm{HA}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{A}^{-}+\mathrm{H}_{3} \mathrm{O}^{+} \\ \mathrm{K}=\dfrac{\left[\mathrm{A}^{-}\right]\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]}{\left.[\mathrm{HA}] \mathrm{H}_{2} \mathrm{O}\right]} \quad \mathrm{K}_{\mathrm{a}}=\dfrac{\left[\mathrm{A}^{-}\right]\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]}{[\mathrm{HA}]} \\ \mathrm{BH}^{+}+\mathrm{H}_{2} \mathrm{O} \rightleftharpoons \mathrm{B}+\mathrm{H}_{3} \mathrm{O}^{+} \\ \mathrm{K}=\dfrac{[\mathrm{B}]\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]}{\left[\mathrm{BH}^{+}\right]\left[\mathrm{H}_{2} \mathrm{O}\right]} \quad \mathrm{K}_{\mathrm{a}}=\dfrac{[\mathrm{B}]\left[\mathrm{H}_{3} \mathrm{O}^{+}\right]}{\left[\mathrm{BH}^{+}\right]} \\ \mathrm{HA}+\mathrm{B} \rightleftharpoons \mathrm{A}^{-}+\mathrm{BH}^{+} \\ \mathrm{K}_{\mathrm{c}}=\dfrac{\left[\mathrm{A}^{-}\right]\left[\mathrm{BH}^{+}\right]}{[\mathrm{HA}][\mathrm{B}]} \end{array}$$

Hello! I am not able to understand these equations for acid base equilibrium please. I have attached the images in the following post.

Last edited:

## Answers and Replies

Huzaifa

Mentor
What you posted is a definition - nothing to understand here, it just is what it is, all you have to do is to accept it - and some algebraic rearranging, that is not difficult to follow if you have a basic understanding of HS math. You have not explained what it is that you don't understand.

Huzaifa
What you posted is a definition - nothing to understand here, it just is what it is, all you have to do is to accept it - and some algebraic rearranging, that is not difficult to follow if you have a basic understanding of HS math. You have not explained what it is that you don't understand.
I don't understand A, B, and H, and the positive and negative signs in superscripts. I assume that A is Acid, and B is Base, and H is Hydrogen. Please correct me if I am wrong.

I also don't understand HA and BH+? Also there is no OH-.

Mentor
the positive and negative signs in superscripts
These are ion charges. I suggest you you get back to basics, otherwise you won't be able understand the acid/base equilibrium.

A and B are often used as general symbols for acid (base) residue. HA means just any monoprotic acid.

jim mcnamara
Hillbillychemist
The first equation shows the reversible transfer of the H+ ion from the acid HA to water H2O to form the hydronium ion H3O+ and the anion A-. The second shows the calculation of the equilibrium constant K for this reaction. Since the concentration of water is very high (~55 molar) it does not change significantly and is neglected in the calculation of the acid equilibrium constant Ka. Thus the equilibrium constant is equal to the concentration of the anion times the concentration of the hydronium ion divided by the concentration of the acid and of the water.
The second set of equations refers to the reaction of a basic salt such as the ammonium ion NH4+ with water and the third set refers to the reaction of an acid HA and a base such as ammonia NH3. These equilibrium constants cover 14 powers of ten from 101 to 10-14 so it is more convenient to use their negative logarithms: 0 to 14.