LO 29 - 35 Flashcards
What does the Arrhenius equation relate?
relates the rate constant k to temperature in kelvin
What are the variables of the following equation?
What does it relate?
K = A · e^ (-Ea ∕ R·T)
K - rate constant
A - frequency factor
Ea - activation energy
R - 8.314 J/mol · K
T - kelvin
This equation relates the rate constant k to temperature
What does (A) in the Arrhenius equation represent?
frequency factor which is the number of times the reactants approach the activation barrier per unit time
What does (Ea) in the Arrhenius equation represent?
activation energy needed to achieve the activation complex
The greater the activation energy needed, the ______ the reaction rate
slower
What is the exponential factor in the Arrhenius equation?
a number between 0 and 1 that represent the fraction of molecules that have enough energy to make it past the activation barrier on a given approach
Low activation energy + high temperature = _______ exponential factor
small, negative so the value approaches 1 (e^-0 = 1)
When the exponential factor approaches 0 what does that signal about the reaction?
since e^-0 = 1, it means that every approach is successful, lots of products
High activation energy + low temperature = ______ exponential factor
large, negative so the value approaches 0 (e^-∞ = 0)
When the exponential factor approaches a large number what does that signal about the reaction?
since e^-∞ = 0, it means there are no successful approaches, no products
The sensitivity of reaction rates to temperature depends on the ___________ of the activation barrier.
magnitude
The greater the activation barrier, the ________ the temperature sensitivity of reaction rate
greater
The frequency factor for most gaseous reaction tends to be _____ than the # of collisions per second.
Smaller
How is the Arrhenius equation
(K = A · e^ (-Ea ∕ R·T) modified for gaseous reactions
K = PZ · e^ (-Ea ∕ R·T)
P - collision frequency
Z - orientation factor to represent fraction collisions with proper orientation
An orientation factor (Z) close to _____ means that the reaction is unspecific, while Z close to ________ would be very specific
1 , 0
The reaction mechanism is the …
series of individual, elementary steps by which a reaction occurs
In a given elementary step, the number of reactants is represented by the ______
molecularity
The ________ is much slower than other elementary steps and therefore determines the ________ for the overall reaction
rate determining step
rate law
What are the conditions of the rate determining step?
- the elementary steps in the mechanism must sum to the overall reaction
- the rate law predicted must be consistent with experimentally determined results
What happens when the first elementary step in mechanism is a “fast” step?
it means a subsequent step containing intermediates is the rate determining step and we thus have to express concentrations of intermediates in terms of reactants
Reaction rates can be increased using a ________ which is NOT consumed during the reaction
catalyst
Catalysts provide an ________ typically with a lower Ae for the rate determining step
alternative mechanism
the catalyst exists in the same phase as the reactants
homogenous
the catalyst exists in a different phase than the reactants
heterogenous
What is hydrogenation?
the catalyzed addition of H to alkene double bonds to make single bonds
What are the steps of catalyzation in the presence of a metal?
- adsorption of reactants onto metal surface
- diffusion of reactants across the surface until they reach each other
- reaction
- desorption of products off the metal surface into the gas phase
Most of the reactions we need to survive occur too slowly at room temperature, so we rely on ______________ to speed up reactions
enzymes as biological catalysts
The concentrations of reactants and products in equilibrium are described by the ____________
equilibrium constant K
A large K value indicates a high concentration of __________.
A small K value indicates a high concentration of _________.
Products
Reactants
_____________ for a chemical reaction is the condition in which the rate forward equals the rate of the reverse reaction
dynamic equilibrium
_________ remain constant at equilibrium unless there is a change in _________
concentrations
temperature
equilibrium constant K is described by the law of mass action in which ….
K = [C]^c · [D]^d / [A]^a · [B]^b
concentrations of products OVER reactants raised to their stoichiometric coefficients
Given a chemical equation, if we reverse the reaction, we _____ K
invert
K reverse = (1/ K forward)
Given a chemical equation, if we multiple coefficients by some factor x then we ________
raise K to the same factor
K^x
Given multiple chemical equations, when adding to get the overall reaction we ________ the corresponding K values
multiply
K 1 · K 2 = K overall
For gaseous reactions we express K in terms of ________
partial pressure
Kc means _________
Kp means ________
K in terms of concentration
K in terms of partial pressure
How do you go from Kc to Kp?
Kp = Kc (R · T) ^ Δn
where Δn is the mols gaseous products - mols gaseous reactants
Is Kp the same as Kc?
NO! they’re only equal if Δn = 0
