for a reaction to occur, the reactants must collide with each other in the correct spatial orientation and with sufficient kinetic energy for the reaction to happen, a threshold which is known as?
- the activatione energy (Ea) of a reaction
the rate of a reaction corresponds to?
- how many such collisions take place over a given unit of time
the intuition is mathematically codified in the Arrhenius equation which defines k, the rate constant:
- k can be thought of in this context as corresponding to the rate of a reaction
- k = Ae-Ea/RT
- k is the rate constant
- A is the frequency factor whcih corresponds to the frequency of collisions between reactants in proper spatial orientation
- Ea is the activation energy
- R is the ideal gas constant
- T is temperature in Kelvins
- e is a mathematical constant
- k = Ae-Ea/RT
the rate constant can only meaningfully be affected by?
- the activation energy and temperature
- if Ea/RT decreases, the rate constant will increase, and if it increases, the rate constant will decrease
- larger values of Ea make -(Ea/RT) more negative, demonstrating an inverse relationship between activation energy and the reaction rate
- lower activation energies correspond to faster reaction rates\temperature has a direct relationship with reaction rate; higehr values of T make Ea/RT smaller, meaning that -(Ea/RT) is less negative or larger
- chemically, higehr temperatures have the effect of both increasing the overall collision rate and the proportion of those collisions that will have KE values exceeding the Ea
- if Ea/RT decreases, the rate constant will increase, and if it increases, the rate constant will decrease
The reacton rate of gaseous reactions can be affected by?
- pressure
- higher pressures increase the reaction rate for essentially the same reason that higher temps do (more collisions)
reactions that collide with sufficient energy in the proper orientation form a short-lived?
- transition complex
- they are not isolable and occur at the highest-energy point of the reaction
what is the slowest step of a reaction called?
- the rate limiting step
what is a reaction coordinate diagram?
exergonic vs endergonic graph:
what are catalysts?
- substances that increase the rate of a reaction by reducing its activation energy
- stabilize the TS, weakening bonds within the reactants, changing the orientation of the reactants to facilitate effective collisions and donating electron density to the reactants so catalysts provide an alternative reaction mechanism
catalysts do not affect what parameters of the overall reaction?
- thermodynamic
what is heterogenous catalysis vs homogenous catalysis?
- In heterogenous catalysis, the catalyst is in a different phase from the reactants; typically a solid and the reactants are liquids or gases (depends on SA)
- Homogenous catalysis occurs when the catalyst and reactamts are in the same phase
what is the rate law of a reaction?
- is given as a function of the rate constant k and the concentration of some or all of the reactants
- the rate law of a general reaction of the form aA + bB → cC + dD is rate = k[A]x[B]y
- the rate itslef has units of M/s or mol/L•s
- [A] and [B] are the concentrations of the reactants in units of M or mol/L
- the exponents x and y must be experimentally determined
- the rate law of a general reaction of the form aA + bB → cC + dD is rate = k[A]x[B]y
how is the order of a reaction defined?
- as the sym of the exponents in the rate law; x and y
- if the exponents sum to 0, the reaction is zero-order
- if the exponents sum to 1, the reaction is first-order
- if the exponents sum to 2, the reaction is second-order
- if the exponents sum to 3, the reaction is third-order
how can the rate law be determined?
- experimentally using the method of initital rates
- in this method, multiple trials are run with variations in the concentration of individual reactants. the initial rate is then measured, with the goal of identifying how changes in the concentration of a reactant affect the rate
what are the units of k?
- zero order: M•s-1, M/s, mol•L•s-1, mol/(L•s)
- first order: s-1, 1/s, s-1, 1/s
- second order: M-1•s-1, 1/(M•s), mol-1•L•s-1, L/(mol/s-1)
- third order: M-2•s-1, 1/(M2•s), mol-2•L2•s-1, L2/(mol2•s)
physiologically, zero-order reactions are exemplified by?
- enzyme-catalyzed reactions in which the enzyme is saturated
first order reactions are exemplified by?
- radioactive decay and SN1 reactions that are dependent on carbocation formation
second order reactions physically involve?
- collisions between two reactant moelcules as in SN2 reactions
graphs for the rate laws:
- zero is linear because the rate of the reaction never changes based on the concentration of the reactants
- first oder is non-linear, but transforming the graph to be a plot of the natural logarithm of the reactant as a function of time results in a linear plot
- for a second order reaction in which the rate is dependent on one reactant, a graph of [A] versus time is again non-linear, but transforming the graph to a/[A] versus time results in a linear graph
