Unit 1: Reaction Kinetics Flashcards
What is Rate of Reaction?
The change in concentration of a species over an interval of time.
How are the rates of formation and disappearance measured?
Experimentally, therefore they do not include stoichiometric coefficients.
Rates of formation and disappearance are always equal to the rate of reaction.
False, they are not necessarily equal.
The Rate of Reaction is always positive.
True, rates of disappearance are negative.
Are rates of formation and disappearance the same amongst different species?
No, they are not the same.
What is Instantaneous rate of reaction?
The change in concentration of a species at a specific time.
What is one way the rate of reaction can be determined?
By dividing the rates of formation and disappearance by their respective coefficients.
Reaction rates are dependent on concentration.
True, but the rate constant and order of the reaction are independent.
How is the order of a rate law equation determined?
Experimentally by looking at inital rates of rxn at differing concentrations and then by taking the sum of n and m.
What is a rate law?
It defines how the rate of a reaction depends on a species.
What does the rate constant depend on?
The reaction and temperature (larger k = faster reaction).
When looking at initial rates you should account for every species that is monitored.
True, products can be monitored and eventually appear in the rate law expression.
What is the rate law of a zero order reaction?
rate = k
In a zero order reaction what is the units for k?
M/s
In a zero order reaction what is the equation for half-life?
[A]0/2k
In a zero order reaction what is the integrated rate law?
[A]t = -k*t + [A]0
What is the graph of a zero order reaction? Is the slope + or -?
[A]t vs. t and the slope is negative
What is the rate law of a first order reaction?
rate = k[A]
In a first order reaction what are the units for k?
1/s
In a first order reaction what is the equation for half-life?
ln2/k
In a first order reaction what is the integrated rate law?
ln[A]t = -k*t + ln[A]0
What is the graph of a first order reaction? Is the slope + or -?
ln[A]t vs t and the slope is negative
What is the rate law of a second order reaction?
rate = k[A]^2
In a second order reaction what are the units for k?
1/Ms
In a second order reaction what is the equation for half-life?
1/k[A]0
In a second order reaction what is the integrated rate law?
1/[A]t = k*t + 1/[A]0
What is the graph of a second order reaction? Is the slope + or -?
1/[A]t vs t and the slope is positive
What order of reaction is radio active decay?
First order
Surface reactions and many enzyme-catalyzed reactions follow zero order reaction kinetics?
True
What is the Arrhenius equation?
k = Ae^(-Ea/RT)
What is A and Ea in the equation?
A = frequency factor (pre-exponential factor) and Ea = activation energy
What is the exponential factor?
The fraction of molecules with enough/sufficient energy to react. It is a # between 0 and 1.
What is the exponential factor equation?
f = e^(-Ea/RT)
What happens with a low Ea and high T?
The exponential factor approaches 1.
What happens with a high Ea and low T?
The exponential factor becomes small, it approaches 0.
What is the frequency factor?
The # of times the reactants approach the activation barrier per unit time.
What is the linearized Arrhenius equation?
lnk = -Ea/R * 1/T +lnA
What is the collision theory?
A reaction occurs after a sufficiently energetic collision between two reactant molecules.
What is needed for a collision to be effective?
Ea has to be met and the molecules need to be properly orientated.
With collision theory what is the rate constant?
k = pZf , where p = steric (orientation factor) and Z = # of collisions per unit volume
What is the transition state model?
It describes how reactants become products assuming favorable collisions occur. It focuses on the change of kinetic energy to potential energy as particles collide.
What happens during collisions?
-Bonds in the reactants stretch and become weaker.
-New intermolecular bonds start to form (still weak at this stage).
-The transition species is known as the transition state (activation complex).
If E reactants > E products the reaction is?
Exothermic
If E reactants < E products the reaction is?
Endothermic
What are the characteristics of the transition state?
-High energy
-Short-lived (unstable)
-Exact structure is unknown
-In equilibrium (low concentrations) with reactants and products
What is reaction mechanisms?
The theoretical concept that describes how a reaction occurs through a series of elementary reactions.
It MUST be consistent with the overall stoichiometry of the reaction AND account for the experimentally determined rate law.
What can the mechanism involve?
-Several steps
-Intermediate species
What are intermediate species?
A species that appears as a product in one step and then is consumed in another.
What is molecularity?
The # of species that must collide simultaneously.
It CANNOT be fractional or negative.
Elementary rate law?
rate = k[species]^n , where n is the stoichiometric coefficient.
What are the 4 types of mechanisms?
- Slow step followed by fast step(s)
- Fast step(s) followed by a slow step
- Steady state (approximation)
- Too complex
What is the rate-determining step
The slowest step
Does each step in a mechanism have its own transition state?
Yes
What is a catalyst?
A substance that increases the rate but is not consumed in the overall reaction.
What does a catalyst do?
-Decreases Ea
-No influence on enthalpy
-No influence on the amount of product
-It allows equilibrium to be reached faster (k is not changed)
What is a Homogeneous Catalyst?
The catalyst is the same phase as the reactant.
What is a Heterogeneous Catalyst?
The catalyst is a different phase as the reactant, often it is solid.
The reactant(s) adsorb onto the solid surface, which weakens the bonds.
What is an example of a heterogeneous catalyst?
Hydrogenation, which means adding hydrogen.
