Kinetics Of Chemical Reactions Flashcards
What is a first order (unimolecular) rate law?
r=k[A] where k is a first order rate constant, units time^-1).
The rate is proportional to the concentration raised to the power 1.
What is a second order (bimolecular) rate law?
r=k[A]^2, where k is a second order rate constant, units conc^-1 time^-1.
The rate is proportional to the concentration raised to the power of 2.
Why is knowing the rate law of a reaction important?
- The rate can be predicted for a particular set of concentrations.
- Can aid understanding of mechanism.
Give an example of a reaction whose rate constant decreases with increasing temperature.
2NO + O2 —>2NO2
What is the Arrhenius equation?
k(T)= Aexp(-Ea/RT)
Where A is the pre-exponential factor, often independent of temperature and with the same units as k (exponential term is dimensionless). Plotting this equation as a straight line shows that A can be identified as the rate constant at infinite temperature (i.e. when RT»Ea).
How are rate constants related to the equilibrium constant of a reaction?
The equilibrium constant is the ratio of the rate constants of the forward and backward reactions.
What does a potential energy surface show?
It gives the potential energy as a function of the positions of all the atoms in the system.
Essentially the course of a reaction can be considered using MO theory, since the energies of the reactant molecules, the transition state and product molecules can be approximated using the energies of the MOs present. However this energy path is not always the same for a reaction- it depends on precisely how the interacting atoms are positioned and so how the orbitals interact. Thus for one reaction there can be a multitude of different ways in which the reactants will approach one another. Each of these ways (representing atomic arrangements) can be mapped to generate a 3D surface.
Stable molecules (i.e. reactants and possible products) exist in wells called POTENTIAL ENERGY MINIMA. The most favoured path requires the least expenditure of energy to traverse. The highest energy point on this path is the TRANSITION STATE (not a molecule/intermediate as doesn’t exist in a minima).
What is the Boltzmann distribution?
exp(-Ea/RT) , the fraction of molecules with the energy needed to traverse the minimum energy pathway. This is the energy between the reactants and transition state.
Any change in the energy greatly influences the number of molecules as it is an exponential function, hence why reacting molecules go to products via the minimum energy route.
What is a reaction coordinate?
The minimum energy pathway from reactants to products. Plotting this against energy gives the reaction profile.
I.e. in the reaction H+H—>H2, the reaction coordinate is the vector between the hydrogen atoms.
What assumptions are made in gas kinetic theory?
Particles are objects whose size is much less than the typical distance between them.
Collisions between one another and the walls of the container are elastic (i.e. kinetic energy conserved).
This means energies and speed are distributed according to the maxwell distribution.
A downside of this theory is that molecules are modelled as spheres, rather than structured molecules that must be oriented in a certain way when they react.
Why does collision theory generally overestimate the value of the A-factor compared with those obtained experimentally?
It assumes all collisions with sufficient energy lead to reaction, but orientation is important.
The fraction of sufficiently energetic collisions which lead to reaction is called the steric factor, p=Aexperiment/Acolltheory
Give an example of a reaction where p is greater than one (i.e. collision theory UNDERestimates the A-factor).
Most involving halogens:
K+Br2–>KBr+Br
It suggests that the collision cross section sigma is greater than pi(rK+rBr2)^2. In other words the molecules interact over greater distances than their radii and don’t necessarily need to collide (this reaction occurs via ‘Harpoon mechanism’).
For first order kinetics, you only need a quantity which is proportional to concentration to work out the rate constant (proportionality constant drops out).
For second order, you need to know absolute concentrations.
.
Pseudo first order meaning?
Rate law obtained using the isolation method to describe more complex relationships.
What are problems with the isolation method?
- Putting one reagent in large excess may make the rate too fast.
- Such conditions could alter the mechanism of a complex reaction.
What are the drawbacks of the differential method?
Must plot rates, and these are much harder to measure than concentrations
