IGCSE Chemistry 0620/42 October/November 2023 Study Notes

Atomic Structure and Electronic Configurations

• The following electronic configurations represent various atoms and ions:

  • A: $2,5$
  • B: $2,8$
  • C: $2,8,2$
  • D: $2,8,4$
  • E: $2,8,5$
  • F: $2,8,6$
  • G: $2,8,18,7$

• Identifications based on electronic configurations:

  • Noble gas atom: B (Neon, with a full outer shell of $8$ electrons).
  • Atom of an element in Group VI: F (Sulfur, having $6$ valence electrons).
  • Atom with an atomic number of 14: D (Silicon, where the total electrons $2 + 8 + 4 = 14$).
  • Atoms from the same group: A and E (Both have $5$ valence electrons, placing them in Group V).
  • Halogen atom: G (Chlorine/Bromine family, specifically having $7$ valence electrons).
  • An atom of an element which is a good conductor of electricity: C (Magnesium, a metal with $2$ valence electrons).
  • A stable ion of a Group V element: B (Group V atoms like A or E gain $3$ electrons to achieve the stable $2,8$ configuration).
  • An atom that forms an ion with a $2-$ charge: F (Needs to gain $2$ electrons to complete its outer shell).

Transition Elements: Cobalt and Copper

• Properties of Transition Elements:

  • They are significantly harder than Group I metals like Lithium.
  • They have much higher densities than Group I metals.
  • They possess significantly higher melting points and boiling points.

• Isotopes and Subatomic Particles:

  • Copper ($Cu$) has two naturally occurring isotopes: ${^{63}Cu}$ and ${^{65}Cu}$.
  • Cobalt ($Co$) has one naturally occurring isotope: ${^{59}Co}$.
  • Table 2.1: Particle Counts:
  • ${^{59}Co}$ atom: Protons = $27$, Neutrons = $32$ ($59 - 27$), Electrons = $27$.
  • ${^{65}Cu^{2+}}$ ion: Protons = $29$, Neutrons = $36$ ($65 - 29$), Electrons = $27$ ($29 - 2$).

• Relative Atomic Mass (RAM) Calculation:

  • Data: ${^{63}Cu}$ relative abundance = $70\%$, ${^{65}Cu}$ relative abundance = $30\%$.
  • Calculation: $\frac{(63 \times 70) + (65 \times 30)}{100} = \frac{4410 + 1950}{100} = 63.6$.
  • Relative atomic mass of copper = $63.6$.

• Water of Crystallisation:

  • Definition: The water molecules that are chemically bonded into the crystalline structure of a salt.
  • Hydrated Cobalt(II) chloride crystals:
  • Colour: Pink.
  • Formula: $CoCl_2 \cdot 6H_2O$.
  • Anhydrous Copper(II) sulfate water test:
  • Adding water causes a color change from white to blue.
  • Reversibility: This change can be reversed by heating the hydrated salt to evaporate the water.

Extraction of Iron in the Blast Furnace

• Starting Materials:

  • Coke (Carbon source).
  • Iron ore (Hematite).
  • Limestone (Calcium carbonate).
  • Hot Air (Oxygen source).

• Chemical Processes:

  • Main Ore: Hematite (contains $Fe_2O_3$).
  • Reduction Equation: $Fe_2O_3 + 3CO \rightarrow 3CO_2 + 2Fe$.
  • Oxidation Number Change: Iron changes from $+3$ in $Fe_2O_3$ to $0$ in $Fe$.
  • Reduction Explanation: The decrease in oxidation number (or the gain of electrons) confirms that the iron has been reduced.

• Removal of Impurities (Silicon(IV) oxide):

  • Equation 1: $CaCO_3 \rightarrow CaO + CO_2$ (Type: Thermal Decomposition).
  • Equation 2: $CaO + SiO_2 \rightarrow CaSiO_3$ (Type: Neutralization / Acid-Base reaction).

• Steel and Corrosion:

  • Alloy: A mixture of a metal with other elements (e.g., steel).
  • Stainless Steel: Made by mixing iron with Carbon and Chromium (or Nickel).
  • Rust: Chemically known as hydrated iron(III) oxide.
  • Protection Methods:
  • Galvanizing: The process of coating steel with Zinc.
  • Barrier Methods: Painting, greasing/oiling, or coating with plastic to prevent contact with water and oxygen.
  • Sacrificial Protection: Zinc is more reactive than iron. It loses electrons more readily and corrodes preferentially, protecting the underlying steel.

Lead(II) Chloride and Electrolysis

• Preparation of Insoluble Salt (PbCl2):

  • Soluble Salts: Lead(II) nitrate ($Pb(NO_3)_2$) and Sodium chloride ($NaCl$) (or Potassium chloride).
  • Ionic Equation: $Pb^{2+}(aq) + 2Cl^-(aq) \rightarrow PbCl_2(s)$.
  • Steps for Preparation:
  1. Mix the two aqueous solutions in a beaker.
  2. Filter the mixture to collect the precipitate.
  3. Wash the residue with distilled water to remove impurities.
  4. Dry the pure sample (e.g., in a warm oven or with filter paper).

• Electrolysis of Molten Lead(II) Chloride:

  • Conductivity: Must be molten so that the ions are mobile and free to move to the electrodes.
  • Anode Reaction (Oxidation): $2Cl^-(l) \rightarrow Cl_2(g) + 2e^-$.
  • Chlorine Test: Damp blue litmus paper turns red and then bleaches white.
  • Cathode Observation: Formation of a silvery liquid bead of molten Lead metal.

Chemical Energetics and Rates

• Enthalpy Change ($\Delta H$): The term used for the transfer of thermal energy during a reaction.

• Reaction Equilibrium: $CCl_4(g) + 2H_2O(g) \rightleftharpoons CO_2(g) + 4HCl(g)$ ($\Delta H = -130\,kJ/mol$).

• Collision Theory Application:

  • If concentration of $CCl_4$ increases, the rate of the forward reaction increases.
  • Explanation: There are more particles per unit volume, leading to a higher frequency of successful collisions.

• Pressure and Equilibrium:

  • Increasing pressure shifts the equilibrium to the left (reactant side).
  • Reason: The left side has fewer moles of gas ($3$ moles) compared to the right side ($5$ moles). The system moves to oppose the change by favoring the side with lower pressure.

• Activation Energy ($E_a$):

  • Definition: The minimum energy that colliding particles must possess for a reaction to occur.
  • Manipulation: The activation energy can be lowered by adding a catalyst.

• Bond Energy Calculation:

  • Bond Energies: $C-Cl = 340\,kJ/mol$, $H-O = 460\,kJ/mol$, $C=O = 805\,kJ/mol$.
  • Energy to break bonds: $(4 \times 340) + (4 \times 460) = 1360 + 1840 = 3200\,kJ$.
  • Energy released from $CO_2$ formation: $2 \times 805 = 1610\,kJ$.
  • Energy change formula: $\Delta H = \text{Energy In} - \text{Energy Out}$.
  • $-130 = 3200 - (1610 + 4(H-Cl))$
  • $1720 = 4(H-Cl)$
  • $H-Cl$ bond energy = $430\,kJ/mol$.

Organic Chemistry: Homologous Series

• Characteristics of a Homologous Series:

  1. Same general formula.
  2. Same functional group.
  3. Similar chemical properties.
  4. Subsequent members differ by a $-CH_2-$ unit.

• Examples:

  • Compound A ($C_{12}H_{24}$): Belongs to the Alkenes.
  • Compound B (Tetradecane): Belongs to the Alkanes.
  • Compound C ($C_3H_7COOH$): Belongs to the Carboxylic acids.
  • Name: Butanoic acid.
  • Structure: A chain of 4 carbons with the terminal carbon double-bonded to Oxygen and single-bonded to an $-OH$ group.

• Amino Acids and Polyamides:

  • Side Chain Calculation:
  • Base structure ($H_2N-CH-COOH$) mass: $N(14) + H(2) + C(12) + H(1) + C(12) + O(16) + O(16) + H(1) = 74$.
  • Total $M_r = 103$.
  • $R$ group mass = $103 - 74 = 29$.
  • Formula of $R$: $C_2H_5$ ($(2 \times 12) + 5 = 29$).
  • Proteins: The natural polyamides formed from amino acid monomers.