Chem #5 Flashcards
what does a change in Gibbs free energy tell us?
will tell us whether or not a reaction will occur by itself without outside assistance.
o Spontaneous: negative ΔG,
o Almost all biochemical reactions that allow life to persist are spontaneous but would be so slow without enzymes.
intermediate
molecule that appears in the steps of the mechanism but not the overall mechanism.
rate determining step
the rate of the whole reaction is only as fast as this step.
the slowest step in the reaction mechanism
The rate is always related to the concentrations of ____
the reactants in the rate-determining step.
collision theory of chemical kinetics
the rate of a reaction is proportional to the number of collisions per second between the reacting molecules.
Each reaction is all or nothing. Either it has enough energy to occur or it does not.
The reaction with the ____ activation energy will have the fastest rate.
lowest
activation energy
or energy barrier: the minimum energy of collision necessary for a reaction to take place
not all collisions have enough energy
rate = Z x f
• Z: total total # collisions per second and f: fraction of collisions that are effective
Can also be expressed as the Arrhenius equation
• Frequency factor (attempt frequency): a measure of how often molecules in a certain reaction collide, with the units 1/s.
o can be increased by increasing the number of molecules in a vessel.
frequency factor
a measure of how often molecules in a certain reaction collide, with the units 1/s.
o can be increased by increasing the number of molecules in a vessel.
how does transition state and intermediates differ
transition state is at top of peak in reaction coordinate and intermediates are if the peak comes back down and goes up again before forming products.
transition state
state (activated complex): highest energy structure of the reaction, theoretical, old bonds are weakened while new bonds are being formed.
NOT intermediates as they are just theoretical.
Transition state energy requirement is the activation energy
Once at the transition state can either become products or revert back to reactants.
can the transition state revert back to reactants?
yes
are transition state connections real?
no
free energy change of the reaction ΔGrxn
): the difference between the free energy of the products and the free energy of the reactants.
Exergonic (+ΔG): energy is released
Endergonic (+ΔG): energy is absorbed.
what does an exergonic vs. endergonic reaction diagram look like?
exergonic, products have less energy than reactants (products minus reactants is negative)
exergonic reaction = _____
spontaneous
what are 4 factors that affect the rate of a reaction?
reactant concentration, temperature, medium, catalysts
how does reaction concentration affect the reaction rate?
increasing concentration increases reaction rate (more collisions)
Zero-order reactions do not increase because of this
For reactions in gaseous state, the PP of the gases serve as measure of concentration
how does temperature affect the reaction rate?
: increasing temperature increases the kinetic energy of particles and thus particles gain enough energy to surpass the activation energy
At a certain temperature, for biological systems, the reaction rate will begin to decrease (there is an optimal zone)
how does the medium affect the reaction rate?
affects the reaction in different ways. polar solvents are often preferred.
how does a catalyst affect the reaction rate?
can do many things to increase the rate of reaction: increase frequency of collisions, change the relative orientation of reactants, donate electron density, reduce intramolecular bonding so easier to break, etc.
homogenous catalysts
the catalyst is in the same phase (solid, liquid, gas) as the reactants
heterogeneous catalysts
the catalyst is in a distinct phase than the reactants.
WHAT IS VERY IMPORTANT TO REMEMBER ABOUT CATALYSTS
NOTE: only affect the Ea, have no effect on the equilibrium position or the measurement of Keq.
• Affect Ea in forward and reverse direction by the same factor.
compare the rate law and equilibrium expression
The expression for equilibrium includes the concentrations of all species in the reaction, both reactants and products. The expression for chemical kinetics (rate law) includes only the reactants. The Keq indicates where the reaction’s equilibrium position lies, whereas the rate indicates how quickly the reaction will get there.
how does Keq compare the forward and reverse rate laws
Keq = kforward/kreverse
the rate constant is specific for _______
a specific reaction at a specific temperature
zero order reaction
one in which the rate of formation of product is independent of changes in concentrations of any of the reactants.
Rate is solely dependent on k
Only way to change is depends on addition of catalyst or increase in temperature.
first order reaction
: has a rate that is directionally proportional to only one reactant. Doubling the concentration of this reactant doubles the rate.
Suggests that a molecule undergoes chemical change all by itself.
order of the reaction
add up the exponents (experimentally determined) of the rate law
second order reaction
: a rate that is proportional to either the concentration of two reactants or to the square of the concentration of a single reactant.
Can suggest a collision between two reactants.
higher order reactions
very rare because you need 3 molecules colliding at once.
mixed order reactions
reactions with rate orders that vary over the course of the reaction.
Be wary that at different concentrations of a reactant, the rate order can take on different values.
mechanism
the steps of a reaction
proposed pathways for a reaction
do all collisions result in a chemical reaction?
No. They must have proper orientation and sufficient energy.
calculate the rate of a chemical reaction (formula)
rate = Z x f
Z: total number of collisions occurring per second and f is the fraction of collision that are effective
write out the arrhenius equation
Ae^(-Ea/RT)
T in Kelvin
R is ideal gas constant
A is frequency factor (how often molecules in a certain reaction collide, units are inverse seconds)
what relationships exist between variables in the arrhenius equation?
As A increases, k increases
As T increases, k increases
As Ea decreases, k increases
in a reaction diagram, which structure has the highest energy?
transition state
in a reaction diagram, when the transition state forms, what happens next?
can proceed to products or revert to reactants
can transition states be isolated?
no, they are only theoretical.
______ solvents are preferred for reactions because their molecular dipole tends to polarize the bonds of the reactants which weakens them
polar
what is involved in a reaction but is not used up?
catalyst
by what factor do catalysts change the forward and reverse rates of a reaction
the same factor
transition state theory vs. collision theory
both involve the requirement of meeting the activation energy for a reaction to occur.
transition state theory: after the activation energy is reached, a high energy activated complex is formed which can either proceed to products or revert back to reactants.
collision theory: collisions between reactants are what makes reactions occur and they must occur with sufficient energy and proper orientation
what is the difference between rate constant and rate law
rate law: rate = k[A][B]
rate constant: k
are the orders of a reaction the same as the stoichiometric coefficients?
NO
2 exceptions:
1. the reaction mechanism is a single step
2. the complete reaction mechanism is given, the rate determining step is identified, and there are no intermediates as reactants (or it gets more complicated)
is the rate constant actually a constant?
only for a specific reaction at a specific temperature
is it possible to change the rate of a 0 order reaction?
yes, by changing the temperature or adding a catalyst (just can’t be changed based on concentrations of the reactants)
can a rate constant be changed for a particular reaction?
yes by changing the temperature
broken order reaction
non-integer orders (fractions)
