Chemistry Equilibrium of weak acid

1. Equilibrium of weak
acids
Zumra Ahmad
Urwa Arshad

2. Contents
 Weak acid
 Equilibrium
 Equilibrium of weak acid
 Acid dissociation constant
 Table of weak acids(Ka values)
 Dissociation of weak acid
 Methods to calculation PH by using Ka values
 Examples
 Three ways to measure PH
 Calculating Ka from PH
 Percent dissociation

3. Weak acid or equilibrium
• Weak Acid: Weak acid is an acid that partially dissociates into its ions in an aqueous
solution or water.
 Weak acids having higher pH value.
 Weak acid is a weak electrolyte.
e.g: methanoic acid, benzoic acid, hydrofluoric acid, nitrous acid, phosphoric acid.
 When HF dissolve in water, only a fraction of the molecules ionize.
 HF(aq) + H2O(l) ⇌ H3O+(aq) + F-(aq)
• Equilibrium: When the rate of forward reaction takes place at the rate of reverse
reaction, the composition of the reaction mixture remains constant.

4. Equilibrium of weak acids
• When an uncharged weak acid is added to water, a homogenous
equilibrium forms in which aqueous acid molecules, HA(aq), react with
liquid water to form aqueous hydronium ions and aqueous anions, A-(aq)

5. Acid dissociation constant(1)
The dissociation of weak acid in water is characterized by an equilibrium
equation.
The equilibrium constant for the dissociation reaction, denoted by Ka is
called the acid-dissociation constant.
 HA(aq) + H2O(l) ⇌ H3O+(aq) + A-(aq)
Water is a base that react with acid HA, A is the conjugate base of the acid
HA and the hydronium ion is the conjugate acid of water.

6. Acid dissociation constant(2)
The acid dissociation constant; Ka qualifies the extent of dissociation of a
weak acid.
The smaller the value of Ka, the weaker the acid and vice versa.
The strength of weak acid depends on how much it dissociates; the more
it dissociates, the stronger the acid.
 Ka =
𝑯𝟑𝑶+ [𝑨−]
[𝑯𝑨]

7. Ka values of weak acids
Ka of Weak Acids
hydrocyanic HCN 6.2 x 10-10
hydrofluoric HF 6.3 x 10-4
hydrogen peroxide H2O2 2.4 x 10-12
hydrogen sulfate ion HSO4
- 1.2 x 10-2

8. Dissociation of weak acid
Weak acids ionize only partially, and the ionization reaction is reversible.
The ionization constant increase as the strengths of the acids increase.
Thus weak acids solutions contain multiple charge and uncharged species in
dynamic equilibrium.
In this article, we will discuss acid dissociation reaction and the related
equilibrium constant; Ka the acid dissociation constant.

9. Method to calculate PH using Ka
1) Write the chemical equation for the ionization equilibrium.
2) Write the equilibrium constant expression.
3) Set up a table for initial/change in/Equilibrium Concentration to
determine equilibrium concentrations as a function of charge (x)
4) Substitute equilibrium concentrations into the equilibrium constant
expression and solve for x. (make assumptions if possible)

10. Calculating PH from Ka
Calculate the PH of a 0.30Msolution of acetic acid, HC2H3O2, at 25∘C.
HC2H3O2(aq) +H2O(l) = H3O+(aq) + C2H3O2-(aq)
Ka for acetic acid at 25°C is 1.8× 10 ∗ −5
Ka =
𝑯𝟑𝑶+ [𝑪𝟐𝑯𝟑𝑶𝟐−]
[𝑯𝑪𝟐𝑯𝟑𝑶𝟐]

11. Three ways to measure PH
A PH meter
An indicator
Litmus paper
 Red to blue (basic)
o PH > 8
 Blue to red (acidic)
o PH < 5

12. Litmus Paper

13. Calculating Ka from PH
The PH of 0.100M solution of formic acid, HCOOH, at 25∘C is 2.38.
Calculate Ka for formic acid at this temperature.
HCOOH(aq) + H2O(l) = H3O+(aq) HCOO-(aq)
We know that; Ka =
𝑯𝟑𝑶+ [𝑯𝑪𝑶𝑶−]
[𝑯𝑪𝑶𝑶𝑯]
To calculate Ka, we need the equilibrium concentrations of all three
things.
We can find [H3O+], which is same as [HCOO-], from the PH.

14. Calculating percent dissociation
%Dissociation = 𝑪𝒐𝒏𝒄𝒆𝒏𝒕𝒓𝒂𝒕𝒊𝒏 𝒊𝒐𝒏𝒊𝒛𝒆𝒅
𝑶𝒓𝒊𝒈𝒏𝒂𝒍 𝒄𝒐𝒄𝒆𝒏𝒕𝒓𝒂𝒕𝒊𝒐𝒏 ×100
 %Dissociation =
𝑪𝒐𝒏𝒄𝒆𝒏𝒕𝒓𝒂𝒕𝒊𝒐𝒏 𝒐𝒇 𝒉𝒚𝒅𝒓𝒐𝒏𝒊𝒖𝒎
𝑪𝒐𝒏𝒄𝒆𝒏𝒕𝒓𝒂𝒕𝒊𝒐𝒏 𝒐𝒇 𝒕𝒉𝒆 𝒘𝒆𝒂𝒌 𝒂𝒄𝒊𝒅
×100
Lets the example of formic acid;
• [H3O+]eq = 4.2 × 10 ∗ −3M
• [HCOOH]initial = 0.10M
%Dissociation =
4.2×10∗−3
0.10
×100 =4.2%

15. THANK YOU!