Module 5

• Equivalence point


  The volume of one solution that reacts excatly with the volume of the other solution
• End point of a titration


  When [HIn] = [In-] (colour is a mixture of two colours in indicator equation)
• Nitric acid formula


  HNO3
• Nitrous acid formula


  HNO2
• Zero order


  When the concentration of a reactant has no effect on the rate
• First order


  When the rate depends on its concentration raised to the power of one (if x doubles, y doubles)
• Second order


  When the rate depends on its concentration raised to the power of two (if x tripled, y increases by nine) x2
• Rate equation


  Rate = k [A]^m [B]^n
• rate constant (k)


  Number that mathematically converts between rate of reaction and concentrations
• Overall order of reaction


  Overall effect of the concentrations of all reactants on the rate of reaction (sum of powers in equation)
• Units for overall order = 0


  Mol dm-3 s-1
• Units for overall order = 1

  S-1
• Units for overall order = 2


  Dm3 mol-1 s-1
• Units for rate


  Mol dm-3 s-1
• Initial rate of reaction

  Rate when t = 0 (draw tangent to curve at t = 0)
• Two methods for continuous monitoring of reactions

  Gas collection

  Mass loss

  Colour change
• concentration time graph gradient

  rate of reaction
• zero order line on conc time graph

  straight line, negative gradient

  value of gradient = rate constant k
• first order line on a conc time graph

  downward curve with decreasing gradient over time

  time taken for conc of reactant to half is constant (confirm first order using half lives)
• second order line on a conc time graph

  downward curve with a steeper gradient at start which tails off more slowly

  hard to distinguish between first and second order so use value of half lives

  if constant its first order if they're not constant its second order
• half life (t1/2)

  time taken for half of a reactant to be used up
• exponential decay

  when first order reactions have a constant half life with the concentration of reactant halving every half life
• [x]0

  initial concentration of x when t = 0
• use can use a tangent on a conc-time graph to measure...

  rate
• how to calculate rate constant k from using tangents on a conc-time graph

  rearrange equation

  substitute value for rate and the conc at which this rate was measured
• k rate constant units

  s-1
• calculating k using half life equations

  k = ln2/t1/2
• why are rate conc graphs so important

  offer a route into the direct link between rate and conc in the rate equation
• zero order line on a rate conc graph

  horizontal straight line with zero gradient

  rate = k

  intercept on y axis gives k
• first order line on a rate conc graph

  straight line graph through origin

  rate directly proportional to conc

  rate constant = gradient of straight line
• second order line on a rate conc graph

  upward curve with increasing gradient

  rate constant cannot be obtained directly from graph

  plot second graph of rate against concentration squared - straight line through orgin

  gradient of this second graph is equal to rate constant k
• initial rate


  instantaneous rate at the start of a reaction when time = 0
• clock reaction


  more convenient way of obtaining initial rate of a reaction by taking single measurement

  time (t) from start of experiment is measured for a visual change to be observed e.g. colour/precipitate

  assumed that average rate of reaction over this time is equal to initial rate

  initial rate proportional to 1/t

  repeated several times at different concentrations and values of 1/t calculated
• iodine clock


  formation of iodine

  starch usually added as gives more distinct colour change from orange to black

  sodium thiosulfate Na2S2O3 removes iodine as it forms then when used up iodine forms

  graph of 1/t against conc plotted

  thiosulfate ions react with iodine forming I- ions which are not coloured when all thiosulphate reactions I- will build up in solution cauing colour change as i2 is formed
• accuracy of clock reactions


  in these reations you are measuring average rate during first part of the reaction

  over this time you can assume that average rate of reaction is constant and thus same as initial rate

  the shorter the period of time measured the less the rate changes and hence more accurate

  reasonably accurate providing less than 15% of reaction has taken place
• reaction mechanism


  series of steps that make up overall reaction
• rate determining step


  slowest step in a multi step reaction
• rules for rate equation/rate determining step

  rate equation only involves species involved in the rate determining step

  orders in the rate equation match the number of species involved in the rate determining step
• effect of temperature on rate constant

  as temp increases, rate increases and so k will increase

  each 10 degrees rise in temp rate constant doubles
• two factors which contribute to higher rate constant at higher temps

  Boltzmann distribution shifted to the right increasing proportion of particles that exceed Ea

  particles move faster and collide more frequently (higher ke) this factor is comparatively small compared with right shift so change in rate mainly governed by Ea