Unit 1 - Lesson 6: Unit 1 Comprehensive Review & Problem Solving
This capstone mastery review provides a rigorous synthesis of the foundational themes of Unit 1 in Prep 2 Science, sharpening analytical proficiency in element deduction, chemical equation balancing, and quantitative stoichiometric calculations. Students demonstrate mastery in determining exact periodic table coordinates from atomic configurations, calculating atomic numbers of adjacent elements in periods and groups, and mapping chemical reactivity trends. Comprehensive equation workshops systematically review metal and non-metal oxidation, alkaline hydroxide formation, and halogen displacements. Furthermore, volumetric stoichiometry in Hofmann voltameter electrolysis and the technological applications of industrial isotopes are reinforced, establishing absolute readiness for digital milestone examinations in alignment with national educational standards.
1. Element Position Deduction Strategies
Rules for locating elements and calculating atomic numbers:
- Period Number = Number of occupied principal energy levels in the neutral atom.
- Group Number (A-notation) = Number of outermost valence electrons (Group 1A to 7A; Group 18 for full shell).
- Element in the same period following immediately: Add
1to the atomic number. - Element in the same group following immediately: Add
8to the atomic number (or2if following Helium).
2. Master Chemical Equation Compendium
| Reaction Type | Balanced Symbolic Equation | Key Observations |
|---|---|---|
| Magnesium with Acid | Mg + 2HCl ➔ MgCl₂ + H₂ ↑ | Hydrogen gas evolved (pops with flame) |
| Magnesium Combustion | 2Mg + O₂ ➔(heat)➔ 2MgO | White basic oxide powder formed |
| Basic Oxide Dissolution | MgO + H₂O ➔ Mg(OH)₂ | Alkaline solution turns litmus blue |
| Carbon Combustion | C + O₂ ➔(heat)➔ CO₂ | Acidic oxide gas produced |
| Acidic Oxide Dissolution | CO₂ + H₂O ➔ H₂CO₃ | Carbonic acid turns litmus red |
| Sodium with Water | 2Na + 2H₂O ➔ 2NaOH + H₂ ↑ | Exothermic reaction with popping flame |
| Halogen Displacement | Cl₂ + 2KBr ➔ 2KCl + Br₂ | Chlorine displaces reddish bromine |
| Water Electrolysis | 2H₂O ➔(electricity/acid)➔ 2H₂ ↑ + O₂ ↑ | Cathode (H₂) is double Anode (O₂) |
3. Comparative Summary of Major Families
| Family | Block | Valency | Valence Shell | Chemical Reactivity Trend | Molecular Form |
|---|---|---|---|---|---|
| Alkali Metals (1A) | s-block (far left) | Monovalent (+1) | 1 electron | Highly reactive metals; reactivity increases down the group | Monoatomic solids |
| Halogens (7A) | p-block (far right) | Monovalent (-1) | 7 electrons | Highly reactive non-metals; reactivity decreases down the group | Diatomic (Cl₂, Br₂, I₂) |
| Noble Gases (18) | p-block (far right) | Zero | 8 electrons (He has 2) | Completely inert under normal conditions | Monoatomic gases |
Exercise 1: Successive Elements Problem
Problem: Element X lies in Period 2 and Group 4A. Find: (a) its atomic number | (b) atomic number of Y following it in the same period | (c) atomic number of Z following it in the same group.
• (a) Period 2 ⟹ K=2, L=4 ⟹ Atomic number of X = 2 + 4 = 6 (Carbon).
• (b) Y following in the same period: 6 + 1 = 7 (Nitrogen).
• (c) Z following in the same group: 6 + 8 = 14 (Silicon).
Exercise 2: Comprehensive Voltameter Gas Calculation
Problem: In Hofmann voltameter electrolysis, the total volume of both evolved gases combined was 30 cm³. Calculate the individual volumes of oxygen and hydrogen.
• Volume ratio of Hydrogen to Oxygen = 2 : 1.
• Total parts = 2 + 1 = 3 parts.
• Volume of 1 part (Oxygen at anode) = 30 ÷ 3 = 10 cm³.
• Volume of Hydrogen at cathode = 2 × 10 = 20 cm³.
Exercise 3: Displacement Feasibility
Problem: Does bromine react with potassium iodide solution? Write the equation and explain.
• Yes, because bromine precedes iodine in the halogen group and is more reactive.
• Equation: Br₂ + 2KI ➔ 2KBr + I₂.
Exercise 4: Distinguishing Solutions Experimentally
Problem: How can you practically differentiate between a solution formed by dissolving magnesium oxide and one formed by dissolving carbon dioxide?
• Add drops of violet litmus indicator to each:
• Magnesium oxide solution turns blue (alkaline magnesium hydroxide).
• Carbon dioxide solution turns red (acidic carbonic acid).
Exercise 5: Amphoteric Behavior Equations
Problem: State word equations illustrating why aluminum oxide Al₂O₃ is classified as an amphoteric oxide.
• Aluminum oxide + Hydrochloric acid ➔ Aluminum chloride salt + Water (acts as a base).
• Aluminum oxide + Sodium hydroxide ➔ Sodium aluminate salt + Water (acts as an acid).
Exercise 6: Density Transition at 4°C
Problem: Why does water density increase as temperature cools down to 4°C, but decrease below 4°C?
• Above 4°C, cooling causes thermal contraction, increasing density.
• Below 4°C, hydrogen bonding organizes molecules into spacious hexagonal ice crystals, expanding volume and lowering density.
Exercise 7: Identifying an Alkali Metal
Problem: An alkali metal lies in Period 3 and forms a chloride of formula MCl. What is its atomic number and oxide formula?
• Alkali metal in Period 3 ⟹ K=2, L=8, M=1 ⟹ Atomic number = 11 (Sodium).
• Since M is monovalent and O is divalent, its oxide formula is M₂O (Na₂O).
Exercise 8: Identifying a Halogen
Problem: A halogen element lies in Period 2. Calculate its atomic number and state its molecular formula in the free state.
• Halogen in Period 2 ⟹ 2 energy levels K=2, L=7 ⟹ Atomic number = 2 + 7 = 9 (Fluorine).
• Molecular formula in free state: F₂.
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