TOPIC 16: Kinetics 2 Flashcards
(18 cards)
Measure rate in experiments (5)
- electric conductivity as there is a change in number of ions
- amount of mass lost as gas in fume cupboard
- volume of gas produced using a gas syringe
- pH of reaction change if H+ ions involved, intervals and a meter
- titration for concentration changes in small samples (aliquot) at intervals, and slow reaction: dilute with deionised water, cool down or quench (add chemical to stop reaction)
- colorimeter measuring absorbance of light
Rate equations
Rate=k[A]^ax[B]^b
Rate (moldm^-3s^-1)
k rate constant
a and b orders of reaction
[A] and [B] concentration of substance
Orders of reaction definition
Powers to which a concentration is raised two in the rate equation = how concentration of a substance affect the rate
Only determined experimentally
Zero, first and second order
How change in concentration… change in rate
0: no
1: proportional
2 Squared proportional
Rate constant k
Constant at particular temperature (increase provokes increase)
Larger value means faster rate
Clock experiments
How long it takes for reaction to occur
Quicker=faster initial rate
Assume = constant rate
- constant temperature
- concentration doesn’t change significantly
- reaction hasn’t proceeded too far
Iodine clock reaction
H2O2 + 2H+ + 2I- -> 2H2O + I2
Add sodium thiosulfate(reacts immediately with I2) and starch (indicator) to excess H2O2
Then I2 reacts with starch (black/blue colour)
2S2O3^2- + I2 -> 2I- + S4O6^2-
Varying [I2] or [H2O2] and all constant = different time so work out order
Rate concentration graph
0 order - flat line (constant gradient in []/t) concentration unchanged
1 order - straight diagonal line (constant change in gradient curve in []/t) concentration changes as rate
2 order - curved line
concentration changes as rate squared
Half life
Time takes for half of reactant used.
0 order: half life decreases
1st: half life constant
2nd: half life increases
Rate determining step
Slowest step in multi-step reaction
Reactants that appear in rate equation affect the rate (must appear in rate determining step)
Catalysts can too
Calculations with rate determining step:
Rate equation
Rate determining step
- slowest and its reactants have to appear
- bingo! Start from step1 and cancel rate molecules when appear as reactants
Propanone and iodine
CH3COCH3 + I2 + H+ -> CH3COCH2I + H+ + I-
Rate=k[CH3COCH3][H+]
Step 1 SLOW
H3CCOCH3 + H+->H3CC+(OH)CH3
Step 2 FAST
H3CC+(OH)CH3->H2CC(OH)CH3 + H+
Step 3 FAST
H2CC(OH)CH3 + I2 -> H2ICCI(OH)CH3
Step 4 FAST
H2ICCI(OH)CH3-> H2ICCOCH3 + H+ + I-
Halogenolakanes types and 2 properties
Hydrolysed (split) by hydroxide ions.
alkyl groups attached to Carbon with halogen
Primary: 1
Secondary: 2
Tertiary:3
Polar bond (halogens negative) so attacked by nucleophiles (electron pair donor)
Nucleophilic substitution
Warm aqueous sodium hydroxide under reflux
RX + NaOH -> ROH + NaX
Sn1 Tertiary and secondary
(1 molecule in rate determining)
Sn2 primary and secondary
(2 molecules in rate determining)
Sn2
Rate= k[CH3CH2Br][OH-] 1st order
Intermediate C has 5 bonds (1 step)
- space around carbon as H are small
Sn1
Rate= k[(CH3)3CBr] one molecule and 1st order
Carbocation formed in rate determining step = intermediate
- no space around carbon as large alkyl groups
Arrhenius equation
k=Ae^(-Ea/RT)
Rate constant=Arrhenius constant x exponential of -activation energy (J) over (gas constant (JK-1mol-1) x temperature (K))
Ea decreases/T increases, k increases
Arrhenius plots
lnk = lnA - Ea/RT
y=mx + c
Graph lnk against 1/T
Gradient -Ea/R
