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📘 Contents — Strength of Acids and Bases
(Learn Chemistry by Inam Jazbi)
🔹 Introduction to Strength of Acids and Bases
🔥Ionization and dissociation concepts
🔥Meaning of strength
⚗️ Strength of Acids
🔥Definition
🔥Acid ionization and Ka expression
🔥Strong and weak acids (with examples)
🔥Factors affecting acid strength
🧪 Strength of Bases
🔥Definition
🔥Base ionization and Kb expression
🔥Strong and weak bases (with examples)
🔄 Relationship Between Ka, Kb, and Kw
🔥Mathematical relation
🔥Example calculations
🔥Conjugate acid-base pairs
💧 pH and Strength Relationship
🔥Formulae for pH, pOH, and Kw
🔥pH ranges for strong and weak acids/bases
🔥Example table
🧠 Comparison Table: Strong vs Weak Acids and Bases
🔥Ionization percentage
🔥Electrical conductivity
🔥pH range differences
💡 Tricks to Remember
🔥“Strong means full ionization — Weak means partial ionization”
🔥Shortcut patterns for remembering examples
🧲 Practical Examples
🔥Everyday acids and bases
🔥Real-life applications in industry and labs
🧾 Summary Points
🔥Key formulas and takeaways
📚 MCQs and Practice Questions (Optional Section)
🔥Concept-based questions for exams and MDCAT
Strength of Acids and Bases
Definition of Strength
In the light of Bronsted-Lowry concept, the strength of an acid is measured from its tendency to donate a proton to water and that of a base from its tendency to accept a proton from water. Different acids differ in their ability to donate protons. Similarly different bases differ in their capacity to accept protons. [i.e. some acids are better proton donors than others; likewise, some bases are better proton acceptors than others].
“The strength of an acid or a base is defined as its extent or capacity to ionize (dissociate) in aqueous solution into H⁺ and OH⁻ ions respectively” i.e. the more an acid or base ionizes, the stronger it is. Thus, they (i.e. acids or bases) are classified as strong or weak, depending on whether they are fully or partially ionized in aqueous solution.
It has been found that the more readily an acid gives up a proton, the less readily its conjugate base accepts a proton. Similarly, the more readily a base accepts a proton, the less readily its conjugate acid gives up a proton. In other words, the stronger an acid, the weaker its conjugate base; the stronger a base, the weaker its conjugate acid. In every acid-base reaction, the position of the equilibrium favours transfer of the proton from the stronger acid to the stronger base.
Ways of Expressing Strength
1. Strength
in terms of degree of dissociation
2. Strength
in terms of dissociation constants
3. Strength in terms of pH
Strength in terms of degree
of dissociation (α)
According to Arrhenius Theory, all the acids and bases ionize in water to
yield H⁺ and OH⁻ ions respectively.
HCl(aq)
⇌ H⁺(aq) + Cl⁻(aq)
NaOH(aq) ⇌ Na⁺(aq) + OH⁻ (aq)
Another measure of the strength of acid and base is their degree of ionization and percent ionization. In practice, strength of acids and bases are expressed in terms of degree of dissociation (denoted as α) or percentage dissociation (denoted as %α), which is the ratio of the number of molecules ionized to the total number of dissolved molecules of a substance. The ratio when multiplied by 100 gives percentage dissociation.
The percent
ionization of a weak acid is the ratio of the concentration of the ionized
acid to the initial acid concentration, times 100 (Because the ratio includes
the initial concentration, the percent ionization for a solution of a given
weak acid varies depending on the original concentration of the acid, and
actually decreases with increasing acid concentration):
in aqueous solution in terms of their degree of dissociation. Thus acids and bases which completely ionize in aqueous solution having high %age dissociation (30% or more) are considered to be strong and those which partially ionize in aqueous solution with low %age dissociation (less than 30%) are considered to be weak.
Types of acids and
bases according to strength or % Dissociation
Acids and bases are classified as strong or weak, depending on whether
they are fully or partially ionized in aqueous solution in terms of their
degree of dissociation.
Strong Acids
A strong acid is one that is almost completely dissociated (strong electrolyte) producing large number of H⁺ ions in aqueous solution having high percentage dissociation usually 30% or above. Strong acids are strong electrolytes that fully ionize and transfer their almost all protons to water. Strong acids have very weak conjugate bases. There are total seven strong acids. e.g.
HCl(aq)+H₂O ⇌ H₃O⁺(aq)+ Cl⁻(aq)(95% ionization in dilute solution)
Characteristics of Strong Acids
1. The concentration
of H+ ions is more.
2. The pH of
the solution in water is nearly zero.
3. They
dissociate completely in water hence α= 1 or nearly
equal to 1.
4. They have a
high value of dissociation constant Ka usually greater than 1.
5. They have very weak conjugate bases.
Weak Acids
A weak acid is one that is only partially dissociated (weak electrolyte) producing only a small number of H+ ions in aqueous solution having low percentage dissociation usually below 30%. Weak acids are weak electrolytes that only partially ionize and transfer only a small fraction of their protons to water. [Weak acids have very strong conjugate bases].
Characteristics of Weak Acids
1. The
concentration of H+ ions is low.
2. The pH of
the solution in water is greater than zero.
3. They
dissociate partially in water hence α < 0.3.
4. They have a
small value of dissociation constant Ka usually smaller than 10-3.
5. They have very strong conjugate bases.
Strong Bases
A strong base is one that is almost completely dissociated (strong
electrolyte) producing large number of OH– ions in aqueous solution
having high percentage dissociation usually 30% or above. Strong bases are
strong electrolytes that fully ionize and accept almost all protons. Strong
bases have very weak conjugate acids. e.g.
Characteristics of Strong Bases
1. The
concentration of OH- ions is more.
2. The pH of
the solution in water is nearly 14.
3. They
dissociate completely in water hence α= 1 or nearly
equal to 1.
4. They have a
high value of dissociation constant Kb usually greater than 1.
5. They have
very weak conjugate acids.
Weak Bases
A weak base is one that is only partially dissociated (weak electrolyte) producing only a small number of OH⁻ ions in aqueous solution having low percentage dissociation usually below 30%. Weak bases are weak electrolytes that weakly ionize and accept only a small fraction of protons. Weak bases have very strong conjugate acids. e.g. NH4OH etc.
Characteristics of Weak Bases
1. The
concentration of OH⁻ ions is low.
2. The pH of
the solution in water is less than 14.
3. They
dissociate partially in water hence α < 0.3.
4. They have a
small value of dissociation constant Kb usually smaller than 10-3.
5. They have
very strong conjugate acids.
Strength in terms of dissociation
constants (Ionization Constants
of Acids and Bases)
In the light of Bronsted-Lowry Concept, the strength of an acid is measured from its tendency to donate a proton to water and that of a base from its tendency to accept a proton from water. The relative strengths of acids and bases are indicated by their Ionization Constants (Kₐ and Kb) in the light of Bronsted-Lowry concept. The higher the value of Kₐ and Kb, the stronger is the acid and base.
The relative strengths of acids are indicated by acid ionization
constant or acid dissociation
constant represented by
Kₐ which is equilibrium constant for acid
dissociation reaction that represents the extent to which an
acid is dissociated and thus it is a quantitative measure of
the strength of the acid in a given solvent (e.g. water). The larger the value of Kₐ,
stronger is the acid and vice versa. Strong
acids have Kₐ values much greater than 1 while weak acids have Kₐ values much
smaller than 1.
For example; Acetic acid is a weaker acid than formic acid because of its smaller Kₐ value (1.0 × 10−5) than formic acid with Kₐ value of 1.8 × 10−4.
Criteria of Deciding Strong and Weak acids
Strength in terms of dissociation
constants (Ionization Constants
of Acids and Bases)
In the light of Bronsted-Lowry Concept, the strength of an acid is measured from its tendency to donate a proton to water and that of a base from its tendency to accept a proton from water. The relative strengths of acids and bases are indicated by their Ionization Constants (Kₐ and Kb) in the light of Bronsted-Lowry concept. The higher the value of Kₐ and Kb, the stronger is the acid and base.
The relative strengths of acids are indicated by acid ionization
constant or acid dissociation
constant represented by
Kₐ which is equilibrium constant for acid
dissociation reaction that represents the extent to which an
acid is dissociated and thus it is a quantitative measure of
the strength of the acid in a given solvent (e.g. water). The larger the value of Kₐ,
stronger is the acid and vice versa. Strong
acids have Kₐ values much greater than 1 while weak acids have Kₐ values much
smaller than 1.
For example; Acetic acid is a weaker acid than formic acid because of its smaller Kₐ value (1.0 × 10−5) than formic acid with Kₐ value of 1.8 × 10−4.
Criteria of Deciding Strong and Weak acids
Derivation of Kₐ
The ionization of an acid or a base in aqueous solution is a reversible
process. The equilibrium mixture of these acids possess unionized molecules of
acid (HA), conjugate base (A⁻) and conjugate acid (H3O⁺).
To derive acid ionization constant (Kₐ), consider acid ionization equilibrium of a general protonic acid HA upon which law of mass action can be applied to calculate Kc. In the dilute solution of an acid, we assume that the concentration of liquid water remains constant. [At the equilibrium state, the concentration of water is almost the same as at the initial stage because it has been taken in large excess. A reasonable approximation, therefore, is to take the concentration of water to be effectively constant and take it on the left hand side with Kc]. Therefore:
HA(aq) + H2O(l) ⇌ H3O+(aq)
+ A–(aq)
Where Kₐ is the acid ionization constant. The greater the value of Kₐ, the stronger is the acid and vice versa. Same is true for base. This equation can be used to calculate Kₐ for any acid if the pH or [H⁺] of that solution and the initial concentration of acid [HA] dissolved are known. The above equation can also be used to calculate the equilibrium concentration of H3O⁺ and A⁻ produced if Kₐ and initial concentration of acid HA are known.
Ka
for Polyprotic Acids
acids that can lose, and bases that can
pick up, more than one H⁺ (e.g. diprotic H₂A and triprotic H₃A acids), ionization
occurs stepwise, and each step has its own Kₐ.
Note: (i) Kₐ₁ > Kₐ₂ > Kₐ₃ Always true for polyprotic acids, i.e., each
ionization step is more difficult because it is more difficult to remove H+
from a molecule as its negative charge increases.
But this
does not apply for ionic salts of these acids, e.g., Na₃PO₄, Na₂SO₄, etc. They dissociate 100% in one step.
Na₃PO₄ (aq) → 3Na⁺(aq) +
PO₄³⁻(aq)
The conjugate bases are polyprotic bases, i.e., PO₄³⁻ is a triprotic base – it can pick up 3H⁺.
Derivation of Kb
Unlike strong bases, weak Bronsted bases which are proton acceptors,
usually consist of molecules or ions. They react with water, removing a proton
from it and generate OH⁻ ions. To derive base ionization constant (Kb),
consider base ionization equilibrium of a general weak Bronsted base (B) upon
which law of mass action can be applied to calculate Kc:
Where Kb is the base ionization constant. This equation can be
used to calculate Kb for any base if the pOH or [OH⁻] of that
solution and the initial concentration of base [B] dissolved are known. The
above equation can also be used to calculate the equilibrium concentration of
BH⁺ and OH⁻ produced if Kb and initial concentration of base B are
known. The greater the value of Kb, the stronger is the base and
vice versa. Same is true for acids.
pKa
Ka values for weak acids are small numbers usually expressed in negative exponential form and hence they become the cause of inconvenience during calculations. It is convenient to convert them into whole numbers by taking their negative logarithm. The negative logarithm of Kₐ of an acid is called pKₐ of the acid which is the measure of strength of an acid. i.e. smaller the value of pKₐ, stronger is the acid while large the value of pKₐ, weaker is the acid. pKₐ value is inversely proportional to the Kₐ. Smaller the value of Kₐ, greater would be the value of pKₐ and vice versa.
pKb
Kb values for weak bases are small numbers usually expressed
in negative exponential form and hence they become the cause of inconvenience
during calculations. It is convenient to convert them into whole numbers by
taking their negative logarithm. The negative logarithm of Kb of a base is called pKb
of the base which is the measure of strength of a base. i.e. smaller the value of pKb, stronger is the base while
large the value of pKb, weaker is the base. pKb value is
inversely proportional to the Ka. Smaller the value of Kb,
greater would be the value of pKb and vice versa.
Strength in terms of pH
Strength of acids and bases may also be expressed in terms of pH values.
Acids with low pH values (<7) are considered to be strong and bases with
high pH values (>7) are considered to be strong.
Strong Acid → Low pH (1–3)
Weak Acid → Moderate pH (4–6)
Strong Base → High pH (12–14)
Weak Base → Moderate pH (8–11)
Factors Affecting Acid Strength
Bond Strength: Weaker H–A bond → stronger acid
Electronegativity: Higher electronegativity of A → stronger acid
Polarity of Bond: Greater polarity → easier ionization
Solvent Effect: Polar solvents like H₂O enhance ionization
🚀💡 Quiz: Acids, Bases, and Salts
1. Which of the following is a strong acid?A) HClO
B) HCl
C) HF
D) H₂CO₃
Answer: B) HCl
Explanation: Hydrochloric acid (HCl) completely ionizes in water, making it a strong acid.
2. Which of the following is a weak base?
A) NaOH
B) KOH
C) NH₃
D) Ca(OH)₂
Answer: C) NH₃
Explanation: Ammonia (NH₃) partially ionizes in water, so it is a weak base.
3. Which of the following is a salt formed by a strong acid and a strong base?
A) NaCl
B) NH₄Cl
C) CH₃COONa
D) Al(OH)₃
Answer: A) NaCl
Explanation: NaCl is formed from HCl (strong acid) and NaOH (strong base) and is neutral in solution.
4. Which indicator turns red in acidic solution?
A) Methyl orange
B) Phenolphthalein
C) Litmus
D) Both A & C
Answer: D) Both A & C
Explanation: Acidic solutions turn methyl orange red and litmus paper red.
5. Which of the following is amphoteric?
A) NaOH
B) Al(OH)₃
C) HCl
D) NaCl
Answer: B) Al(OH)₃
Explanation: Amphoteric substances can act as both acid and base. Aluminum hydroxide reacts with acids and bases.
6. Which gas is released when an acid reacts with a metal?
A) O₂
B) CO₂
C) H₂
D) N₂
Answer: C) H₂
Explanation: Metals like Zn or Mg react with acids to release hydrogen gas.
7. Which of the following is a weak acid?
A) HCl
B) H₂SO₄
C) CH₃COOH
D) HNO₃
Answer: C) CH₃COOH
Explanation: Acetic acid (CH₃COOH) partially ionizes in water, making it weak.
8. Which of the following salts is basic in nature?
A) NaCl
B) KNO₃
C) Na₂CO₃
D) NH₄Cl
Answer: C) Na₂CO₃
Explanation: Sodium carbonate is basic because it hydrolyzes in water to form OH⁻ ions.
9. Which base is commonly used in soap making?
A) NH₃
B) NaOH
C) Ca(OH)₂
D) KOH
Answer: B) NaOH
Explanation: Sodium hydroxide reacts with fats to produce soap in a process called saponification.
10. Which salt is acidic in nature?
A) NaCl
B) NH₄Cl
C) K₂SO₄
D) Na₂CO₃
Answer: B) NH₄Cl
Explanation: Ammonium chloride hydrolyzes in water to produce H⁺ ions, making the solution acidic.
11. Which of the following statements is true?
A) All acids taste bitter
B) All bases taste sour
C) Acids react with metals to produce H₂ gas
D) Salts are always basic
Answer: C) Acids react with metals to produce H₂ gas
Explanation: This is a characteristic property of acids.
12. Litmus turns _____ in a basic solution.
A) Red
B) Blue
C) Green
D) Yellow
Answer: B) Blue
Explanation: Litmus paper turns blue in the presence of a base.
13. Which of the following is a property of bases?
A) Corrosive
B) Turns red litmus blue
C) Tastes bitter
D) All of the above
Answer: D) All of the above
Explanation: Bases have multiple characteristic properties, including corrosiveness and bitterness.
14. What is the common name of NaHCO₃?
A) Baking soda
B) Washing soda
C) Table salt
D) Lime
Answer: A) Baking soda
Explanation: Sodium bicarbonate is commonly called baking soda and is slightly basic.
15. Which reaction represents neutralization?
A) HCl + NaOH → NaCl + H₂O
B) Na₂CO₃ + H₂O → NaOH + HCO₃⁻
C) H₂SO₄ + Zn → ZnSO₄ + H₂
D) CH₃COOH + H₂O → CH₃COO⁻ + H₃O⁺
Answer: A) HCl + NaOH → NaCl + H₂O
Explanation: Neutralization is the reaction between an acid and a base to form salt and water.
16. Which of the following acids is found in lemons?
A) Hydrochloric acid
B) Sulfuric acid
C) Citric acid
D) Acetic acid
Answer: C) Citric acid
Explanation: Citric acid gives lemons their sour taste.
17. Which of the following is used as an antacid?
A) HCl
B) NaOH
C) Mg(OH)₂
D) H₂SO₄
Answer: C) Mg(OH)₂
Explanation: Magnesium hydroxide neutralizes excess stomach acid and relieves acidity.
18. Which of the following is a characteristic of strong acids?
A) Fully ionize in water
B) Partially ionize in water
C) Have a bitter taste
D) Are slippery
Answer: A) Fully ionize in water
Explanation: Strong acids completely dissociate into ions in water.
19. Which of the following salts is neutral?
A) NaCl
B) NH₄Cl
C) Na₂CO₃
D) AlCl₃
Answer: A) NaCl
Explanation: Salt of a strong acid and strong base is neutral in aqueous solution.
20. Which of the following bases is least soluble in water?
A) NaOH
B) KOH
C) Ca(OH)₂
D) Ba(OH)₂
Answer: C) Ca(OH)₂
Explanation: Calcium hydroxide is sparingly soluble in water compared to NaOH or KOH.
🧠
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