Kinetics 1 Flashcards
What factors affect reaction rates?
- Concentration
- Temperature
reaction pathway
the minimum energy route to get from reactants to products
reaction coordinate
abstract coordinate to show progress along a reaction route
activation energy
the minimum energy for a reaction to occur - energy barrier
transition state
as molecules cross the energy barrier, they proceed via a transition state
intermediate molecules
a reaction may proceed by a number of steps, via intermediate molecules
When are transition states seen on a diagram?
at the energy maxima
When are intermediates seen on a diagram?
at an energy minima
Reaction rate equation when characterised by rate of disappearance of reactant
rate = - d[reactant]/ dt
Reaction rate equation when characterised by the appearance of a product
rate = d[product]/ dt
Methods for measuring reaction rates
- ‘stopwatch’ method
- continuous flow
- stopped flow
- relaxation methods
- flash photolysis
Overall order of a reaction
the sum of the orders of the substances in the rate equation
Rate equation of a first order reaction
rate = -d[A]/ dt = d[P]/dt = k[A]
What does integrated rate laws do?
- gives us concentrations as a function of time
- allows us to determine the order of reaction
What do half-lives do?
- another way of characterising reaction rates
What to always plot for integrated rate laws?
f[A] vs t, not [P] vs t
What is the half-life of a reactant?
the time taken for its concentration to drop to half its original value
What is the half life for a first-order reaction?
a constant
What is the half life for a second-order reaction?
it increases as reaction proceeds
What does the arrhenius equation tell us?
a small increase in temperature will produce a significant increase in the magnitude of the rate coefficient of the reaction
What does the A stand for in the arrhenius equation for a bimolecular, gas-phase reaction? k = Aexp(−E/RT)
it is related to the probability of molecules meeting in the correct orientation to react
What does the exponential relate to in the arrhenius equation for a bimolecular, gas-phase reaction? k = Aexp(−E/RT)
the probability molecules have enough energy to react
What does the A stand for in the arrhenius equation for a unimolecular, gas-phase reaction? Aexp(−E/RT)
related to the frequency of the vibration
What does the exponential relate to in the arrhenius equation for a unimolecular, gas-phase reaction?
the probability molecules have enough energy to break bond
What is the units of A in the arrhenius equation?
the same units as k
What does a catalyst do?
they give an alternative reaction pathway with lower activation energy
What is the molecularity of an elementary reaction step?
it is the number of reactant molecules involved
How would you know an intermediate is highly reactive?
k 2 is very large
- its concentration remains very low
- it rapidly reaches a steady state - its rate of production balances its rate of removal
assume: d[I]/ dt = 0
What is the rate determining step in terms of energetics?
the step with the highest activation energy
What is kinetic control?
the major product is from the fastest reaction (irreversible conditions)
What is thermodynamic control?
the major product is the more stable one (equilibrium, reversible conditions)
What is the rate referred to in enzymolysis?
velocity, v
What is competitive enzyme inhibition?
when the substrate cannot bind to the enzyme at the same time as the inhibitor
What is non-competitive inhibition?
when the inhibitor doesn’t prevent enzyme from binding substrate but prevents formation of the product
