In this video, we will focus on displacement reaction, a chemical reaction that Group VII elements, which we call the halogens, undergo.

Displacement reaction

Halogens are very reactive non-metals. The reactivity decreases down the Group. Fluorine is the most reactive non-metal in the Periodic Table. Chlorine is more reactive than bromine, and bromine is more reactive than iodine.

Halogens undergo displacement reactions with halides solutions. A more reactive halogen can displace a less reactive halogen from its halide solution.

Let’s focus on these 6 possible scenarios – adding 3 different halogens, namely chlorine, bromine and iodine, into 3 different halide solutions.

For simplicity, let’s make all sodium halide solutions.

Scenario 1

Aqueous chlorine added to bromide solution Let’s start with chlorine. When chlorine is added to bromide solution, what will happen and will what we observe? Now we know that reactivity of halogens decreases down the Group. Chlorine is more reactive than bromine. Hence, chlorine will be able to displace bromine from bromide solution. So, chlorine will take the place of bromine, forming sodium chloride. Put in chemistry terms, the more reactive chlorine displaces less reactive bromine from aqueous sodium bromide, forming sodium chloride and bromine. What will we observe? Aqueous chlorine is light yellow. Both halide solutions are colourless. Aqueous bromine is reddish brown. Hence, in this reaction, we can say, when light yellow aqueous chlorine is added to colourless sodium bromide solution, a reddish brown solution of aqueous bromine is formed.

Scenario 2

Aqueous chlorine added to iodide solution When chlorine is added to iodide solution, what will happen and will what we observe? Reactivity of halogens decreases down the Group. Chlorine is more reactive than iodine. Hence, chlorine will be able to displace iodine from iodide solution. So, chlorine will take the place of iodine, forming sodium chloride. Put in chemistry terms, the more reactive chlorine displaces less reactive iodine from aqueous sodium iodide, forming sodium chloride and iodine. What will we observe? Aqueous chlorine is light yellow. Again, both halide solutions are colourless. Aqueous iodine is brown. Hence, in this reaction, we can say, when light yellow aqueous chlorine is added to colourless sodium iodide solution, a brown solution of aqueous iodine is formed.

Scenario 3

Aqueous bromine added to chloride solution Do you expect any reaction when aqueous bromine is added to sodium chloride? Bromine is less reactive than chlorine. Hence, bromine is unable to displace chlorine from chloride solution. Hence, there is no reaction.

Scenario 4

Aqueous bromine added to iodide solution When bromine is added to iodide solution, what will happen and will what we observe? Bromine is more reactive than iodine. Hence, bromine will be able to displace iodine from iodide solution. So, bromine will take the place of iodine, forming sodium bromide. Hence, we can say, the more reactive bromine displaces less reactive iodine from aqueous sodium iodide, forming sodium bromide and iodine. What will we observe? Aqueous bromine is reddish brown. Halide solutions are colourless. Aqueous iodine is brown. Hence, in this reaction, we can say, when reddish brown aqueous bromine is added to colourless sodium iodide solution, a brown solution of aqueous iodine is formed.

Scenario 5

Aqueous iodine added to chloride solution What will happen if aqueous iodine is added to sodium chloride? Iodine is less reactive than chlorine. Hence, iodine is unable to displace chlorine from chloride solution. Hence, there is no reaction.

Scenario 6

Aqueous iodine added to bromide solution What will happen if aqueous iodine is added to sodium bromide? Iodine is less reactive than bromine. Hence, iodine is unable to displace bromine from bromide solution. Hence, there is no reaction.

Topic: The Periodic Table, O Level Chemistry, Singapore

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