Acids and Bases Chemistry

Introduction

• Acidic and basic are two extremes that describe chemical properties of substances.
• Mixing acids and bases can neutralize each other.
• A neutral substance is neither acidic nor basic.
• The pH scale measures how acidic or basic a substance is, ranging from 0 to 14.
• A pH of 7 is neutral.
• A pH less than 7 indicates an acidic substance.
• A pH greater than 7 indicates a basic substance.
• The pH scale is logarithmic; for pH values below 7, each whole pH value is ten times more acidic than the next higher value e.g., pH 4 is ten times more acidic than pH 5.
• For pH values above 7, each whole pH value is ten times more alkaline than the next lower value e.g., pH 10 is ten times more alkaline than pH 9.
• Pure water is neutral but mixtures with chemicals can become acidic or basic.
• Examples of acidic substances include vinegar and lemon juice.
• Examples of basic substances: lye, milk of magnesia, and ammonia.

Acids

• An acid donates hydrogen ions when dissolved in water.
• An acid is a proton (H⁺) donor.
• Example reaction: HCl (aq) → H⁺(aq) + Cl⁻(aq).
• Hydrochloric acid (HCl) is found in gastric juice.
• Carbonic acid (H₂CO₃) is found in soft drinks.
• Ascorbic acid (vitamin C) is found in lemons and many fruits.
• Citric acid is found in oranges and lemons.
• Acetic acid is found in vinegar.
• Tannic acid is found in tea.
• Nitric acid (HNO₃) and Sulphuric acid (H₂SO₄) are used in laboratories.

Bases

• A base accepts hydrogen ions.
• A base is a proton (H⁺) acceptor.
• A base dissolved in water is referred to as an Alkali, liberating hydroxide ions (OH⁻).
• Example reaction: NaOH (aq) → Na⁺(aq) + OH⁻(aq).
• The term 'alkali' is often used for water-soluble bases.
• Sodium hydroxide (NaOH), or caustic soda, is used in washing soaps.
• Potassium hydroxide (KOH), or potash, is used in bathing soaps.
• Calcium hydroxide (Ca(OH)₂) is used in white wash (lime water).
• Magnesium hydroxide (Mg(OH)₂), or milk of magnesia, is used to control acidity.
• Ammonium hydroxide (NH₄OH) is used in hair dyes.

Reactions of Acids

• Acid + Metal = Salt + H₂
• Acid + Metal Hydroxide = Salt + H₂O
• Acid + Metal Oxide = Salt + H₂O
• Acid + Metal Carbonate = Salt + H₂O + CO₂
• Acid + Metal Hydrogen Carbonate = Salt + H₂O + CO₂
• Acidic Oxide + Base = Salt + H₂O

Basicity of Acids

• Basicity is the number of hydrogen ions produced per molecule of acid when dissolved in water.
• Monobasic acids produce one H⁺ ion.
• Dibasic acids produce two H⁺ ions.
• Tribasic acids produce three H⁺ ions.

Acid Anhydrides

• Acid anhydrides are compounds reacting with water to form an acid, generally non-metal oxides.
• Oxides of period three elements show periodicity with respect to acidity, becoming more acidic across the period.
• Sodium and magnesium oxides are alkaline.
• Aluminium oxides are amphoteric, reacting as both a base and an acid.
• Silicon, phosphorus, sulfur, and chlorine oxides are acidic.
• Nitrous oxide (N₂O) and carbon monoxide (CO) do not display acid/base characteristics.
• Carbon dioxide is the anhydride of carbonic acid: CO₂(g) + H₂O(l) → H₂CO₃ (aq).
• Sulfur dioxide reacts to form sulfurous acid, a weak acid: SO₂(g) + H₂O (I)→ H₂SO₃ (aq).
• Sulfur trioxide forms sulfuric acid, a strong acid: SO₃(g) + H₂O(l) → H₂SO₄ (aq).

Properties of Acids and Bases

Properties of Acids

• Release hydrogen ions in water.
• Acids feel like water.
• Acids taste sour/tart.
• pH < 7.
• Acids turn blue litmus red.
• Acids conduct electricity.

Properties of Bases

• Release hydroxide ions into water.
• Bases feel slippery to the touch.
• Bases taste bitter.
• pH > 7.
• Bases turn red litmus blue.
• Alkalis conduct electricity.

Strength of Acids and Alkalis

• The strength depends on the degree of ionization in water.
• A strong acid is fully ionized, with high concentration of H⁺ ions; examples include Hydrochloric acid (HCl), sulfuric acid (H₂SO₄), and nitric acid (HNO₃).
  • Example reaction: HCl(aq) → H⁺(aq) + Cl⁻(aq).
• A weak acid is only partially ionized, resulting in a low concentration of H⁺ ions; example is ethanoic acid (CH₃COOH).
  • Example reaction: CH₃COOH(aq) ⇌ CH₃COO⁻(aq) + H⁺(aq).
• A strong alkali is fully ionized, with a high concentration of OH⁻ ions; Potassium hydroxide (KOH) and sodium hydroxide (NaOH) are strong alkalis.
  • Example reaction: NaOH(aq) → Na⁺(aq) + OH⁻(aq).
• A weak alkali is partially ionized, with a low concentration of OH⁻ ions; ammonia (NH₃) is an example.
  • Example reaction: NH₃(g) + H₂O(l) ⇌ NH₄⁺(aq) + OH⁻(aq).

Strong Acids

• Acids that completely dissociate in water are called strong acids; nitric acid is one example.
• Reaction: HNO₃(aq) → H⁺(aq) + NO₃¯(aq)
• There are only seven strong acids:
• HCl (Hydrochloric Acid)
• HBr (Hydrobromic Acid)
• HI (Hydroiodic Acid)
• HClO₄ (Perchloric Acid)
• HClO₃ (Chloric Acid)
• H₂SO₄ (Sulphuric Acid)
• HNO₃ (Nitric Acid)

Weak Acids

• Acids that partially dissociate in water are called weak acids.
• Organic acids like acetic acid and some inorganic acids are weak acids.
• Since dissociation is only partially, it is depicted by double half arrows
• Reaction: HF(aq) ⇌ H⁺(aq) + F⁻(aq)
• The double arrows indicates that the aqueous solution of hydrofluoric acid not only contains H⁺ (aq) and F⁻(aq) ions but also the undissociated acid HF(aq).
• Examples:
• CH₃COOH Ethanoic (acetic) acid
• HF Hydrofluoric acid
• HCN Hydrocynic acid
• C₆H₅COOH Benzoic acid

Type of oxides

• Oxides can be metallic or non-metallic.
• Metallic oxides can be basic (e.g., Na₂O, CaO) or amphoteric (e.g., ZnO, Al₂O₃).
• Non-metallic oxides can be acidic (e.g., CO₂, SO₂) or neutral (e.g., H₂O, CO).

Indicators

• Indicators show different colors in acidic and basic mediums.
• Litmus is a natural dye that turns red in acidic solutions and blue in basic solutions.
• Phenolphthalein and methyl orange are other indicators.

Salts

• A salt is a compound formed when some or all hydrogen ions in an acid are replaced by metal or ammonium ions.
• The metal or ammonium ions can come from the metal itself, a base, a carbonate, or a hydrogencarbonate.
• Salts are classified into normal and acid salts.

Classification of Salts

• Normal salts form when all H⁺ ions in an acid are replaced, e.g., potassium sulfate.
  • Example reaction: 2KOH(aq) + H₂SO₄(aq) → K₂SO₄(aq) + 2H₂O(I)
• Acid salts form when H⁺ ions in an acid are only partially replaced, e.g., potassium hydrogensulfate.
  • Example reaction: KOH(aq) + H₂SO₄(aq) → KHSO₄(aq) + H₂O(I)
• Only dibasic and tribasic acids can form acid salts.
• A normal salt is produced when 2 mol of potassium hydroxide reacts with 1 mol of sulfuric acid.
• An acid salt is produced when 1 mol of potassium hydroxide reacts with 1 mol of sulfuric acid.

Methods of Preparing Salts

• To determine the method of preparation, it is necessary to know if the salt is soluble in water.
• The decision tree involves reacting with acids, precipitation reactions, and choosing the starting material.
• Soluble salts use titration or reaction with acids (metal, base, carbonate).
• Insoluble salts use precipitation reaction with acids using insoluble substances.
• Titration involves acid-alkali or acid-carbonate solutions.
• Reactions with insoluble substances use excess metal, excess insoluble base, or insoluble carbonate.
• Key steps: concentrating, crystallizing, and filtering the salt solution to obtain salt crystals.

Precipitation Method Steps

• Mix two solutions of soluble substances in a beaker (Step 1).
• Filter the precipitate (Step 2).
• Wash the filtered precipitate with deionized water (Step 3).
• Scrape the insoluble salt off the filter paper into a dish and dry it in an oven (Step 4).