Kinetics 2

Question 1

Rate of reaction

Answer 1

Change in amount of reactants or products per unit time

Question 2

Orders

Answer 2

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

Question 3

Overall order of reaction

Answer 3

Sum of the orders of all the reactants

Question 4

Half life

Answer 4

Time taken for the concentration of a reactant to decrease by half
Used in conjunction with continuous rate method practicals (colorimetric, sampling reaction mixture)

Question 5

Clock reaction

Answer 5

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

Question 6

Continuous monitoring

Answer 6

Measurements taken over the duration of the reaction

Question 7

Zero order graph half life

Answer 7

Decreases over time at a constant rate

Question 8

First order graph half life

Answer 8

Constant

Question 9

2nd order graph half life

Answer 9

Increases over time at a constant rate

Question 10

Relationship between half life and rate constant

Answer 10

K= ln2/half life

(ln2 is natural log 2)

Question 11

Rate determining step

Answer 11

Slowest step in the multi step mechanism for a reaction

Question 12

Arrhenius equation (to use in exam)

Answer 12

lnK=-Ea/RT +lnA
K= rate constant
A= Arrhenius constant
Ea= activation energy, J/mol
R= gas constant
T= temperature, K

Question 13

RT

Answer 13

Average kinetic energy

Question 14

Ea/RT

Answer 14

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

Question 15

e

Answer 15

Exponent (inverse of natural log)
As temperature increases, the rate constant increases exponentially

Question 16

Arrhenius equation y=mx+c

Answer 16

y= lnK
m= -Ea/R
x=1/T
c= lnA

Question 17

calculating A from graph

Answer 17

e^lnA
May need to extrapolate line to y intercept

Question 18

Calculating activation energy from graph

Answer 18

Gradient x R

Question 19

Rate expression

Answer 19

A mathematical representation of how changing concentrations affects the rate of reaction

Question 20

SN1

Answer 20

Nucleophilic substitution
One molecule involved in the RDS
tertiary haloalkanes

Question 21

SN2

Answer 21

Nucleophilic substitution
2 molecules involved in the RDS
primary haloalkanes

Question 22

Primary haloalkane- C2H5Cl

Answer 22

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

Question 23

Tertiary haloalkane, C4H9Cl

Answer 23

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

Question 24

Why do tertiary haloalkanes undergo SN2

Answer 24

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

Question 25

Homogenous catalyst

Answer 25

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

Question 26

Heterogenous catalyst

Answer 26

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)

Question 27

Heterogenous catalysts- solids, poisons

Answer 27

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

Question 28

Iodine + propanone

Answer 28

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

Question 29

Iodine clock

Answer 29

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

Question 30

Determining Ea

Answer 30

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

Question 31

Table layout for determining Ea

Answer 31

Temp/oC. Time/s. Temp/K. 1/T (K). lnt

Question 32

Explaining why data isn’t an order of …

Answer 32

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 ….