Analía Bellizzi – Chemistry Classes

Ronald Reagan Senior High School

HALOGEN NOTES

The physical properties of the group 17 elements

  1. Colors and Volatility Trends of Chlorine, Bromine, and Iodine:

    • Chlorine (Cl₂): Chlorine gas appears as a greenish-yellow gas. It has moderate volatility, existing as a gas at room temperature and pressure.
    • Bromine (Br₂): Bromine is a reddish-brown liquid at room temperature, which vaporizes readily to form a reddish-brown gas. It has higher volatility than chlorine.
    • Iodine (I₂): Iodine is a dark purple solid at room temperature. It sublimes easily to form a violet vapor. Iodine exhibits the highest volatility among these halogens due to its weaker intermolecular forces.


    Volatility Trend Explanation:
    The trend in volatility can be attributed to the strength of the intermolecular forces between the molecules. As we move down the group in the halogens, the van der Waals forces between molecules decrease due to increased atomic size and electron shielding. This results in higher volatility because the molecules can more easily escape into the gas phase.

  2. Trend in Bond Strength of Halogen Molecules:

      • The bond strength of halogen molecules generally decreases down the group from fluorine to iodine.
      • Explanation: This trend is primarily due to the increase in atomic size and the corresponding increase in the number of electron shells. As we move down the group, the atomic radius increases, leading to increased electron-electron repulsion between the valence electrons, which weakens the bond strength.
  3. Interpreting Volatility in Terms of Instantaneous Dipole–Induced Dipole Forces:

      • Instantaneous dipole-induced dipole forces, also known as London dispersion forces, arise due to temporary fluctuations in electron density within molecules.
    • Interpretation: In halogens, despite being non-polar diatomic molecules, there are temporary shifts in electron density creating instantaneous dipoles. These induced dipoles in one molecule can then induce dipoles in neighboring molecules, leading to attractive forces between them. As the size of the halogen atoms increases down the group, the electron cloud becomes more easily polarizable, resulting in stronger instantaneous dipole-induced dipole forces. Consequently, this increases volatility because molecules can escape the liquid or solid phase more readily when the forces holding them together are weaker. Therefore, iodine, being the largest halogen with the most easily polarizable electron cloud, exhibits the highest volatility among chlorine, bromine, and iodine.

The chemical properties of the halogen elements and the hydrogen halides

    1. Relative Reactivity of the Elements as Oxidizing Agents:

The reactivity of elements as oxidizing agents depends on their ability to gain electrons and undergo reduction in a chemical reaction. Generally, elements with higher electronegativity and a greater tendency to gain electrons are more effective oxidizing agents. For example:

        • Fluorine (F₂) is the most reactive oxidizing agent among the halogens because it has the highest electronegativity and readily gains electrons to form fluoride ions.
        • Chlorine (Cl₂) is less reactive than fluorine but still acts as a strong oxidizing agent, especially in the presence of heat or light.
        • Bromine (Br₂) is less reactive than chlorine but can still act as an oxidizing agent under suitable conditions.
        • Iodine (I₂) is the least reactive among the halogens and exhibits weak oxidizing properties compared to fluorine, chlorine, and bromine.

2. Reactions of the Elements with Hydrogen and Their Relative Reactivity:

The reactivity of elements with hydrogen depends on their electronegativity and ability to form stable compounds with hydrogen. Generally, elements with higher electronegativity tend to form more stable hydrides with hydrogen and exhibit greater reactivity. For example:

      • Fluorine (F₂) reacts violently with hydrogen to form hydrogen fluoride (HF) gas due to its high electronegativity and strong tendency to gain electrons.
      • Chlorine (Cl₂) reacts with hydrogen to form hydrogen chloride (HCl) gas, but the reaction is less vigorous compared to fluorine.
      • Bromine (Br₂) reacts slowly with hydrogen to form hydrogen bromide (HBr) gas, and the reaction requires heat or light to proceed.
      • Iodine (I₂) reacts sluggishly with hydrogen to form hydrogen iodide (HI) gas, and the reaction typically requires heating.

3. Relative Thermal Stabilities of Hydrogen Halides and Explanation in Terms of Bond Strengths:

The thermal stability of hydrogen halides decreases down the group from fluorine to iodine due to differences in bond strengths. The strength of the hydrogen-halogen bond increases down the group, leading to greater thermal stability of the compounds. For example:

      • Hydrogen fluoride (HF) has the strongest hydrogen-halogen bond among the hydrogen halides due to the high electronegativity of fluorine and its ability to form strong covalent bonds. As a result, HF is the most thermally stable hydrogen halide.
      • Hydrogen chloride (HCl) is less thermally stable than HF but still relatively stable due to the strong hydrogen-chlorine bond.
      • Hydrogen bromide (HBr) and hydrogen iodide (HI) have weaker hydrogen-halogen bonds compared to HF and HCl, leading to lower thermal stability. HI is the least thermally stable hydrogen halide due to the weakest hydrogen-iodine bond.

Overall, the relative thermal stabilities of hydrogen halides can be explained by differences in bond strengths, which depend on the electronegativity and atomic size of the halogen atoms.

Some reactions of the halide ions

  1. Relative Reactivity of Halide Ions as Reducing Agents:

The relative reactivity of halide ions () as reducing agents increases down the group from fluoride () to iodide (). This trend is due to the decreasing strength of the X-H bond and the increasing ease with which the halide ions are oxidized. Thus, iodide ions are the most effective reducing agents among the halogens

2. Reactions of Halide Ions:

Reaction with Aqueous Silver Ions Followed by Aqueous Ammonia:

Silver nitrate solution is then added to give:

ion presentobservation
Fno precipitate
Clwhite precipitate
Brvery pale cream precipitate
Ivery pale yellow precipitate

The chloride, bromide and iodide precipitates are shown in the photograph:

Ag+(aq)  +  Cl(aq)    AgCl(s)

Ag+(aq)  +  Br(aq)    AgBr(s)

Ag+(aq)  +  I(aq)    AgI(s)

Silver fluoride is soluble, and so you don’t get a precipitate.

Confirming the precipitate using ammonia solution

Carrying out the confirmation

Ammonia solution is added to the precipitates.

original precipitate

observation
AgClprecipitate dissolves to give a colourless solution
AgBrprecipitate is almost unchanged using dilute ammonia solution, but dissolves in concentrated ammonia solution to give a colourless solution
AgIprecipitate is insoluble in ammonia solution of any concentration

Reaction with Concentrated Sulfuric Acid:

  1. Halide ions react with concentrated sulfuric acid () to produce hydrogen halides and sulfur dioxide gas ():

    • represents the hydrogen halide gas formed (e.g., for chloride ions, for bromide ions, and for iodide ions).

These reactions illustrate the redox properties of halide ions, where they act as reducing agents and undergo oxidation themselves. The specifics of the reactions depend on the identity of the halide ion and the conditions of the reaction.

 

 

The reactions of chlorine

  1. Reaction of Chlorine with Aqueous Sodium Hydroxide:

    When chlorine gas () reacts with cold aqueous sodium hydroxide (), it undergoes disproportionation, meaning that chlorine simultaneously undergoes oxidation and reduction:

    This reaction involves the following changes in oxidation number:

    • Chlorine is reduced from an oxidation state of 0 in to -1 in .
    • Chlorine is oxidized from an oxidation state of 0 in to +1 in .

    Similarly, when chlorine gas reacts with hot aqueous sodium hydroxide, the same disproportionation reaction occurs but is more vigorous due to the increased temperature.

  2. Use of Chlorine in Water Purification:

    Chlorine is commonly used in water purification due to its ability to kill bacteria, viruses, and other harmful microorganisms. Chlorine reacts with water () to produce hypochlorous acid () and hypochlorite ions (), which are effective disinfectants:

    In this reaction, hypochlorous acid () is the active species responsible for disinfection. It penetrates the cell walls of bacteria and disrupts their metabolic processes, leading to their destruction. Hypochlorite ions () also contribute to disinfection but are less effective than hypochlorous acid.

    The equilibrium between chlorine, hypochlorous acid, and hypochlorite ions ensures a continuous supply of active disinfectants in the water, providing effective protection against harmful pathogens.

Overall, the use of chlorine in water purification exploits its ability to generate active disinfectants, such as hypochlorous acid and hypochlorite ions, which effectively kill bacteria and other microorganisms present in water.