Group 17: Chemistry 9701 (Cambridge International AS & A Level)

A resealable glass ampoule found in a school stock cupboard has lost its label. At room temperature it contains a lustrous grey-black solid. When the ampoule is warmed gently in a water bath, the solid does not melt but instead gives off a distinctive violet vapour, which condenses back into dark solid crystals on the cooler glass above.

Which halogen is inside the ampoule?

A student sets up three test-tubes, each containing 2 cm$^3$ of a colourless aqueous solution: tube 1 contains sodium chloride, tube 2 contains sodium bromide, and tube 3 contains sodium iodide. The student adds a few drops of orange bromine water to each tube and shakes gently.

(a) Predict and describe what would be observed in each of the three tubes. [3]

(b) Write an ionic equation, including state symbols, for any reaction that occurs in tube 3. [2]

(c) Explain, in terms of atomic radius and oxidising power, why using chlorine water instead of bromine water in this experiment would give a different result in tube 2. [2]

Chlorine gas is bubbled into cold, dilute sodium hydroxide solution to make a solution used as household bleach. In a separate experiment, chlorine gas is bubbled into hot, concentrated sodium hydroxide solution.

(a) Write a balanced symbol equation, including state symbols, for the reaction between chlorine and cold, dilute sodium hydroxide. [2]

(b) State the oxidation number of chlorine in each of the three chlorine-containing species in your equation from (a), and use these values to explain why this reaction is described as disproportionation. [2]

(c) Write a balanced symbol equation for the reaction between chlorine and hot, concentrated sodium hydroxide, and state the oxidation number of chlorine in the new chlorine-containing product that is not formed under cold conditions. [3]

(d) A 250 cm$^3$ sample of the cold sodium hydroxide bleach solution made in (a) is found to have a sodium chlorate(I), $\text{NaOCl}$, concentration of 0.160 mol dm$^{-3}$. Calculate the minimum mass of chlorine gas, $\text{Cl}_2$, that must have reacted to produce this concentration of $\text{NaOCl}$, giving your answer to 3 significant figures. [3]

A technician has three unlabelled bottles, each containing a colourless aqueous solution of a different sodium halide: sodium chloride, sodium bromide and sodium iodide. To identify the solution in bottle Q, the technician first acidifies a sample with dilute nitric acid, then adds aqueous silver nitrate. A yellow precipitate forms. Excess concentrated aqueous ammonia is then added to this precipitate, and it does not dissolve.

(a) Identify the halide ion present in bottle Q, giving your reasoning. [2]

(b) Write an ionic equation, including state symbols, for the formation of the precipitate in bottle Q. [2]

(c) Describe how the results of this same test (dilute nitric acid, then aqueous silver nitrate, then aqueous ammonia) would allow the technician to distinguish between solutions of sodium chloride and sodium bromide, referring to both the colour of each precipitate and its solubility in ammonia. [3]

(d) Explain why dilute nitric acid is added to each sample before the silver nitrate solution. [1]

Solid sodium iodide is warmed with concentrated sulfuric acid in a fume cupboard. Among the products formed is hydrogen sulfide gas, $\text{H}_2\text{S}$, identified by its smell of rotten eggs.

What is the change in the oxidation number of sulfur between $\text{H}_2\text{SO}_4$ and $\text{H}_2\text{S}$, and what does this indicate about the behaviour of the iodide ion in this reaction?

Chlorine gas is used to disinfect water supplies. When chlorine gas is bubbled directly into pure water (rather than into sodium hydroxide solution), it sets up a reversible reaction, forming a mixture of hydrochloric acid and chloric(I) acid, $\text{HOCl}$.

(a) Write a balanced equation, including state symbols and an equilibrium sign, for the reaction of chlorine gas with water. [2]

(b) State the oxidation number of chlorine in $\text{Cl}_2$, in $\text{HCl}$, and in $\text{HOCl}$, and explain why this reaction is classified as a disproportionation reaction. [3]

(c) $\text{HOCl}$ is the active bactericidal species responsible for killing microorganisms in the treated water. Suggest one reason, in terms of the trend in volatility down Group 17, why chlorine (rather than bromine or iodine) is the halogen most commonly dosed directly as a gas into water supplies for large-scale disinfection. [2]

A technician prepares three test-tubes of freshly made aqueous halogen solutions: chlorine water, bromine water and iodine solution.

Which row correctly gives the colour of each solution?

A student separately treats solid sodium chloride and solid sodium bromide, each with cold concentrated sulfuric acid, in a fume cupboard.

(a) Write a balanced equation, including state symbols, for the reaction between solid sodium chloride and concentrated sulfuric acid. State what is observed, and explain why no redox reaction occurs in this case. [3]

(b) With solid sodium bromide, a redox reaction also occurs. Write a balanced equation, including state symbols, for the overall reaction, given that the products include bromine and sulfur dioxide. State the oxidation number of sulfur in the sulfuric acid and in the sulfur dioxide formed, and describe what would be observed. [4]

(c) Explain, in terms of ionic radius, why the reducing power of the halide ions increases in the order $\text{Cl}^- < \text{Br}^- < \text{I}^-$. [2]

A student adds a small volume of chlorine water to separate aqueous solutions of potassium bromide and potassium iodide, in two test-tubes labelled P and Q respectively.

(a) State the colour change observed in the aqueous layer of tube P, and write an ionic equation, including state symbols, for the reaction that occurs. [3]

(b) State the colour change observed in the aqueous layer of tube Q, and write an ionic equation, including state symbols, for the reaction that occurs. [3]

(c) The student then adds a few cm$^3$ of cyclohexane to each tube, shakes both gently, and allows the layers to separate. Describe the colour of the upper cyclohexane layer in tube P and in tube Q, and explain why the halogen formed moves into this layer rather than remaining in the aqueous layer. [3]

A technician wants to identify the halide ion present in an unknown colourless solution. Before adding aqueous silver nitrate, a dilute acid must first be added, to remove any carbonate or hydroxide ions that would otherwise also form a precipitate with silver ions and give a false result.

Which dilute acid should be used, and why is dilute hydrochloric acid unsuitable for this purpose?