Chemical Properties of Acid

Reaction of Acids with Metals

Acids react with reactive metals to produce hydrogen gas and a salt. Only metals that are more reactive than hydrogen (those above hydrogen in the electrochemical series) can displace hydrogen from acids.

General Equation

Acid + Metal → Salt + Hydrogen gas

Detailed Examples

1. Hydrochloric acid with zinc:

2HCl(aq) + Zn(s) → ZnCl₂(aq) + H₂(g)↑

Observation: Zinc dissolves with bubbles of colorless gas (hydrogen) produced. The solution becomes warm.

2. Tetraoxosulphate(VI) acid with magnesium:

H₂SO₄(aq) + Mg(s) → MgSO₄(aq) + H₂(g)↑

Observation: Magnesium ribbon dissolves rapidly with vigorous effervescence. The reaction is exothermic.

3. Trioxonitrate(V) acid with aluminium:

6HNO₃(aq) + 2Al(s) → 2Al(NO₃)₃(aq) + 3H₂(g)↑

Note: With very dilute nitric acid, hydrogen is produced. However, concentrated nitric acid produces nitrogen dioxide (NO₂) instead of hydrogen.

4. Ethanoic acid with iron:

2CH₃COOH(aq) + Fe(s) → (CH₃COO)₂Fe(aq) + H₂(g)↑

Observation: Reaction is slower because ethanoic acid is a weak acid. Iron dissolves gradually with gentle bubbling.

Important Points About Metal-Acid Reactions

• Reactive metals only: Metals like potassium, sodium, calcium, magnesium, aluminium, zinc, and iron react with acids. Metals below hydrogen like copper, silver, and gold do NOT react with dilute acids.
• Hydrogen gas test: The gas produced burns with a “pop” sound when a lighted splint is brought near it. This is the confirmatory test for hydrogen.
• Rate of reaction: Strong acids (HCl, H₂SO₄) react faster than weak acids (CH₃COOH). More reactive metals (like magnesium) react faster than less reactive ones (like iron).
• Temperature effect: These reactions are exothermic – they produce heat, making the container warm.
• Safety warning: Never add very reactive metals like sodium or potassium to acids. The reaction is explosive and dangerous.

Reaction of Acids with Bases (Neutralization)

When an acid reacts with a base or alkali, they neutralize each other to form salt and water only. This is called a neutralization reaction. The hydrogen ions (H⁺) from the acid combine with hydroxide ions (OH⁻) from the base to form water.

General Equation

Acid + Base → Salt + Water

Detailed Examples

1. Hydrochloric acid with sodium hydroxide:

HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

Ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l)

Observation: No visible change, but the mixture becomes warm. If an indicator is present, it changes color (e.g., phenolphthalein turns from pink to colorless).

2. Tetraoxosulphate(VI) acid with potassium hydroxide:

H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l)

Observation: Heat is evolved. The solution can be evaporated to obtain crystals of potassium tetraoxosulphate(VI).

3. Trioxonitrate(V) acid with calcium hydroxide:

2HNO₃(aq) + Ca(OH)₂(aq) → Ca(NO₃)₂(aq) + 2H₂O(l)

Observation: The cloudy calcium hydroxide solution becomes clear as neutralization occurs.

4. Ethanoic acid with ammonia solution:

CH₃COOH(aq) + NH₃(aq) → CH₃COONH₄(aq)

Note: Ammonia (NH₃) is a weak base. The product is ammonium ethanoate, which dissolves in water.

Key Points About Neutralization

• Products are always salt and water: No gas is produced in neutralization (unlike with metals or carbonates).
• Heat evolution: All neutralization reactions are exothermic. The temperature rises during the reaction.
• pH change: The acidic solution (pH < 7) becomes neutral (pH = 7) or slightly alkaline depending on the amounts used.
• Indicator changes: Litmus paper changes from red (acidic) to purple/blue (neutral/alkaline). Methyl orange changes from red to yellow.
• Practical uses: Farmers add lime (calcium hydroxide) to neutralize acidic soil. NAFDAC approves antacids containing magnesium hydroxide to neutralize excess stomach acid.

Reaction of Acids with Carbonates and Hydrogen Carbonates

Acids react with carbonates (CO₃²⁻) and hydrogen carbonates (HCO₃⁻) to produce carbon dioxide gas, water, and a salt. This reaction is used as a test to identify carbonates in the laboratory.

General Equations

Acid + Carbonate → Salt + Water + Carbon dioxide

Acid + Hydrogen carbonate → Salt + Water + Carbon dioxide

Detailed Examples

1. Hydrochloric acid with calcium carbonate (limestone):

2HCl(aq) + CaCO₃(s) → CaCl₂(aq) + H₂O(l) + CO₂(g)↑

Observation: Limestone dissolves with vigorous effervescence (bubbling). The gas produced turns limewater milky.

2. Tetraoxosulphate(VI) acid with sodium carbonate:

H₂SO₄(aq) + Na₂CO₃(s) → Na₂SO₄(aq) + H₂O(l) + CO₂(g)↑

Observation: Sodium carbonate dissolves with fizzing. Colorless, odorless gas is produced.

3. Trioxonitrate(V) acid with sodium hydrogen carbonate (baking soda):

HNO₃(aq) + NaHCO₃(s) → NaNO₃(aq) + H₂O(l) + CO₂(g)↑

Observation: Baking soda dissolves with gentle effervescence. This is why baking soda neutralizes bee stings (which contain methanoic acid).

4. Ethanoic acid (vinegar) with calcium carbonate (eggshell):

2CH₃COOH(aq) + CaCO₃(s) → (CH₃COO)₂Ca(aq) + H₂O(l) + CO₂(g)↑

Observation: Eggshell dissolves slowly with bubbles. This is why vinegar can remove limescale from kettles.

Test for Carbon Dioxide Gas

The gas produced in these reactions can be confirmed as carbon dioxide using these tests:

1. Limewater test: Bubble the gas through limewater (calcium hydroxide solution). It turns milky due to formation of calcium carbonate.

   CO₂(g) + Ca(OH)₂(aq) → CaCO₃(s) + H₂O(l)

   Note: With excess CO₂, the milkiness disappears as soluble calcium hydrogen carbonate forms.

2. Burning splint test: The gas extinguishes a burning splint (carbon dioxide does not support combustion).
3. Physical properties: Colorless, odorless gas, slightly heavier than air.

Reaction of Acids with Metal Oxides

Metal oxides are basic oxides. They react with acids to form salt and water only, similar to the reaction with bases.

General Equation

Acid + Metal oxide → Salt + Water

Examples

1. Hydrochloric acid with copper(II) oxide:

2HCl(aq) + CuO(s) → CuCl₂(aq) + H₂O(l)

Observation: Black copper(II) oxide dissolves to form a blue-green solution of copper(II) chloride.

2. Tetraoxosulphate(VI) acid with zinc oxide:

H₂SO₄(aq) + ZnO(s) → ZnSO₄(aq) + H₂O(l)

Observation: White zinc oxide dissolves to form a colorless solution.

3. Trioxonitrate(V) acid with iron(III) oxide (rust):

6HNO₃(aq) + Fe₂O₃(s) → 2Fe(NO₃)₃(aq) + 3H₂O(l)

Observation: Brown rust dissolves to form a yellow-brown solution. This is why acidic cleaners remove rust stains.

Comparison Table of Acid Reactions

Common Exam Mistakes to Avoid

Based on WAEC Chief Examiner reports, students commonly make these mistakes:

1. Wrong products in equations: Students write acid + metal → salt + oxygen instead of salt + hydrogen. Remember: metals give H₂, carbonates give CO₂.
2. Unbalanced equations: Many students forget to balance chemical equations. For example, writing HCl + Zn → ZnCl₂ + H₂ instead of 2HCl + Zn → ZnCl₂ + H₂. Always count atoms on both sides.
3. Confusing gas tests: Students mix up the tests for hydrogen (pop sound) and carbon dioxide (limewater turns milky). Learn these tests thoroughly.
4. Wrong metal reactivity: Writing equations with unreactive metals like copper or silver reacting with dilute acids. Only metals above hydrogen in the electrochemical series react with dilute acids.
5. Incomplete product identification: When acids react with carbonates, students often forget to mention water as one of the products. The products are salt + water + CO₂, not just salt + CO₂.
6. Confusing neutralization with other reactions: Students describe neutralization as producing gas. True neutralization (acid + base) produces ONLY salt and water, no gas.
7. Poor explanations: When asked to “explain” observations, students merely describe what they see without stating the chemical reason. For example, saying “bubbles appear” without explaining that the bubbles are hydrogen gas produced from the reaction.
8. Wrong salt names: Forgetting to change acid names to salt names correctly (e.g., hydrochloric acid → chloride, not hydrochlorate; sulphuric acid → sulphate, not sulphurate).

Practice Questions

Multiple Choice Questions

1. When dilute hydrochloric acid is added to zinc metal, the gas evolved is:
   1. Oxygen
   2. Chlorine
   3. Hydrogen ✓
   4. Carbon dioxide

   Answer: C. Acids react with reactive metals to produce hydrogen gas.

2. Which of the following metals will NOT react with dilute tetraoxosulphate(VI) acid?
   1. Magnesium
   2. Zinc
   3. Iron
   4. Copper ✓

   Answer: D. Copper is below hydrogen in the electrochemical series, so it cannot displace hydrogen from acids.

3. The reaction between an acid and a base to produce salt and water only is called:
   1. Oxidation
   2. Neutralization ✓
   3. Decomposition
   4. Displacement

   Answer: B. Neutralization is the reaction between an acid and a base producing salt and water.

4. When hydrochloric acid reacts with calcium carbonate, the gas produced turns limewater:
   1. Blue
   2. Red
   3. Milky ✓
   4. Yellow

   Answer: C. Carbon dioxide produced turns limewater milky due to formation of calcium carbonate.

5. Which product is NOT formed when ethanoic acid reacts with sodium hydroxide?
   1. Water
   2. Sodium ethanoate
   3. Heat
   4. Carbon dioxide ✓

   Answer: D. Neutralization (acid + base) produces only salt and water, no gas.

Essay/Theory Questions

1. Describe what you would observe when dilute hydrochloric acid is added to: (a) magnesium ribbon (b) solid sodium carbonate. Write balanced chemical equations for each reaction. (10 marks)

   Answer Guide:

   (a) Magnesium ribbon reaction (5 marks):

   • Observation: Magnesium ribbon dissolves with vigorous effervescence/bubbling (1 mark). Colorless gas is produced which burns with a pop sound (1 mark). The solution becomes warm (1 mark).
   • Equation: 2HCl(aq) + Mg(s) → MgCl₂(aq) + H₂(g) (2 marks)

   (b) Sodium carbonate reaction (5 marks):

   • Observation: Sodium carbonate dissolves with effervescence (1 mark). Colorless, odorless gas is produced (1 mark). The gas turns limewater milky (1 mark).
   • Equation: 2HCl(aq) + Na₂CO₃(s) → 2NaCl(aq) + H₂O(l) + CO₂(g) (2 marks)

   Examiner’s tip: Always mention the gas test (pop sound for H₂, limewater test for CO₂) for full marks.

2. State three differences between the reaction of dilute acids with metals and the reaction of dilute acids with bases. (6 marks)

   Answer Guide:

   • Acid + metal produces hydrogen gas while acid + base produces no gas, only water (2 marks)
   • Acid + metal reaction can be violent and dangerous with very reactive metals while acid + base neutralization is safer (2 marks)
   • In acid + metal, the metal must be above hydrogen in the reactivity series, but acid + base works with all bases (2 marks)

   Alternative acceptable differences:

   • Acid + metal requires reactive metal only; acid + base works with any base
   • Acid + metal is a displacement reaction; acid + base is a neutralization reaction

   Examiner’s tip: Use “while” or “whereas” to show clear contrasts between the two reactions.

3. A student added dilute tetraoxosulphate(VI) acid to substance X. Effervescence occurred and a gas was produced which turned limewater milky.
   1. Identify substance X. (1 mark)
   2. Name the gas produced. (1 mark)
   3. Write a balanced chemical equation for the reaction, assuming X is calcium compound. (3 marks)
   4. State one everyday use of this type of reaction. (2 marks)

   Answer Guide:

   • (a) Substance X is a carbonate or hydrogen carbonate (1 mark)
   • (b) Carbon dioxide (1 mark)
   • (c) H₂SO₄(aq) + CaCO₃(s) → CaSO₄(aq) + H₂O(l) + CO₂(g) (3 marks – 1 mark for correct formulas, 1 mark for correct products, 1 mark for balancing)
   • (d) Removing limescale from kettles and bathroom tiles (1 mark for stating the use, 1 mark for brief explanation that acids dissolve calcium carbonate deposits)

   Other acceptable answers for (d): Using baking soda to neutralize bee stings; using vinegar to dissolve eggshells in science experiments.

   Examiner’s tip: When identifying unknown substances based on reactions, mention the class of compounds (carbonate) not just “substance X is calcium carbonate” – it could be any carbonate.

4. Explain why concentrated trioxonitrate(V) acid does not produce hydrogen gas when it reacts with copper, even though the reaction is vigorous. (4 marks)

   Answer Guide:

   • Copper is below hydrogen in the electrochemical series/reactivity series (1 mark)
   • Therefore, copper cannot displace hydrogen from acids under normal conditions (1 mark)
   • However, concentrated trioxonitrate(V) acid is a strong oxidizing agent (1 mark)
   • It oxidizes copper to copper(II) ions, producing nitrogen dioxide (NO₂) gas instead of hydrogen (1 mark)

   Expected equation (may earn extra credit): Cu(s) + 4HNO₃(conc) → Cu(NO₃)₂(aq) + 2NO₂(g) + 2H₂O(l)

   Examiner’s tip: This question tests deeper understanding. Mention both the reactivity series concept AND the oxidizing nature of concentrated nitric acid.

Memory Aids

Remember the four main reactions: MBCO

• Metals → Salt + Hydrogen (H₂)
• Bases → Salt + Water
• Carbonates → Salt + Water + Carbon dioxide (CO₂)
• Oxides (metal) → Salt + Water

Gas identification:

• “Pop goes hydrogen” – Hydrogen burns with a pop sound
• “Milky means CO₂” – Carbon dioxide turns limewater milky

Products pattern:

• If you see METAL → expect H₂ gas
• If you see CO₃ or HCO₃ → expect CO₂ gas
• If you see OH or oxide → expect water only (no gas)

Salt naming: “Change the acid ending”

• Hydrochloric acid → Chloride
• Sulphuric acid → Sulphate
• Nitric acid → Nitrate
• Ethanoic acid → Ethanoate

Reactivity reminder: “Please Send Charlie’s Mangy Alligator Zinc In Heaven”

• Potassium, Sodium, Calcium, Magnesium, Aluminium, Zinc, Iron, Hydrogen
• Only metals BEFORE hydrogen react with dilute acids

Related Topics

• Classification of Acids – Learn about different types of acids (strong/weak, organic/inorganic, mono/di/tribasic)
• pH Scale and Indicators – Understand how to measure acidity and track neutralization reactions
• Electrochemical Series – Study the reactivity order of metals with acids
• Salts and Salt Preparation – Explore methods of preparing different salts from acid reactions
• Bases and Alkalis – Learn about the substances that neutralize acids