WIP Temperature dependent Kinetics and complex reactions

Give the arrhenius equation with units

Linearise the arrhenius equation, when can we do this

Draw out a boltzmann graph of high and low temperature, state the equation for the fraction of molecules with energy greater than Ea

state two ways an energy barrier can be reached

• Can overcome the barrier with heat 

  • Need high T experiments 

• Can lower the barrier with catalysis (or enzymes) 

  • Complex reactions (Barrier is not elementary) 

what makes a reaction arrheinious like and what does this mean

• Having to break strong bonds means a reaction is arrhenious like

  • this will mean that there will be a strong dependance on temperature

What is a complex reaction

Multi step reactions where the rate equation doesnt always reflect stoichiometry

Describe the Isolation method

• Create a great excess of all reactants but one making them all essentially constant. As such we can analyse the rate equation in respect to just one of the reactants.

• from here we can find what order that reactant is.

In sequential reactions what should you look to identify first

The slowest step of the reaction

What is the rate determining step

• This is the slowest step

• It will not matter how fast subsequent steps are in the reaction sequence, only the RDS and the steps prior to it will effect the obsereved rate.

Use a three step reaction to help describe how RDS effects order

A → B → C → D

• Just [A] is first order if A→B is RDS

• Just [A] and [B] are second order if B→C is RDS

• [A] [B] and [C] are third order if C→D is RDS

Write out a catalysed version of A→B

A + cat → A-cat → B-cat → B + cat

What happens when the critical conversion step at the catalyst is also the RDS

• it will be zeroth order meaning rate is independant of concentration

• This is because few of the active sites are not consumed at this point aswell as the fact that reagent A is in large excess

Draw a reaction graph of catalysed vs non-catalysed

State what an elementary reaction is

Occur at atomic level, observed order = molecularity, straightforward kinetics

State what a bulk reaction is

Represents the overall stoichiometry. occur via many elementary reaction steps and can have a misleading order. May have very complicated kinetics.

What does steady state approximation mean, and how do we use it

• Analysis of complex sequential reactions from its ‘steady state’, where rate of formation = rate of loss of the intermediate

• As such the intermediate is not changing with time

• As such we have the key assumption of d[intermediate] / dt = 0

Complete a Steady State approxiamtion of this simple complex reaction

for gas phase reactions what assumptions do we make and when are they good/bad

• assume that : gases behave statistically and uses ideal gas behaviours

• This works well for Gases like helium that have weak forces and are hard monoatomics, Works poorly for larger molecules with great many forces

When are 2nd order reactions bimolecular

when they are elementary

Give K units for gas phase

• 1st order , K is s_^-1

• Pseudo 1st order , Keff is s^-1

• 2nd order , K is conc^-1time^-1 or cm³molecule^-1seconds^-1

Give the collision rate coefficient equation, stating what each part means as well as what it implies

How can we estiamte the rate of gas phase radical equations

• We look at collision theory to estimate K.

• In general at T=298K, Kcoll = ~2×10^-10 cm³molecule^-1s^-1

• However sterics and orientation can alter these values

• True K is =< Kcoll

Describe Discharge flow for gas-phase

• Inititation by mixing, the DF will mix within ms

• Time resolution dependant on distance from the detector

• Monitors radical A sensitively, in steady state at the end of the tube, as such no fast detectors are needed. (UV-Vis, Flouresence, Mass-Spec)

• Monitoring unchanging [B] accurately (dilution, GC, NMR, FTIR)

give equation for rection time when using fast flow tubes

reacton time , t (s) = distance to detector (m) / gas flow velocity (m/s)

why requires more energy bond dissociation or or activation energy

• BDE »»» Ea

What does boltzmann graphs display and give the equation for the fraction of molecules capable of reacting

• Boltzmann distributions statistically describe how molecules of a gas all exist at different energies

• The fraction of molecules with energy greater than the Ea is e^-Ea/RT

what happens to arrhenious plot over large range of T

It becomes curved

What is the significance of A in bimolecular reactions

• A is related to the collision rate, k_coll

give equation for k_coll

what other factors does A relate too

A = k_coll x P

• it relates to a steric factor (P) aswell

• It also as we see via its relation to k_coll depends on T

Why was transition state theory developed

As a way of using quasi-equilibrium to understand massive reaction mechanisms like octane and diesel

Providing a more detailed