Chemistry Year 12 - Exam Notes

Strand 1: General Chemistry

  • Uncertainty of measurements is the doubt that exists about the result of any measurement.

  • Uncertainty tells us about the quality of a particular measurement.

  • All measurements have a degree of uncertainty regardless of precision and accuracy.

  • Uncertainties in measurements are caused by systematic and random errors.

  • Common Metric Conversion:

    • Deci (d): 10^{-1}
    • Centi (c): 10^{-2}
    • Milli (m): 10^{-3}
    • Micro ($\mu$): 10^{-6}

  • Systematic errors:

    • Arise from incorrect procedure, incorrect use of instruments, or failure of some value to be what it is assumed to be.
    • Are consistent in magnitude and/or direction.
    • If the magnitude and direction of the error is known, accuracy can be improved by carrying out necessary corrections.

  • Random errors:

    • Also known as human error.
    • Is determined by the experimenter’s skill or ability to perform the experiment and read scientific measurements.
    • Vary from measurement to measurement.

  • General formula for conversions:
    Desired\ unit = Given\ unit \times \frac{Desired\ unit}{Given\ unit}

  • Dimensional analysis involves:

    • Finding appropriate conversion factors.
    • Multiplying values such that units cancel by having equal units in the numerator and the denominator.
    • Units on the top and bottom of an expression cancel out.

  • Accuracy: Closeness of a measured value to a standard (known or true) value.

  • Precision: Closeness of two or more measurements to each other.

  • Precision is independent of accuracy.

Strand 2: Investigating Matter

  • Electronegativity:

    • Measure of the strength of an atom to attract the shared pair of electrons in forming a covalent bond.
    • Increases across the period (left to right) and decreases down the group (for the first twenty elements).

  • Types of Forces:

    • Intramolecular forces:
      • Forces that hold atoms within a molecule.
      • Strong covalent bonds.
    • Intermolecular forces:
      • Forces that hold molecules together.
      • Generally much weaker than covalent bonds.

  • Discrete Molecular Solids:

    • Have simple structures.
    • Generally soft.
    • Molecules are held together by weak intermolecular forces (van der Waals forces).
    • Sublime easily or have low melting and boiling points because not much energy is required to break the weak intermolecular forces of attraction.
    • Do not conduct electricity because there are no ions or free-moving electrons.

  • Constructing Lewis Structures:

    1. Calculate the total number of valence electrons available for bonding by adding up all the valence electrons of individual atoms in the molecule.
    2. Identify the central atom and place all other atoms around it.
    3. Place a pair of electrons between all the atoms to form a bond.
    4. Distribute the remaining valence electrons to the atoms surrounding the central atom to satisfy the octet rule:
      • Subtract the number of electrons used to form the bonds from the total valence electrons.
      • Distribute the remaining electrons as pairs to all the atoms to satisfy the octet rule.
      • If the octet rule is still not satisfied on an atom, it suggests that a multiple bond is present.
      • To obtain a multiple bond, move one or two electron pairs from a surrounding atom to the bond connecting the central atom.

  • Carbon tetrachloride (CCl₄) is non-polar because its symmetrical tetrahedral geometry causes the dipole moments of the four polar carbon-chlorine bonds to cancel each other out.

  • To charge a plastic rod, rub it briskly on the sleeve of the shirt/blouse/hair or a piece of fur.

  • The deflection of polar liquids occurs due to the interactions between the induced dipoles in the liquid and the electric field produced by the charged rod.

Strand 3: Reactions

  • Oxidation is the increase in oxidation number.

  • Molar mass (M) is the mass of one mole of a substance.

  • One requirement for an effective collision in a reaction is that reactants must have sufficient energy to start the reaction.

  • Calculations for hydrated salt:

Mass\ of\ water = mass\ of\ hydrated\ salt - mass\ of\ anhydrous\ salt

  • Importance of performing three trial runs: To obtain concordant results/ results which agree with each other.

  • Molecular Formula Calculation:
    Molecular\ Formula = X(EF)
    X = \frac{mm}{em}
    where:

  • EF = Empirical Formula

  • mm = molecular mass

  • em = empirical mass

  • Le Chatelier’s Principle: If a change is applied to a system at dynamic equilibrium, the position of the equilibrium shifts to counteract the change and reestablish equilibrium.

    • Temperature:
      • An increase in temperature favors the endothermic reaction.
      • A decrease in temperature favors the exothermic reaction.

  • pH is a measure of how acidic or basic a substance is.

  • Activation Energy (Ea) is the minimum amount of energy required by the reactants to start up a chemical reaction; measured from the reactant energy level to the peak of the curve.

  • Enthalpy Change is the energy change occurring during a reaction.

    • Exothermic Reaction: Enthalpy change (\DeltaH) is negative since \DeltaH = H{products}- H{reactants}, \DeltaH < 0
    • Endothermic reaction: Enthalpy change (\DeltaH) is positive since \DeltaH = H{products}- H{reactants}, \DeltaH > 0

Strand 4: Materials

  • Most oxides of non-metallic elements are covalent and acidic.
  • Chemical Reactions of Alkenes and Alkynes:
    • The most characteristic reaction of alkenes and alkynes is addition to the carbon-carbon double bond or carbon to carbon triple bond.
    • The double bond is broken and in its place single bonds form to two new atoms or groups of atoms.
    • The triple bond is broken and in its place single bonds form to four new atoms or groups of atoms.

Strand 5: Consumer Chemistry

  • Insecticide: Harmful pesticides/chemicals that controls insects.
  • Herbicide: Harmful pesticides/chemicals that controls weed.
  • Condensation reaction: A chemical reaction whereby two or more reactants joined together to form a large molecule or loss of water molecule or water molecule becomes the product.
  • Hydrolysis reaction: A chemical reaction that involves the use of water to break down a large molecule into smaller molecule.
  • Amino acids consist of both an amine group (NH2) and a carboxyl group (COOH).
  • Detergents are not affected by hard water because the magnesium/calcium salts which forms from the reaction of detergents are soluble.
  • Soap do not work well in hard water as the calcium ions (Ca2+) and magnesium ions (Mg2+) causes the precipitation of the insoluble calcium and magnesium salts of the fatty acids known as ‘scum’. This prevents formation of lather.
  • Fats: normally solid at room temp/ high mpt/ more stable relatively more saturated
  • Oils: remains liquid at room temp/ low mpt/less stable and relatively more unsaturated.
  • The smoke point of various fats is important to note because a fat is no longer good for consumption after it has exceeded its smoke point and has begun to break down.