chapter 6

Question 1

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?

Answer 1

• the activatione energy (E_a) of a reaction

Question 2

the rate of a reaction corresponds to?

Answer 2

• how many such collisions take place over a given unit of time

Question 3

the intuition is mathematically codified in the Arrhenius equation which defines k, the rate constant:

Answer 3

• 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
    • E_a is the activation energy
    • R is the ideal gas constant
    • T is temperature in Kelvins
    • e is a mathematical constant

Question 4

the rate constant can only meaningfully be affected by?

Answer 4

• 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

Question 5

The reacton rate of gaseous reactions can be affected by?

Answer 5

• pressure
  
  • higher pressures increase the reaction rate for essentially the same reason that higher temps do (more collisions)

Question 6

reactions that collide with sufficient energy in the proper orientation form a short-lived?

Answer 6

• transition complex
  
  • they are not isolable and occur at the highest-energy point of the reaction

Question 7

what is the slowest step of a reaction called?

Answer 7

• the rate limiting step

Question 8

what is a reaction coordinate diagram?

Answer 8

Question 9

exergonic vs endergonic graph:

Answer 9

Question 10

what are catalysts?

Answer 10

• 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

Question 11

catalysts do not affect what parameters of the overall reaction?

Answer 11

• thermodynamic

Question 12

what is heterogenous catalysis vs homogenous catalysis?

Answer 12

• 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

Question 13

what is the rate law of a reaction?

Answer 13

• 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

Question 14

how is the order of a reaction defined?

Answer 14

• 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

Question 15

how can the rate law be determined?

Answer 15

• 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

Question 16

what are the units of k?

Answer 16

• 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)

Question 17

physiologically, zero-order reactions are exemplified by?

Answer 17

• enzyme-catalyzed reactions in which the enzyme is saturated

Question 18

first order reactions are exemplified by?

Answer 18

• radioactive decay and S_N1 reactions that are dependent on carbocation formation

Question 19

second order reactions physically involve?

Answer 19

• collisions between two reactant moelcules as in S_N2 reactions

Question 20

graphs for the rate laws:

Answer 20

• 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