Topic 16 Kinetics II Flashcards
(29 cards)
What is the unit for rate of reaction? Usually
moldm-3s-1
What is the definition of rate of reaction?
Change in measurable quantity over time (s)
What is the rate constant?
k
What is the definition of half life?
Time taken for concentration of a reactant to half
It is a first order reaction
What is the activation energy used for?
Breaking bonds
What is a rate limiting step?
the slowest step in chemical reactions, which determines the overall rate of the other reactions in the pathway
What is a zeroth order reaction?
Change in concentration with no change in rate
What is a first order reaction?
Change in conc is proportional to change in rate
E.g. double conc = double rate
In graphs, goes through the origin
Constant half life if decreasing
What is a second order reaction?
Rate = (change in conc)^2
Double the conc, quadruple the rate
How to find overall order of a reaction?
Find rate equation
E.g. k[x][y][z]^2
Add them up = 4 overall order
What is the Arrhenius Equation?
Finding activation energy (Ea) using
- temp T (K)
- rate constant k (s-1)
Find gradient (m) where x axis: 1/T, y axis: ln k
m should be negative, UNIT K
m x -8.31 %1000
Ans should be positive (kJ mol-1)
What does a mop do?
E.g. in CP13
Thiosulfste (mop) reacts with Iodine (product of reaction we study, reactant twitch mop) as it forms
You time how long it takes for a certain SAME mole of iodine is formed
Same amount of mop added, change other concs
What is the purpose of a mop?
Makes reaction measurable
Thiosulfate eeacts with I2 as it forms so
So solution doesn’t turn blue-black immediately
When all thiosulfate is used up, solution turns blue black
To time how long it takes to make a certain number of moles of I2
What is SN1 and its’ rate equation?
1st order Nucleophilic substitution
rate = k[halogenoalkane]
Is SN1 or SN2 faster?
SN2 because it’s 2nd order
What is SN2 and its’ rate equation?
2nd order nucleophilic substitution
Rate = k[halogenoalkane] [nucleophile]
How many molecules involved in the RDS of SN1 and SN2 mechanisms?
1 molecule in SN1 Rate determining step
2 molecules in SN2 RDS
What reacts with iodine?
Sodium thiosulfate !!!
How to calculate rate constant?
To find k
Initial rate = k [x] [y]
Power if second order, don’t include if zeroth
NOTHING TO DO WITH MOLES
Explain the presence of starch in the iodine clock reaction as it is not involved in the ionic equation. (2 marks)
- indicator
- time taken for formation of blue-black to find reaction rate
Explain how a graph plotting time against [H+(aq)] against time that the order with respect to H+ is zero. (2 marks)
Change in [H+] / change in time
= gradient = rate of reaction
Since a straight line down
Change in concentration not affecting rate
So RoR is the same at all conc
So zeroth order wrt [H+]
What is an autocatalysis reaction?
When one of the products catalyses the reaction
so as soon as products are formed, reaction is catalysed
learn graphs
When heating water in a beaker by boiling methanol under, which system is endothermic and exothermic?
Water gaining heat so it is endothermic
Methanol losing heat, so it is exothermic
Explain why a reaction is neither zeroth nor first nor second order. (3 marks)
- not zeroth as rate is not constant
- not first order as half life not equal
- not second as rate is not proportional to the square of the concentration of one reactant
