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

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Question 1

Paper 2Medium5 marks

An aqueous sodium hydrogencarbonate solution is tested with a pH probe. At $25\,^\circ\mathrm{C}$ the pH is 8.30. At this temperature, neutral water has $[\mathrm{H^+}]=1.00\times10^{-7}\ \mathrm{mol\,dm^{-3}}$

A student concludes that $\mathrm{HCO_3^-}$ can act only as a Brønsted–Lowry base because the solution is alkaline.

(a)

Calculate $[\mathrm{H^+}]$ in the solution, in $\mathrm{mol\,dm^{-3}}$ to two significant figures.

[1]

(b)

Using your answer to (a), determine whether the solution is acidic, neutral or alkaline.

[1]

(c)

Write equations with state symbols to show $\mathrm{HCO_3^-}$ acting as a Brønsted–Lowry acid and as a Brønsted–Lowry base in water. Use the equations to assess the student's conclusion.

[3]

Question 2

Paper 2Medium6 marks

A student investigates displacement reactions between halogens and halide ions in aqueous solution. The results are shown below. Chlorine, bromine and iodine occur in this order from top to bottom in Group 17.

Halogen added	Halide ions present	Result
Cl₂(aq)	Br⁻(aq)	Reaction
Br₂(aq)	I⁻(aq)	Reaction
I₂(aq)	Br⁻(aq)	No reaction

(a)

Write the balanced ionic equation for the reaction between Br₂(aq) and I⁻(aq).

[2]

(b)

Using your equation in (a), identify the species oxidized.

[1]

(c)

A student predicts that I₂(aq) reacts with Cl⁻(aq). Predict whether this reaction occurs. Use the results and the positions of the elements in Group 17 to justify your answer.

[3]

Question 3

Paper 2Medium6 marks

CFC-12, CF₂Cl₂, can reach the stratosphere. UV radiation can break one of its C—Cl bonds. In a simplified ozone-depletion sequence, a chlorine radical reacts with ozone to form a chlorine monoxide radical and oxygen. The chlorine monoxide radical then reacts with an oxygen radical: ClO• + O• → Cl• + O₂

(a)

Write the equation for the UV-induced breaking of one C—Cl bond in CF₂Cl₂, showing UV above the arrow. State whether this is an initiation, propagation or termination step.

[2]

(b)

Represent the bond being broken as C—Cl. Draw two single-barbed (fish-hook) arrows to show the movement of electrons during this bond breaking.

[1]

(c)

Write the balanced equation for the reaction of a chlorine radical with ozone described in the question.

[1]

(d)

Combine the reactions in the question and in part (c) to give the overall reaction. State why the chlorine radical can continue the sequence.

[2]

Question 4

Paper 2Medium6 marks

In aqueous solution, bromoethane reacts with hydroxide ions. Figure 1 shows the reactants; the products are not shown.

(a)

On Figure 1, draw one curly arrow to show the movement of the bonding pair in the C—Br bond during the reaction.

[1]

(b)

Write a balanced ionic equation for the reaction, using structural formulae for the organic compounds and including state symbols.

[2]

(c)

Using Figure 1 and your equation in (b), deduce the type of reaction and explain the electron-pair changes at outcome level. Do not give a detailed mechanism.

[3]

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