A more reactive halogen displaces a less reactive one
Definition
Displacement reaction
A reaction in which a more reactive metal takes the place of a less reactive metal in a compound.
Definition
Halide ion
The negative ion formed when a halogen atom gains one electron.
- A halogen is added to a solution containing the halide ion of another halogen.
- If the added halogen is more reactive, it takes the place of the one in the compound.
- The displaced halogen is set free and colours the solution.
- If the added halogen is less reactive, nothing happens at all.
- Testing every pair in turn puts the halogens in order of reactivity.
Key Idea
A colour appearing means a reaction; no change is an equally useful result.
What the displacement tests show
- Chlorine displaces bromine, turning the solution orange: Cl2+2KBr→2KCl+Br2\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2Cl2+2KBr→2KCl+Br2
- Chlorine displaces iodine, turning the solution brown: Cl2+2KI→2KCl+I2\text{Cl}_2 + 2\text{KI} \rightarrow 2\text{KCl} + \text{I}_2Cl2+2KI→2KCl+I2
- Bromine displaces iodine, again turning the solution brown: Br2+2KI→2KBr+I2\text{Br}_2 + 2\text{KI} \rightarrow 2\text{KBr} + \text{I}_2Br2+2KI→2KBr+I2
- Iodine displaces neither chlorine nor bromine, so those mixtures show no change.
- Bromine does not displace chlorine, which completes the picture.
Example
- Chlorine water with potassium bromide: colourless to orange.
- Chlorine water with potassium iodide: colourless to brown.
- Iodine solution with potassium chloride: no change.
Reactivity decreases down the group
- Chlorine displaces both of the others, so it is the most reactive of the three.
- Bromine displaces iodine but not chlorine, so it sits between them.
- Iodine displaces neither, so it is the least reactive of the three.
- The order chlorine, bromine, iodine is therefore one of decreasing reactivity.
- This runs opposite to Group 1, where reactivity increases down the group.
Common Mistake
The two groups trend in opposite directions, because one loses an electron and the other gains one.
Predicting astatine
- Astatine lies below iodine, so it is less reactive than iodine.
- It would therefore displace none of chlorine, bromine or iodine from their compounds.
- Any of the three would displace astatine from a solution of an astatide.
- Chlorine and potassium astatide would react: Cl2+2KAt→2KCl+At2\text{Cl}_2 + 2\text{KAt} \rightarrow 2\text{KCl} + \text{At}_2Cl2+2KAt→2KCl+At2
- Fluorine, above chlorine, would displace every one of the others.
Note
A prediction here rests on position in the group, and is stated with that reason attached.
Electronic configuration explains the trend
Definition
Electronic configuration
The arrangement of an atom's electrons in its shells, written as the number in each shell from the innermost outwards.
- Every halogen atom has seven outer electrons, and reacting means gaining one more.
- Down the group each atom has more shells, so the outer shell sits further from the nucleus.
- The inner shells also shield the outer shell from the nucleus's positive charge.
- Distance and shielding together mean an incoming electron is attracted less strongly.
- An electron that is gained less easily makes the halogen less reactive, which is the pattern observed.
Self review
- What is seen when chlorine water is added to potassium bromide solution?
- Write the equation for bromine reacting with potassium iodide.
- Why does adding iodine to potassium chloride solution give no change?
- Predict whether astatine would displace bromine, and say why.
- Why does reactivity decrease down Group 7 when it increases down Group 1?
