ES almn: Risks & benefits of chlorine; iodine-thiosulfate titrations; hydrogen halides

Question 1

What specific risks does chlorine pose to humans?

Answer 1

Toxic:

• Irritation of eyes, skin, respiratory system
• Reacts in lungs to form HCl, causing drowning

Question 2

Chlorine is transported in pressurised tanks which are usually lined with steel. Why is it essential that the interior of a tank is dry?

Answer 2

Chlorine reacts with water to produce corrosive acids.

Question 3

What are the 2 main uses of chlorine?

Answer 3

• Sterilising water by killing bacteria
• Making bleach

Question 4

What is the formula of sodium thiosulfate?

Answer 4

Na₂S₂O₃

2 Na⁺ ionically bonded to S₂O₃^2-

Question 5

An iodine-thiosulfate titration involves what type of reaction?

Answer 5

Redox

Question 6

What are iodine-thiosulfate titrations used for?

Answer 6

Finding the concentration of a chemical which is a strong enough oxidising agent to oxidise iodide ions.

Question 7

The equilibrium below was investigated:

H_2(g) + I_{2 (g)} ⇌ 2HI_(g)

Mixtures of known masses of hydrogen and iodine were allowed to react in sealed tubes until equilibrium was established. The tubes were rapidly cooled. The iodine present was analysed.

• Suggest why the flasks are cooled
• Describe how a sealed tube could be investigated to measure the mass of iodine it contained

Answer 7

• To preserve equilibrium position
• Titrate with thiosulfate of known concentration. Mass I₂ = 0.5 x mol thiosulfate x 253.8

Question 8

What equation is used to work out atom economy?

Answer 8

(Mr of desired product x 100) / total Mr of reactants/products

Question 9

Explain the difference between a co-product and by-product.

Answer 9

• Co-product: from intended reaction. Useful
• By-product: from side reactions. Not useful

Question 10

How is hydrogen chloride produced industrially?

Answer 10

• Done at plants producing Cl₂ and H₂ from brine by electrolysis
• H_{2 (g)} + Cl_{2 (g)} → 2HCl_(g)

Question 11

What is the strongest oxidising agent in group 7?

Answer 11

Fluorine

Question 12

Give equations, with state symbols, to show how the following halides react with sulfuric acid.

• Sodium fluoride
• Sodium chloride
• Sodium bromide
• Sodium iodide

Answer 12

Sodium fluoride + chloride:

Sodium fluoride/chloride + sulfuric acid → hydrogen fluoride/chloride + sodium hydrogen sulfate

NaF(s) + H₂SO₄(aq)→ HF(aq) + NaHSO₄(aq)​

NaCl(s) + H₂SO₄(aq)→ HCl(aq) + NaHSO₄(aq)

Sodium bromide:

Br^- ions are strong enough reducing agents to reduce sulfuric acid to sulfur dioxide:

2NaBr(s) + H₂SO₄(aq) + 2H⁺(aq) → Br₂(l) + SO₂(g) + 2H₂O(l) + 2Na⁺(aq)

This reaction occurs, but only to a small extent: NaBr(s) + H₂SO₄(aq) → HBr(g) + NaHSO₄

Sodium iodide:

I^- ions are strong enough reducing agents to reduce sulfuric acid to hydrogen sulfide:

8NaI(s) + H₂SO₄(aq) + 8H⁺(aq) → 4I₂(s) + H₂S(g) + 4H₂O(l) + 8Na⁺(aq)

This reaction occurs, but only to a small extent: NaI(s) + H₂SO₄(aq) → HI(g) + NaHSO₄

Question 13

• Why is adding concentrated sulfuric acid to sodium bromide not a good way to produce hydrogen bromide?
• What is used instead?

Answer 13

• They react to produce some HBr, but Br^- ions reduce sulfuric acid to sulfur dioxide. Gas produced is an impure mixture of HBr, SO₂ + Br₂
• Phosphoric acid

Question 14

• Why is adding concentrated sulfuric acid to sodium iodide not a good way to produce hydrogen iodide?
• What is used instead?

Answer 14

• They react to produce some HI, but I^- ions reduce sulfuric acid to hydrogen sulfide. Gas produced is an impure mixture of HI + H₂S
• Phosphoric acid

​I₂ is produced in the reactions, but is a solid so isn’t part of the gas mixture

Question 15

State and explain the trend in thermal stability of the hydrogen halides, observing down the group.

Answer 15

• Thermal stability decreases down group
• Bond enthalpies decrease because:
  
  • Halide ion becomes larger
  • Outermost bonding electrons become further from nucleus
  • Weaker electrostatic attraction

Question 16

Suggest and explain what would be observed when each of the hydrogen halides were heated.

Answer 16

• HF: no change; not broken down
• HCl: no change; not broken down
• HBr: some brown Br₂ gas produced
• HI: much purple I₂ gas produced

Question 17

Describe the hydrogen halides’ acidity in solution.

Answer 17

All strongly acidic (almost 100% dissociation), apart from HF.

H-F has highest bond enthalpy since F smallest

Question 18

• Write a general equation for when the hydrogen halides react with ammonia
• Name what is produced
• Use your answer to suggest a test for hydrogen halides

Answer 18

• NH₃(g) + HX(g)→ NH₄X(s)
• Ammonium salts
• Dip a glass rod into ammonia solution, then into a hydrogen halide gas. White cloud of ammonium halide forms