Ch 5 - Chemical Kinetics

Question 1

What does the change in Gibbs free energy determine?What does a negative/positive Gibbs free energy correlate with?

Answer 1

• whether or not a reaction is spontaneous
• +G = endergonic = energy absorbed
• -G = exergonic = energy given off

Question 2

What do chemical mechanisms propose?

Answer 2

a series of steps that make up the overall reaction

Question 3

What are intermediates?

Answer 3

molecules that exist within the course of a reaction but are neither reactants nor products overall

Question 4

What is the rate-determining step?

Answer 4

• slowest step
• determines the overall rate of the reaction because the reaction can only proceed as fast as this step
• rate related to the concentration of the reactants in the rate determining step

Question 5

What is the collision theory?

Answer 5

states that a reaction rate is proportional to the number of effective collisions between the reacting molecules
- focuses on energy and orientation of reactants, and considers each potential reaction to be all or nothing (either enough energy to form products or not)
rate = Z x f

Question 6

What must happen in order for a collision theory to be effective?

Answer 6

molecules must be in the proper orientation and have sufficient kinetic energy to exceed the activation energy

Question 7

What is the Arrhenius equation?

Answer 7

a mathematical way of representing collision theory
k = Ae^(-Ea/RT)
- low Ea and high temperature make the negative exponent of the equation smaller in magnitude and increase the rate constant k

Question 8

What is the transition state theory?

Answer 8

forms high energy activated complex that can proceed forward or backward, forming products or reverting to reactants

Question 9

Where does the reaction proceed from the transition state?

Answer 9

toward products or revert back to reactants

Question 10

Where is the transition state on a free energy reaction diagram?

Answer 10

the highest point

Question 11

How does the concentration of the reactant affect the reaction rate?

Answer 11

increasing the concentration of reactant will increase reaction rate (except for zero-order reactions) because there are more effective collisions per time

Question 12

How does the temperature affect the reaction rate?

Answer 12

increasing the temperature will increase reaction rate because the particles’ kinetic energy is increased

Question 13

How can changing the medium affect the reaction rate?

Answer 13

changing the medium can increase or decrease the reaction rate, depending on how the reactant interact with the medium

Question 14

How does a catalyst affect the reaction rate?

Answer 14

adding a catalyst increases reaction rate because it lowers the activation energy

Question 15

What is the difference between a homogenous and heterogeneous catalyst”

Answer 15

• homo: same phase as the reactants

- hetero: different phase

Question 16

How are reactions rates measured?

Answer 16

in terms of the rate of disappearance of a reactant or appearance of a product

Question 17

What is the form for rate laws?

Answer 17

rate = k[A]^x[B]^y[C]^z
determine how rate is affected during trials
change in rate = change in […]^*

Question 18

How are rate laws determined?

Answer 18

must be from experimental data

Question 19

What is the rate order?

Answer 19

• the sum of all individual rate orders in the rate law

- usually do not match the stoichiometric coefficient

Question 20

What are zero-order reactions and how are they graphed?

Answer 20

• have a constant rate that does not depend on the concentration of reactant
  rate = k[A]^0[B]^0
• concentration v time curve is a straight line; the slope is equal to -k

Question 21

What affects the rate of a zero-order reaction?

Answer 21

only changing the temperature or adding a catalyst

• lower temperature = decreased rate
• added catalyst = increased rate

Question 22

What are first order reactions and how are they graphed?

Answer 22

• has a rate that is directly proportional to only one reactant such that doubling the concentration of that reactant results in a doubling of the rate of formation of product
  rate = k[A]^1 or rate = [B]^1
• concentration v time curve is non linear
• the slope of a ln[A] v time plot is -k

Question 23

What is a second order reaction and how are they graphed?

Answer 23

• has a rate that is proportional to either the concentrations of 2 reactants or to the square of the concentrations of a single reactant
  rate = k[A]^1[B]^1 or rate = k[A]^2 or rate = k[B]^2
• concentration v time curve is non linear
• slot of a 1/[A] v time plot is k

Question 24

What are broken order reactions?

Answer 24

those with non integer orders

Question 25

What are mixed order reactions?

Answer 25

those that have a rate order that changes over time

Question 26

What is activation energy?

Answer 26

the minimum energy required for a chemical reaction to occur - on the graph of reaction coordinate v free energy, Ea represented by the distance on the y axis from the energy of the reactants to the peak energy prior to formation of products

Question 27

What affects the rate of a first order reactions?

Answer 27

- lower temperature = decreased rate - all reactants' concentrations double = rate doubled - catalyst added = rate increased

Question 28

What affects the rate of a second order reactions?

Answer 28

- lower temperature = decreased rate - all reactants' concentrations double = rate x 4 - catalyst added = rate increased

Question 29

What type of reaction is a certain equilibrium process if the activation energy of the forward reaction (Gf) is greater than the Ea of the reverse reaction (Gr) and why?

Answer 29

nonspontaneous reaction - if the Ea of the forward reaction is greater than the Ea of the reverse reaction, then the products must have a higher free energy and the overall energy of the system is higher at the end that it was at the beginning, meaning the net free energy is positive (nonspontaneous)

Question 30

How do saturated catalysts affect reactions?

Answer 30

saturated solutions containing a catalyst have a maximum turnover rate and cannot increase the rate constant or the reaction rate any higher by adding more reactant molecules

Question 31

What are exergonic reactions?

Answer 31

- if energy is released by the reaction - the net energy change is negative, and the free energy of the final products is lower than the free energy of the initial reactants