Chemguide: Support for CIE A level Chemistry

Learning outcome 11.3

Group 17 (previously Group 7)

Some reactions of the halide ions

Learning outcome 11.3.1

This statement is about the relative reactivity of halide ions as reducing agents.

In statement 11.2.1, we looked at halogens as oxidising agents, taking electrons from other things to make halide ions.

X2 + 2e-    2X-

The ability to remove electrons from other things decreases down the group. Fluorine does it best; iodine does it least well.

In other words, the oxidising ability falls as you go down the group.

This statement reverses this and looks at it from the point of view of the halide ions.

2X-    X2 + 2e-

If the element fluorine is very good at taking electrons away from other things, then it follows that fluoride ions will be pretty reluctant to give electrons away again.

On the other hand, if iodine molecules don't accept electrons very readily, it is reasonable to suppose that iodide ions will probably be much more ready to lose those electrons again.

Reduction is gain of electrons. A reducing agent is something which gives electrons to something else, and therefore reduces it.

Fluoride ions aren't good at releasing electrons again, and so will be poor reducing agents.

Iodide ions will release electrons much more readily and so are much better reducing agents.

Reducing ability of the halide ions increases as you go down the group.

You will soon come across a specific example of the reducing ability of the halide ions in their reactions with concentrated sulfuric acid in the next statement (11.3.2).

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© Jim Clark 2020