2013 Chains, Energy and Resources

Question 1

The branched-chain isomer B has a lower boiling point than the straight chain alkane A

Answer 1

• B has less surface (area of) contact
• B has fewer van der Waals’ forces OR B has weaker
van der Waals’ forces
• So less energy needed to break the intermolecular forces in B

Question 2

When cracking takes place, a large number of products is formed

Suggest why a large number of products are formed

Answer 2

• Any carbon–carbon bond (in the chain) can

break

Question 3

Explain the term homologous series

Answer 3

• A series of organic compounds
• With the same functional group
• But with each successive group differing by CH2

Question 4

What is meant by the term stereisomers

Answer 4

• Same structural formula
• Different arrangement (of atoms) in space OR different
spatial arrangement

Question 5

What is meant by the term enthalpy change of combustion

Answer 5

• Enthalpy change that occurs when one mole of substance

* Completely combusts OR reacts fully with oxygen

Question 6

Describe the mechanism of the reaction that forms iodomethane and hydrogen bromide

Answer 6

• Radical substitution  
Initiation 
• IBr -> Br + I
• Homolytic fission
Propagation 
• Br + CH4 -> HBr + CH3
• Br + CH4 -> HBr + CH3
Termination
• I + I -> I2

Question 7

Describe and explain, using equations, how the concentration of ozone in the stratosphere is maintained

Answer 7

• O3 -> O2 + O
• O + O2 -> O3
• Rate of ozone decomposition (almost) equals rate of ozone formation

Question 8

Give an everyday use for infrared spectroscopy

Answer 8

• Monitor air pollution

* Breathalysers

Question 9

State one type of a biodegradable alternative to CFCs

Answer 9

• HCFCs OR hydrocarbons OR HFCs

Question 10

Why does the petroleum industry process straight-chain alkanes into cyclic hydrocarbons

Answer 10

• Cyclic hydrocarbons promote efficient combustion

Question 11

Compound I is a alkene. It is shaken with aqueous bromine. What colour change will you see

Answer 11

• Orange to colourless

Question 12

Why is the reaction of 1-bromobutane with hot KOH(aq) slower than the reaction of 1-iodobutane with hot KOH(aq)

Answer 12

• C−I bond is weaker than C−Br bond

* C−I bond is easier to break than C−Br bond

Question 13

C3H6(g) + NH3(g) 0.5O2 -> CH2CHCN(g) + 3H2O(g)
∆H = -540

Using le Chateliers principle describe the effect on the position of the equilibrium.
A temperature of 450oC
A pressure above 2.5
No catalyst

Answer 13

• (As temperature rises) position of equilibrium changes to minimise effect of temperature rise by absorbing energy
• As temperature rises the position of equilibrium shifts to the left AND increase in pressure shifts the equilibrium to the left
• Relates change with temperature to the
(forward) reaction being exothermic
• Change with pressure because there are fewer moles of reactants
• Removing the catalyst does not change the position of
equilibrium

Question 14

Using Boltzmann diagrams explain the effect on the rate of a reaction of:

• Increasing the temperature
• Adding a catalyst

Answer 14

• Axes labelled
(number of) molecules on y-axis and energy on x-axis
• Idea that activation energy is lowered with a catalyst
Collision theory
• Reaction is faster with catalyst AND when temperature is increased
• Greater proportion of molecules with energy above
activation energy
• More effective collisions OR more successful collisions