12.2 Reactivity of Halogens | Halogens | Chemistry 11

This section explores the chemical reactivity of the halogens (Group 17 elements), focusing on their trends as oxidizing agents, their reactions with hydrogen, and the properties of the resulting hydrogen halides and halide ions.

Halogens→

Reactivity and Oxidizing Power of Halogens

The reactivity of halogens is defined by their ability to gain an electron to form a halide ion (e.g., $F^{-}$ , $C l^{-}$ , $B r^{-}$ , $I^{-}$ ). This tendency is highest for fluorine and decreases down the group.

Reactivity Trend: Reactivity decreases down Group 17. This is because electronegativity decreases and atomic size increases, making it less favorable for larger atoms to attract and gain an electron.

Oxidizing Power: Halogens are strong oxidizing agents because they readily accept electrons. Their oxidizing strength follows the same trend as their reactivity.

The order of decreasing oxidizing power is: $F_{2} > C l_{2} > B r_{2} > I_{2}$

A more reactive halogen can displace a less reactive halide ion from its aqueous solution.

Displacement Reactions:

Fluorine is the most reactive halogen and can displace chloride, bromide, and iodide ions. $F_{2 (g)} + 2 K C l_{(a q)} \to 2 K F_{(a q)} + C l_{2 (g)}$ $F_{2 (g)} + 2 K B r_{(a q)} \to 2 K F_{(a q)} + B r_{2 (l)}$ $F_{2 (g)} + 2 N a I_{(a q)} \to 2 N a F_{(a q)} + I_{2 (s)}$
Chlorine can displace bromide and iodide ions.
Bromine can only displace iodide ions.
Iodine is the least reactive and cannot displace any other halide ions.

Reactions of Halogens with Hydrogen

Halogens react with hydrogen gas to form gaseous hydrogen halides ( $H X$ ).

General Reaction: $H_{2} + X_{2} \to 2 H X (where X = F, C l, B r, I)$

The vigor of this reaction decreases down the group as the reactivity of the halogen decreases.

Fluorine ( $F_{2}$ ): Reacts explosively with hydrogen, even in cold and dark conditions.
Chlorine ( $C l_{2}$ ): Reacts explosively with hydrogen in the presence of sunlight or a flame.
Bromine ( $B r_{2}$ ): Reacts with hydrogen only upon ignition, causing a mild explosion.
Iodine ( $I_{2}$ ): Reacts with hydrogen partially and reversibly, even with continuous heating, forming an equilibrium.

Properties of Hydrogen Halides ( $H X$ )

Thermal Stability

Thermal stability refers to the ability of a compound to resist decomposition upon heating. The stability of hydrogen halides depends on the strength of the H-X bond.

Trend: The H-X bond strength decreases down the group as the halogen atom size increases and electronegativity decreases.

Order of Stability: $H F > H C l > H B r > H I$

HF is the most thermally stable, and HI is the least.

Acidic Strength of Halogen Acids

In aqueous solution, hydrogen halides form hydrohalic acids. Their strength is determined by how easily they can donate a proton ( $H^{+}$ ).

Trend: The acidic strength is inversely related to the H-X bond strength. A weaker bond allows for easier dissociation of the proton.

Order of Acidic Strength: $H I > H B r > H C l > H F$

HI is the strongest hydrohalic acid, and HF is the weakest.

Reactivity of Halide Ions ( $X^{-}$ )

Halide ions can act as reducing agents by losing an electron. Their reducing power increases down the group.

Reducing Power Trend: As the ion size increases down the group, the attraction between the nucleus and the valence electrons weakens, making it easier for the ion to lose an electron. $I^{-} > B r^{-} > C l^{-} > F^{-}$

Iodide ( $I^{-}$ ) is the strongest reducing agent, and fluoride ( $F^{-}$ ) is the weakest.

Reactions of Halide Ions with Concentrated Sulfuric Acid

The reaction depends on the reducing strength of the halide ion.

Fluoride ( $F^{-}$ ) and Chloride ( $C l^{-}$ ): These ions are weak reducing agents and do not reduce concentrated $H_{2} S O_{4}$ . They undergo a simple acid-base reaction. $NaF + H_{2} S O_{4} \to NaHS O_{4} + HF$ $NaCl + H_{2} S O_{4} \to NaHS O_{4} + HCl$
Bromide ( $B r^{-}$ ): Strong enough to reduce concentrated $H_{2} S O_{4}$ to sulfur dioxide ( $S O_{2}$ ). $NaBr + H_{2} S O_{4} \to NaHS O_{4} + HBr$ $2 HBr + H_{2} S O_{4} \to B r_{2} + S O_{2} + 2 H_{2} O$
Iodide ( $I^{-}$ ): The strongest reducing agent, capable of reducing concentrated $H_{2} S O_{4}$ to hydrogen sulfide ( $H_{2} S$ ). $NaI + H_{2} S O_{4} \to NaHS O_{4} + HI$ $8 HI + H_{2} S O_{4} \to 4 I_{2} + H_{2} S + 4 H_{2} O$

Reaction with Silver Nitrate (Silver Nitrate Test)

This is a classic qualitative test to identify halide ions in an aqueous solution. Adding silver nitrate ( $AgN O_{3}$ ) solution produces a silver halide precipitate ( $AgX$ ) with a characteristic color and solubility in ammonia.

General Reaction: $Ag_{(a q)}^{+} + X_{(a q)}^{-} \to AgX_{(s)}$

Halide Ion	Precipitate Color	Solubility in Aqueous Ammonia
Chloride ( $C l^{-}$ )	White	Soluble
Bromide ( $B r^{-}$ )	Cream (Off-white)	Partially soluble
Iodide ( $I^{-}$ )	Yellow	Insoluble

Reaction of Chlorine with Sodium Hydroxide

Chlorine undergoes a disproportionation reaction with sodium hydroxide, where it is simultaneously oxidized and reduced. The products depend on temperature.

Cold, Dilute NaOH: Forms sodium hypochlorite ( $NaOCl$ ), used as a bleaching agent. $C l_{2} + 2 NaOH \to NaOCl + NaCl + H_{2} O$ In this reaction, $C l_{2}$ is both oxidized (to $C l^{+}$ in $N a O C l$ ) and reduced (to $C l^{-}$ in $N a C l$ ).
Hot, Concentrated NaOH: Forms sodium chlorate ( $NaCl O_{3}$ ). $3 C l_{2} + 6 NaOH \to NaCl O_{3} + 5 NaCl + 3 H_{2} O$

Summary of Trends Down Group 17

Property	Trend Down Group 17
Reactivity / Oxidizing power	Decreases: $F_{2} > C l_{2} > B r_{2} > I_{2}$
Vigor of reaction with $H_{2}$	Decreases
Thermal stability of $H X$	Decreases: $H F > H C l > H B r > H I$
Acidic strength of $H X$	Increases: $H I > H B r > H C l > H F$
Reducing power of $X^{-}$	Increases: $I^{-} > B r^{-} > C l^{-} > F^{-}$

Halogens→

Reactivity and Oxidizing Power of Halogens

Oxidizing Power: Halogens are strong oxidizing agents because they readily accept electrons. Their oxidizing strength follows the same trend as their reactivity.

The order of decreasing oxidizing power is: $F_{2} > C l_{2} > B r_{2} > I_{2}$

A more reactive halogen can displace a less reactive halide ion from its aqueous solution.

Displacement Reactions:

Fluorine is the most reactive halogen and can displace chloride, bromide, and iodide ions. $F_{2 (g)} + 2 K C l_{(a q)} \to 2 K F_{(a q)} + C l_{2 (g)}$ $F_{2 (g)} + 2 K B r_{(a q)} \to 2 K F_{(a q)} + B r_{2 (l)}$ $F_{2 (g)} + 2 N a I_{(a q)} \to 2 N a F_{(a q)} + I_{2 (s)}$
Chlorine can displace bromide and iodide ions.
Bromine can only displace iodide ions.
Iodine is the least reactive and cannot displace any other halide ions.

Reactions of Halogens with Hydrogen

Halogens react with hydrogen gas to form gaseous hydrogen halides ( $H X$ ).

General Reaction: $H_{2} + X_{2} \to 2 H X (where X = F, C l, B r, I)$

The vigor of this reaction decreases down the group as the reactivity of the halogen decreases.

Fluorine ( $F_{2}$ ): Reacts explosively with hydrogen, even in cold and dark conditions.
Chlorine ( $C l_{2}$ ): Reacts explosively with hydrogen in the presence of sunlight or a flame.
Bromine ( $B r_{2}$ ): Reacts with hydrogen only upon ignition, causing a mild explosion.
Iodine ( $I_{2}$ ): Reacts with hydrogen partially and reversibly, even with continuous heating, forming an equilibrium.

Properties of Hydrogen Halides ( $H X$ )

Thermal Stability

Thermal stability refers to the ability of a compound to resist decomposition upon heating. The stability of hydrogen halides depends on the strength of the H-X bond.

Trend: The H-X bond strength decreases down the group as the halogen atom size increases and electronegativity decreases.

Order of Stability: $H F > H C l > H B r > H I$

HF is the most thermally stable, and HI is the least.

Acidic Strength of Halogen Acids

In aqueous solution, hydrogen halides form hydrohalic acids. Their strength is determined by how easily they can donate a proton ( $H^{+}$ ).

Trend: The acidic strength is inversely related to the H-X bond strength. A weaker bond allows for easier dissociation of the proton.

Order of Acidic Strength: $H I > H B r > H C l > H F$

HI is the strongest hydrohalic acid, and HF is the weakest.

Reactivity of Halide Ions ( $X^{-}$ )

Halide ions can act as reducing agents by losing an electron. Their reducing power increases down the group.

Iodide ( $I^{-}$ ) is the strongest reducing agent, and fluoride ( $F^{-}$ ) is the weakest.

Reactions of Halide Ions with Concentrated Sulfuric Acid

The reaction depends on the reducing strength of the halide ion.

Fluoride ( $F^{-}$ ) and Chloride ( $C l^{-}$ ): These ions are weak reducing agents and do not reduce concentrated $H_{2} S O_{4}$ . They undergo a simple acid-base reaction. $NaF + H_{2} S O_{4} \to NaHS O_{4} + HF$ $NaCl + H_{2} S O_{4} \to NaHS O_{4} + HCl$
Bromide ( $B r^{-}$ ): Strong enough to reduce concentrated $H_{2} S O_{4}$ to sulfur dioxide ( $S O_{2}$ ). $NaBr + H_{2} S O_{4} \to NaHS O_{4} + HBr$ $2 HBr + H_{2} S O_{4} \to B r_{2} + S O_{2} + 2 H_{2} O$
Iodide ( $I^{-}$ ): The strongest reducing agent, capable of reducing concentrated $H_{2} S O_{4}$ to hydrogen sulfide ( $H_{2} S$ ). $NaI + H_{2} S O_{4} \to NaHS O_{4} + HI$ $8 HI + H_{2} S O_{4} \to 4 I_{2} + H_{2} S + 4 H_{2} O$

Reaction with Silver Nitrate (Silver Nitrate Test)

General Reaction: $Ag_{(a q)}^{+} + X_{(a q)}^{-} \to AgX_{(s)}$

Halide Ion	Precipitate Color	Solubility in Aqueous Ammonia
Chloride ( $C l^{-}$ )	White	Soluble
Bromide ( $B r^{-}$ )	Cream (Off-white)	Partially soluble
Iodide ( $I^{-}$ )	Yellow	Insoluble

Reaction of Chlorine with Sodium Hydroxide

Chlorine undergoes a disproportionation reaction with sodium hydroxide, where it is simultaneously oxidized and reduced. The products depend on temperature.

Cold, Dilute NaOH: Forms sodium hypochlorite ( $NaOCl$ ), used as a bleaching agent. $C l_{2} + 2 NaOH \to NaOCl + NaCl + H_{2} O$ In this reaction, $C l_{2}$ is both oxidized (to $C l^{+}$ in $N a O C l$ ) and reduced (to $C l^{-}$ in $N a C l$ ).
Hot, Concentrated NaOH: Forms sodium chlorate ( $NaCl O_{3}$ ). $3 C l_{2} + 6 NaOH \to NaCl O_{3} + 5 NaCl + 3 H_{2} O$

Summary of Trends Down Group 17

Property	Trend Down Group 17
Reactivity / Oxidizing power	Decreases: $F_{2} > C l_{2} > B r_{2} > I_{2}$
Vigor of reaction with $H_{2}$	Decreases
Thermal stability of $H X$	Decreases: $H F > H C l > H B r > H I$
Acidic strength of $H X$	Increases: $H I > H B r > H C l > H F$
Reducing power of $X^{-}$	Increases: $I^{-} > B r^{-} > C l^{-} > F^{-}$