Acids and Bases: Double Displacement and Neutralisation

Double Displacement Reactions

  • Definition: Double displacement reactions involve the exchange of partners in a chemical reaction.
      - General Form: AB + CD → AD + BC

  • Common Types of Double Displacement Reactions:
      - Neutralisation
      - Precipitation

Neutralisation

  • Definition: Neutralisation is a type of double displacement reaction where an acid reacts with a base to form salt and water.
      - Importance of Neutralisation: It stops the corrosive properties of acids or bases, effectively "neutralising" their effects.

Bases

  • Definition: Bases are substances that have a bitter taste and feel slippery or soapy to the touch.

  • Characteristics of Bases:
      - Corrosiveness: Some bases are very corrosive (e.g., caustic soda).
      - Applications: Used in cleaning agents like soap, shampoo, toothpaste, and more.

  • Alkalis: Bases that are soluble in water.
      - Examples of Strong Bases:
        - Potassium hydroxide (KOH)
        - Sodium hydroxide (NaOH)
        - Barium hydroxide (Ba(OH)2)
      - Examples of Weak Bases:
        - Ammonia (NH3)
        - Calcium carbonate (CaCO3)
        - Sodium carbonate (Na2CO3)

  • Measurement of Strength: The strength of an acid or base is measured using the pH scale. Strong bases react more vigorously than weak bases.

pH Scale

  • Range: The pH scale ranges from 0 to 14, reflecting the concentration of hydrogen ions in a solution.
      - Low pH (0-7): Indicates an acidic solution.
      - pH 7: Neutral solution.
      - High pH (7-14): Indicates a basic solution.

Neutralisation Reactions and Their Products

  • When an acid reacts with a base, the products formed are:
      - A salt
      - Water

  • Such reactions often occur in solution.

Common Acids and Their Uses

Acid

Uses

Hydrochloric acid (HCl)

Cleaning surfaces of iron during manufacture, food processing, chemical manufacture, oil recovery

Nitric acid (HNO3)

Manufacture of fertilizers, dyes, drugs, and explosives

Sulfuric acid (H2SO4)

Manufacture of fertilizers, plastics, paints, drugs, detergents, and paper; petroleum refining and metallurgy

Citric acid (C6H8O7)

Used in the food industry; present in citrus fruits

Carbonic acid (H2CO3)

Present in fizzy drinks, formed when CO2 dissolves in water

Ethanoic acid (CH3COOH)

Found in vinegar; used in chemical production, including aspirin

Common Bases and Their Uses

Base

Uses

Sodium hydroxide (NaOH)

Manufacture of soap, cleaning agent

Ammonia (NH3)

Manufacture of fertilizers and cleaning agents

Sodium bicarbonate (NaHCO3)

Used in baking to make cakes rise

Example Reactions

  • Stomach Acid Example:
      - Hydrochloric acid in the stomach aids digestion. Excess acid may require antacids like milk of magnesia (contains magnesium oxide).
      - Chemical Reaction:
        - extMgO(s)+2HCl(aq)→MgCl2(aq)+extH2extO(l)ext{MgO(s) + 2HCl(aq) → MgCl}_2(aq) + ext{H}_2 ext{O}(l)

  • Bicarbonate Reaction:
      - Sodium bicarbonate reacts with hydrochloric acid to produce carbon dioxide, salt, and water.
      - Chemical Reaction:
        - extNaHCO3(s)+extHCl(aq)→NaCl(aq)+CO2(g)+extH2extO(l)ext{NaHCO}_3(s) + ext{HCl(aq) → NaCl(aq) + CO}_2(g) + ext{H}_2 ext{O}(l)

Summary of Neutralisation Reactions

  • General Forms:
      - Metal hydroxide + acid → salt + water
      - Metal oxide + acid → salt + water
      - Metal carbonate + acid → salt + carbon dioxide + water
      - Metal hydrogen carbonate + acid → salt + carbon dioxide + water

Base

Acid

Resulting Salt

Salt Formula

Sodium hydroxide

Sulfuric acid

Sodium sulfate

Na2SO4

Magnesium oxide

Hydrochloric acid

Magnesium chloride

MgCl2

Sodium oxide

Ethanoic acid

Sodium ethanoate

CH3COONa

Copper(II) oxide

Nitric acid

Copper(II) nitrate

Cu(NO3)2

Case Study: Different Types of Salts

  • Not all salts are edible like table salt (sodium chloride).
      - Characteristics: Many salts can be brightly colored and some are highly poisonous.
      - Examples of Color:
        - Copper salts are blue.
        - Nickel salts are pale green.
        - Chromium salts can be green or orange.
        - Cobalt salts are pink.
        - Manganese salts are black.

Handling Spills in Laboratory

  • Safety Procedures: Accidental acid or base spills should be reported immediately.
      - Methods of Cleanup: Could involve neutralisers, absorption pads, or mops.
      - Warning: Do not attempt to neutralise a strong acid with a base due to the risk of violent reactions.

Indicators

  • Purpose: Indicators help determine whether a solution is acidic or basic by changing colors.
      - Common Indicators:
        - Universal indicator: Turns red in acidic conditions and blue in basic conditions.
        - Bromothymol blue: Turns yellow in acid and blue/purple in base.

  • pH Scale:
      - The scale ranges from 0 (most acidic) to 14 (most basic). Neutral pH is 7. The color changes indicate the strength of an acid or base:
        - Strong acid: pH around 0-1.
        - Neutral: pH 7.
        - Strong base: pH around 13-14.

Investigation: Exploring Neutralisation Reactions

  • Aim: To identify the products of a reaction between an acid and a base.

Extension: Strength vs. Concentration

  • Definitions:
      - Strength: Refers to a substance's ability to fully dissociate into ions in water (strong vs. weak).
      - Concentration: Measures the amount of active substance in a solution (concentrated vs. dilute).
      - Strong Acid Example: Hydrochloric acid (HCl) - fully dissociates as extH++extCl−ext{H}^+ + ext{Cl}^-.
      - Strong Base Example: Sodium hydroxide (NaOH) - fully dissociates as extNa++extOH−ext{Na}^+ + ext{OH}^-.

  • Illustration: Figure displaying concentrated and dilute strong and weak acids.