16 - kinetics 2

Question 1

rate equation =

Answer 1

change in conc of product/time

• change in conc of reactant/time
  (negative as the reactant conc is decreasing so the negative sign makes rate positive)

Question 2

what methods are there for measuring the rate of reaction?

Answer 2

• measuring volume of gas evolved
• measuring change in mass of a reaction mixture
• monitoring the change in intensity of colour of a reaction mixture (colorimetry)
• measuring change in conc of a reactant or product using titration
• measuring the change in pH of a solution
• measuring the change in electrical conductivity of a reaction mixture

Question 3

how do you measure the volume of gas evolved?

Answer 3

1 collection over water into a measuring cylinder
or
2 collection using a gas syringe

gas syringe is more accurate however for large volumes the difference in degree of measurement is so small that either instrument is sufficient

Question 4

how do you measure the change in mass of a reaction mixture?

Answer 4

(gas is evolved)
reaction flask and contents are placed on a digital balance and the decrease in mass is measured as the reaction proceeds
technique is most precise when the gas given off has a high density

Question 5

how do you measure a colour change?

Answer 5

uses a naked eye
greater precision can be obtained by using a colorimeter as it can detect far more subtle changes than the human eye can observe and provides a quantitative measurement

Question 6

method of analysis by titration

Answer 6

remove aliquots at regular intervals
reaction in aliquot either stopped by adding another substance or slowed to zero by immersing in an ice bath
aliquot titrated to determine the conc of reactant or product
if aliquot contains an acid it can be titrated against sodium hydroxide

Question 7

analysis by titration core prac chemicals

Answer 7

reaction between iodine and propanone
titred against sodium hydroxide
sodium hydrogencarbonate added to aliquot to remove acid catalyst and hence stop the reaction
remaining iodine titrated against sodium thiosulfate

Question 8

how to measure electrical conductivity?

Answer 8

total number or types of ions in solution changes during a reaction it might be possible to follow the reaction by measuring changes in electrical conductivity of a solution using a conductivity meter

Question 9

how do you measure any other physical property that shows significant change?

Answer 9

• changes in volume of liquid (dilatometry)
• chirality
• refractive index

Question 10

what are the units for zero and first order rate constants?

Answer 10

0 - mol dm-3 s-1
1 - s-1

Question 11

what is the slowest step of a reaction?

Answer 11

rate determining step

Question 12

how do you determine the rate equation using the continuous method?

Answer 12

one reaction mixture made and aliquots removed a regular intervals
conc determined through experimental technique e.g. titration
- draw conc-time graph
- find the half life (check with multiple half lives)
if all half lives are the same then the reaction is first order
if half life doubles it it second orddr

Question 13

what does it mean if a conc time graph is a straight line with a negative gradient?

Answer 13

zero order

Question 14

how do you calculate the rate from a conc time or volume time graph?

Answer 14

draw a tangent (make a triangle/find gradient)

Question 15

how do you determine the rate equation from the initial rate method?

Answer 15

several reaction mixtures made and the initial rate is measured
calculate mathematical relationship between the rate of reaction and the conc of reactants

Question 16

how do you determine the order from a rate conc graph?

Answer 16

initial rate of reaction is proportional to 1/time
zero - straight horizontal line
first - directly proportional
second - curve
y - 1/t x - conc

Question 17

what are intermediates?

Answer 17

species made during a reaction that do not appear in the overall equation

Question 18

what is an elementary reaction?

Answer 18

single collision between two reactant particles
assume if there are two single reactants that they would both be first order and the overall equation would be second
we can deduce the rate law directly from the stoichiometric equation

Question 19

what kind of mechanism is alkaline hydrolysis of haloalkanes?

Answer 19

nucleophilic substitution
hydroxide ion acts as nucleophile
(SN2?????)

Question 20

which sn is hydrolysis of a tertiary haloalkane?

Answer 20

SN1

Question 21

which sn is hydrolysis of a primary haloalkane?

Answer 21

SN2

Question 22

what are homogeneous catalysts?

Answer 22

all species in the same phase
(many aqueous solutions are catalysed by the hydrogen ion)

Question 23

what are heterogeneous catalysts?

Answer 23

species in different phases

important uses in industry:
- the Haber process - solid iron
- the Contact process - solid vanadium oxide

Question 24

what are the three stages involving surface catalysis?

Answer 24

adsorption - reactants adsorbed onto the surface of the catalyst
reaction - reactant molecules held in positions that enable them to react together
desorption - the product molecules leave the surface

Question 25

how is a heterogeneous catalyst used in cars?

Answer 25

three-way catalytic converter employed in the exhaust of cars
converts unburned hydrocarbons into water and carbon dioxide
carbon and carbon monoxide into carbon dioxide
nitrogen oxides into oxygen and nitrogen

Question 26

how is ethanedioic acid oxidised by manganate (VII) ions?

Answer 26

in autocatalysis the reaction is catalysed by one of its products
very slow at room temp but catalysed by manganese ions
reaction rate increases