Kinetics Flashcards
What is the rate of a chemical reaction?
The rate at which products are formed, or reactants used up
rate= Δconc/time
rate= d[conc] / dt
What are elementary reactions and the three types?
Reactions which occur in a single step. All chemical reactions can be made up of a series of elementary steps
Unimolecular: isomerisation or disassociation, contain only 1 reactant
Bimolecular: collision between 2 species
Termolecular: collision between three species, very rare and require very high pressures
What is a complex reaction?
A reaction involving more than one elementary step
Most reactions, e.g Haber-Process
What is the activation energy of a reaction and how can it be understood?
An energetic barrier which must be overcome for particles to successfully reaction
The energy difference between the reactants and transition state
How has the equation for rate of reaction from simple collision theory been derived?
3 key factors:
- collision frequency
- energetic requirement > Ea
- orientation of the collision / sterics correct
How does a catalyst increase rate of reaction?
Lowering the activation energy of a reaction
From the rate formula, catalysts change the exponential term exp(-Ea/RT), as the power will become less negative
What are the failures of simple collision theory?
Not all of the kinetic energy of the reactants is available for a reaction, only KE relative to motion contributes
Energy stored in internal degrees of freedom (rotation, vibration) of reactants is ignored
So often experimental K and that of the formula do not align
What is the formula for A, the pre-exponential factor?
How do you write the differential equation form for rate of reaction e.g for the Haber process?
N₂(g) + 3H₂(g) ⇌ 2NH₃(g)
-d[N₂]/dt= -1/3 d[H₂]/dt = 1/2 d[NH₃]/dt
1/coefficient
- signs for reactants as their concs are decreasing, so becomes positive
What is a rate law? What is an order and overall order?
An expression which relates reaction rates to the concentration of species in the reaction mixture
Can include catalysts, reactants, products, but not intermediates
Order: The power a concentration is raised to
Overall: sum of individual orders
What is a simple rate law? And a complex rate law? What do they indicate?
Simple rate laws can be written directly from the reaction equation, e.g elementary reactions, complex reactions sometimes
rate= k [A] [B]
Complex: more complicated dependence on conc, often with multiple constants
e.g rate= (k1[A]^1/2 [B])/ (1 + K’[B])
A complex rate law always indicates a complex reaction
What does molecularity mean? How does this relate to elementary reactions?
The number of each species in an elementary step reflects the order of the species in the rate equation
How do you convert between pressure/ time and conc?
pV=nRT
n/V= p/RT
Divide by RT
What are the units of both K for this reaction? When can rate constants be compared?
When adding/subtracting, the species must have the same units
If these added species are used to divide, they count as one unit
How do you calculate the integrated rate law for zeroth and first order reactions?
How do you calculate the integrated rate law for second order reactions?
You don’t need to divide by a half, as these are not elementary steps
How do you calculate integrated rate laws for n order reactions?
What is the half life of a substance? What is the general approach for finding the equation for half life of a reaction based on order?
The time taken for the concentration of a substance to fall to half of its initial value
Substitute t=t1/2 and [A] = [A]0/2 into the integrated rate law
What are the half lives of zeroth, first, and second order reactions?
zeroth: [A]₀ / 2k
first: ln2/K
second: 1/K[A]₀
How would you calculate the half life of an n order reaction?
Actually 2^n-1 -1
How can the isolation method be used to determine rate laws?
Keep one reagent in such a large excess its concentration is effectively constant for the reaction
This means the change in rate is due to the decrease of the other reactant over time
A + B –> P
Rate=k[A]^n [B]^m
If B in a large excess
Keff= [B] * K
Rate= keff[A]^n
So now pseudo first order
How can differential methods be used to determine rate laws?
After isolation, rate dependent on 1 concentration
rate = k[A]^a
log(rate)= logK + alog([A])
Plot the graph, gradient=a, and log K = y intercept
Can be acquired through multiple initial rates methods calculation of rate from conc-time monitoring several times
How can integral methods be used to determine rate laws?
Using the integrated rate laws: literally just the equations they give
zeroth: [A] vs t linear
first: ln[A] vs t linear
second: 1/[A] vs t linear
How can half-lives be used to determine rate laws?
Using the integrated rate laws for half life equations:
zeroth order: successive half lives half each time
first: constant half life
second: successive half lives double
What is the Arrhenius equation and how can it be used?
k= Aexp(-Ea/RT)
lnk= -Ea/RT + lnA
Plot a graph of lnk against 1/T,
gradient = -Ea/RT, y intercept= lnA
When does the Arrhenius equation break down? Why?
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
How can the Arrhenius equation be applied for elementary and complex reactions? How can activation energy vary?
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
Why may a reaction have a negative activation energy?
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
What is the general process for measuring the rate of a chemical reaction?
- Mix the reactants on a timescale negligible to the reaction
- Monitor the conc of a reactants as a function of time
- Ensure temperature is constant
What is the difference between batch techniques and continuous techniques?
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
How can continuous flow techniques be used to initiate reactions? And the time span?
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
How can stopped flow techniques be used to initiate reactions? And the time span?
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
How can flash photolysis techniques be used to initiate reactions? And the time span? How about pump probes?
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
What are relaxation methods for measuring rate?
Disturbing a system at equilibrium, e.g via temperature change e.g microwave discharge
Already well mixed
Sometimes measured with laser-induced fluorescence, proteins
What are the calculations behind the relaxation method, equilibrium?
How can shock tubes be used to initiate reactions? And the time span?
Using a high pressure gases released into a system to cause production of reactive species from a precursor, rapid heating
But not very selective
What are the main ways for monitoring the concentrations in slow reactions?
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
How can fast reactions have their concentrations monitored?
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
How can temperature be controlled when measuring the rate of reactions?
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
