Topic 11 Flashcards
Rate of reaction
change in concentration of a substance in unit time
How to find the rate of reaction with concentration time graph
Gradient
Measure change in volume of gas (when use and how)
when there is change in number of moles of gas
use a syringe to measure volume or the water trough
Measure change of mass (when use and how)
when there is gas produced
use a measuring balance
Titrating samples of reaction mixture with acid/alkali/sodium thiosulphate (how)
small samples are removed from reaction quenched and titrated with a suitable reagent
Colourimetry (when and how)
if one of the reactants or products is coloured
measure the change in colour of reacting mixture
Measuring change in electrical conductivity (when)
When there is a change in the number of ions in reaction mixture
Measuring of optical activity (when and how)
change in optical activity through reaction
use a polarimeter
rate equation
rate = k[A]^n[B]^m
total order=n+m
Rate and concentration graph
Zero order: straight horizontal line
First order: straight line passing through origin (directly proportional)
Second order: curve
(think of y=mx)
Half life
Zero order: concentration time graph negative gradient straight line
First order: curve decreasing, half lives are constant (independent of the concentration and is constant)
Second order: half life increases
Effect of temperature on rate constant
As temperature increases, value of the rate constant k increases
k=Ae^(-Ea/RT) lnk=lnA-Ea/RT
Rate-determining step
Slowest step, controlling the overall rate of reaction
Molarity of molecules in the slowest step will be the same as the order of reaction of each substance
Rate of reaction between propanone and iodine
- react propanone and iodine in acidic condition. (acid is the catalyst)
- remove small samples from reaction mixture with a pipette, quench the solution with sodium hydrogen carbonate to neutralise the acid catalyst to stop reaction. sample is then titrated with sodium thiosulphate with starch added into the volumetric flask
- 2S2O32- + I2 –> 2I- + S4O62-
rate = k[propanone][H+] –> 2 step reaction as I2 not in the rate equation meaning that order of reaction with respect to I2 is 0
SN1 and SN2
Sn1: one molecule in the RDS (for tertiary halogenoalkanes as the carbocation formed is stable) –> will have intermediate
Sn2: 2 molecules in the RDS (for primary halogenoalkanes as the carbocation formed is unstable)
Heterogenous catalyst
A catalyst in different phase from the reactants
Effect of pressure on heterogenous catalyst
Increasing pressure has limited effect on the rate of heterogenous catalysed reactions as the reaction takes place on surface of catalyst. Active sites on the catalyst surface are already saturated with reactant molecules. increasing pressure does not help
