Comprehensive Study Guide for Group 17 Halogen Chemistry

Physical States and Appearance of Group 17 Elements

The halogens exhibit distinct physical properties at room temperature, varying in both color and state as their atomic weight increases:

• Chlorine: A yellow-green gas.
• Bromine: A red-brown liquid.
• Iodine: A grey solid.

Volatility Trends in Halogens

The volatility of halogens follows a clear trend as one moves down the group from chlorine to bromine to iodine:

• Trend: Volatility decreases as you go down the group.
• Explanation:
  • The number of electrons per molecule increases as the atoms get larger.
  • This lead to the formation of stronger van der Waals forces between molecules.
  • Because these intermolecular forces are stronger, more energy is required to overcome them during phase changes.
  • Consequently, the boiling point increases, and the volatility decreases.

Oxidising Ability and Reactivity Trends

Halogens are well-known for their role as oxidising agents. An oxidising agent is defined as a species which brings about oxidation by gaining electrons. In the process, the oxidising agent is itself reduced.

Trend in Relative Reactivity

• Trend: Reactivity and oxidising ability decrease moving down Group 17.
• Explanation:
  • As you move down the group, the nuclear charge, atomic radius, and shielding all increase.
  • The increase in atomic radius and shielding outweighs the increase in nuclear charge, resulting in a decrease in nuclear attraction for incoming electrons.
  • Because it is harder for the halogen atom to attract and gain an electron to complete its outer shell, it becomes less effective as an oxidising agent.

Investigation of Reactivity via Displacement Reactions

The reactivity of halogens as oxidising agents can be demonstrated through displacement reactions with other halides in solution:

• Mechanism: A halogen will displace a halide ion from a solution only if that halide is located below it in the periodic table (meaning the halide is less reactive).
• Observation: If a displacement reaction occurs, a visible color change will be observed in the solution.

Reactions of Halogens with Hydrogen

The decrease in reactivity down Group 17 is further evidenced by the differing conditions required for halogens to react with hydrogen ($H_2$):

• Fluorine ($F_2$): Reacts explosively with hydrogen, even under cold atmospheric conditions.
• Chlorine ($Cl_2$): Reacts with hydrogen when exposed to sunlight or when lightly heated.
• Bromine ($Br_2$): Reacts with hydrogen if heated directly with a flame.
• Iodine ($I_2$): Only partially reacts with hydrogen, and even then, only when subjected to constant heating.

Thermal Stability of Hydrogen Halides ($HX$)

The thermal stability of hydrogen halides decreases from $H-F$ to $H-I$ based on the following factors:

• Bond Enthalpy: Bond enthalpy decreases significantly as you go down the group.
• Molecular Factors: As the halogen atoms get larger, the atomic radius and shielding increase. This leads to a weaker electrostatic attraction between the nucleus and the shared bonding electrons.
• Result: Because the hydrogen-halogen bond is weaker, less energy is required to break it, causing thermal stability to decrease moving down the group.

Qualitative Analysis of Halide Ions

Halide ions can be identified through sequential testing with silver nitrate ($AgNO_3$) followed by ammonia ($NH_3$):

Reactions of Sodium Halides with Concentrated Sulfuric Acid

The reactions of solid halides with concentrated sulfuric acid ($H_2SO_4$) demonstrate the varying reducing abilities of halide ions.

Sodium Fluoride ($NaF$) and Sodium Chloride ($NaCl$)

These halides are not strong enough reducing agents to reduce sulfuric acid; only acid-base reactions occur:

• Fluoride Reaction: $NaF + H_2SO_4 \rightarrow NaHSO_4 + HF$
• Chloride Reaction: $NaCl + H_2SO_4 \rightarrow NaHSO_4 + HCl$
• Observation: Both $HF$ and $HCl$ are observed as misty fumes.

Sodium Bromide ($NaBr$)

Bromide ions are stronger reducing agents than chloride ions, allowing for further redox reactions:

• Initial Step: $NaBr + H_2SO_4 \rightarrow NaHSO_4 + HBr$
• Redox Step: $2HBr + H_2SO_4 \rightarrow Br_2 + SO_2 + 2H_2O$

Sodium Iodide ($NaI$)

Iodide ions are very strong reducing agents, leading to multiple reduction steps for sulfur:

• Initial Step: $NaI + H_2SO_4 \rightarrow NaHSO_4 + HI$
• Redox Step 1: $2HI + H_2SO_4 \rightarrow I_2 + SO_2 + 2H_2O$
• Redox Step 2: $6HI + SO_2 \rightarrow 3I_2 + H_2S + 2H_2O$

Chlorine Disproportionation Reactions

When chlorine reacts with sodium hydroxide ($NaOH$), it undergoes disproportionation, a reaction where the same element is simultaneously oxidised and reduced. The products depend on the temperature and concentration of the alkali.

Reaction with Cold Dilute Sodium Hydroxide

• Equation: $2NaOH + Cl_2 \rightarrow NaCl + NaClO + H_2O$
• Oxidation States: Chlorine is oxidised from $0$ in $Cl_2$ to $+1$ in $NaClO$. It is simultaneously reduced from $0$ in $Cl_2$ to $-1$ in $NaCl$.

Reaction with Hot Concentrated Sodium Hydroxide

• Equation: $6NaOH + 3Cl_2 \rightarrow 5NaCl + NaClO_3 + 3H_2O$
• Oxidation States: Chlorine is oxidised from $0$ in $Cl_2$ to $+5$ in $NaClO_3$. It is simultaneously reduced from $0$ in $Cl_2$ to $-1$ in $NaCl$.

Industrial Importance and Water Purification

Halogens and their derivatives have significant industrial applications:

Role of Chlorine in Water Purification

Chlorine is used extensively to treat water for the following reasons:

• It kills bacteria that cause diseases.
• It eliminates unpleasant tastes and odors.
• It removes discolouration from water.
• It prevents the growth of algae.
• The persistent presence of residual chlorine prevented reinfection over time.
• Concerns: Chlorine is toxic and can react with organic matter to produce potential carcinogens. However, because the amount added is so small, the health benefits are generally considered to outweigh the risks.

Other Industrial Uses

• Bleaches: Halogens are primary components in bleach manufacturing.
• Polyvinyl Chloride (PVC): Halogens are essential in PVC production, which is used for windows and drain pipes. The addition of plasticisers allows PVC to be used for electrical cable insulation and clothing.
• Halogenated Hydrocarbons: These are utilized as solvents, refrigerants, and propellants in aerosols.