7.1 Acids and Bases
Acids and bases have been known to mankind for centuries. Acids are characterized by their sour taste (like lemon) and ability to change the color of litmus paper from blue to red. Bases have a bitter taste, slippery touch, and turn litmus paper from red to blue.
Acids
Taste: Sour
Litmus: Blue → Red
Examples: Lemon, Vinegar
Bases
Taste: Bitter
Litmus: Red → Blue
Examples: Soap, Baking Soda
KEY NOTES: Types of Acids
NATURAL/ORGANIC ACIDS
• Obtained from natural sources
• Generally weak acids
• Examples: Citric acid (lemon), Acetic acid (vinegar), Lactic acid (curd)
MINERAL ACIDS
• Prepared from minerals
• Man-made, strong acids
• Examples: HCl, H₂SO₄, HNO₃
• Highly corrosive and dangerous
ALKALIS
• Bases soluble in water
• Produce OH⁻ ions in water
• Examples: NaOH, KOH, Ca(OH)₂
• Caustic (can burn skin)
Common Organic Acids and Sources
| Organic Acid | Natural Source | Properties |
|---|---|---|
| Acetic acid | Vinegar | Weak acid, sour taste |
| Ascorbic acid | Amla, Guava, Citrus fruits | Vitamin C, antioxidant |
| Citric acid | Lemon, Orange | Sour taste, food preservative |
| Lactic acid | Sour milk, Curd | Fermentation product |
| Formic acid | Ant sting, Nettle | Causes burning sensation |
| Oxalic acid | Tomato, Spinach | Simplest diprotic organic acid |
| Tartaric acid | Tamarind, Grapes | Used in baking powder |
Activity 7.1: Testing Substances with Litmus
Procedure: Test the following substances with blue and red litmus paper strips to determine if they are acidic or basic:
| Substance | Blue Litmus | Red Litmus | Nature |
|---|---|---|---|
| Tap water | No change | No change | Neutral |
| Battery water (H₂SO₄) | Turns red | No change | Acidic |
| Rain water (pH ~5.6) | Turns red | No change | Acidic |
| Soap solution | No change | Turns blue | Basic |
| Toothpaste | No change | Turns blue | Basic |
| Shampoo | May turn red | May turn blue | Depends on type |
| Bleach | No change | Turns blue | Basic |
Memory Trick:
“ACID = Sour like LEMON, BASE = Bitter like SOAP”
Acids turn Blue litmus Red (ABR: Acid Blue to Red). Bases turn Red litmus Blue (BRB: Base Red to Blue).
7.2 Arrhenius Concept of Acids and Bases
Svante Arrhenius (1859-1927)
Swedish chemist who proposed that acids and bases can be classified based on their behavior in water. Awarded Nobel Prize in Chemistry in 1903 for his electrolytic theory of dissociation.
Arrhenius proposed a molecular-level definition of acids and bases based on their behavior in aqueous solutions:
KEY NOTES: Arrhenius Definitions
ARRHENIUS ACID
• Substance that dissociates in water to give H⁺ ions
• Actually produces H₃O⁺ (hydronium ions)
• Examples: HCl, H₂SO₄, HNO₃, HCN
ARRHENIUS BASE
• Substance that dissociates in water to give OH⁻ ions
• Examples: NaOH, KOH, Ba(OH)₂
NEUTRALIZATION (Arrhenius)
• Acid + Base → Salt + Water
• Actually: H₃O⁺ + OH⁻ → 2H₂O + Heat
• Salt remains as hydrated ions in solution
Ionization Examples
Strong Acid (Complete Ionization):
Weak Acid (Partial Ionization):
Only 1.32 molecules out of 1000 dissociate for glacial acetic acid!
Diprotic Acid (Two-step Ionization):
Memory Trick:
“Arrhenius Acids give H⁺ in water, Arrhenius Bases give OH⁻ in water”
Remember: H⁺ from Acids, OH⁻ from Bases → Combine to make H₂O (neutralization).
Interesting Information: Stomach Acidity
Our stomach produces hydrochloric acid (HCl) to digest food. Sometimes, excess acid production causes hyperacidity (heartburn).
Solution: Antacids contain weak bases like Ca(OH)₂ or Mg(OH)₂ that neutralize excess stomach acid:
The OH⁻ concentration in antacids is low enough to neutralize acid without harming the stomach lining.
7.3 Bronsted-Lowry Concept
Johannes N. Brønsted (1879-1947)
Danish chemist who independently developed the proton transfer theory of acids and bases with Lowry in 1923.
Thomas M. Lowry (1874-1936)
English chemist who developed the proton transfer concept simultaneously with Brønsted.
Brønsted-Lowry concept overcomes Arrhenius limitations by focusing on proton transfer, not requiring water as solvent:
KEY NOTES: Brønsted-Lowry Definitions
BRØNSTED-LOWRY ACID
• Proton (H⁺) DONOR
• Must have removable H⁺
• Examples: HCl, H₂O, NH₄⁺
• All Arrhenius acids are Brønsted-Lowry acids
BRØNSTED-LOWRY BASE
• Proton (H⁺) ACCEPTOR
• Must have lone pair to accept H⁺
• Examples: OH⁻, NH₃, H₂O, CO₃²⁻
• NOT all are Arrhenius bases
CONJUGATE PAIRS
• Acid → Conjugate Base (loses H⁺)
• Base → Conjugate Acid (gains H⁺)
• Differ by one H⁺
• Strong acid → Weak conjugate base
Examples of Brønsted-Lowry Reactions
Example 1: HCl in water
Acid: HCl (donates H⁺), Base: H₂O (accepts H⁺)
Conjugate Acid: H₃O⁺, Conjugate Base: Cl⁻
Example 2: NH₃ in water
Acid: H₂O (donates H⁺), Base: NH₃ (accepts H⁺)
Conjugate Acid: NH₄⁺, Conjugate Base: OH⁻
Example 3: Amphoteric water
Water acts as both acid (donates H⁺) and base (accepts H⁺) – amphoteric!
Memory Trick:
“Brønsted-Lowry: Acids DONATE protons (like giving a gift), Bases ACCEPT protons (like receiving a gift)”
Conjugate pairs: What’s left after donating/accepting H⁺.
7.4 Properties of Acids and Bases
KEY NOTES: Acid Properties
1. WITH BASES/METAL OXIDES
• Acid + Base → Salt + Water (Neutralization)
2. WITH REACTIVE METALS
• Acid + Metal → Salt + H₂ gas
Non-reactive metals: Cu, Ag, Au, Pt don’t produce H₂
3. WITH CARBONATES/BICARBONATES
• Acid + Carbonate → Salt + H₂O + CO₂↑
Activity 7.2: Zinc with Dilute H₂SO₄
Procedure: Take zinc granules in test tube, add dilute H₂SO₄, heat gently.
Observation: Bubbles of hydrogen gas evolve. Burning matchstick produces ‘pop’ sound near mouth of test tube.
Test for H₂: ‘Pop’ sound with burning splint confirms hydrogen gas.
KEY NOTES: Base Properties
1. NEUTRALIZATION WITH ACIDS
• Base + Acid → Salt + Water
2. WITH AMMONIUM SALTS
• Base + Ammonium salt → Salt + H₂O + NH₃↑
Test for NH₃: Pungent smell, turns moist red litmus blue
3. ALKALIS (SOLUBLE BASES)
• NaOH, KOH: Highly soluble, strong bases
• Ca(OH)₂: Sparingly soluble (lime water)
• Cu(OH)₂: Insoluble base
Practical Application: Cleaning Blocked Drain
Method 1 (Natural):
1. Pour ½ cup sodium bicarbonate (NaHCO₃) into drain
2. Add ½ cup vinegar (CH₃COOH)
3. Cover, wait 30 minutes
4. Pour boiling water to flush
Method 2 (Chemical):
Use caustic drain cleaners (NaOH) that dissolve grease, hair, food. Wait 30 minutes, then flush with water.
Solved Exercises with Explanations
(i) Which acid is not used as a food or mixed with food?
Show ExplanationExplanation: Formic acid (HCOOH) is found in ant stings and causes burning sensation. It is NOT used in foods. Tartaric acid is used in baking powder, ascorbic acid is Vitamin C (in fruits), citric acid is used as food preservative and flavoring agent.
Key point: Food acids are generally weak organic acids that are safe for consumption.
(ii) While baking, which gas is responsible for raising the bread and making it soft?
Show ExplanationExplanation: Baking powder contains sodium bicarbonate (NaHCO₃) and a weak acid (like tartaric acid). When mixed with water and heated:
The CO₂ gas bubbles cause the dough to rise, making bread soft and spongy.
(iii) Predict the main characteristics of the reactions of metals with acids.
Show ExplanationExplanation: When reactive metals (Mg, Zn, Fe) react with acids:
1. Metals dissolve in the acid
2. Metals are converted to their salts (Mg → MgCl₂, Zn → ZnSO₄)
3. Hydrogen gas is evolved (test with burning splint gives ‘pop’ sound)
Exception: Less reactive metals (Cu, Ag, Au, Pt) don’t react with dilute acids.
(iv) How many hydroxide ions, calcium hydroxide will release in water?
Show ExplanationExplanation: Calcium hydroxide Ca(OH)₂ dissociates in water as:
Each formula unit of Ca(OH)₂ releases 2 hydroxide ions (OH⁻). However, Ca(OH)₂ is only sparingly soluble in water (0.16g/100mL at 20°C), so the actual concentration of OH⁻ is low.
(v) In a neutralization reaction between KOH and H₃PO₄, how many molecules of KOH will react with one molecule of H₃PO₄?
Show ExplanationExplanation: H₃PO₄ (phosphoric acid) is a triprotic acid – it has 3 ionizable H⁺ ions. Neutralization requires 3 OH⁻ ions to react with all 3 H⁺ ions:
Each KOH provides 1 OH⁻ ion, so 3 molecules of KOH are needed to neutralize 1 molecule of H₃PO₄.
(vi) Which acid is used in the preparation of soap?
Show ExplanationExplanation: Soap is prepared by saponification – reaction of fats/oils (triglycerides) with strong base (NaOH). Fats contain fatty acids like stearic acid (C₁₇H₃₅COOH), palmitic acid, oleic acid. When these react with NaOH:
Stearic acid gives sodium stearate, a common soap component.
(viii) Which of the following contains oxalic acid?
Show ExplanationExplanation: Oxalic acid (HOOC-COOH) is found in:
• Tomato, spinach, rhubarb (plants)
• Simplest diprotic organic acid
• Used commercially for bleaching straw/leather, removing rust/ink stains
Orange contains citric acid, tamarind contains tartaric acid, sour milk contains lactic acid.
i. Choose Arrhenius Acids among the following compounds. HF, NH₃, H₂SO₄, SO₂, H₂S, H₂O
Answer:
Arrhenius acids: HF, H₂SO₄, H₂S
Explanation:
• HF (hydrofluoric acid): Dissociates in water to give H⁺ and F⁻
• H₂SO₄ (sulfuric acid): Strong acid, completely ionizes to give H⁺
• H₂S (hydrosulfuric acid): Weak acid, partially ionizes to give H⁺
Not Arrhenius acids:
• NH₃: Arrhenius base (gives OH⁻ in water)
• SO₂: Forms H₂SO₃ in water (sulfurous acid) but SO₂ itself doesn’t give H⁺ directly
• H₂O: Amphoteric, not typically classified as Arrhenius acid or base
ii. How does calcium metal react with dilute H₂SO₄?
Answer:
Calcium metal reacts vigorously with dilute H₂SO₄ to produce calcium sulfate and hydrogen gas:
Observations:
1. Calcium dissolves in the acid
2. Bubbles of hydrogen gas are evolved
3. Solution may become warm (exothermic reaction)
4. White precipitate of CaSO₄ may form (calcium sulfate is slightly soluble)
Test for H₂: Burning splint produces ‘pop’ sound near gas.
iii. Which salt is formed when HCl reacts with BaCO₃?
Answer:
Hydrochloric acid reacts with barium carbonate to form barium chloride, water, and carbon dioxide:
Details:
• Salt formed: Barium chloride (BaCl₂)
• Type of reaction: Acid-carbonate reaction (produces CO₂ gas)
• Observations: Effervescence (bubbling) due to CO₂ evolution, solid BaCO₃ dissolves
• Test for CO₂: Turns lime water (Ca(OH)₂) milky due to formation of CaCO₃
iv. How will you justify that HSO₄⁻ is a Bronsted-Lowry acid?
Answer:
HSO₄⁻ (hydrogen sulfate or bisulfate ion) is a Bronsted-Lowry acid because it can donate a proton (H⁺).
Evidence:
In this reaction:
1. HSO₄⁻ donates H⁺ to H₂O → Acts as Bronsted-Lowry acid
2. H₂O accepts H⁺ → Acts as Bronsted-Lowry base
3. Forms conjugate base SO₄²⁻ and conjugate acid H₃O⁺
Additional proof: HSO₄⁻ is the conjugate base of H₂SO₄ and the conjugate acid of SO₄²⁻, showing it can both donate and accept protons (amphiprotic).
i. What chemical name will you give to soap as a compound?
Answer:
Soap is chemically known as sodium or potassium salt of long-chain fatty acids.
More specifically:
1. Hard soap: Sodium salts of fatty acids (e.g., sodium stearate C₁₇H₃₅COONa)
2. Soft soap: Potassium salts of fatty acids (e.g., potassium palmitate C₁₅H₃₁COOK)
Preparation (Saponification):
Example: Sodium stearate from stearic acid:
iii. Why does Na₂CO₃ behave like a base in water?
Answer:
Na₂CO₃ (sodium carbonate) behaves as a base in water due to hydrolysis of carbonate ion (CO₃²⁻).
Explanation:
1. Na₂CO₃ dissociates completely in water:
2. Carbonate ion (CO₃²⁻) hydrolyzes with water:
3. This produces hydroxide ions (OH⁻), making the solution basic (pH > 7).
From Bronsted-Lowry perspective: CO₃²⁻ acts as a base by accepting protons from water.
Key point: Although Na₂CO₃ doesn’t contain OH⁻ groups, it produces OH⁻ through hydrolysis, explaining its basic behavior.
iv. Is NaHCO₃ a base or an acid? Justify your answer.
Answer:
NaHCO₃ (sodium bicarbonate) is amphoteric – it can act as both an acid and a base depending on conditions.
1. As a base (more common behavior):
• Accepts H⁺ (Bronsted-Lowry base)
• Used as antacid to neutralize stomach acid
2. As an acid:
• Donates H⁺ (Bronsted-Lowry acid)
• Weak acid (Ka = 4.7 × 10⁻¹¹)
3. In water (hydrolysis):
Overall: NaHCO₃ solution is slightly basic (pH ~8.3) because basic hydrolysis dominates slightly over acidic hydrolysis.
v. What is the difference between a strong acid and a concentrated acid?
Answer:
| Strong Acid | Concentrated Acid |
|---|---|
| Definition: Extent of ionization in water | Definition: Amount of acid dissolved in given volume of solution |
| Measure: Degree of dissociation (α) | Measure: Molarity (mol/L) or percentage |
| Examples: HCl, H₂SO₄, HNO₃ (100% ionized) | Examples: Concentrated HCl (~12M), Concentrated H₂SO₄ (~18M) |
| Opposite: Weak acid (e.g., CH₃COOH, 0.1% ionized) | Opposite: Dilute acid (e.g., 0.1M HCl) |
| Key Point: Can have dilute strong acid (e.g., 0.01M HCl) | Key Point: Can have concentrated weak acid (e.g., glacial acetic acid) |
Example clarification:
• Strong and concentrated: 12M HCl (completely ionized, high concentration)
• Strong but dilute: 0.01M HCl (completely ionized, low concentration)
• Weak but concentrated: Glacial acetic acid (0.1% ionized, pure acid)
• Weak and dilute: 0.1M CH₃COOH (0.1% ionized, low concentration)