Chapter 5: Chemical Kinetics Flashcards
Reactions can be spontaneous or nonspontaneous; the change in ________ ________ ____________ (∆G) determines whether or not a reaction will occur by itself without outside assistance.
Gibbs free energy
Many reactions proceed by more than one step, the series of which is known as the ________________ of a reaction, and the sum of which gives the overall reaction.
mechanism
The slowest step in any proposed mechanism is called the ________-____________ step.
rate-determining
The ____________ theory of ____________ ____________ states that the rate of a reaction is proportional to the number of collisions per second between the reacting molecules.
collision theory of chemical kinetics
The minimum energy of collision necessary for a reaction to take place is called the ____________ ____________.
activation energy
Rate of reaction can be expressed with which formula?
rate = Z x f
Z = # of collisions per second
f = fraction of effective collisions
What equation more accurately represents collision theory? What is it?
Arrhenius equation
k = Ae-Ea/RT
k = rate constant
A = frequency factor
R = ideal gas constant
Ea = activation energy
T = temp
The ____________ ____________ is a measure of how often molecules in a certain reaction collide, with the units s-1
frequency factor
As the exponent in the Arrhenius equation becomes less negative (more positive), the rate constant (decreases/increases?)
increases
How can the frequency factor of a reaction be increased?
By increasing the number of molecules in a vessel
When molecules collide with energy equal to or greater than the activation energy, they form a ________________ ________ in which the old bonds are weakened and the new bonds begin to form.
transition state
How much energy does the transition state have compared to reactants and products?
Greater energy than both of them
The energy required to reach the transition state is called ?
activation energy
Are transition states and intermediates the same thing?
No, TS are theoretical constructs that exist at the point of max energy, rather than distinct identities with finite lifetimes.
A negative free energy indicates that a ____________ reaction, meaning energy is ____________.
exergonic, given off
A positive free energy change indicates an ________________ reaction, where energy is ____________.
endergonic, absorbed
The difference in free energy between the transition state and reactants is the ____________ ____________.
activation energy
How does reactant concentration affect the number of effective collisions?
Increases!
An increase in the # of effective collisions per unit time (increases/decreases?) the frequency factor (A) of the Arrhenius equation.
increases
For an increase in [reactant], reaction rate will increase for all except which reactions?
Zero-order
For reactions in the gaseous state, what serves as a measure of concentration?
partial pressure
How does temperature affect reaction rate?
Increases it
Which type of solvents will increase reaction rate? Why?
polar solvents, their dipole tends to polarize the bonds of reactants, which lengthens and weakens them
In ____________ catalysis, the catalyst is in the same phase as the reactants.
homogeneous
Catalysts only change what?
Rate of reaction; no effect on free energy
By convention, we place a (+/-?) sign in front of the rate expression for reactants, and a (+/-?) sign for the products.
-, +
For nearly all forward, irreversible reactions, the rate is proportional to what?
the concentration of reactants, with each concentration raised to some experimentally determined exponenet
The stoichiometric coefficients and orders of a reaction usually don’t match on the MCAT. What are the 2 cases where they would?
- When the reaction mechanism is a single step and the balanced overall reaction is the entire chemical process
- When the complete reaction mechanism is given and the rate-determining step is indicated
The stoichiometric coefficients on the reactant side of the ________-________________ step are equal to the orders of reaction.
rate determining
If the rate-determining step involves an intermediate as a reactant, one must derive the intermediate molecule’s concentration by the ________ of ________ ________ for the step that produced the intermediate.
law of mass action
For a reversible reaction, the Keq is equal to what?
The ratio of the rate constant for the forward reaction, k, divided by the rate constant for the reverse reaction, k-1
A ________-________ reactio nis one in which the rate of formation of product C is independent of changes in concentrations of any of the reactants.
Their reaction rate is contant and equal to ____, the rate constant.
zero order; rate constant
How can you speed up a zero order rxn?
Increase rate or add catalyst
In a zero order rxn’s graph, k is equal to what?
-slope
A ________-________ reaction has a rate that is directly proportional to only one reactant, such that doubling the concentration of that reactant results in a doubling of the rate of formation of product.
first-order
The unit for k for a first-order reaction is?
1/s
A first-order rate law with a single reactant suggests that the reaction begins when what?
The molecule undergoes a chemical change all by itself, without a chemical interaction (and usually without any other molecule_
In a first order reaction, plotting what will reveal a straight line?
ln[A] vs. time
In a first order reaction, what is the rate constant k equal to?
-slope in a graph of ln[A] vs. time
A ____________-________ reaction has a rate that is proportional to either the concentrations of 2 reactants or to the square of the concentration of a single reactant
second-order
How can you plot a second-order rxn to yield a linear curve?
1/[A] vs. time
In a second order reaction, what is the rate constant k equal to?
the slope of 1/[A] vs. time
