Chemistry

• State in general terms what determines the distance travelled by a spot in TLC. (1) ?

  solubility in mobile phase and retention by stationary phase
• conditions for cracking in general terms?

  high temperature, slight pressure
• the answer when calculating enthalpy of combustion is always negative
• what allows for stereoisomerism to occur?

  no rotation about the C = C double bond and two different groups attached to each carbon
• where is the positive charge placed in Friedel-crafts acylation?

  on the carbon

  in nitration of arenes?

  on the nitrogen
• what allows hydrogen bonds to form between H and O?

  electron deficient H attracts lone pair on O
• P= pascals

  v= m3

  t= K
• Kc > 1, equilibrium shifts to the right

  Kc < 1, equlibrum shifts to the left
• what is the bond angle in NH2- and why?

  104.5

  it has 4 areas of electron density and 2 lone pairs

  electrons will arrange themselves to minimise repulsion

  repulsion between lone pairs is greater than between lone pair and bonding pair
• how to quench a reaction?

  to measure concentration of a given substance at a given time in the raction, aliquots are taken. the reaction needs to be quenched. this can be done by:

  netralise

  remove catalyst

  immerse in the ice bath

  heavily dilute
• 1. Weigh the sample bottle containing the solid on a (2 dp) balance.
  2. Transfer solid to beaker and reweigh sample bottle.
  3. Record the difference in mass.
  4. Add distilled water and stir with a glass rod until all the solid has dissolved.
  5. Transfer to a volumetric flask with washings.
  6. Make up to the 250cm3 mark with distilled water.
  7. Stopper and invert/shake flask
• conditions for nuc.sub with kcn?

  aqueous alcohol
• reagent and conditions for reduction of nitriles?

  hydrogen and nickel
• why can't NaBH4 reduce alkenes?

  the C = C bond is electron rich so it repels the hydride ion
• how does CO2 cause global warming?

  the C = O bonds can absorb infrared radiation so infrared radiation emitted by earth cant escape
• sulfur trioxide is a simple molecule
• when defining an enthalpy change start with?

  the enthalpy change when

  or

  heat energy change when
• I2 is a black solid
• how to accurately measure mp?

  heat in a capillary tube in oil bath/ thiele tube

  slowly near melting point
• why enthoyl anydride vs ethanoyl chloride?

  hcl gas is toxic
• recrystallisation:

  min volume - saturated solution

  hot water - to increase yield

  filtered hot - to remove insoluble impurities

  cooled in ice - to increase amount of crystals formed

  wash with cold water - to remove soluble impurities
• why amino acids have different rf values?

  different polarities so different retention on the stationary phase
• reflux?


  A mixture of liquids is heated to boiling point for a prolonged time

  Vapour is formed which escapes from the liquid mixture, is changed back into liquid and returned to the liquid mixture