Chemistry

• Percentage yeild


  Actual mass/Theoretical mass (the mass in grams)
• Atom economy


  .

  
• Electronegativity


  The power of an atom to attract bonded electrons in a covalent bond towards itself
• Hesse's Law


  total enthalpy change for a reaction is independent of the route
• Standard enthalpy change of Formation 


  The enthalpy change that occurs when 1 mole of a substance is formed from its elements under standards conditions and with all substances in their standard states.
• Standard enthalpy change of combustion

  The enthalpy change that occurs when 1 mole of a compound reacts completely with oxygen under standard conditions where all reactants and products are in their standard states.
• Mean bond enthalpy


  Is the enthalpy change when 1 mole of covalent bonds are broken averages over the range of different compounds.
• Bond enthalpy


  Is the energy required to break one mole of a given covalent bond in the molecule in the gaseous state
• Enthalpy change 


  Change in heat measured at constant pressure
• 1st ionisation energy 


  The energy required to remove 1 mole of electrons from 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions
• Linear


  • 2 bonding pairs
  • 0 lone pairs
  • 180 degrees
• Trigonal planar


  • 3 bonding pairs
  • 0 lone pairs
  • 120 degrees
• Tetrahedral


  • 4 bonding pairs
  • 0 lone pairs
  • 109.5 degrees
• Trigonal pyramidal


  • 3 bonding pairs
  • 1 lone pair
  • 107 degrees
• Bent


  • 2 bonding pairs
  • 2 lone pairs
  • 104.5 degrees
• Trigonal bipyramidal


  • 5 bonding pairs
  • 0 lone pairs
  • 120 and 90 degrees
• Octahedral


  • 6 bonding pairs
  • 0 lone pairs
  • 90 degrees
• Fractional distillation

  Oil is pre-heated

  then passed into column.

  The fractions condense at different heights

  The temperature of column decreases upwards

  The separation depends on boiling point.

  Boiling point depends on size of molecules.

  The larger the molecule the larger the van der waals forces

  Similar molecules ( size, bp, mass) condense together

  Small molecules condense at the top at lower temperatures

  and big molecules condense at the bottom at higher temperatures.
• The mass spectrometer can be used to determine all the isotopes present in a sample of an element and to therefore identify elements
• It needs to be under a vacuum otherwise air particles would ionise and register on the detector
• Mass spectrometer essential steps

  1. Ionisation

  2. Acceleration

  3. Flight Tube

  4. Detection
• Electron impact ionisation


  • A vaporised sample is injected at low pressure
  • An electron gun fires high energy electrons at the sample
  • This knocks out an outer electron
  • Forming positive ions with different charges
• Electrospray ionisation


  • The sample is dissolved in a volatile, polar solvent
  • Injected through a fine needle giving a fine mist or aerosol
  • The tip of the needle has high voltage
  • At the tip of the needle the sample molecule, M, gains a proton, H+, from the solvent forming MH+
  • The solvent evaporates away while the MH+ ions move towards a negative plate
• • Electron impact is used for elements and substances with low formula mass
  • Electron Impact can cause larger organic molecules to fragment
• Electrospray ionisation is used preferably for larger organic molecules. The 'softer' conditions of this technique mean fragmentation does not occur
• Acceleration


  • Positive ions are accelerated by an electric field
  • To a constant kinetic energy
• Kinetic energy equation


  KE = 1/2 mv^2
• Relationship between mass and velocity

  Lighter particles have a faster velocity, and heavier particles have a slower velocity
• Flight Tube


  1. The positive ions with smaller m/z values will have the same kinetic energy as those with larger m/z and will move faster

  2. The heavier particles take longer to move through the drift area

  3. The ions are distinguished by different flight times
• Time of flight equation

  t = d/v
• Combining equations

  m/z = (d^2 * 2KE)/t^2
• Detection


  • The ions reach the detector and generate a small current, which is fed to a computer for analysis
  • The current is produced by electrons transferring from the detector to the positive ions
  • The size of the current is proportional to the abundance of the species
• Nucleophilic substitution reactions


  • Substitution: swapping a halogen atom for another atom or groups of atoms
  • Nucleophile: electron pair donator e.g. :OH, :NH, :CN-
• Nucleophilic substitution mechanism


  • The nucleophile attacks the positive carbon atom
  • A curly arrow shows the movement of two electrons
• Factors affecting rate of nucleophilic substitution

  • The rate depends on the strength of the C-X bond
  • The weaker the bond, the easier it is to break and the faster the reaction
  • Iodoalkanes are the fastest, fluoroalkanes are the slowest
• Hydrolysis of halogenoalkanes


  • Splitting of a halogenoalkane by reaction with water
  • Aqueous silver nitrate can be used to compare reactivity of halogenoalkanes
• Nucleophilic substitution with aqueous hydroxide ions

  1. Reagent: potassium (or sodium) hydroxide

  2. Conditions: In aqueous solution; Heat under reflux

  3. Mechanism: Nucleophilic Substitution

  4. Product: alcohol
• The aqueous conditions needed is an important point. If the solvent is changed to ethanol an elimination reaction occurs
• Nucleophilic substitution with tertiary halogenoalkanes


  1. The Br first breaks away from the halogenoalkane to form a carbocation intermediate

  2. The hydroxide nucleophile then attacks the positive carbon
• Nucleophilic substitution with cyanide ions

  1. Reagent: KCN dissolved in ethanol/water mixture

  2. Conditions: Heating under reflux

  3. Mechanism: Nucleophilic Substitution

  4. Product: nitrile