Chlorine is a halogen, an element in group 17 of the periodic table. It can displace elements lower in the group like bromine and iodine from salts like potassium bromide. Potassium bromide is an ionic compound formed from bromine and potassium.
Chlorine has a higher electron affinity than bromine, meaning that more energy is released when chlorine gains an electron than when bromine gains an electron. Consequently, it also has a higher electronegativity, meaning that it can exert a stronger "pull" on electrons. In the ionic compound potassium bromide, the bromine atom has taken an electron from the potassium atom so that both atoms now have equal and opposite charges and are thus attracted to each other.
When chlorine gas is bubbled through a solution of potassium bromide, the chlorine will oxidize the bromide or take electrons from it. Each atom of chlorine gains an electron to become a chloride ion, while each bromide ion loses an electron to become elemental bromine. The bromine atoms now react with each other to form Br2 or bromine gas, which bubbles out from the surface of the solution.
Since the chlorine has displaced the bromine, if this solution were heated to evaporate the water, potassium chloride salt would be left behind. This reaction is a fairly straightforward way to produce bromine gas in the lab, although since chlorine gas is highly toxic (it was used as poison gas during the First World War), it's not recommended that you try this at home.
- "Chemical Principles, the Quest for Insight, 4th Edition"; Peter Atkins and Loretta Jones; 2008.
- Journal of Chemical Education Online: Reaction of Chlorine with Potassium Bromide
- GCSE Science: Halogen Displacement Reactions
- Photo Credit Test Kit image by Cinneman from Fotolia.com
Potassium Bromide & Water
Potassium bromide is an ionic salt, written as KBr for scientific purposes. It is manufactured by the reaction of potassium carbonate with...