7.1 Acids and Bases
A What are Acids?
- Sour taste (lemon, vinegar, tamarind)
- Turn blue litmus red
- pH less than 7
- Release H⁺ ions in water
B What are Bases?
- Bitter taste, slippery touch
- Turn red litmus blue
- pH more than 7
- Release OH⁻ ions in water
- Alkalis: Bases soluble in water
🍋 Natural Organic Acids
| Acid | Natural Source | Properties |
|---|---|---|
| Citric Acid | Lemon, Orange | Food preservative |
| Acetic Acid | Vinegar | Food seasoning |
| Lactic Acid | Sour milk, Curd | Food fermentation |
| Ascorbic Acid | Amla, Guava | Vitamin C, antioxidant |
| Formic Acid | Ant sting | Defensive chemical |
| Oxalic Acid | Tomato | Cleaning agent |
| Tartaric Acid | Tamarind | Baking powder |
Uses: Fertilizers, batteries, cleaning, industrial processes
7.2 Arrhenius Concept
⚡ Acid Ionization Examples
⚡ Base Ionization Examples
💪 Strong vs Weak Acids
- Strong: Complete ionization (HCl, H₂SO₄, HNO₃)
- Weak: Partial ionization (CH₃COOH 0.132%, HCOOH 1.06%)
- Strong Bases: Complete ionization (NaOH, KOH)
- Weak Bases: Partial ionization (NH₄OH, Al(OH)₃)
Excess HCl in stomach → Take antacids (Ca(OH)₂, Mg(OH)₂) to neutralize
7.4 Properties of Acids & Bases
🧪 Acid Reactions
⚗️ Base Reactions
- With acids: Neutralization reaction
- With ammonium salts: Base + NH₄⁺ salt → Salt + H₂O + NH₃↑
- With metals: Generally no reaction
NaHCO₃ + CH₃COOH → CH₃COONa + H₂O + CO₂↑ (gas pressure clears block)
7.3 Bronsted-Lowry Concept
🎯 Definitions
- Acid: Proton (H⁺) donor
- Base: Proton (H⁺) acceptor
- No requirement for water as solvent
- Includes more substances than Arrhenius
🔄 Conjugate Acid-Base Pairs
Base: NH₃ Acid: H₂O
Conjugate Acid: NH₄⁺ Conjugate Base: OH⁻
🌊 Amphoteric Water
- Can act as both acid and base
- As acid: H₂O + NH₃ → NH₄⁺ + OH⁻
- As base: H₂O + HCl → H₃O⁺ + Cl⁻
Examples of Bronsted-Lowry Bases: OH⁻, NH₃, CN⁻, CO₃²⁻
Metal Oxides as Bases
🛡️ Basic Oxides
- Metal oxides react with acids to form salts
- Contain oxide ion (O²⁻) which reacts with water to form OH⁻
- Examples: Na₂O, CaO, MgO, ZnO
⚖️ Alkali vs Base
- All alkalis are bases
- Not all bases are alkalis
- Alkali: Base soluble in water (NaOH, KOH)
- Base: Any substance accepting H⁺
Exercise Questions
Multiple Choice Questions:
i) Which acid is NOT used as food or mixed with food?
Answer: (d) Formic acid
Explanation: Formic acid is found in ant stings and is irritating/painful, not used in foods. Others are food acids.
ii) While baking, which gas raises bread and makes it soft?
Answer: (b) Carbon dioxide
Explanation: Baking powder contains tartaric acid + NaHCO₃ → CO₂ gas bubbles expand dough.
iii) Predict main characteristics of metals with acids.
Answer: (d) All characteristics are true
Explanation: Metals dissolve, form salts, and evolve H₂ gas with acids (except Cu, Ag, Au, Pt).
iv) How many OH⁻ ions does Ca(OH)₂ release in water?
Answer: (b) 2
Explanation: Ca(OH)₂ → Ca²⁺ + 2OH⁻ (two hydroxide ions per formula unit).
v) In neutralization between KOH and H₃PO₄, how many KOH molecules react with one H₃PO₄?
Answer: (c) 3
Explanation: H₃PO₄ has 3 replaceable H⁺ ions: H₃PO₄ + 3KOH → K₃PO₄ + 3H₂O
Short Answer Questions:
i. Choose Arrhenius acids: HF, NH₃, H₂SO₄, SO₃, H₂S, H₂O
Answer: HF, H₂SO₄, H₂S
Explanation: These donate H⁺ in water. NH₃ accepts H⁺, SO₃ forms acid but doesn’t contain H, H₂O is amphoteric.
ii. How does calcium metal react with dilute H₂SO₄?
Answer: Ca + H₂SO₄ → CaSO₄ + H₂↑
Explanation: Calcium displaces hydrogen from acid forming calcium sulfate and hydrogen gas.
Constructed Response Questions:
i. What chemical name for soap as a compound?
Answer: Sodium or potassium salt of long-chain fatty acids
Explanation: Soaps are salts formed from saponification: Fat + NaOH → Glycerol + Soap (R-COONa).
iii. Why does Na₂CO₃ behave like a base in water?
Answer: Na₂CO₃ hydrolyzes in water: CO₃²⁻ + H₂O ⇌ HCO₃⁻ + OH⁻
Explanation: The carbonate ion accepts protons from water, producing OH⁻ ions, making solution basic.
Descriptive Questions:
i. Explain Arrhenius concept of acids and bases.
Answer: Arrhenius concept (1884): Acid = substance dissociating in water to give H⁺ or H₃O⁺ ions. Base = substance dissociating in water to give OH⁻ ions. Requires water as solvent. Neutralization: H₃O⁺ + OH⁻ → 2H₂O + heat.
ii. Compare Arrhenius and Bronsted-Lowry concepts.
Answer:
Arrhenius: Requires water, acid gives H⁺, base gives OH⁻
Bronsted-Lowry: Acid = H⁺ donor, Base = H⁺ acceptor, no water requirement, includes more substances
Relationship: All Arrhenius acids/bases are Bronsted-Lowry, but not vice versa.
Investigative Questions:
ii. What happens when CO₂ is passed through lime water?
Answer:
(i) Short duration: CO₂ + Ca(OH)₂ → CaCO₃↓ + H₂O (white precipitate)
(ii) Long duration: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂ (clear solution, calcium bicarbonate soluble)