Deck 6: kinetics

Question 1

rate of reaction definition

Answer 1

change in concentration of a particular product or reactant per unit time

Question 2

average rate of reaction definition

Answer 2

average rate during a specific time interval is the change in concentration of a particular reactant or product over that time interval

Question 3

conditions for average rate ≈ initial rate

Answer 3

time interval is small enough
time interval starts from t=0

Question 4

given aA + bB = cC + dD, what is the rate of the reaction and what is the rate of formation of D

Answer 4

Question 5

what does a quenching agent do

Answer 5

if u add ice cold water
- dilute and cools mixture

if u add excess quenching agents
- reacts immediately with one of the reactants or the catalyst

ensures that the composition of the reaction mixture does not continue to change

Question 6

rate for clock reaction

Answer 6

if its a reactant rate is proportional to Vclock/t

if its a product rate is proportional to 1/t

Question 7

what kind of clock reaction occurs between thiosulfate and hydrogen

Answer 7

formation of sulfur to obscure a black cross

Question 8

why is volume of water varied in each experiment

Answer 8

to keep the total volume of the reaction mixture constant for the different experiments so that the initial concentration of each reactant in the reaction mixture is directly proportional to its volume used

Question 9

why must use same beaker for clock reaction

Answer 9

ensure depth remains the same so the same amount of sulfur needs to be precipitated before the cross is obscured

Question 10

what is the relationship between the initial rate of reaction and the time t taken for the cross to be obscured

Answer 10

initial rate is proportionate but NOT EQUAL to 1/t

Question 11

why only add a small amount of thiosulfate for clock reaction

Answer 11

so that time of the reaction is very short so that average rate ≈ initial rate

Question 12

why some reactions need heating

Answer 12

reaction involves cleavage of ____ bond and heating is needed to increase the temperature of the reaction mixture so that more reactant particles can have energy greater than or equal to the activation energy of the reaction to increase the rate of reaction

Question 13

why reaction between HCl and NaOH is immediate

Answer 13

NaOH and HCl are strong bases and acids and dissociate completely in water to give OH- and H+

these oppositely charged ions would have a natural tendency to attract each other to form H2O hence the reaction occurs readily

Question 14

why increase concentration increase rate of reaction (collision theory)

Answer 14

as conc increases, reactant particles come closer together and frequency of effective collisions increase

effective collision means correct collision geometry and sufficient energy

Question 15

rate equation definition just read

Answer 15

mathematical expression that relates the rate of reaction to the concentration of each reactant raised to the appropriate power

shows the exact dependence of the reaction rate on the concentrations of all the reactants

Question 16

rate constant definition and what causes it to increase

Answer 16

constant of proportionality in the rate equation of the reaction

rise in temperature and catalyst

Question 17

what is the arrhenius equation

Answer 17

Question 18

order of reaction definition

Answer 18

with respect to a given reactant power to which the concentration of that reactant is raised in the rate equation

Question 19

overall order of a reaction definition

Answer 19

sum of the powers of the concentration terms in the rate equation

Question 20

half life definition

Answer 20

time taken for the concentration of a reactant to decrease to half its initial value

Question 21

equation for half life for first order reaction

Answer 21

Question 22

what causes pseudo order reaction (3)

Answer 22

1. presence of a large excess of a reactant
   - [____] will hardly change during a reaction relative to the change in [____]
   - [____] is effectively constant throughout the reaction
2. reactant is also the solvent
   - eg. water present in large excess, concentration remains effectively constant throughout the reaction
3. catalyst
   - eg. HCl increases the rate of the reaction but it is not consumed by the reaction as it is regenerated
   - hence [HCl] is essentially constant throughout the reaction

Question 23

definition of elementary step

Answer 23

distinct step in a reaction mechanism which describes a single molecular event that involves breaking and/or making bonds

Question 24

molecularity definition

Answer 24

of an elementary step number of reactant particles taking part in that step

Question 25

intermediate definition

Answer 25

species that is formed in one step of a reaction mechanism and consumed in a subsequent step

Question 26

activation energy definition

Answer 26

minimum amount of energy that the reactant particles must possess before their collisions can result in a reaction

Question 27

boltzmann distribution curve

Answer 27

Question 28

change in rate of reaction for higher temp + boltzmann distribution curve

Answer 28

- when temp increase, average ke of reactant particles increase - more reactant particles have energy greater than or equal to the activation energy as shown by the larger shaded area at the higher temperature - increase in frequency of effective collisions and rate of reaction increase - higher temp also means larger rate constant higher temp as shown by larger sh

Question 29

change in rate of reaction with catalyst + boltzmann curve

Answer 29

- catalyst increase rate of reaction by providing an alternative reaction pathway, one of the lower activation energy than the uncatalysed reaction - more reactant molecules have energy greater than or equal to the activation energy - increase in frequency of effective collisions and increase rate of reaction - lower Ea also results in a larger rate constant

Question 30

catalyst definition

Answer 30

substance which increases the rate of reaction without itself undergoing any permanent chemical change

Question 31

just look

Answer 31

Question 32

how does Fe2+ and Fe3+ catalyse peroxodisulfate ions and iodide ions

Answer 32

in uncatalysed reaction, there is direct reaction between the 2 anions and the repulsion causes the reaction to have a high activation energy but after the addition of the catalyst, each step involves oppositely charged ions which have a natural tendency to attract each other, lowering the activation energy

Question 33

homogeneous and hetero catalysis definition

Answer 33

homo: catalyst and reactant are in the same phase hetero: catalyst and reactants are in different phases

Question 34

explain hetero catalysis

Answer 34

- reactant molecules need to be adsorbed onto the active site of the catalyst surface - weakens the covalent bonds within the reactant molecules and hence lowers the activation energy for the reaction - adsorption increases the conc of reactant at the catalyst surface and allows them to come into closer contact w the proper orientation for reaction - products then undergo desorption

Question 35

for catalytic converter, what are the 3 reactions and their catalysts?

Answer 35

Question 36

Autocatalysis definition

Answer 36

type of catalytic reaction where the product of a reaction acts as a catalyst for the reaction

Question 37

graph for autocatalysis and explanation

Answer 37

at the start, reaction is slow due to high Ea as the ____ is produced, it increases the rate of reaction by acting as an autocatalyst and providing an alternative reaction pathway with lower activation energy towards the end of the reaction, the conc of the reactants has fallen to a low level and rate of reaction falls even though there is sufficient catalyst

Question 38

what is a reaction that NO2 catalyses?

Answer 38

SO2 + 1/2O2 --> SO3

Question 39

enzymes definition

Answer 39

proteins which catalyse chemical reaction in living systems

Question 40

graph for rate of reaction against substrate concentration

Answer 40

at constant [enzyme], rate of reaction increases with increasing substrate conc until a max is reached - first order wrt [substrate] when the active sites of the enzyme become saturated with substrate, a further increase in [substrate] will not have any effect on reaction rate - zero order

Question 41

just read

Answer 41

in mechanism A, the slow step is a bimolecular elementary reaction involving one molecule of H2O2 and one I- ion. the rate equation is rate =k[H2O2][I-], which fits the rate equation