## Grade 12 Semester 1 HL Paper 1A & 1B Test Notes This document provides comprehensive notes for the Grade 12 Semester 1 HL Paper 1A & 1B test, covering key concepts and addressing common misconceptions. ### Atomic Structure and Periodicity **1. Hydrogen Emission Spectrum:** * **Question:** How are the lines in the emission spectrum of hydrogen produced? * **Correct Answer:** B. Electrons move to lower energy levels releasing photons. * Remember that emission spectra are produced when electrons *fall* from higher to lower energy levels, releasing energy as photons of light. * When an electron in a hydrogen atom transitions from the n=3 to n=2 energy level, it emits a photon of specific energy, corresponding to a specific line in the visible spectrum. * **Incorrect Answers:** A, C, and D describe absorption, not emission. **2. Ionization Energies:** * **Question:** The first three ionization energies for two elements, X and Y, are given. Which pair of elements represent X and Y? * **Concept:** Ionization energy increases across a period and decreases down a group. Large jumps in ionization energy indicate the removal of an electron from a new shell. * Analyze the differences between successive ionization energies. A large jump suggests the removal of an electron from a core shell. * If the ionization energies show a significant increase after the second ionization, it suggests the element has two valence electrons (like an alkaline earth metal). * The specific ionization energies would be needed to answer this question. **3. Electron Affinity:** * **Question:** Which change represents electron affinity? * **Correct Answer:** D. O⁻(g) + e⁻ → O²⁻(g) * Electron affinity is the energy change when a neutral atom gains an electron. * **Incorrect Answers:** A is bond dissociation, B is ionization energy, and C is a reduction reaction but not electron affinity. ### Coordination Chemistry **4. Color of Coordination Complexes:** * **Question:** [Cr(OH₂ )₆]³⁺ is violet and [Cr(NH₃)₆]³⁺ is yellow. What is correct? (The Color Wheel is provided) * **Concept:** The color of a coordination complex depends on the ligand field strength. Stronger ligands cause a larger energy gap between d-orbitals, resulting in absorption of higher energy (shorter wavelength) light. * Use the color wheel to determine the complementary color. The absorbed color is opposite the observed color. * If a complex absorbs blue light, it will appear orange (the complementary color). * The specific answer depends on the color wheel provided. **5. Effect of Stronger Ligand:** * **Question:** What is the effect of a stronger ligand? * **Answer:** Stronger ligands increase the crystal field splitting energy (Δ), leading to a larger energy difference between d-orbitals, resulting in absorption of higher energy light and a shift in color towards the blue end of the spectrum. **6. Oxidation State and Charge of Complex Ion:** * **Question:** What is the oxidation state of the metal ion and charge of the complex ion in [Co(NH₃)₄Cl₂]Cl? * **Answer:** The oxidation state of Co is +3, and the charge of the complex ion is +1. ### Bonding and Molecular Structure **7. Delocalized Electrons:** * **Question:** Which species has delocalized electrons? * **Correct Answer:** D. CO₃²⁻ (carbonate ion) * Delocalized electrons are spread over multiple atoms. * **Incorrect Answers:** OH⁻, H₂CO, and CO₂ do not have delocalized electrons. **8. Resonance Structures:** * **Question:** Which species does not have resonance structures? * **Correct Answer:** B. NH₄⁺ (ammonium ion) * Resonance structures occur when electrons can be distributed in multiple ways. * **Incorrect Answers:** C₆H₆, CO₃²⁻, and O₃ all exhibit resonance. **9. Sigma (σ) and Pi (π) Bonds in HCN:** * **Question:** How many sigma (σ) and pi (π) bonds are present in hydrogen cyanide, HCN? * **Answer:** 2 σ bonds and 2 π bonds. **10. Hybridization in Methanol:** * **Question:** What is the hybridization of carbon and oxygen in methanol? * **Answer:** Carbon is sp³ hybridized, and oxygen is sp³ hybridized. **11. Description of NO₂⁻:** * **Question:** Which combination best describes NO₂⁻? * **Answer:** This requires options to be provided. The description should include bent shape, resonance structures, and polar bonds. **12. Structure of SO₃:** * **Question:** Which is the correct structure of SO₃, based on the lowest formal charge? * **Answer:** This requires options to be provided. The correct structure minimizes formal charges on all atoms. **13. Percentage of Carbon by Mass:** * **Question:** Which compound has the highest percentage of carbon by mass? * **Answer:** This requires calculating the percentage composition of each compound (CH₄, C₂H₆, CO, CO₂). ### Stoichiometry and Chemical Calculations **14. Amount of NO Produced:** * **Question:** 6.00 mol of copper, Cu, are mixed with 12.00 mol of dilute nitric acid, HNO₃(aq). What is the amount, in mol, of nitrogen(II) oxide, NO, produced? * **Answer:** B. 4.00 mol. (Use stoichiometry to determine the limiting reactant and calculate the moles of NO produced.) **15. Volume-Temperature Relationship of Ideal Gas:** * **Question:** Which graph shows the correct relationship between the volume and temperature of an ideal gas at constant pressure? * **Answer:** A linear graph with a positive slope (Charles's Law: V ∝ T). **16. Relative Atomic Mass of Metal M:** * **Question:** Metal M reacts with 16.0 g of sulfur to produce 26.0 g of the compound MS₂. What is the relative atomic mass of M? * **Answer:** C. 20 (Use stoichiometry and the molar mass of sulfur to calculate the molar mass of M). **17. Formula for Enthalpy Change of Reaction:** * **Question:** What is the correct formula for calculating enthalpy change of reaction, ΔH°reaction? * **Correct Answer:** ΔH°reaction = ΣΔH°f(products) – ΣΔH°f(reactants) * Standard enthalpy change of reaction is the difference between the standard enthalpies of formation of products and reactants. **18. Standard Enthalpy Change Calculation:** * **Question:** Standard enthalpy changes of reaction are provided for three reactions. What is the standard enthalpy change of a fourth reaction? * **Answer:** This requires using Hess's Law to manipulate the given equations to obtain the target equation and then calculate the enthalpy change. **19. Lattice Enthalpy Calculation (Born-Haber Cycle):** * **Question:** Consider the Born-Haber cycle for the formation of sodium oxide. What is the lattice enthalpy? * **Answer:** This requires using the Born-Haber cycle to calculate the lattice enthalpy using the given values. ### Thermodynamics and Equilibrium **20. Gibbs Free Energy and Temperature:** * **Question:** The table shows the variation of standard Gibbs energy with temperature for a reversible reaction. What can be concluded about the reaction? * **Answer:** This requires analyzing the relationship between ΔG°, ΔH°, ΔS°, and temperature. **21. Temperature Dependence of Spontaneity:** * **Question:** Which reaction becomes more spontaneous as temperature increases? * **Answer:** A. CaCO₃(s) → CaO(s) + CO₂(g) (positive ΔS°) **22. Equilibrium Constant Calculation:** * **Question:** Kc for 2N₂O(g) ⇌ 2N₂(g) + O₂(g) is 7.3 × 10³⁴. What is Kc for N₂(g) + ½O₂(g) ⇌ N₂O(g)? * **Answer:** B. 1/√(7.3 × 10³⁴) (Manipulate the equilibrium expression). **23. Role of Catalyst:** * **Question:** Which statement describes a role that a catalyst might have in increasing the rate of reaction by providing an alternative mechanism? * **Correct Answer:** C. It increases the proportion of molecules colliding in the correct orientation. **24. Second-Order Reaction Graph:** * **Question:** Which graph represents a second-order reaction with respect to X? * **Answer:** A graph showing 1/[X] vs. time as a straight line. **25. Equilibrium Constant and Concentrations:** * **Question:** The value of Kc for the equilibrium between H₂(g) and I₂(g) is 51 at 720 K. Which combination is correct? * **Answer:** This requires options to be provided. The correct combination will use the equilibrium expression and the given Kc value. **26. Equilibrium Concentration Calculation:** * **Question:** At equilibrium, the concentrations of chlorine and iodine are both 0.02 mol dm⁻³. What is the concentration of iodine monochloride, ICl? * **Answer:** B. 454 × 0.02 (Use the equilibrium expression and the given Kc value). **27. Equilibrium Constant from Partial Pressures:** * **Question:** Iodine and bromine gases were mixed and allowed to reach equilibrium. What is the value of the equilibrium constant? * **Answer:** This requires partial pressures or concentrations to be provided. ### Acid-Base Equilibria and Titrations **28. Buffer Dilution:** * **Question:** When a 1.0 M NH₄⁺/NH₃ buffer (pH = 9.2) is diluted to 0.5 M with water, which statements are correct? * **Correct Answer:** A. I and II only (Dilution decreases buffer capacity but not pH). **29. Acid Strength Comparison:** * **Question:** At 298 K, the pKa of ethanoic acid is 4.76 and the pKa of chloroethanoic acid is 2.87. Which statement is correct? * **Correct Answer:** D. The ethanoate ion is a stronger base than the chloroethanoate ion. (Lower pKa indicates stronger acid, and stronger acid means weaker conjugate base). **30. Titration Curve Identification:** * **Question:** What type of titration is represented by the titration curve shown? * **Answer:** This requires the titration curve to be provided. **31. pKb Estimation from Titration Curve:** * **Question:** A weak base is titrated with a strong acid. Which value of pKb can be estimated from this titration curve? * **Answer:** This requires the titration curve to be provided. The pKb is estimated from the half-equivalence point. **32. Buffer Region in Weak Acid-Strong Base Titration:** * **Question:** Where is the buffer region for the titration of a weak acid with a strong base? * **Answer:** The buffer region is around the half-equivalence point. **33. Titration Experiment Analysis:** **(a.i) Uncertainty in Titrant Volumes:** * **Question:** Comment on the uncertainty recorded for the titrant volumes. * **Answer:** The uncertainty of ±0.05 cm³ is appropriate for a 25 cm³ burette. The significant figures in the recorded volumes should reflect this uncertainty. **(a.ii) Initial % Content of Fe²⁺:** * **Question:** Calculate the initial % content of Fe²⁺ in the raw spinach. * **Answer:** This requires the data from the table to be provided and then using stoichiometry to calculate the mass of Fe²⁺ and then the percentage. **(a.iii) Significance of Difference in Fe²⁺ Content (Day 4 and 5):** * **Question:** Comment on the significance of the difference in Fe²⁺ content measured for day 4 and 5. * **Answer:** This requires the data from the table to be provided and then comparing the values to determine if the difference is significant or within experimental error. **(b.i) Flaw in Experimental Design:** * **Question:** Suggest two flaws in the design that could have contributed to the random error. * **Answer:** Examples include: inconsistent spinach sample preparation (size of pieces), incomplete extraction of Fe²⁺ from spinach, and parallax error in burette readings. **(b.ii) Error from Not Standardizing KMnO₄:** * **Question:** Suggest what type of error this may have caused, giving your reasons. * **Answer:** This would cause a systematic error. If the KMnO₄ concentration is unknown, the calculated Fe²⁺ content will be inaccurate. All results would be affected proportionally.