Topic 16 - Chemical Kinetics Flashcards
Definition of chemical kinetics
Study of the factors affecting the reaction rate
Definition of rate of reaction
Change in concentration of the products/reactants per time unit
Factors affecting rate of reaction (4)
a) Particle size/Physical state
b) Temperature
c) Concentration
d) Catalyst
How does particle size/physical state affect reaction rate?
The more finely divided a solid/liquid reactant, the greater its surface area, the more contact it makes with the other, and the faster the reaction occurs.
How does temperature affect reaction rate? (2)
a) The particles move faster (KE), so there will be more collisions per second.
b) More colliding particles will possess the necessary Ea for successful collisions.
How does concentration affect reaction rate?
Increasing the concentration increases the frequency of collisions and thus the frequency of successful collisions.
How does adding a catalyst affect reaction rate?
It lowers the activation energy of both forward and reverse reactions by using an alternative pathway.
Definition of instantaneous rate
The rate at a particular instant during the reaction (Slope of tangent)
Expressing the Reaction Rate in terms of reactants and products
Rate= -(∆[R])/∆T = (∆[P])/∆T
(-1/a) d[A]/dt = (1/b) d[B]/dt
How can concentration changes be monitored? (3)
a) Spectrometric methods
b) Conductometric methods
c) Manometric methods
Spectrometric methods
a) As the concentration of the colored compound increases, it absorbs more light, so less is transmitted
b) Spectrometer measures the concentration of a component that absorbs specific wavelengths of light
Conductometric methods
a) The total electrical conductivity of a solution depends on the total concentration of its ions and on their charges. If this changes, it indicates a change in the concentration of ions.
b) Conductivity can be measured directly using a conductivity meter in a solution
Manometric methods
a) Measures the pressure change in a vessel due to a reaction that changes the number of moles of gas
b) Reaction vessel of fixed volume and temperature
Why is the gradient of a concentration vs time graph higher at the beginning?
a) There is a higher concentration of reactant particles, so there is a higher frequency of successful collisions.
b) The rate of reaction (gradient) decreases over time as time as the concentration of reactant particles decreases
Rate law (k[A]^x[B]^y)
Mathematical relationship of reaction rate with reactant concentration
Rate constant k
Constant for a particular reaction at a particular temperature
Overall order
Sum of x and y
How is the rate law determined?
It is only determined experimentally, and not from the stoichiometric equations.
Integrated rate laws for:
a) Zero order
b) First order
c) Second order
a) [A] = -kt + [Ao]
b) ln[A] = -kt + ln[Ao]
c) 1/[A] = kt + 1/[Ao]
Half life for first order reaction
ln2 / k
Collision Theory
For a reaction between two particles to occur 3 conditions must be met:
a) They must collide
b) They must collide with sufficient energy to bring about the reaction.
c) They must collide with the appropriate geometry
Effect of temperature on collision energy of particles
A rise in temperature increases the average kinetic energy of the reactant particles and enlarges the fraction of collisions with enough energy to exceed Ea
Formula of the fraction of molecules with energy equal or greater than Ea
f=e^(-Ea/RT)
Frequency factor A
Fraction of molecules that have the appropriate orientation to react
Arrhenius Equation
k=Ae^(-Ea/RT)
Arrhenius equation to calculate Ea
ln (k1/k2) = Ea/R (1/T2 - 1/T1)
Transition State Theory
Every reaction goes through its own transition state (high energy species)
How is the Ea of a particle collision used?
a) As two molecules approach each other, repulsions between their e-clouds increase
b) KE is converted to PE by pushing them together to overcome the repulsions and surpass Ea
c) An unstable species exists at the highest potential (Transition state)
Features of reaction energy diagram (5)
a) Change in the PE as the reaction proceeds
b) Reactants => Transition state => Products
c) The rate-determining step has a larger Ea than the other step
d) Intermediates are higher than products or reactants
e) Transition states are at the peak of each elementary reaction
Definition of reaction mechanism
Sequence of steps by which a chemical reaction occurs
Definition of elementary reactions
Reactions that occur in a single step – single molecular event
Principle for finding the rate law of elementary reactions
Its rate law is based directly on its molecularity
Definition of intermediates
Substances that are consumed in one elementary reaction and consumed in the next
Rules for writing mechanisms (3)
a) The elementary steps must add up to the overall balanced equation
b) The elementary steps must be reasonable (1/2)
c) The mechanism must correlate with the rate law
Rate determining step
Elementary reaction with the slowest rate
Features of catalysts (2)
a) Speeds up both the forward and reverse reaction (Same yield)
b) Not consumed but rather used and regenerated
Types of catalysts
a) Homogeneous
b) Heterogenous
Examples of homogenous catalysts
a) NO in SO3 production from SO2/O2
b) HBr in H2O2 decomposition
Examples of heterogenous catalysts
a) Hydrogenation (Ni/Pd/Pt)
b) Catalytic converter
Biological catalysts (Enzymes)
a) Provide a surface on which one reactant is immobilized to wait until the other reactant lands
b) Incredibly efficient in the number of reactions catalyzed per time (Stabilize transition state
c) Highly specific due to active site
Decomposition of O3 Layer by CF2Cl2
O3 + Cl => ClO + O2
ClO + O => O2
