Neutralisation and salt preparation methods
Why This Matters
# Neutralisation and Salt Preparation Methods ## Summary This lesson examines neutralisation reactions between acids and bases to form salts plus water, alongside practical methods for salt preparation including precipitation, titration, and direct combination. Students learn to select appropriate preparation methods based on salt solubility, write balanced equations for salt formation, and understand the practical techniques required for producing pure, dry salt samples. These concepts are fundamental to IGCSE Chemistry examinations, featuring prominently in both structured questions on ionic equations and practical assessments requiring accurate titration techniques and crystallisation procedures.
Key Words to Know
Core Concepts & Theory
Neutralisation is a chemical reaction between an acid and a base that produces a salt and water only. The general equation is:
Acid + Base → Salt + Water
A salt is an ionic compound formed when the hydrogen ion (H⁺) of an acid is replaced by a metal ion or ammonium ion (NH₄⁺). The type of salt produced depends on the acid and base used.
Key acid-base reactions:
-
Acid + Metal Oxide → Salt + Water
- Example: 2HCl + CuO → CuCl₂ + H₂O
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Acid + Metal Hydroxide → Salt + Water
- Example: H₂SO₄ + 2NaOH → Na₂SO₄ + 2H₂O
-
Acid + Metal Carbonate → Salt + Water + Carbon Dioxide
- Example: 2HNO₃ + CaCO₃ → Ca(NO₃)₂ + H₂O + CO₂
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Acid + Ammonia → Ammonium Salt
- Example: HCl + NH₃ → NH₄Cl
Salt nomenclature: The first part of the salt name comes from the metal/ammonium, the second part from the acid:
- Hydrochloric acid → chloride salts
- Sulfuric acid → sulfate salts
- Nitric acid → nitrate salts
Memory Aid (NACHO): Nitric-nitrate, Acidic-acid, Chloride-hydrochloric, Hydrochloride-HCl, Oxide-sulfuric-sulfate
pH changes: During neutralisation, pH moves toward 7 as H⁺ ions react with OH⁻ ions to form H₂O. This is an exothermic reaction, releasing heat energy.
Detailed Explanation with Real-World Examples
Real-world applications of neutralisation:
Agriculture: Farmers add calcium hydroxide (lime) or calcium carbonate to acidic soils to neutralise excess acidity, allowing crops to grow optimally. The reaction: Ca(OH)₂ + soil acids → calcium salts + water. Think of this like adding antacid medicine to your stomach—both neutralise excess acid!
Indigestion relief: Antacid tablets contain bases like magnesium hydroxide or calcium carbonate that neutralise excess hydrochloric acid in the stomach. The reaction Mg(OH)₂ + 2HCl → MgCl₂ + 2H₂O relieves discomfort immediately.
Wasp and bee stings: Wasp stings are alkaline, so vinegar (ethanoic acid) neutralises them. Bee stings are acidic, so baking soda (sodium hydrogencarbonate, a weak base) provides relief. This demonstrates the principle that acids neutralise bases and vice versa.
Industrial processes: The Haber process produces ammonia, which reacts with acids to make fertilisers like ammonium nitrate: NH₃ + HNO₃ → NH₄NO₃. These ammonium salts provide essential nitrogen for plant growth.
Analogy for understanding: Think of neutralisation like mixing hot and cold water. The acid (hot) and base (cold) combine to reach a neutral middle ground (lukewarm/pH 7). Just as hot + cold = warm + energy released, acid + base = salt + water + heat.
Environmental applications: Limestone slurry (calcium carbonate suspended in water) neutralises acidic gases like sulfur dioxide in power station flue gases, preventing acid rain formation.
Worked Examples & Step-by-Step Solutions
Example 1: Write a balanced equation for making copper(II) sulfate
Question: Copper(II) oxide reacts with dilute sulfuric acid. Write the balanced equation.
Step 1: Write the word equation Copper(II) oxide + Sulfuric acid → Copper(II) sulfate + Water
Step 2: Write formulae (use valency rules) CuO + H₂SO₄ → CuSO₄ + H₂O
Step 3: Balance the equation CuO + H₂SO₄ → CuSO₄ + H₂O (already balanced)
Examiner note: State symbols earn marks: (s), (aq), (l)
Example 2: Calculate mass of salt produced
Question: What mass of sodium chloride forms when 4.0 g of sodium hydroxide reacts with excess hydrochloric acid? (Ar: Na=23, O=16, H=1, Cl=35.5)
Step 1: Balanced equation NaOH + HCl → NaCl + H₂O
Step 2: Calculate moles of NaOH Mr(NaOH) = 23+16+1 = 40 moles = 4.0/40 = 0.1 mol
Step 3: Use mole ratio (1:1) moles NaCl = 0.1 mol
Step 4: Calculate mass Mr(NaCl) = 23+35.5 = 58.5 mass = 0.1 × 58.5 = 5.85 g
Example 3: Identify the salt
Question: Name the salt formed: zinc carbonate + nitric acid
Solution: Metal = zinc → zinc... / Acid = nitric → ...nitrate Answer: Zinc nitrate (+ water + carbon dioxide)
Command word "Name": Give the chemical name only, no formula needed.
Common Exam Mistakes & How to Avoid Them
Mistake 1: Forgetting products in carbonate reactions
Error: Writing "CaCO₃ + 2HCl → CaCl₂ + H₂O" (missing CO₂)
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Cambridge Exam Technique & Mark Scheme Tips
Command word mastery:
"State" (1 mark): Brief answer, no explanation. Example: "State the products of neutralis...
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Exam Tips
- 1.Always remember the general equation: Acid + Base → Salt + Water. This is your foundation!
- 2.When asked to describe salt preparation, specify whether the base is soluble (alkali) or insoluble, as this affects the method (e.g., using an indicator for soluble bases, or adding excess and filtering for insoluble bases).
- 3.Be precise with your terminology: 'evaporate to dryness' is often incorrect for pure crystal preparation; instead, 'heat gently until saturation/crystals appear, then cool slowly'.
- 4.Practise writing balanced chemical equations for different neutralisation reactions (e.g., acid + metal, acid + carbonate, acid + base).
- 5.Understand why each step in a salt preparation method is important (e.g., filtering removes unreacted solid, slow cooling produces larger crystals).