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Acid Base Equilibrium


The equilibrium constants of acids and bases which absorb in the ultra violet region may be determined spectrophotometrically easily and accurately. Acid base equilibrium itself in a more general manner and apply the results in several different situations.

Chemical reactions tend towards an equilibrium, in which all reactants and products are present in varying amounts. The equilibrium is dynamic, that is there is a continuous interchange of individual reactant and product species but with no net change, at a given temperature. 
When acid-base equilibria are treated with a thermodynamic approach, the equilibrium constant values allow to calculate the buffer properties, such as pH and buffering capacity. Equilibrium present a new way to view chemical reactions. Single arrow reactions that are assumed to go to completion use proportional reasoning or dimensional analysis to solve. 


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A very important type of reaction that occurs in aqueous solution is the transfer of a proton from an acid to a base. In aqueous solutions, water can act as an acid or a base. In aqueous solutions, water can act as an acid or a base. In the presence of an acid, symbolized HA water acts as a base by accepting a proton.
The equilibrium constant for this transfer of a proton  from an acid to a water molecule is called acid ionization constant (Ka):

$HA(aq) + H2O(l) \rightleftharpoons A-(aq) + H3O+(aq)$

$Keq$ = $\frac{[A^{-}]_{eq} [H_{3}O^{+}]_{eq}}{[HA]_{eq}}$ = $Ka$

For instance, when acetic acid an example of a weak acid dissolves in water, some acetic acid molecules transfer protons to water to produce acetate anions and hydronium ions. 

Acid Base Equilibrium

An equilibrium expression is a mathematical relationship between reactants and products. It is often used to determine how much of a substance at equilibrium is present. An equilibrium expression begins with the capital letter K, which stands for the equilibrium constant. 

In summary K relates to:
1) A specific chemical equation (which is usually given, but might have o be written and balanced).
2) A numerical value (found usually with no units).
3) A mathematical equilibrium expression (created).
4) A chemical species or topic represented by a subscript. 

Practise Problems

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Since acids are substances capable of losing a proton, the tendency to lose a proton would be a measure of its acid strength. Similarly the strength of a base is determined by its tendency to accept a proton. 

Some of the problems based on acid base equilibrium is given below.

Solved Examples

Question 1: A buffer can be prepared using H2PO4- and its conjugate base. Write the acid-base reaction equation for H2PO4- and H2O reacting to form the conjugate base of H2PO4- and H3O+.
The conjugate base of H2PO4- has one less H+ than H2PO4-. So the formula of the two reactants and one of the two products are given below.

H2PO4- + H2O $\rightleftharpoons$ HPO42- + H3O+  

An increase in the concentration of H3O+ causes the rate of the reverse reaction to increase.
H3PO4- + H2O $\rightarrow$ HPO42- + H3O+

Question 2: Calculate the pH of a solution that contains 1.0 $\times$ 10-5 mol of H+/L.
By definition, pH is the negative logarithm of the hydrogen ion concentration when this concentration is expressed in moles of H+/L.
pH = - log[H+] = - log 10-5 = 5.0