Physical chemistry

• why do you need to ionise atoms before they can be analysed in a TOF mass spectrometer

  accelerated by electric field and creates current when hitting the detector
• how ions are formed in a TOF mass spectrometer

  1. bombarded by high energy electrons so sample loses an electron and becomes M+
  2. high voltage is applied and gains a proton forming MH+
• how is the TOF mass spectrometer able to separate two species to give two peaks.

  accelerated by an electric field to a constant kinetic energy. they will have the same kinetic energy but the lighter ion will have a faster velocity
• explain how permanent dipole-dipole forces arise

  a difference in electronegativity leads to bond polarity. attraction between ß+ on one molecule and ß- on another
• first ionisation energy 


  the energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms to form 1 mole of gaseous 1+ ions
• number of particles equation

  number of moles*avogadro’s constant
• standard solution

  1. place weighing boat on balance and weigh mass of solid. tip into beaker, reweigh and calculate difference in mass
  2. add distilled water to beaker and stir until solid has dissolved
  3. tip solution into volumetric flask
  4. rinse glassware washings into flask
  5. top up flask to 250cm3 and make sure bottom of miniscus is at the graduation mark
  6. stopper and invert
• percentage yield


  actual/theoretical * 100
• percentage yield uses


  tells you how wasteful a process is - how much product is lost
• atom economy uses 


  how wasteful the reaction is - desired product out of everything
• atom economy equation 


  Mr of desired product/Mr of all reactants * 100
• how do van der waals happen

  • electrons move which creates a temporary dipole
  • induces another temporary dipole in a neighbouring atom
  • 2 dipoles are attracted to each other
  • electrons move again so dipole is lost
• ice density 


  as liquid water cools, molecules make more hydrogen bonds and arrange themselves into a regular lattice structure. H2O molecules are further apart so ice is less dense than liquid water
• enthalpy change 


  the heat energy transferred in a reaction at constant pressure (kJ mol-1)
• standard enthalpy of formation

  the enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions
• standard enthalpy of combustion

  the enthalpy change when 1 mole of a substance is completely burned in oxygen under standard conditions
• enthalpy change equation 


  q=mcT then q/-1000 then divide by moles
• hess’s law 


  the total enthalpy change of a reaction is independent of the route taken
• area under maxwell-boltzmann curve

  equal to the total number of molecules
• peak of the maxwell-boltzmann curve

  represents the most likely energy of any single molecule
• where is the mean energy of all the particles on a maxwell-boltzmann

  on the right of the peak
• reaction rate 


  the change in concentration of a reaction or product over time
• why does a small increase in temperature lead to a large increase in reaction rate

  more molecules have at least the activation energy so react and molecules move faster so they collide more
• catalyst


  increases the rate of a reaction by providing an alternative reaction pathway with a lower activation energy and remains chemically unchanged
• reaction compromise

  • lower temps favour forwards reaction so better yield but means slower rate of reaction - compromise between reasonable yield and faster reaction
  • higher temps favour forward reaction so better yield but is expensive - compromise between reasonable yield and lowest possible cost
• why does temperature change Kc

  • if temp change leads to more product at equilibrium, Kc rises
  • if temp change leads to less product at equilibrium, Kc decreases
• oxidation state of oxygen in peroxides

  -1
• oxidation state of hydrogen in metal hydrides

  -1
• why is the value for enthalpy of combustion of a liquid alkane different to a value obtained


  alkane isn't gaseous
• what happens to particles when concentration decreases

  amount of gas has been reduced so particles are spread further apart
• why could enthalpy of combustion be less exothermic than actual value

  incomplete combustion
• enthalpy change 


  heat change at constant pressure
• disadvantages of using glass beer and tripod and gauze

  • glass is a poorer conductor
  • tripod and gauze would reduce heat transfer
• Explain how the experimental method and use of apparatus can be improved to provide more accurate data.

  Describe how this data from the improved method can be used to determine an accurate value for the temperature change

  • measure and record initial temp for a few mins before addition
  • measure and record temp after addition at regular intervals until trend is observed
  • plot a graph of temp against time
  • determine change in temp at point of addition
  • extrapolate
• how to reduce percentage uncertainty while using same apparatus

  increase concentration/mass