IB Chemistry HL Reactivity 3: What are the mechanisms of chemical change? questions

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Question 1

Paper 2Medium6 marks

Chlorine reacts with propane when irradiated with ultraviolet (UV) light. Figure 1 shows the displayed structural formula of propane.

CCCHHHHHHHH
(a)

Determine the number of different monochlorinated structural isomers that can form from propane.

[1]
(b)

Write the equation for the initiation step, using single-barbed arrows to show the movement of electrons.

[2]
(c)

For the formation of 2-chloropropane:

(i)

Write the first propagation equation, in which a hydrogen atom is transferred from propane to a chlorine radical.

[1]
(ii)

Using the propan-2-yl radical, CH₃CH•CH₃, write the second propagation equation.

[1]
(d)

Write one equation for a termination step in this chain reaction.

[1]

Question 2

Paper 2Medium6 marks

Boron trifluoride forms a 1:1 adduct with ammonia:

Reaction I: BF₃ + NH₃ → BF₃·NH₃

A proton reacts with water to form a hydronium ion:

Reaction II: H⁺ + H₂O → H₃O⁺

(a)

Identify the electrophile in reaction I and state which reactant supplies both electrons in the new B–N bond.

[2]
(b)

Identify the electrophile in reaction II and explain the origin of both electrons in the new O–H bond.

[2]
(c)

A student claims that an electrophile must be positively charged. Assess this claim using both reactions.

[2]

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