unit 6

Question 1

reaction mechanism

Answer 1

sequence of bond breaking and bond making to make reactions happen
- a series of small steps

Question 2

rate of reaction

Answer 2

describes how quickly rxn happens
- change in concentration of reactant or product in respect to time

Question 3

the 2 ways of making a reaction occur faster

Answer 3

• inc. # of collisions
• inc. the fraction of effective collisions

Question 4

how does temp affect reaction rate

Answer 4

• temp: average kinetic energy of molecules
• more kinetic energy=faster moving molecules-> more collisions increasing rate of reaction

Question 5

temp increase of what to double the rate of rxn

Answer 5

10C

Question 6

how does concentration affect reaction rate

Answer 6

• conc: amount of material in given space
• higher conc=inc. change that 2 molecules will collide -> inc. rate of rxn

Question 7

how does surface area/particle size affect reaction rate

Answer 7

• SA/particle size: a sample of matter that is able to react + visible to other particles
• more SA=more places of contact for reactants to collide

Question 8

how does pressure affect reaction rate (GASES)

Answer 8

• high pressure compresses gas, inc. concentration -> inc. frequency of collisions

Question 9

how does a catalyst affect reaction rate

Answer 9

• can lower the amount of energy needed to break bonds
• can also help line up reactants in the right orientation and guide collision

Question 10

catalyst

Answer 10

substance that inc. the rate of rxn without being consumed. or consumed but reformed

Question 11

stoichiometry in reaction rates

Answer 11

• no indication of rates of rxn but tells us relative rate of consumption of reactants and formation of products

Question 12

how to find rate of consumption/rate of formation when given one or the other

Answer 12

• look at the stoichiometric ratios or use rate expression

Question 13

rate graph: reactant concentration vs time graph

Answer 13

• decreases quickly, then decreases slowly (exponential)

Question 14

rate graph: product concentration vs time graph

Answer 14

• increases quickly, then increases slowly (exponential)

Question 15

when is the rate of reaction fastest? why

Answer 15

when t=0
- there are no products yet, so all the collisions occur bw reactants, all being successful
- more products made = higher fraction of ineffective collisions

Question 16

what do effective collisions depend on

Answer 16

• sufficient energy
• correct orientation/alignment

Question 17

what is the unstable activation complex, when does it occur

Answer 17

• short-lived high energy, transition state where old chemical bonds break and new ones form
• peak of activation energy barrier

Question 18

why doe maxwell boltzmann energy distribution curves never touch the x axis (except at 0)

Answer 18

• there is always a possibility that a molecule can have a higher average kinetic energy

Question 19

activation energy

Answer 19

minimum collision energy required for a successful reaction
- energy required to form an activated complex from reactants

Question 20

most probable speed vs. average speed on maxwell-boltzmann curve

Answer 20

found at the peak of the curve vs. found slightly to the right of the peak (bcus of the asymptote)

Question 21

axises of maxwell-boltzmann energy distribution graph

Answer 21

x: kinetic energy
y: number of molecules (with energy)

Question 22

how does temperature affect a maxwell-boltzmann energy distribution graph

Answer 22

• at higher temps, peak is lower and moves to the right (molecules more distributed)
• higher amount of molecules w enough kinetic energy to overcome activation energy barrier

Question 23

activation energy graph axises

Answer 23

x: reaction pathway
y: potential energy (kJ/mol)

Question 24

relationship bw activation energy and chemical nature of reactants

Answer 24

• higher activation energy = less reactive (less molecules w enough Ea)
• lower activation energy = more reactive (more molecules w enough Ea)

Question 25

what do catalysts do

Answer 25

- speed up a reaction by providing an alternative lower energy pathway from reactants to products (diff mechanism/steps)

Question 26

how to find instantaneous rate of rxn

Answer 26

- draw line tangent to curve at any timepoint, calculate slope

Question 27

the only accurate data for the relationship bw concentration of reactant and reaction rate

Answer 27

at point where there is no product yet (t=0) - after that, it becomes uncertain

Question 28

how to determine how concentration of reactants affects rate of rxn

Answer 28

- must perform multiple experiments with different initial concentrations then graph initial rates in a linear line

Question 29

do we look at reactants or products when determining rate law equation

Answer 29

reactants

Question 30

what is k

Answer 30

- proportionality constant (rate law constant) - different for each rxn at any given temperature

Question 31

what does k tell us

Answer 31

- speed of rxn - large k = fast rxn - small k = slow rxn - units can tell overall order of rxn

Question 32

are m and n affected by temp

Answer 32

- no

Question 33

zero order rate(y)-concentration(x) curve, what it look like?

Answer 33

- rate is not affected by the concentration of the reactant r=k - straight horizontal line

Question 34

first order rate(y)-concentration(x) curve, what it look like?

Answer 34

- rate is proportional to concentration of a reactant r=k[A] - linear line from (0,0)

Question 35

second order rate(y)-concentration(x) curve, what it look like?

Answer 35

- rate is proportional to the square of the concentration of a reactant r=k[A]^2 - exponential-> starting at (0,0) increase slowly at first then quickly

Question 36

zero order concentration(y)-time(x) curve for reactants, what it look like?

Answer 36

- reactants turn directly into products - linear line downwards towards x-axis

Question 37

first order vs second order concentration(y)-time(x) curve, what it look like?

Answer 37

- flipped exponential line, where 1st order is steeper than 2nd order - hard to distinguish, can use integrated rate law to represent as linear lines instead - when this is done first order will have neg slope and 2nd order will have positive slope

Question 38

name of each step in reaction mechanism

Answer 38

elementary step

Question 39

reaction intermediates

Answer 39

- substances formed in one step and used up in another

Question 40

rate determining step

Answer 40

- step with the highest activation energy (slowest step) - determines how fast a reaction can go

Question 41

where are the reaction intermediates found on a potential energy diagram

Answer 41

- troughs of graph

Question 42

molecularity

Answer 42

number of species involved in an elementary step

Question 43

what can the rate determining step (RDS) help with determining

Answer 43

- if a proposed reaction mechanism is valid - its rate law should match the rate law for the overall rxn if valid

Question 44

criteria for a proposed mechanism to be valid

Answer 44

- each step adds up to the overall balanced chemical equation - each step is reasonable/likely (unimolecular/bimolecular) - RDS rate law same as experimental rate law

Question 45

what does the arrhenius constant represent

Answer 45

- aka frequency factor/pre-exponential factor - the frequency of reactions and likelihood of correct molecular orientation - number of collisions per second with the proper orientation to react

Question 46

what does the exponential factor represent

Answer 46

- the fraction of the reactant particles that have enough energy to react - dimensionless (no units)

Question 47

how does adding a catalyst affect k

Answer 47

increases k (suggests a faster rxn)

Question 48

how does increasing the concentration of a reactant affect k and rate

Answer 48

- no effect on k (k is temp dependent not conc dependent) - increases the rate

Question 49

how does increasing the temp affect k and rate

Answer 49

- increases k and rate