Definitions - Chemistry Unit 1 and 2

• Half-life


  time taken for half the atoms in a radioactive isotope to decay
• Standard 1st molar ionisation energy


  the energy required to remove one mole of gaseous atoms under standard conditions
• Relative Atomic mass


  average mass of one atom of the element relative to 1/12th of the mass of one atom of carbon-12
• Relative isotopic mass

  average mass of one atom of the isotope relative to 1/12th of the mass of one atom of carbon-12
• Relative formula mass, Mr


  Avg. mass of a formula relative to 1/12 the mass of one atom of carbon-12
• Relative molecular mass


  Avg. mass of a molecule relative to 1/12th the mass of one atom of carbon-12
• Empirical formula


  Simplest whole number ratio for each element in a compound
• Mole


  Amount of substance that contains the same number of particles as there are grams in exactly 12g of carbon-12.
• Ionic bond


  A chemical bond resulting from the attraction between oppositely charged ions.
• Coordinate bond


  A shared pair of electrons which has been provided by one of the bonding atoms only
• Electronegativity


  The ability of an atom to attract a bonding electron in a covalent bond
• VSEPR theory


  Valence-shell electron-pair repulsion theory

  Used to predict and explain the shapes of simple molecules/ions with 3+ atoms as they assume the position of minimum repulsion
• Linear is the VSEPR theory shape for 2 bonding pairs with a bond angle of 180°. Trigonal Planar is the shape for 3 bonding pairs and has a bond angle of 120°. If a molecules has 4 bonding pairs it assumes the tetrahedral shape with a bond angle of 109°. Finally if a molecule has 6 bonding pairs it has an octahedral shape with bond angles of 90°.
• Le Chatelier's Principle

  States that if a system at dynamic equilibrium experiences a change in its conditions then the equilibrium will shift to try and minimise change
• Dynamic equilibrium


  A reversible reaction where the rate of the forward reaction is equal to the rate of the backwards reaction.
• The molar enthalpy change of combustion means the enthalpy change when one mole of a substance is burned completely in excess oxygen under standard conditions.
• The standard molar enthalpy change of formation is the enthalpy change when one mole of a substance is formed from its constituent elements in their standard states, under standard conditions.
• Standard conditions are defined as 25°C (or 298K) under 1 atm of pressure and substances in standard states.
• The enthalpy change of reaction, $Δ rH$ is the amount of energy lost or gained during the reaction.
• Hess' Law states that the enthalpy change of the reaction is independent of the route of the reaction.
• A catalyst provides an alternate route of lower activation energy in order to speed up the rate of reaction.
• The key principles of green chemistry include;

  • Plants - they are biomass and are therefore carbon neutral. The carbon dioxide they absorb during their lifespans in photosynthesis makes up for the carbon dioxide produced in burning them for fuel.
  • Atom economy - An 100% atom economy means no waste product.

  • Toxic chemicals - using up toxic chemicals is a benefit however producing them is a drawback.
  • Energy consumption - higher temperatures and pressures use more energy (produced through fossil fuels). It is also more dangerous for the workers.
• Structural isomerism refers to compounds with the same molecular formula but a different structural formula.
• An electrophile is a species that accepts a pair of electrons from an electron dense region, e.g. electrophilic addition. While a nucleophile is a species that donates a pair of electrons to an electron deficient region, e.g. nucleophilic substitution.
• A radical is a species with a single unpaired electron. They are formed through homolytic fission where a covalent bond breaks evenly meaning each atom receives on electron each. The opposite of the is heterolytic fission where the covalent bond breaks unevenly and one atom receives both electrons while the other receives neither.
• E-Z isomerism is caused when both carbons in a carbon to carbon double bond have two different atoms or groups attatched. Double bonds consist of a sigma bond ($\sigma$) and a pi bond ($\pi$). The pi bond is formed from the side to side overlap of p-orbitals which forms two lobes which restrict rotation in the molecule.
• In E-Z isomerism, the E isomer is where the different groups or atoms are on different planes, the Z isomer is where the groups are in the same plane.
• An acid is an aqueous solution with a H+ donor. Weak acids partially dissociate in water and strong acids fully dissociate in water.
• Equations for finding energy from wavelength of light emissions;

  $E=$$hf$

  $f=$$c/\lambda$