Chemical Kinetics

Question 1

Transition state

Answer 1

• (Also called an activated complex)
• unstable, high-energy state
• briefly exists between a reactant and intermediate, or between a reactant and product.
• At the peak of the hill in a reaction coordinate diagram
• state of the reactant(s) in which old bonds are breaking and new bonds are forming
• both the incoming group and the leaving group are present transiently on the molecule
• cannot be isolated from a reaction mixture

Question 2

What does the collision theory state?

Answer 2

for a chemical reaction to occur:
1. molecules must collide
2. Both molecules must collide with enough energy: molecules must have sufficient energy to overcome the activation energy
3. Both molecules must collide in the propert 3D orientation

Question 3

What increases the collision frequency?

Answer 3

Increasing temperature increases the collision frequency and percentage of high energy collisions

Question 4

What will affect the rate of an irreversible chemical reaction?

Answer 4

Increasing the concentration of the reactants will affect the rate

A chemical reaction occurs when two or more reactant molecules hit each other with enough fore and in the proper orientation. The more molecules hitting each other, the higher the chance of a chemical reaction and the faster the reaction proceeds. Therefore, the higher the concentration of the reactants, the higher the chance for the molecules to strike (collide) each other, causing a reaction and an increase in the rate

Question 5

According to the Arrhenius equation, an increase in temperature of a chemical reaction results in:

Answer 5

Increase in reaction rate
Increase in collision frequency
Increase in rate constant

Question 6

what does chemical kinetics explain?

Answer 6

• the speed of a reaction
• answers the question: “how fast?”
• spontaneity does not affect speed of a reaction

Question 7

energy diagram for an exothermic reaction:

Answer 7

• products are lower in energy than reactants

Question 8

activation energy

Answer 8

• refers to the minimum amount of energy the reactants must possess in order for the reaction to occur
• energy where reactants start to top of the “hill”
• even if a reaction is spontaneous, if the minimum amount of energy is not provided, the reaction will not occur

Question 9

what is a catalyst?

Answer 9

• a substance that lowers the activation energy
• speeds up the rate of the reaction
• provides an alternate pathway for the reaction
• they are not consumed by the reaction
• they are not altered in the process, so they are present at the end of the reaction

Question 10

what are intermediates in energy diagrams?

Answer 10

• species that are formed during the course of the reaction
• they are not part of the reactants or the products
• their energy is less than the transition states energy
• possible to isolate them from a mixture
• there can be more than one

stopovers on a road trip

Question 11

zero-order reaction rate law expression

Answer 11

rate = k[A]⁰ = k(1) = k
* anything raised to the power of 0 = 1

Question 12

zero-order reactions:

Answer 12

• rate = k
• the rate of the reaction does NOT depend on the concentration of the reactants
• increasing/decreasing concentration of reactants does NOT change the rate of the reaction

Question 13

first order reaction rate law expression:

Answer 13

rate = k[A]¹ = k[A]

Question 14

first order reactions

Answer 14

• rate = k[A]
• rate of the reaction is proportional to the concentration of the reactant
• if the concentration of A were to double, the rate would also double

Question 15

second order rate law expression

Answer 15

• rate = k[A]² or
• rate = k[A][B]

Question 16

second order rate law

Answer 16

rate = k[A]²
* the rate of the reaction is proportional to [A]²
* if we double [A], the rate of the reaction would increase by 2²
rate = k[A][B]
* the rate of the reaction is directly proportional to both [A] and [B]
* halving [A] would also halve the rate

Question 17

distinguishing between catalysts and intermediates:

Answer 17

• catalyst: a reactant first and is then regenerated as a product
• intermediate: is a product first and then a reactant

Question 18

arrhenius equation

Answer 18

• describes the temperature dependent nature of reaction rates
• k = Ae^-E_a/RT
• k = rate constant
• A = arrhenius constant
• e = mathematical constant (~2.71)
• E_a = activation energy
• R = universal gas constant
• T = temperature

temp and activation energy are the only ones that can be manipulated

Question 19

what would an increase in T (in the arrhenius equation) result in?

Answer 19

• increase in temperature = molecules move faster and collide more often = increase in collision frequency = increased percentage of high energy collisions - sufficient energy to overcome the activation energy barrier
• increase in temperature = increase in the rate of the reaction = increase in k

k and rate of reaction are DIRECTLY related

Question 20

what would an increase in E_a (in the arrhenius equation) result in?

Answer 20

• increase in activation energy = rate of the reaction slows down = decrease in k

Question 21

how does increasing or decreasing T affect E_a?

Answer 21

• it doesn’t
• temperature and activation energy are independent of each other
• the only thing that can alter the activation energy is a catalyst

Question 22

According to the Arrhenius equation, a decrease in E_a of a chemical reaction results in:

Answer 22

• increase rate constant (k)
• increase in rate of the reaction