Kinetics Flashcards

Question

What is the Arrhenius equation and how can it be used?

Answer 1

k= Aexp(-Ea/RT) lnk= -Ea/RT + lnA Plot a graph of lnk against 1/T, gradient = -Ea/RT, y intercept= lnA

Answer 2

At high temperatures, as A is also temperature dependent, there may be deviations At low temperatures, quantum-mechanical tunnelling through the activation barrier may result in deviations

Answer 3

Elementary: As normal, with the Ea the barrier height, and A collision rate Complex: Effectively an equation for the temperature dependence on rate, rather than having physical meanings If the plot of lnk against 1/T is not linear the equation: Ea= RT² dlnk/dT Can be used, and further things inferred from the T² term Ea can be positive or negative, 0, and temperature dependent

Answer 4

Implies a complex reaction mechanism, and that increasing temperature decreases rate e.g pre equilibrium The forward reaction is exothermic so favoured by lower temperatures, and as this stage occurs first, dominates the reaction Ea no longer represents a physical energy barrier in complex reactions so can be negative, 0 ion-ion

Answer 5

1. Mix the reactants on a timescale negligible to the reaction 2. Monitor the conc of a reactants as a function of time 3. Ensure temperature is constant

Answer 6

Batch: reaction initiated at a well-defined time, concentration followed after initiation Continuous: Continuously initiated, time dependence inferred from different positions in the reaction vessel ie how far along the tube it is

Answer 7

s-ms Reactants introduced into start of tube, and a moveable or fixed detector detects the conc Use t=d/v But needs large quantities of reactants, and very fast flow rates

Answer 8

Fixed volumes rapidly mixed and flow into a reaction chamber via syringe Monitored spectroscopically, as a function of time Needs small volumes - but may react with the walls, heterogenous reactions, so coated in inert halocarbon wax - quantify via varying flow tube diameter

Answer 9

A pulse of light disassociates a precursor molecule producing the reactive species And conc of species measured spectroscopically Enables thorough mixing, and reactants can be central so no reactions with walls fs Laser pump propes: two flashes, one to initiate, one to detect spectroscopically

Answer 10

Disturbing a system at equilibrium, e.g via temperature change e.g microwave discharge Already well mixed Sometimes measured with laser-induced fluorescence, proteins

Answer 11

Using a high pressure gases released into a system to cause production of reactive species from a precursor, rapid heating But not very selective

Answer 12

Real time analysis: withdrawing samples and testing Quenching: Reaction stopped after a certain time, e.g cooling, removal of catalyst, dilution, adding a quencher - but needs to be slow enough for no real reaction progress during quenching Both used together If a gas is produced, measure mass loss, volume collected Ions: conductivity/pH Titrations Colorimetry Spectroscopy Polarimetry (chirality) Mass spec, gas chromatography

Answer 13

Absorption spectroscopy, using beer lambert law, different absorptions Absorbance= e c l Resonance fluorescence: atomic species - precursor, microwave for RS, lamp emits wavelengths as species give off photos as dexcite - light excites atoms of the same species, and detector detects light emitted by these species, intensity proportional to conc Laser induced fluorescence: laser excites species, emits photons, intensity of photons detected

Answer 14

Thermocouples / Thermostats Gas-phase in a vacuum chamber, equilibrium maintained High temps via heating, low via cooling liquids through walls Cryogenic liquids for for very cool e.g nitrogen, helium as liquids Or even cooler, supersonic molecular beams

Answer 15

Agreement with the rate law Product distribution Product stereochemistry Kinetic isotope effects Temperature dependence

Answer 16

We say the rate of the forward and backward reactions is equal in the pre equilibrium SO can use this equation to rearrange for a concentration Effectively K of the equilibrium is much larger than the one direction reaction, little leakage Only in later stages when the equilibrium has applied

Answer 17

A reactive intermediate is used up virtually as soon as it is formed, so its concentration is always very low, and constant for most of the reaction (apart from the end and beginning) We can assume d[intermediate] / dt = 0 Forming simultaneous algebraic rate equations rather than coupled differential equations

Answer 18

Write down the intermediate conc in terms of Ks and [A]/[B]... Set equal to 0, and rearrange Substitute into the rate equation desired Sometimes, rather than rearranging for a certain concentration, it is easier to set the equation for a set e.g k1k2[A][C] = [R][B] if this comes up in another equation If the species in question appears twice either being used up or made, 2 infront of the equation Squared only if being used up - so if dHBr/dt, coefficients depend only upon HBr in the equation (See later for more)

Answer 19

d[A]/dt = k x [reactants that form A] = k[reactants than use up A, can include [A] itself]

Answer 20

Used to describe a unimolecular gas phase reaction with first order kinetics Explains how they acquire enough energy to react, reversible The gas molecule reacts with another molecule (could be the same type) resulting in this molecule gaining energy and the other unchanged The excited state A* goes on to unimolecular react

Answer 21

At high pressures, collisions are more likely, meaning the bimolecular de-excitation of A* is more likely than unimolecular At low pressures, unimolecular isomerisation is more likely than de-excitation of A*

Answer 22

Use a plot of K Deviations at high pressures with K too large A particular degree of freedom is required for energy excitation, not taken into account

Answer 23

Pre-equilibria, as the chance of a 3 body collision is far too low for the rate observed And also in this case, increasing temperature decreases rate so complex Think about when this will be 3rd order, comparing k2 and k-1

Answer 24

Bimolecular forming a vibrationally excited state This excited state transfers some of its energy to a third molecule to enable successful formation of ozone O + O2 → O3* O3* + M → O3 + M

Answer 25

An enzyme is a protein that catalyses a specific biochemical reaction via lowering the activation energy The substrate binds to the enzyme at the active site, causing a shift in substrate geometry closer to that of the transition state, lowering activation energy

Answer 26

Use SSA, and then substitute for [E] with [E]0 + [ES] Then rearrange for [ES], and take out k1 to help determine Km

Answer 27

Chain reactions contain chain carriers, reactive intermediates which react to form further reactive intermediates Cyclic: chain carriers continuously regenerated until removed via a termination step Non cyclic: involve many reactive species and elementary steps Linear: each propagation step produces 1 reactive intermediate Branches: Contains propagation steps which produce more than one reactive intermediate

Answer 28

n = rate of propagation / rate of initiation/termination

Answer 29

3rd step irreversible as HBr < H2

Answer 30

Find the equations for the reactive intermediates Add them, to simplify and rearrange to find the concentrations of both Write the equation for rate of the product HBr Substitute the equations formed before

Answer 31

With chlorine, but faster rate as propagation more efficient, inhibition much slower Iodine too slow to operate at room temps, termolecular reactions Rate of step 2 formation of XBr more endothermic down the group

Answer 32

Thermal: Heat generated by a reaction cannot be dissipated rapidly, increasing rate, further reaction... Chain branching: Each step produces more reactive intermediates, keeps increasing in chain carriers

Answer 33

At lower pressures, collisions with the wall more likely, meaning initiation=termination, step increase in rate with pressure, At higher pressures, more likely to collide before the wall, so propagation + initiation > termination , 1st explosive limit At even higher pressures, the termolecular reaction termination steps is important, balancing initiation and propagation resulting in a steady increase, 2nd explosion limit At even higher pressures, rate increases further, generating higher temperatures, and so more explosive, 3rd explosive limit

Answer 34

Initiation/branching endothermic increasing in rate, whilst termination unaffected / exothermic so reduced So first limit occurs faster, lower limit 2nd increased as needs a higher pressure for termolecular to become important 3rd lower as more heat already produced

Answer 35

Increased rate of initiation and termination, so more termination raising pressure limits UNaffected in the 2nd limit Higher limit for 3rd as increased SA:V means faster heat loss

Answer 36

Decrease mean free path, promotes collisions so propagation, lower first limit Second lower as can act as a 3rd body 3rd lower as reduced heat transfer

Answer 37

Add the Ks in the exponential term