7.1 The Characteristic Properties of Acids and Bases
1. Overview
Acids and bases are chemical opposites that play a fundamental role in chemical reactions. This topic covers how these substances behave in aqueous solutions, their characteristic reactions with other compounds, and how we measure their strength using indicators and the pH scale.
Key Definitions
- Acid: A substance that releases hydrogen ions ($H^+$) in aqueous solution. (Extended: A proton donor).
- Base: A metal oxide or hydroxide that neutralises an acid to form a salt and water. (Extended: A proton acceptor).
- Alkali: A soluble base that releases hydroxide ions ($OH^-$) in aqueous solution.
- Indicator: A chemical that changes colour depending on whether it is in an acidic or alkaline solution.
- Salt: A compound formed when the hydrogen ion of an acid is replaced by a metal ion or ammonium ion.
- Neutralisation: A reaction between an acid and a base/alkali to produce a salt and water.
Core Content
Properties of Acids
Acids have a sour taste, are corrosive, and have a pH of less than 7.
Reactions of Acids:
- With Metals: Acid + Metal → Salt + Hydrogen $(g)$
- Word: Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen
- Symbol: $Mg (s) + 2HCl (aq) → MgCl_2 (aq) + H_2 (g)$
- With Bases (Neutralisation): Acid + Base → Salt + Water $(l)$
- Word: Copper(II) oxide + Sulfuric acid → Copper(II) sulfate + Water
- Symbol: $CuO (s) + H_2SO_4 (aq) → CuSO_4 (aq) + H_2O (l)$
- With Carbonates: Acid + Carbonate → Salt + Water $(l)$ + Carbon dioxide $(g)$
- Word: Calcium carbonate + Hydrochloric acid → Calcium chloride + Water + Carbon dioxide
- Symbol: $CaCO_3 (s) + 2HCl (aq) → CaCl_2 (aq) + H_2O (l) + CO_2 (g)$
Properties of Bases and Alkalis
Bases are typically metal oxides (e.g., $CuO$) or metal hydroxides (e.g., $NaOH$). Alkalis are the subset of bases that dissolve in water.
Reactions of Bases:
- With Acids: (See Neutralisation above).
- With Ammonium Salts: Alkali + Ammonium Salt → Salt + Water $(l)$ + Ammonia $(g)$
- Word: Sodium hydroxide + Ammonium chloride → Sodium chloride + Water + Ammonia
- Symbol: $NaOH (aq) + NH_4Cl (s) → NaCl (aq) + H_2O (l) + NH_3 (g)$
Indicators and pH
- Acids in aqueous solution contain hydrogen ions: $H^+ (aq)$.
- Alkalis in aqueous solution contain hydroxide ions: $OH^- (aq)$.
Colour Changes Table:
| Indicator | Colour in Acid | Colour in Neutral | Colour in Alkali |
|---|---|---|---|
| Litmus | Red | Purple | Blue |
| Thymolphthalein | Colourless | Colourless | Blue |
| Methyl Orange | Red | Orange | Yellow |
Universal Indicator and pH Scale: The pH scale measures the concentration of $H^+$ ions.
- pH 0-2: Strongly acidic (Red)
- pH 3-6: Weakly acidic (Orange/Yellow)
- pH 7: Neutral (Green)
- pH 8-11: Weakly alkaline (Blue)
- pH 12-14: Strongly alkaline (Violet/Purple)
A pH scale from 0-14 showing the colour spectrum of Universal Indicator from red to purple, with 7 marked as green/neutral.
The Neutralisation Reaction
The essential ionic reaction between any acid and alkali to produce water is: $$H^+ (aq) + OH^- (aq) → H_2O (l)$$
Extended Content (Extended Only)
Proton Transfer
- Acids are defined as proton donors. They give away an $H^+$ ion (which is just a single proton).
- Bases are defined as proton acceptors. They receive the $H^+$ ion.
Strong vs. Weak Acids
Strength is determined by the degree of dissociation (splitting into ions) in water.
Strong Acids: Completely dissociate in aqueous solution.
- Example: Hydrochloric acid ($HCl$)
- Equation: $HCl (aq) → H^+ (aq) + Cl^- (aq)$
- Note the single arrow (→) indicating the reaction goes to completion.
Weak Acids: Partially dissociate in aqueous solution.
- Example: Ethanoic acid ($CH_3COOH$)
- Equation: $CH_3COOH (aq) ⇌ H^+ (aq) + CH_3COO^- (aq)$
- Note the reversible arrow (⇌) indicating that only some molecules split into ions.
Key Equations
| Reaction Type | General Equation |
|---|---|
| Acid + Metal | $Acid + Metal → Salt + H_2 (g)$ |
| Acid + Base | $Acid + Base → Salt + H_2O (l)$ |
| Acid + Carbonate | $Acid + Carbonate → Salt + H_2O (l) + CO_2 (g)$ |
| Alkali + Ammonium Salt | $Alkali + NH_4^+ Salt → Salt + H_2O (l) + NH_3 (g)$ |
| Neutralisation (Ionic) | $H^+ (aq) + OH^- (aq) → H_2O (l)$ |
Common Mistakes to Avoid
- ❌ Wrong: Writing $H$ for hydrogen gas.
- ✓ Right: Always write hydrogen gas as a diatomic molecule: $H_2 (g)$.
- ❌ Wrong: Confusing "Strong/Weak" with "Concentrated/Dilute".
- ✓ Right: Strong/Weak refers to dissociation (how many ions are made). Concentrated/Dilute refers to the amount of acid dissolved in a volume of water.
- ❌ Wrong: Forgetting state symbols in equations.
- ✓ Right: Pure water is always $(l)$, precipitated salts or carbonates are often $(s)$, and acids/alkalis are always $(aq)$.
Exam Tips
- Command Word - "State": If a question asks you to "state the colour of methyl orange in acid," a single word ("Red") is sufficient.
- Command Word - "Name": If asked to name a gas, do not write the formula. Write "carbon dioxide," not $CO_2$.
- Gas Identification: Remember that the reaction of a base with an ammonium salt produces ammonia gas ($NH_3$). This is often used in exams to identify an unknown substance. Ammonia turns damp red litmus paper blue.
- Contexts: You may be asked to suggest how to treat acidic soil. Since bases neutralise acids, adding a base like Calcium oxide ($CaO$, lime) or Calcium hydroxide ($Ca(OH)_2$, slaked lime) is the standard real-world application.