Kinetics 2 Flashcards
Rate of reaction
Change in amount of reactants or products per unit time
Orders
Tell you how a reactants concentration affects the rate
Officially: with respect to a given substance, is the number/ exponent that describes the effect that it’s change in concentration has on the initial rate of reaction
Overall order of reaction
Sum of the orders of all the reactants
Half life
Time taken for the concentration of a reactant to decrease by half
Used in conjunction with continuous rate method practicals (colorimetric, sampling reaction mixture)
Clock reaction
Type of initial rates method
Measure how the time taken for a set amount of product to form changes as you vary the concentration of one of the reactants
Observable end point
Continuous monitoring
Measurements taken over the duration of the reaction
Zero order graph half life
Decreases over time at a constant rate
First order graph half life
Constant
2nd order graph half life
Increases over time at a constant rate
Relationship between half life and rate constant
K= ln2/half life
(ln2 is natural log 2)
Rate determining step
Slowest step in the multi step mechanism for a reaction
Arrhenius equation (to use in exam)
lnK=-Ea/RT +lnA
K= rate constant
A= Arrhenius constant
Ea= activation energy, J/mol
R= gas constant
T= temperature, K
RT
Average kinetic energy
Ea/RT
Ratio between activation energy and average kinetic energy
Lower Ea, higher Ek, faster rate of reaction. Small ratio= faster reaction
Fraction of molecules possessing enough energy to react
e
Exponent (inverse of natural log)
As temperature increases, the rate constant increases exponentially
Arrhenius equation y=mx+c
y= lnK
m= -Ea/R
x=1/T
c= lnA
calculating A from graph
e^lnA
May need to extrapolate line to y intercept
Calculating activation energy from graph
Gradient x R
Rate expression
A mathematical representation of how changing concentrations affects the rate of reaction
SN1
Nucleophilic substitution
One molecule involved in the RDS
tertiary haloalkanes
SN2
Nucleophilic substitution
2 molecules involved in the RDS
primary haloalkanes
Primary haloalkane- C2H5Cl
Permanent dipole between C and Cl
OH- with lone pair attracted to delta + carbon atom
Intermediate
Alcohol produced
RDS is the OH- attacking the haloalkane so both molecules involved in rate equation
Tertiary haloalkane, C4H9Cl
Heterolytic fission just occurs, no OH- needed
Forms carbocation which is attacked by OH-
Normal substitution occurs to produce alcohol
RDS is the Heterolytic fission of the haloalkane so just this molecule involved in the rate equation
Why do tertiary haloalkanes undergo SN2
The R groups next to the carbocation release electrons that stabilise it (electron inducing effect)
The R groups also (physically) prevent the OH- from attacking the delta positive carbon atom
Homogenous catalyst
Same state as reactant
Reactants combine with the catalyst to make an intermediate species which then reacts to form the products and reform the catalyst
Heterogenous catalyst
Different physical state from reactants
Reactant molecules arrive at surface of catalyst and bond with the solid catalyst (adsorption)
2. The bonds between the reactant atoms are weakened and broken up to form radicals which get together and make new molecules
3. New molecules are then detached from the catalyst (desorption)
Heterogenous catalysts- solids, poisons
Solid ones provide a surface for the reaction to occur, so fine powders increase SA
Can be easily separated from products and leftover reactants
Poison is a substance that climbs to the catalysts surface more strongly than the reactant, preventing the catalyst from being involved in the reaction
Iodine + propanone
I2+CH3COCH3+H+—-> CH3COCH2I +2H+ + I-
Mix known vol and conc of propanone and sulfuric acid in a beaker
Add known vol and conc of iodine and swirl and immediately start a timer
At approx 3 mins use a pipette to extract (10cm3) sample of reaction mixture and transfer to conical flask, immediately adding sodium hydrogen carbonate (quenching)
Titration sample with sodium thiosulfate (burette) and starch indicator to find concentration of iodine in the conical flask at that time
Repeat every 3mins
Graph of concentration of iodine against time
Zero order
Rate= gradient
Iodine clock
Hydrogen peroxide + iodide + hydrogen ions= water and iodine, starch indicator
Very fast reaction
Add sodium thiosulfate to reaction mixture before the iodide
Start timer when add iodide, thiosulfate react with iodine to remake iodide to slow reaction
When thiosulfate ions run out, the iodide ions as normal are converted into iodine and the solution turns and remains blue/black (stop timer)
Increases accuracy by reducing percentage uncertainty as time increased
Graph of 1/t against concentration of hydrogen peroxide
First order
Determining Ea
Use reaction where you can find the initial rate
Repeat over range of temperatures
Graph of lnt against 1/T where gradient x R= Ea
Table layout for determining Ea
Temp/oC. Time/s. Temp/K. 1/T (K). lnt
Explaining why data isn’t an order of …
Not zero order as rate not constant
Not 1st order as half lives not constant or rate doesn’t change in proportion to change in concentration of ….
