Halogens and hydrogen halides
Halogens and hydrogen halides
- Each halogen gains an electron to form a $1-$ ion — so it is an oxidising agent.
- Its oxidising power decreases down the group.
- The hydrogen halides get less stable down the group.
Practice
A halogen acts as an oxidising agent because it:
Halogens gain an electron (so they oxidise other species); this power falls down the group.
Oxidising power and displacement
- A halogen acts as an oxidising agent (it gains one electron). This power falls down the group (larger atoms attract the extra electron less).
- A more reactive halogen displaces a less reactive one from its salt:
$$\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2$$
Practice
Chlorine displaces bromine from potassium bromide because chlorine is:
A more reactive halogen pushes out a less reactive one: Cl₂ + 2KBr → 2KCl + Br₂.
Reactions with hydrogen
- Each halogen forms a hydrogen halide: $\text{H}_2 + \text{Cl}_2 \rightarrow 2\text{HCl}$.
- The reaction gets less vigorous down the group (chlorine explosive in light; iodine slow and partial).
Practice
The reaction of the halogens with hydrogen:
Chlorine reacts explosively in light, bromine needs heat, and iodine reacts slowly and partly.
Thermal stability of HX
- The thermal stability of the hydrogen halides decreases down the group.
- The H–X bond gets weaker as the halogen gets larger, so HI breaks apart on gentle heating while HCl is very stable.
Practice
The thermal stability of the hydrogen halides:
The H–X bond weakens as X gets larger, so HI decomposes easily while HCl is very stable.
You've got it
Key idea
- halogens are oxidising agents (gain 1 electron); power decreases down the group
- a more reactive halogen displaces a less reactive one ($\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2$)
- reactions with hydrogen get less vigorous down; HX thermal stability decreases down (HI least stable)