Chapter 4. Chemical Reactions

4.1  Introduction for Reactions

4.1.1

  1. Iron Nail Rusting (Chemical Change) – The reddish-brown coating is iron oxide, a new substance formed through oxidation, making this a chemical reaction.

  2. Sugar Dissolving in Tea (Physical Change) – The sugar molecules mix with the liquid but do not undergo a chemical transformation. Since no new substance is created, this is a physical change.

  3. Formation of a Yellow Precipitate (Chemical Change) – Mixing silver nitrate with potassium iodide results in silver iodide (AgI), an insoluble solid, which is a sign of a chemical reaction.

  4. Iodine Sublimation (Physical Change) – Iodine changes directly from solid to gas without altering its chemical structure. Since this is only a phase change, it is a physical change.

  5. Burning a Log (Chemical Change) – Combustion transforms wood into ash, carbon dioxide, and other gases, creating new substances and making it a chemical change.

Detailed explanation here.

4.2  Net Ionic Equations

4.2.1

4.2.2

4.2.3

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

Detailed explanation here.

H₂SO₄ (aq) + 2 NaOH (s) → Na₂SO₄ (aq) + 2 H₂O (l)

Detailed explanation here.

4 Al (s) + 3 O₂ (g) → 2 Al₂O₃ (s)

Detailed explanation here.

2 KI (aq) + Pb(NO₃)₂ (aq) → PbI₂ (s) + 2 KNO₃ (aq)

Detailed explanation here.

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)

Detailed explanation here.

Each element has the same number of atoms on both sides of the equation, demonstrating the Law of Conservation of Mass.

Detailed explanation here.

PART A: Balanced Chemical Equation

2Na₃PO₄(aq) + 3CaCl₂(aq) → Ca₃(PO₄)₂(s) + 6NaCl(aq)

Detailed explanation here.

PART B: Overall Ionic Equation

6Na⁺(aq) + 2PO₄³⁻(aq) + 3Ca²⁺(aq) + 6Cl⁻(aq) → Ca₃(PO₄)₂(s) + 6Na⁺(aq) + 6Cl⁻(aq)

Detailed explanation here.

PART C: Spectator Ions

Na⁺, Cl⁻

Detailed explanation here.

4.3  Representations of Reactions

4.3.1

The incorrect statement is (c): ❌ "The reaction produces two different products."

The diagram shows that only one type of product is formed, contradicting statement (c). Hence, this is the statement that does not correctly describe the reaction.

Detailed explanation here.

4.4  Physical and Chemical Changes

4.4.1

4.4.2

Physical Changes (No New Substances Formed)

  • H₂O (s) → H₂O (l) (Melting Ice):

    • Type of Change: Physical

    • Forces Affected: Intermolecular forces broken (Hydrogen bonding)

  • C₆H₁₂O₆ (s) → C₆H₁₂O₆ (aq) (Dissolving Sugar in Water):

    • Type of Change: Physical

    • Forces Affected: Intermolecular forces broken and formed (Hydrogen bonding with water)

Chemical Changes (New Substances Formed)

  • 2Na (s) + Cl₂ (g) → 2NaCl (s) (Formation of Sodium Chloride):

    • Type of Change: Chemical

    • Forces Affected: Intramolecular bonds broken (Na–Na and Cl–Cl), new ionic bonds formed

  • N₂ (g) + 3H₂ (g) → 2NH₃ (g) (Formation of Ammonia):

    • Type of Change: Chemical

    • Forces Affected: Intramolecular bonds broken (N≡N, H–H), new covalent bonds (N–H) formed

  • CaCO₃ (s) → CaO (s) + CO₂ (g) (Decomposition of Calcium Carbonate):

    • Type of Change: Chemical

    • Forces Affected: Intramolecular bonds broken and formed

Detailed explanation here.

Boiling Water (H₂O → H₂O gas): This is a physical change because no chemical bonds are broken, only hydrogen bonds (IMFs), and the water remains H₂O in a different phase.

Detailed explanation here.

Dissolving Sugar in Water (C₁₂H₂₂O₁₁ dissolves): A physical change occurs since no chemical bonds are broken, only intermolecular forces, and the sugar molecules remain intact but dispersed in water.

Detailed explanation here.

Melting Ice (H₂O solid → H₂O liquid): This is a physical change where hydrogen bonds (IMFs) are broken, but no chemical bonds are affected, and water remains H₂O.

Detailed explanation here.

Dissolving Calcium Chloride in Water (CaCl₂ dissolves, releasing heat): This is a physical change where ionic bonds in CaCl₂ break, forming Ca²⁺ and Cl⁻ ions in solution, but no new substances are created.

Detailed explanation here.

Sublimation of Dry Ice (CO₂ solid → CO₂ gas): A physical change occurs as only dispersion forces (IMFs) are broken, and CO₂ remains the same chemical substance, just in a different phase.

Detailed explanation here.

4.5  Stoichiometry

4.5.1

4.5.2

4.5.3

3.97 × 10²³ nitrogen atoms

This means that when 2.00 grams of hydrogen gas react with excess nitrogen gas, the reaction produces approximately 3.97 × 10²³ nitrogen atoms in the ammonia (NH₃) formed.

Detailed explanation here.

53.9 grams of H₂O

This means that when 34.0 grams of NH₃ react with excess O₂, the reaction produces approximately 53.9 grams of water (H₂O).

Detailed explanation here.

The theoretical yield of rust (Fe₂O₃) is 21.45 grams.

  • Iron (Fe) is the limiting reactant because it produces less Fe₂O₃ than oxygen does.

  • The maximum amount of rust (Fe₂O₃) that can form is 21.45 grams.

Detailed explanation here.

4.6  Introduction to Titration

4.6.1

PART A.

Determine the Volume of KOH Used = 0.03930 L

Detailed explanation here.

PART B.

Molarity of HF = 0.570 M

Detailed explanation here.

PART C. The calculated concentration of HF would be lower than the actual value due to the loss of HF before titration.

Detailed explanation here.

4.7  Types of Chemical Reactions

4.7.1

4.7.2

4.7.3

4.7.4

4.7.5

HSO₄⁻ acting as an acid:

HSO₄⁻ (aq) + H₂O (l) → SO₄²⁻ (aq) + H₃O⁺ (aq)

HSO₄⁻ acting as a base:

HSO₄⁻ (aq) + H₃O⁺ (aq) → H₂SO₄ (aq) + H₂O (l)

Detailed explanation here.

PART A. Zn (s) + CuSO₄ (aq) → ZnSO₄ (aq) + Cu (s)

Detailed explanation here.

PART B. Zinc starts at an oxidation state of 0 and increases to +2, meaning it is oxidized and loses 2 electrons. Copper(II) starts at +2 and decreases to 0, meaning it is reduced and gains 2 electrons. Sulfate remains unchanged as a spectator ion. The reaction involves the transfer of 2 electrons from Zn to Cu²⁺, confirming it is a redox process.

Detailed explanation here.

Mg (s) + 2 AgNO₃ (aq) → Mg(NO₃)₂ (aq) + 2 Ag (s)

In this redox reaction, magnesium (Mg) is oxidized, losing two electrons to form Mg²⁺, while silver ions (Ag⁺) are reduced, gaining electrons to form solid silver (Ag). Magnesium acts as the reducing agent, and silver ions act as the oxidizing agent. The nitrate ion (NO₃⁻) remains unchanged as a spectator ion. This reaction demonstrates electron transfer, where magnesium donates electrons, and silver ions accept them, leading to the formation of silver metal.

Detailed explanation here.

PART A.

Final oxidation numbers:

  • Carbon (C) = +4

  • Oxygen (O) = -2

Detailed explanation here.

PART B.

Final oxidation numbers:

  • Sulfur (S) = +6

  • Fluorine (F) = -1

Detailed explanation here.

PART C.

Final oxidation numbers:

  • Nitrogen (N) = +5

  • Oxygen (O) = -2

Detailed explanation here.

PART A. K₂SO₄ (aq) + BaCl₂ (aq) → BaSO₄ (s) + 2KCl (aq)

Precipitate: BaSO₄ (s)

Detailed explanation here.

PART B. Since both products remain soluble, no precipitate forms.

Detailed explanation here.

PART C. CaCl₂ (aq) + Na₂CO₃ (aq) → CaCO₃ (s) + 2NaCl (aq)

Precipitate: CaCO₃ (s)

Detailed explanation here.

4.8  Introduction to Acid-Base Reactions

4.8.1

4.8.2

4.8.3

Brønsted-Lowry Acid: H₂O (water) → donates H⁺
Brønsted-Lowry Base: NH₃ (ammonia) → accepts H⁺

Conjugate Acid: NH₄⁺ (ammonium ion)
Conjugate Base: OH⁻ (hydroxide ion)

Detailed explanation here.

First Ionization of H₂SO₄ (Strong Acid Behavior)

H₂SO₄(aq) + H₂O(l) → H₃O⁺(aq) + HSO₄⁻(aq)

Second Ionization of HSO₄⁻ (Weak Acid Behavior)

HSO₄⁻(aq) + H₂O(l) ⇌ H₃O⁺(aq) + SO₄²⁻(aq)

Water acts as a Brønsted-Lowry base in both steps by accepting H⁺ ions and forming H₃O⁺.

The oxygen atom in H₂O has lone pairs, which attract and stabilize the H⁺ protons, allowing the ionization process to occur efficiently.

Detailed explanation here.

PART A.

HBr is the stronger Brønsted-Lowry acid because it fully dissociates and has a more stable conjugate base (Br⁻).

Detailed explanation here.

PART B.

F⁻ is the stronger Brønsted-Lowry base because it is more electronegative and holds onto H⁺ more tightly compared to Cl⁻.

Detailed explanation here.

4.9  Oxidation-Reduction (Redox) Reactions

4.9.1

4.9.2

Cr₂O₇²⁻ + 14H⁺ + 6Fe²⁺ → 2Cr³⁺ + 7H₂O + 6Fe³⁺

Detailed explanation here.

3C₂O₄²⁻ + 2MnO₄⁻ + 8OH⁻ → 6CO₃²⁻ + 2MnO₂ + 4H₂O

Detailed explanation here.