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.
