Ch. 5: Chemical Kinetics Flashcards
(38 cards)
what does change in Gibbs free energy (delta G) denote?
determines whether or not a reaction will occur by itself without outside assistance (spontaneous or not)
defn: reaction mechanism
proposed pathways/ the series of steps that proceed in order for a reaction to occur
sum of the steps gives the overall reaction
defn: rate-determining step
the slowest step in any proposed mechanism
acts like a kinetic bottleneck, preventing the overall reaction from proceeding any faster than that slowest step
defn + char + implications: collision theory of chemical kinetics
the rate of a reaction is proportional to the number of collisions per second between the reacting molecules
- not all collisions result in a chemical reaction
- an effective collision (one that leads to product formation) only occurs if the molecules collide with each other in the correct orientation and with sufficient energy to break their existing bonds and form new ones
defn + aka: activation energy (Ea)
part of the collision theory of chemical kinetics
the minimum energy of collision necessary for a reaction to take place
aka: energy barrier
eqn: rate of a rxn (collision theory of chemical kinetics)
rate = Z x f
Z = total number of collisions occurring per second
f = the fraction of collisions that are effective
eqn + defn: Arrhenius equation (collision theory of chemical kinetics) + defn: frequency factor
a more quantitatively rigorous analysis of the collision theory
k = Ae^ (-Ea/RT)
k = rate constant of a reaction
A = the frequency factor
Ea = activation energy of the reaction
R = ideal gas constant
T = temperature (Kelvin)
defn: frequency factor = the attempt frequency of the reaction = a measure of how often molecules in a certain reaction collide (unit = s ^-1)
what relationships can you infer from the Arrhenius equation? (k = Ae^(-Ea/RT)
As A increases, k increases
reaction rate increases with temperature because exponent becomes smaller (becomes less negative)
how can the frequency factor (A) be increased?
by increasing the number of molecules in a vessel
more molecules = more opportunities for collision
expl: transition state theory
when molecules collide with energy equal to or greater than the activation energy, they form a transition state in which the old bonds are weakened and the new bonds begin to form
the transition state then dissociates into products, fully forming the new bonds
defn: reaction coordinate (transition state theory)
traces the reaction from reactants to products, through the transition state
defn + aka + char: transition state
aka: activated complex
a theoretical intermediate between reactants and products
has greater energy than both the reactants and the products
is denoted by sort of a double cross symbol
once it is formed it can either dissociate into products or revert to reactants without any additional energy input
what is the difference between transition states and reaction intermediates?
transition states are theoretical constructs that exist at the point of max energy, rather than reaction intermediates which are distinct identities with finite timelines
defn: free energy diagram
illustrates the relationship between the activation energy, the free energy of the reaction, and the free energy of the system
defn: free energy of the reaction (deltaGrxn)
the difference between the free energy of the products and the free energy of the reactants
defn: exergonic and endergonic reaction (in terms of free energy)
EXERGONIC REACTION = a negative free energy change // energy is given off
ENDERGONIC REACTION = a positive free energy change // energy is absorbed
where is the transition state in the energy diagram?
at the peak of the energy diagram
what are the four factors affecting reaction rate in chemical kinetics?
- reaction concentrations
- temperature
- medium
- catalysts
how does reaction concentration affect reaction rate?
the greater the reactants’ concentrations, the greater the number of effective collisions per unit time
this leads to an increase in frequency factor (A)
SO reaction rate will increase for all but zero-order reactions
how does temperature affect reaction rate?
for nearly all reactions, the reaction rate will increase as the temperature increases
substance temperature = a measure of the particles’ average kinetic energy
increasing the temperature increases the average kinetic energy of the molecules
also the proportion of reactants gaining enough energy to surpass Ea (and thus can undergo reaction) increases with higher temperature
ALL REACTIONS ARE TEMPERATURE DEPENDENT AND EXPERIENCE AN OPTIMAL TEMPERATURE FOR ACTIVITY
what temperature is an enzymatic reaction optimal at? what happens after this optimal point?
35 - 40 deg. C (body temperature)
after 40 deg, the curve falls sharply because denaturation has occurred
are polar or nonpolar solvents generally preferred in terms of reaction rate?
polar –> bc the molecular dipole tends to polarize the bonds of the reactants, thereby lengthening and weakening them, allowing the rxn to occur faster
defn + char: catalyst
defn: substances that increase reaction rate without themselves being consumed in the reaction
char:
- interact with the reactants by adsorption or through the formation of intermediates
- stabilize reactants so as to reduce the activation energy necessary for the reaction to proceed
- return to their original chemical state upon product formation
how can catalysts affect reaction rate?
- increase the frequency of collisions between the reactants
- change the relative orientation of the reactants (making higher % of the collisions effective)
- donate electron density to the reactants
- reduce intramolecular bonding within reactant molecules
