L3 - Activity Series

objectives

Understandings

• An oxidizing agent is reduced and a reducing agent is oxidized.
• The activity series ranks metals according to the ease with which they undergo oxidation.

Applications

• Identification of the species oxidized and reduced and the oxidizing and reducing agents, in redox reactions.
• Deduction of the feasibility of a redox reaction from the activity series or reaction data.

Definitions

žRedox reactions ALWAYS involve the simultaneous oxidation of one reactant with the reduction of another through the transfer of electrons.

žThe reactant causing the oxidation of the other reactant is called the oxidizing agent. (It is the one that was reduced.)

žThe reactant causing the reduction of the other reactant is called the reducing agent. (It is the one that is oxidized.)

Activity

It is possible to organise a group of similar chemicals that undergo either oxidation or reduction according to their relative reactivity.

Oxidation (and reduction) is a competition for electrons. The oxidising species (agents) remove electrons from other species and can force them to become reducing agents (releasers of electrons)

A good example of this competition for electrons is the behaviour of metals. Metals always react by losing electrons (oxidation) they are then reducing agents. However if a metals is in competition with metal ions the more reactive metal can oblige the less reactive metal (in the form of ions) to accept electrons. This is called a displacement reaction.

Observing Reactions 1

Example: Zinc reacts with a solution containing copper ions.

Zn(s) → Zn2+(aq) + 2e

Cu2+(aq) + 2e- → Cu(s)

The zinc metal is more reactive than copper metal and so it can force the copper metal ions to accept electrons and become metal atoms.

The zinc metal passes its electrons to the copper ions. We observe that the zinc develops a pink layer of copper on its surface and the blue copper ion solution fades in colour.

We say that the zinc displaces the copper ions from solution.

Reduction of Metal Oxides by Metals

metal A + metal B oxide → metal A oxide + metal B

When a metal A is heated with a metal B oxide there will be a reaction if the free metal A is more reactive than the metal B of the metal B oxide. This is because the metal B in the metal B oxide is in the form of a metal ion - it has already lost electrons.

There is a competition between the metal ion (in the oxide) and the free metal for the electrons. The more reactive of the two metals will win the competition. Consequently if there is a reaction between a metal and a metal oxide then this tells us that the free metal is more reactive than the metal in the metal oxide.

Reactivity Series - ordered by Reducing Power

Reactivity Series involving Non-Metals

Metals react by losing electrons - they are reducing agents. Non-metals react by gaining electrons - they are oxidising agents.

In the same way that metals can be ordered in terms of reducing strength, the non-metals can be ordered in terms of their oxidising strength.

The halogens are a typical example of a non-metal reactivity series.

Reactivity Series - Ordered by Oxidising Power

Do not confuse this order of reactivity with that of the metals - these are non-metals, their reactivity is in terms of oxidising power - i.e. chlorine is the best oxidising agent out of chlorine, bromine and iodine.

1. Chlorine will displace bromine from solutions containing bromide ions

Cl2 + 2Br- --> Br2 + 2Cl-

In this reaction the chlorine is oxidising the bromide ions by removing an electron from them. Bromine is liberated from the solution and may be detected by its orange/red colour

2. Bromine will displace iodine from solutions containing iodide ions

Br2 + 2I- --> I2 + 2Br-

In this reaction the bromine is oxidising the iodide ions by removing an electron from them. Iodine is liberated from the solution and may be detected by its orange/brown colour which turns blue/black in the presence of starch indicator.

It is predictable, then, that chlorine will also displace iodine from a solution containing iodide ions

Reactivity Series - Ordered by Oxidising Power