2.2 Rates of Reaction

Question 1

what are 6 factors which affect the rate of reaction?

Answer 1

1. pressure (only gases)
2. temperature
3. light intensity
4. catalyst presence
5. concentration
6. surface area (only solids)

Question 2

how does concentration affect the rate of reaction?

Answer 2

the higher the conc, the more reactant particles present per unit vol and so a greater possibility of successful collisions occurring per second therefore the faster the reaction.

Question 3

how does pressure affect the rate of reaction?

Answer 3

the higher the pressure, the closer the reactant particles are together and so a greater possibility of successful collisions per second and therefore the faster the reaction

Question 4

describe the rate of reaction in a usual graph and explain

Answer 4

-rate fastest at the start of the rxn since each reactant is at its highest conc
-slows down as rxn proceeds bc conc of reactants decreases
-becomes 0 when rxn stops (1 of the reactants become used up)

Question 5

how does a high Ea affect rate?

Answer 5

fewer no.of molecules that will have the sufficient energy to react and therefore rate decreases

Question 6

how do you find the gradient/rate? units?

Answer 6

change in conc/times
moldm-3s-1

Question 7

Label an exothermic reaction profile (book)

Question 8

Label an endothermic reaction profile (book)

Question 9

what must you always use when explaining the effect of changing conditions on reaction rates?

Answer 9

always use the collision theory

Question 10

Draw a Maxwell-Boltzman distribution graph. Add a line for a lower temperature. Add a line for a higher temperature. (book)

Question 11

what does a lower temperature on the Maxwell-Boltman distribution graph mean?

Answer 11

fewer molecules with an energy greater than or equal to the activation energy

Question 12

what does a higher temperature on the Maxwell-Boltman distribution graph mean?

Answer 12

greater proportion of molecules with an energy greater or equal to the activation energy

Question 13

which molecules are able to react? (in terms of Maxwel-Boltzman graph)

Answer 13

only those with an equal energy to or greater than the activation energy

Question 14

what is the area under a maxwell-boltzman graph proportional to?

Answer 14

the number of molecules

Question 15

what are the two types of catalysts?

Answer 15

homogenous, heterogenous

Question 16

what is a homogenous catalyst, give an example

Answer 16

catalyst in the same physical state as reactants
eg acid catalysed hydrolysis of esters
CH3COOH (aq) + H2SO4 (c) + H2O(l) ⇌ CH3COOH (aq) + CH3OH (aq)

Question 17

What is a heterogenous reaction and give an example

Answer 17

catalyst in a different physical state to reactants
eg catalytic hydrogenation of an alkene
C2H4 (g) + H2(g) ->Ni(c) C2H6(g) Nickel in the hydrogenation of unsaturated oils in the production of margerine

Question 18

Give the 7 methods for studying reaction kinetics

Answer 18

1. change in colour over time
2. change in vol of a gas produced over time
3. changes in press over time
4. change in pH over time
5. formation of a ppt over time
6. change in mass of a reactant over time
7. chemical analysis

Question 19

Describe the method of measuring a change in colour over time
give an example

Answer 19

measure the amount of EM radiatoon absorbed by substances in the visible region over time
CuSO4 (blue) + Zn -> ZnSO4 (colourless) + Cu

Question 20

Describe the method of measuring a change in volume over time
give an example
(in book draw apparatus)

Answer 20

using a syringe measure the change in volume of a gas produced over time
Mg + 2CHl -> MgCl2 + H2(g)

Question 21

Describe the method of measuring a change in pressure over time
give an example

Answer 21

measure the change in pressure over time using a barometer
2NO2(g) -> 2NO(g) + O2(g)

Question 22

Describe the method of measuring a change in pH over time
give an example
(in book draw apparatus)

Answer 22

measure the change in pH over time using a pH meter
Any formation of H+ or OH- ions

Question 23

Describe the method of measuring the formation of a ppt over time
give an example
(in book draw apparatus)

Answer 23

time how long it takes a cross to disppear under the reaction mixture
Na2S2O3 + 2HCL -> 2NaCl + SO2 + H2O + S

Question 24

Describe the method of chemical analysis
(in book draw apparatus)
(what does quenched mean)

Answer 24

taking samples at regular intervals til the rxn sample is ‘quenched’, the conc of a reactant can be determined by a titration
quenched - put in ice

Question 25

VO2+ ions may also be oxidised to VO2 ions using coloured Ce^*, which is itself reduced to colourless Ce³+. A titration using this reaction does not usually include an indicator and colorimetry is sometimes used. State why an indicator may not be useful in this reaction and explain how colorimetry can be used to find the end point of the titration. [3]

Answer 25

indicator colour may be difficult to see due to coloured compounds / the vanadium compounds change colour during titration colour change of transition metals can be used in place of an indicator for colorimetry find a wavelength OR frequency of light absorbed only by the VO2+ / Ce4+ end point is when there is no absorbance at VO2+ frequency / Ce remains in solution

Question 26

What would be the effect on the initial rate of reaction if the halogen in the halogenoalkane were changed from chlorine to bromine? You should assume that this is the only change made. Explain your answer.

Answer 26

Rate would be faster because C-X bond needs to be broken (1) C-Br is weaker than C-CI / takes less energy to break (1) This outweighs effect of greater dipole in C-CI/chlorine being more electronegative (1)

Question 27

Explain what is meant by a carbon-neutral fuel.

Answer 27

when fuel from renewable / biological source (burns) (1) no net change in atmospheric carbon dioxide levels (1) carbon dioxide released is that absorbed by photosynthesis as the plant grew (1)