Acids/bases & reactions

Acids are substances that produce hydrogen ions (H⁺) when dissolved in water. Bases are substances that neutralize acids, while alkalis are soluble bases that produce hydroxide ions (OH⁻) in solution.

The pH Scale ranges from 0-14, measuring hydrogen ion concentration. pH < 7 indicates acidic solutions, pH = 7 is neutral, and pH > 7 indicates alkaline solutions. Each pH unit represents a tenfold change in H⁺ concentration.

Indicators change color to show pH: litmus (red in acid, blue in alkali), methyl orange (red in acid, yellow in alkali), and phenolphthalein (colorless in acid, pink in alkali). Universal indicator shows a range of colors across the entire pH scale.

Key Acid-Base Reactions:

1. Neutralization: Acid + Base → Salt + Water

   • Example: HCl + NaOH → NaCl + H₂O

2. Acid + Metal: Acid + Metal → Salt + Hydrogen

   • Example: Mg + H₂SO₄ → MgSO₄ + H₂

3. Acid + Carbonate: Acid + Carbonate → Salt + Water + Carbon dioxide

   • Example: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

4. Acid + Metal Oxide: Acid + Metal Oxide → Salt + Water

   • Example: CuO + H₂SO₄ → CuSO₄ + H₂O

Salts are ionic compounds formed when the H⁺ of an acid is replaced by a metal ion or ammonium ion (NH₄⁺). The salt's name derives from the metal and the acid used (hydrochloric acid → chloride, sulfuric acid → sulfate, nitric acid → nitrate).

Understanding acids and bases transforms everyday experiences into chemistry lessons. Stomach acid (hydrochloric acid, pH 1-2) digests food but causes heartburn when excessive. Antacids like Gaviscon contain bases (magnesium hydroxide or calcium carbonate) that neutralize excess acid, demonstrating practical neutralization.

Agricultural chemistry relies heavily on pH management. Farmers test soil pH because plants have optimal ranges: blueberries thrive in acidic soil (pH 4.5-5.5), while asparagus prefers alkaline conditions (pH 7.5-8). Adding lime (calcium carbonate) raises pH in acidic soils, while sulfur lowers pH in alkaline soils—both demonstrating acid-base chemistry in action.

Think of acids as "hydrogen donors" and bases as "hydrogen acceptors"—this mental model helps predict reaction outcomes.

Industrial applications are extensive: the Haber process uses ammonia (a base) in fertilizer production, feeding billions. Sulfuric acid is the world's most-produced chemical, essential for manufacturing fertilizers, detergents, and batteries. Car batteries contain sulfuric acid, while battery corrosion (white powder on terminals) results from acid reacting with metal.

Environmental chemistry involves acid rain (pH < 5.6), formed when sulfur dioxide and nitrogen oxides from fossil fuel combustion dissolve in atmospheric water. This damages limestone buildings and statues through the carbonate reaction, demonstrating how abstract equations manifest in heritage destruction.

Bee stings are acidic while wasp stings are alkaline—explaining why baking soda (sodium bicarbonate, a base) neutralizes bee stings but vinegar (acetic acid) treats wasp stings. This everyday first-aid practice perfectly illustrates neutralization chemistry.

Example 1: Neutralization Calculation

Question: Calculate the mass of calcium carbonate needed to neutralize 100 cm³ of 2 mol/dm³ hydrochloric acid.

Solution:

Step 1: Write the balanced equation CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂

Step 2: Calculate moles of HCl moles = concentration × volume (dm³) = 2 × 0.1 = 0.2 mol

Step 3: Use stoichiometry (1 CaCO₃ : 2 HCl) moles CaCO₃ = 0.2 ÷ 2 = 0.1 mol

Step 4: Calculate mass (Mr of CaCO₃ = 100) mass = moles × Mr = 0.1 × 100 = 10 g

Examiner note: Always show the balanced equation first (1 mark) and unit conversions clearly.

Example 2: Identifying Products

Question: Zinc oxide reacts with sulfuric acid. Name the products and write the balanced equation.

Solution:

Step 1: Identify reaction type: acid + metal oxide Pattern: Salt + water only (no gas)

Step 2: Name the salt

• Metal from oxide: zinc → Zn²⁺
• Acid gives: sulfuric acid → sulfate (SO₄²⁻)
• Salt name: zinc sulfate

Step 3: Write equation ZnO + H₂SO₄ → ZnSO₄ + H₂O

Examiner note: Metal oxide reactions NEVER produce gas—only salt and water.

Example 3: Gas Testing

Question: Describe how to test the gas produced when magnesium carbonate reacts with nitric acid.

Solution:

MgCO₃ + 2HNO₃ → Mg(NO₃)₂ + H₂O + CO₂

Test: Bubble gas through limewater Positive result: Limewater turns milky/cloudy Identifies: Carbon dioxide

Examiner note: Use precise language—"cloudy" scores marks, "changes color" doesn't.