Chemistry Revision

• Covalent radius


  Half the distance between the nuclei of two bonded atoms
• Noble gases do not have a covalent radius because they do not exist as bonded atoms
• Atomic radius


  • Increases as you go down the group due to the increasing number of electron shells, the inner electrons shield the outer electrons meaning there is less of an attraction between the nucleus and the outer electrons
  • Decreases as you go along a period due to increasing number of protons which means there is a stronger attraction between nucleus and outer electrons pulling them in closer
• Electronegativity


  A measure of an atom's attraction for bonding electrons
• Electronegativity


  • Decreases as you go down a group as there are more energy levels meaning the screening effect is greater and there is less of an attraction between the positive nucleus and the outer electrons
  • Increases as you go across a period due to the increasing number of protons, this means there is a greater attraction between the nucleus and the bonding electrons
• Ionisation energy


  The energy required to remove one mole of electrons from one mole of gaseous atoms
• Ionisation energy


  • Decreases down the groups, this is because the outer electron is further away from the positive pull of the nucleus. There will be a greater screening effect as there are more shells and the inner electrons will screen the outer electrons from the positive nucleus
  • Increases across the period due to the increasing protons. This means there is a greater attraction between the nucleus and outer electrons
• Increasing nuclear charge across, screening effect (more energy levels) down
• Bonding continuum


  Range from pure covalent/non-polar to most ionic character
• Most ionic character = biggest difference in electronegativity, least ionic character = least difference
• Most covalent character = least difference in electronegativity, least covalent character = most difference
• Pure compounds with pure covalent bonds


  • Diatomic elements
  • Sulfur (S8)
  • Phosphorus (P4)
• Types of covalent bonding


  • Covalent molecular gases
  • Covalent network (carbon in form of graphite/diamond)
  • Covalent molecular solids (carbon in form of fullerene)
• Metallic bonding


  • Melting point and boiling point decrease as you go down the group as outer electrons are further away, therefore metallic bonding is weaker
• London dispersion forces


  • Electrostatic attractions between a temporary dipole and a temporary induced dipole due to the movement of electrons
  • Very weak, resulting in low melting and boiling points
  • Bigger atom/molecule, more electrons, stronger LDF, more energy, higher melting/boiling point
• Pure covalent molecules (diatomic elements, sulfur, phosphorus, fullerenes) and noble gases have only London dispersion forces
• Permanent dipole-permanent dipole forces


  Attraction between the positive dipoles of one molecule and the negative dipoles of another molecule due to asymmetrical charge and shape
• Hydrogen bonding


  A special type of dipole-dipole attraction between molecules, not a covalent bond, resulting from the attractive force between a hydrogen atom covalently bonded to a very electronegative atom such as N, O, or F
• Hydrogen bonding is the strongest intermolecular force but much weaker than covalent bonding
• Collision theory


  • For successful collisions, molecules need to collide with the correct orientation and energy greater than or equal to the activation energy
  • Increased temperature increases kinetic energy, moving the graph to the right and lowering the height
  • Adding a catalyst lowers the activation energy, moving the activation energy line to the left
  • Increasing concentration increases the number of molecules, increasing the height of the graph
• Reaction profile


  • The ΔH value is not changed when a catalyst is used, ΔH = products - reactants
  • Activation energy is the energy from the bottom of the hill to the top
  • The activated complex is the very brief stage at the point of maximum collision when some bonds are in the process of breaking while new bonds are forming
• Homologous series


  • Alkanes
  • Cycloalkanes
  • Alkenes
  • Alkynes
  • Alcohols
  • Carboxylic acids
  • Esters
  • Aldehydes
  • Ketones
  • Fats and oils
  • Amines
  • Amino acids
  • Proteins
  • Terpenes
• Be able to name and draw members of each homologous series, and know their functional groups by name and be able to draw them
• Functional groups


  • Ester link
  • Peptide/amide link
  • Amino/amine group
• As a general rule, if a molecule contains only C and H atoms it will be non-polar, while if it also contains O or/and N atoms it is more likely to be polar. However, the non-polarity of long hydrocarbon chains/rings can cancel the effect of a single polar group and make the molecule behave as non-polar.
• Ester formation


  1. Carboxylic acid + alcohol -> ester + water (condensation)

  2. Water + ester -> carboxylic acid + alcohol (hydrolysis)

  3. Butanoic acid + ethanol -> ethyl butanoate
• Making an ester


  • Use concentrated sulfuric acid as a catalyst to prevent reverse reaction
  • Use a condenser to cool down reactants
  • Use a base to neutralise excess acid
  • Use a water bath as alcohols are flammable
  • Observe an immiscible layer and strong scent
• Esters


  Low melting and boiling points due to weak intermolecular forces, not very soluble in water as no hydrogen bonding present
• Fats and oils


  • Made up of triglycerides (naturally occurring esters)
  • Fats are mostly saturated (C-C), can pack closer together so LDF is stronger, higher melting point, solid at room temp
  • Oils are mostly unsaturated (C=C), loosely packed so LDF is weaker, lower melting point, liquid at room temp
• Hydrogenation


  To increase the melting point of an oil, addition reactions between the double bonds and hydrogen can take place, reducing unsaturation and causing hardening
• Bromine test


  Can be used to determine the level of unsaturation in a fat or oil
• Proteins


  Large molecules our cells need to function properly, consist of amino acids, the structure and function of our bodies depend on them
• Amino acids are the building blocks of proteins, they have two functional groups and join through condensation polymerisation to make polypeptides (proteins)
• Essential amino acids are not produced by the body so need to come from the diet
• Enzymes


  Made of protein but will denature (change shape) above certain temperatures and can only work at certain pH values
• When proteins are heated


  Intermolecular bonds are broken, allowing the proteins to change shape (denature) and causing the texture to change
• Free radicals


  • Atoms or molecules that are highly reactive due to the presence of unpaired electrons
  • UV light can provide enough energy to break bonds within molecules, causing free radicals to form
  • Free radical scavengers are molecules that react with free radicals to form stable molecules and prevent chain reactions
• UV light can cause sunburn and speed up ageing of the skin, sun-block products prevent UV light reaching the skin
• Types of alcohols


  • Primary (OH on end of chain)
  • Secondary (OH within chain)
  • Tertiary (OH within chain attached to branched carbon)
• Oxidation of alcohols


  Primary alcohols oxidise to aldehydes, secondary alcohols oxidise to ketones, tertiary alcohols do not oxidise