Topic 4: Acids & Bases Flashcards
What is an ‘acid’?
Definition:
- An acid is a hydrogen-containing substance that gives hydrogen ion [H⁺(aq)] as the only positive ion (cation) in its aqueous solution.
Points to note:
- Water must be present for an acid to show its acidic properties.
- An acid ionizes in water to give H⁺(aq) ion and anion(s). H⁺(aq) is responsible for all the acidic properties.
- An acid must be a H-containing covalent compound in DSE chemistry.
Diagrams: Refer to notes p.2 (Part 1)
Examples:
- HCl(g) (^H₂O) → HCl(aq) → H⁺(aq) + Cl⁻(aq)
hydrogen chloride 無H⁺ → hydrochloric acid 有H⁺ → hydrogen ion + chloride ion - CH₃COOH(l) (^H₂O) → CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq)
ethanoic acid 無H⁺ → ethanoic acid 有H⁺ ⇌ ethanoate ion + hydrogen ion
What are some examples of strong acids and weak acids?
Strong acids:
- HCl (hydrochloric acid), HNO₃ (nitric acid), H₂SO₄ (sulphuric acid)
Weak acids: all other acids in DSE
- HOCl (hypochlorous acid), HNO₂ (nitrous acid), H₂SO₃ (sulphurous acid), H₂CO₃ (carbonic acid), H₂C₂O₄ (ethanedioic acid / oxalic acid), H₃PO₄ (phosphoric acid), HCOOH (methanoic acid), CH₃COOH (ethanoic acid)
What are the differences between acids and acidic substances?
Acids are different from acidic substances. Acids must be acidic substances, but acidic substances may not be acids.
- Most non-metal oxides are acidic substances, e.g. CO₂, NO₂, SO₂ and SO₃.
- They form acidic solutions (pH < 7 at 25°C) when dissolved in water.
Points to note: (Diagram: Refer to notes p.3 (part 1))
- Some acidic substances (e.g. CO₂, NO₂, SO₂ and Cl₂) do not contain H, but they can react with water to form one or more acids which produce H⁺(aq).
- Some acidic substances (e.g. NaHSO₄ and NH₄Cl) dissolve in or react with water to produce H⁺(aq), forming acidic solutions, but other cations are also present.
What are some common acids in daily life?
+Acids in the substance
Toilet cleaner:
- Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), Phosphoric acid (H₃PO₄)
Soft drink:
- Carbonic acid (H₂CO₃, 碳酸)
Vinegar:
- Ethanoic acid (CH₃COOH, in vinegar)
Car battery:
- Sulphuric acid (H₂SO₄)
Orange, lemon:
- Citric acid (檸檬酸), Ascorbic acid (vitamin C)
→ Solid at 25°C (room temp.)
Tomatoes:
- Oxalic acid (H₂C₂O₄)
→ Solid at 25°C (room temp.)
Yoghurt, Yakult:
- Lactic acid (乳酸)
What are some common acids in the laboratory?
HCl, HNO₃, H₂SO₄ and CH₃COOH
Note:
- HCl, HNO₃ and H₂SO₄ are called mineral acids because they can be made from minerals.
HCl = from NaCl; HNO₃ = from NaNO₃; H₂SO₄ = from S
What are the characteristics of acids?
- Corrosive (for concentrated acids)
- Irritant (for dilute acids)
- Sour taste^
- Effects on indicators (refer to Card 7)
- Ionization in water
Strong acid = 100% / completely ionizes in water
Weak acid = < 100% / partially ionizes in water - Can conduct electricity (refer to Card 8)
What are the effects of acids on indicators?
Indicator + Colour in acids
- Litmus solution: Red
-
Blue damp litmus paper: Red
(litmus paper is made from litmus solution) -
Universal indicator: Red / Orange / Yellow
(Red: pH < 3, Orange/Yellow: pH = 3-6, Green: Neutral) -
Damp pH paper: Red / Orange / Yellow
(pH paper is made from universal indicator)
Used in titration only:
-
Methyl orange: Red / Orange
(Red: pH < 3.1; Orange: 3.1 < pH < 4.4) - Phenolphthalein: Colourless (pH < 8.2)
Why can acids conduct electricity?
Requirements of a substance to conduct electricity:
- Presence of mobile ion / delocalized e⁻
Acids undergo ionization in water to give H⁺(aq) and the corresponding anions
- → Contains mobile ions
- → Aqueous solutions of acids can conduct electricity
What are dilute acid and concentrated acid?
Dilute acid:
- Acid of low concentration (no. of moles of acid in 1 dm³ of water = small)
Concentrated acid:
- Acid of high concentration (no. of moles of acid in 1 dm³ of water = large)
What are the reactions between common acids and metals?
Metal + Dilute Acid → Salt + Hydrogen gas
Metal reactivity: ([K > Na > Ca > Mg > Al > Zn > Fe > Sn > Pb] > Cu > Hg > Ag) > Pt > Au
- [ ] : React with dil. HCl, dil. H₂SO₄, dil. CH₃COOH + conc. HCl, conc. CH₃COOH → form salts and H₂(g)
- ( ) : React with conc. H₂SO₄, dil. HNO₃, conc. HNO₃ → form salts, SO₂(g)/NO(g)/NO₂(g) and H₂O(l)
Key:
- Dilute acids (except HNO₃) react with metals above Cu in the metal reactivity series (more reactive than Cu) or above H₂ in the electrochemical series, to give salts and H₂(g).
Draw a setup for the reaction between dilute acid and metal.
Refer to notes p.10 (part 1).
What is a ‘salt’?
What are ‘acid salt’ and ‘normal salt’ formed?
A salt = an ionic compound = a metal ion / ammonium ion (NH₄⁺) + anion from an acid
- Acid salt is formed by replacing only part of the hydrogen ions from an acid.
- Normal salt is formed by replacing all of the hydrogen ions from an acid.
Note:
- An ionic compound may not be a salt (e.g. NaOH).
- Some salts only exist theoretically.
What are the equations of ionization of the following acids: HCl, HNO₃, H₂SO₄, H₃PO₄, H₂CO₃, H₂SO₃, HCOOH, CH₃COOH?
Hydrochloric acid (HCl):
- HCl → H⁺ + Cl⁻ (form normal salts)
Nitric acid (HNO₃):
- HNO₃ → H⁺ + NO₃⁻ (form normal salts)
Sulphuric acid (H₂SO₄):
- H₂SO₄ → H⁺ + HSO₄⁻ (form acid salts)
→ 2H⁺ + SO₄²⁻ (form normal salts)
Phosphoric acid (H₃PO₄):
- H₃PO₄ ⇌ H⁺ + H₂PO₄⁻ (form acid salts)
⇌ 2H⁺ + HPO₄²⁻ (form acid salts)
⇌ 3H⁺ + PO₄³⁻ (form normal salts)
Carbonic acid (H₂CO₃):
- H₂CO₃ ⇌ H⁺ + HCO₃⁻ (form acid salts)
⇌ 2H⁺ + CO₃²⁻ (form normal salts)
Sulphurous acid (H₂SO₃):
- H₂SO₃ ⇌ H⁺ + HSO₃⁻ (form acid salts)
⇌ 2H⁺ + SO₃²⁻ (form normal salts)
Methanoic acid (HCOOH):
- HCOOH ⇌ H⁺ + HCOO⁻ (form normal salts)
Ethanoic acid (CH₃COOH):
- CH₃COOH ⇌ H⁺ + CH₃COO⁻ (form normal salts)
Remarks about the reactions between metals (Fe, Ca, Pb, K, Na) and acids.
Fe(s):
Reactions of Fe(s) with dilute acids (except HNO₃) usually gives Fe²⁺(aq), not Fe³⁺(aq).
- Fe(s) + 2CH₃COOH(aq) → (CH₃COO)₂Fe(aq) + H₂(g)
- Fe(s) + H₂SO₄(aq) → FeSO₄(aq) + H₂(g)
[Not Fe₂(SO₄)₃(aq)]
Ca(s):
Ca(s) does not react readily with H₂SO₄(aq) .
Insoluble CaSO₄(s) is formed on the surface of Ca(s). This prevents Ca(s) from further reaction with the H₂SO₄(aq).
- Ca(s) + H₂SO₄(aq) → CaSO₄(s) + H₂(g)
Pb(s):
Pb(s) does not react readily with HCl(aq) and H₂SO₄(aq).
Insoluble PbCl₂(s) and PbSO₄(s) are formed on the surface of Pb(s). This prevents Pb(s) from further reaction with the acids.
- Pb(s) + 2HCl(aq) → PbCl₂(s) + H₂(g)
- Pb(s) + H₂SO₄(aq) → PbSO₄(s) + H₂(g)
K(s) and Na(s):
- Reactions of K(s) and Na(s) with acids are explosive.
Product [H₂(g)]:
- H₂(g) formed can be tested with a burning splint.
Observation: A ‘pop’ sound
What are the acids that should not be used for H₂(g) preparation?
- Concentrated hydrochloric acid, HCl(aq)
The reaction is too vigorous because the reaction rate increases with reactant concentration. In addition, toxic HCl(g) will be given out from conc. HCl(aq). - Concentrated sulphuric acid, H₂SO₄(l)
SO₂(g) is formed instead of H₂(g). - Dilute or concentrated nitric acid, HNO₃(aq)
NO(g) or NO₂(g) is formed instead of H₂(g).
Note:
- Very dil. HNO₃ reacts with metals (more reactive than Cu) to give H₂(g).
- Dil./conc. HNO₃ reacts in a similar way as other acids (e.g. HCl), except its reactions with metals.
