Group 17 Flashcards
(18 cards)
What are the colours and states of chlorine, bromine and iodine in elemental form at room temperature and pressure?
- Chlorine - green/yellow gas
- Bromine - orange/brown liquid
- Iodine - grey/black solid
What is the trend in volatility down group 17?
Volatility decreases (as the melting and boiling points increase)
What is the explanation for the trend in volatility down the group?
- All halogens exist as simple covalent molecules, held together solely by instantaneous-induced dipole forces
- As a consequence, molecules of halogens with more electrons experience greater forces of attraction between one another as the id-id forces are stronger
- The strength of the intramolecular bonds has no bearing on a simple covalent molecule’s volatility and melting point
What is the trend in covalent bond strength down group 17 and what is the explanation for this trend?
- Bond strength decreases
- This is because the atomic radii of the halogens increase down the group, so bonding electrons are held less tightly to the nuclei in a covalent bond
Fluorine is an anomaly; its bond enthalpy is lower than chlorine’s as fluorine atoms are so small that their lone pairs interact in an F₂ molecule, driving the atoms apart
What is the trend in the reactivity of the elements as oxidising agents down group 17 and what is the explanation for this trend?
- It decreases
- This is because the electronegativity of the atoms decreases, meaning they attract the outer electrons of other atoms decreasily readily
- This reduces their ability to act as oxidising agents in redox and displacement reactions
What is the general equation for the reaction between a halogen and hydrogen and what is the trend in reactivity with hydrogen down group 17?
- H₂ (g) + X₂ (g) → 2HX (g)
- The reactivity of halogens with hydrogen decreases down the group
This reaction forms hydrogen halides
What is the trend in the thermal stability of the hydrogen halides down the group?
- It decreases
- This is because the bond strength of the H-X bond decreases, owing to the reduction in the electronegativity of the elements and the increasing atomic radii
Thermal stability is determined by how readily intramolecular forces break to initiate decomposition, while melting point/volatility is determined by how readily a substance undergoes physical changes, which often involves only breaking intermolecular forces
What is the trend in reactivity of the halide ions as reducing agents down the group and what is the explanation for this trend?
- They increase in reactivity
- This is because ionic radii increases, so the outermost electrons are held less tightly to the nucleus
- As a consequence, these electrons can be lost to another atom to oxidise the halide ion more easily
What is the chemical equation for the reaction of halide ions with aqueous silver ions (that are part of silver nitrate) and how can it be used to test for halides?
- AgNO₃ (aq) + X⁻ (aq) → AgX (s) + NO₃⁻ (aq)
- The silver halide will visibly precipitate out
- Its colour can be used to determine its identity: white is chloride, cream is bromide and yellow is iodide
Add dilute nitric acid first
How can ammonia be used to further test for halides after the silver nitrate test?
- Add dilute ammonia; if the precipitate dissolves, it is a chloride precipitate
- Add concentrated ammonia; if the precipitate dissolves, it is either a chloride or a bromide precipitate
- Iodide precipitates are not soluble in either dilute or concentrated ammonia
What is the reaction between chloride and sulfuric acid?
- H₂SO₄ (l) + NaCl (s) → HCl (g) + NaHSO₄ (s)
- The HCl gas produced initially is wet, but can be passed through a conical flask of sulfuric acid to react again to be dried
- The HCl gas is visible as white fumes
What is the reaction between bromide and sulfuric acid, including subsequent oxidation?
- H₂SO₄ (l) + NaBr (s) → HBr (g) + NaHSO₄ (s)
- As HBr is less thermally stable than HCl, it is able to be oxidised via an additional reaction with sulfuric acid, as shown below
- 2HBr (g) + H₂SO₄ (l) → Br₂ (g) + SO₂ (g) + 2H₂O (l)
The bromine gas produced is seen as reddish-brown fumes
What is the reaction between iodide and sulfuric acid, including subsequent oxidation?
- H₂SO₄ (l) + NaI (s) → HI (g) + NaHSO₄ (s)
- As HI decomposes very readily, it is able to be oxidised by sulfuric acid. This can happen in several different ways, as shown below
- 2HI (g) + H₂SO₄ (l) → I₂ (g) + SO₂ (g) + 2H₂O (l)
- 6HI (g) + H₂SO₄ (l) → 3I₂ (g) + S (s) + 4H₂O (l)
- 8HI (g) + H₂SO₄ (l) → 4I₂ (g) + H₂S (s) + 4H₂O (l)
Iodine gas is purple vapour, sulfur is a yellow solid and hydrogen sulfide has a malodorous smell
What is the explanation for the differences between the halide-sulfuric acid reactions, with reference to the nature of the halides and the role of the sulfuric acid?
- The halide ions become weaker reducing agents down the group
- The stability of the hydrogen halides decreases down the group
- As a result, hydrogen bromide and hydrogen iodide are strong enough reducing agents to reduce sulfuric acid once formed and be oxidised
- Thus, sulfuric acid acts simply as an acid in the chloride reaction (as it donates protons), while it acts as an acid and an oxidising agent in the bromide and iodide reactions
What is the reaction of chlorine in cold (15 °C) sodium hydroxide and what type of reaction is it?
- Cl₂ (aq) + 2NaOH (aq) → NaCl (aq) + NaClO (aq) + H₂O (l)
- It is a redox reaction, specifically a disproportionation reaction
- This is because the chlorine is oxidised from 0 to +1 and reduced from 0 to -1
What is the reaction of chlorine in hot (60 °C) sodium hydroxide and what type of reaction is it?
- 3Cl₂ (aq) + 6NaOH (aq) → 5NaCl (aq) + NaClO₃ (aq) + 3H₂O (l)
- It is a redox reaction, specifically a disproportionation reaction
- This is because the chlorine is oxidised from 0 to +5 and reduced from 0 to -1
How is chlorine used in water purification?
- It reacts with water with the equation Cl₂ (aq) + H₂O (l) ⇌ HCl (aq) + HClO (aq)
- HClO (chloric(I) acid) is a sterilising agent, which can dissociate in water to form ClO⁻, which is also a sterilising agent
This is also a disproportionation reaction
What is the ionic equation and ionic half equations for the reaction Cl₂ (aq) + H₂O (l) ⇌ HCl (aq) + HClO (aq)?
- Ionic: Cl₂ (aq) + H₂O → Cl⁻ + ClO⁻ + 2H⁺
- Reduction: 1/2Cl₂ (aq) + e⁻ → Cl⁻
- Oxidation: 1/2Cl₂ (aq) + H₂O → ClO⁻ + 2H⁺ + e⁻
This is to aid your understanding of half-equations; it is not necessary to memorise
