Oxidation and Reduction (2022)

Oxidationand Reduction

Oxidation-Reduction Reactions The Role of Oxidation Numbers in Oxidation-Reduction Reactions Oxidation Numbers vs. The True Charge of Ions
Oxidizing Agents and Reducing Agents Conjugate Oxidizing Agent/Reducing Agent Pairs The Relative Strengths of Metals as Reducing Agents

Oxidation-ReductionReactions

The term oxidation was originally used to describereactions in which an element combines with oxygen.

Example: The reaction between magnesium metal and oxygen toform magnesium oxide involves the oxidation of magnesium.

Oxidation and Reduction (1)

The term reduction comes from the Latin stem meaning"to lead back." Anything that that leads back tomagnesium metal therefore involves reduction.

The reaction between magnesium oxide and carbon at 2000C toform magnesium metal and carbon monoxide is an example of thereduction of magnesium oxide to magnesium metal.

Oxidation and Reduction (2)

After electrons were discovered, chemists became convincedthat oxidation-reduction reactions involved the transfer ofelectrons from one atom to another. From this perspective, thereaction between magnesium and oxygen is written as follows.

2 Mg + O2 Oxidation and Reduction (3) 2 [Mg2+][O2-]

In the course of this reaction, each magnesium atom loses twoelectrons to form an Mg2+ ion.

Mg Oxidation and Reduction (4)Mg2+ + 2 e-

And, each O2 molecule gains four electrons to forma pair of O2- ions.

O2 + 4 e- Oxidation and Reduction (5) 2 O2-

Because electrons are neither created nordestroyed in a chemical reaction, oxidation and reduction arelinked. It is impossible to have one without the other, as shownin the figure below.

Oxidation and Reduction (6)

Practice Problem 1:

Determine which element is oxidized and which is reduced when lithium reacts with nitrogen to form lithium nitride.

6 Li(s) + N2(g) Oxidation and Reduction (7) 2 Li3N(s)

Click here to check your answer to Practice Problem 1

The Role of OxidationNumbers in Oxidation-Reduction Reactions

Chemists eventually extended the idea of oxidation andreduction to reactions that do not formally involve the transferof electrons.

Consider the following reaction.

CO(g) + H2O(g)Oxidation and Reduction (9)CO2(g) + H2(g)

As can be seen in the figure below, the total number ofelectrons in the valence shell of each atom remains constant inthis reaction.

Oxidation and Reduction (10)

What changes in this reaction is the oxidation state of theseatoms. The oxidation state of carbon increases from +2 to +4,while the oxidation state of the hydrogen decreases from +1 to 0.

Oxidation and Reduction (11)

Oxidation and reduction are therefore bestdefined as follows. Oxidation occurs when theoxidation number of an atom becomes larger. Reductionoccurs when the oxidation number of an atom becomes smaller.

Practice Problem 2:

Determine which atom is oxidized and which is reduced in the following reaction

Sr(s) + 2 H2O(l) Oxidation and Reduction (12) Sr2+(aq) + 2 OH-(aq) + H2(g)

Click here to check your answer to Practice Problem 2

Oxidation Numbers Versusthe True Charge on Ions

The terms ionic and covalent describe theextremes of a continuum of bonding. There is some covalentcharacter in even the most ionic compounds and vice versa.

It is useful to think about the compounds of the main groupmetals as if they contained positive and negative ions. Thechemistry of magnesium oxide, for example, is easy to understandif we assume that MgO contains Mg2+ and O2-ions. But no compounds are 100% ionic. There is experimentalevidence, for example, that the true charge on the magnesium andoxygen atoms in MgO is +1.5 and -1.5.

Oxidation states provide a compromise between a powerful modelof oxidation-reduction reactions based on the assumption thatthese compounds contain ions and our knowledge that the truecharge on the ions in these compounds is not as large as thismodel predicts. By definition, the oxidation state of an atom isthe charge that atom would carry if the compound were purelyionic.

For the active metals in Groups IA and IIA, the differencebetween the oxidation state of the metal atom and the charge onthis atom is small enough to be ignored. The main group metals inGroups IIIA and IVA, however, form compounds that have asignificant amount of covalent character. It is misleading, forexample, to assume that aluminum bromide contains Al3+and Br- ions. It actually exists as Al2Br6molecules.

This problem becomes even more severe when we turn to thechemistry of the transition metals. MnO, for example, is ionicenough to be considered a salt that contains Mn2+ andO2- ions. Mn2O7, on the otherhand, is a covalent compound that boils at room temperature. Itis therefore more useful to think about this compound as if itcontained manganese in a +7 oxidation state, not Mn7+ions.

Oxidizing Agents andReducing Agents

Let's consider the role that each element plays in thereaction in which a particular element gains or loses electrons..

When magnesium reacts with oxygen, the magnesium atoms donateelectrons to O2 molecules and thereby reduce theoxygen. Magnesium therefore acts as a reducing agent inthis reaction.

The O2 molecules, on the otherhand, gain electrons from magnesium atoms and thereby oxidize themagnesium. Oxygen is therefore an oxidizing agent.

2 Mg + O2 Oxidation and Reduction (18) 2 MgO
oxidizing
agent

Oxidizing and reducing agents therefore can be defined asfollows. Oxidizing agents gain electrons. Reducingagents lose electrons.

Practice Problem 3:

Identify the oxidizing agent and the reducing agent in the following reaction.

Ca(s) + H2(g) Oxidation and Reduction (19) CaH2(g)

Click here to check your answer to Practice Problem 3

The table below identifies the reducing agent and theoxidizing agent for some of the reactions discussed in this webpage. One trend is immediately obvious: The main group metalsact as reducing agents in all of their chemical reactions.

Typical Reactions of Main Group Metals

Reaction Reducing
Agent
Oxidizing
Agent
2 Na + Cl2 Oxidation and Reduction (20) 2 NaCl Na Cl2
2 K + H2 Oxidation and Reduction (21) 2 KH K H2
4 Li + O2 Oxidation and Reduction (22) 2 Li2O Li O2
2 Na + O2 Oxidation and Reduction (23) Na2O2 Na O2
2 Na + 2 H2O Oxidation and Reduction (24) 2 Na+ + 2 OH- + H2 Na H2O
2 K + 2 NH3 Oxidation and Reduction (25) 2 K+ + 2 NH2- + H2 K NH3
2 Mg + O2 Oxidation and Reduction (26) 2 MgO Mg O2
3 Mg + N2 Oxidation and Reduction (27) Mg3N2 Mg N2
Ca + 2 H2O Oxidation and Reduction (28) Ca2+ + 2 OH- + H2 Ca H2O
2 Al + 3 Br2 Oxidation and Reduction (29) Al2Br6 Al Br2
Mg + 2 H+ Oxidation and Reduction (30) Mg2++ H2 Mg H+
Mg + H2O Oxidation and Reduction (31) MgO + H2 Mg H2O

Conjugate OxidizingAgent/Reducing Agent Pairs

Metals act as reducing agents in their chemical reactions.When copper is heated over a flame, for example, the surfaceslowly turns black as the copper metal reduces oxygen in theatmosphere to form copper(II) oxide.

Oxidation and Reduction (33)

If we turn off the flame, and blow H2 gas over thehot metal surface, the black CuO that formed on the surface ofthe metal is slowly converted back to copper metal. In the courseof this reaction, CuO is reduced to copper metal. Thus, H2is the reducing agent in this reaction, and CuO acts as anoxidizing agent.

Oxidation and Reduction (34)

An important feature of oxidation-reduction reactions can berecognized by examining what happens to the copper in this pairof reactions. The first reaction converts copper metal into CuO,thereby transforming a reducing agent (Cu) into an oxidizingagent (CuO). The second reaction converts an oxidizing agent(CuO) into a reducing agent (Cu). Every reducing agent istherefore linked, or coupled, to a conjugate oxidizing agent, andvice versa.

Every time a reducing agent loses electrons, it forms anoxidizing agent that could gain electrons if the reaction werereversed.

Oxidation and Reduction (35)

Conversely, every time an oxidizing agent gains electrons, itforms a reducing agent that could lose electrons if the reactionwent in the opposite direction.

Oxidation and Reduction (36)

The idea that oxidizing agents and reducing agents are linked,or coupled, is why they are called conjugate oxidizingagents and reducing agents. Conjugate comes from the Latinstem meaning "to join together." It is therefore usedto describe things that are linked or coupled, such as oxidizingagents and reducing agents.

The main group metals are all reducing agents. They tend to be"strong" reducing agents. The active metals in GroupIA, for example, give up electrons better than any other elementsin the periodic table.

The fact that an active metal such as sodium is a strongreducing agent should tell us something about the relativestrength of the Na+ ion as an oxidizing agent. Ifsodium metal is relatively good at giving up electrons, Na+ions must be unusually bad at picking up electrons. If Na is astrong reducing agent, the Na+ ion must be a weakoxidizing agent.

Oxidation and Reduction (37)

Conversely, if O2 has such a high affinity forelectrons that it is unusually good at accepting them from otherelements, it should be able to hang onto these electrons once itpicks them up. In other words, if O2 is a strongoxidizing agent, then the O2- ion must be a weakreducing agent.

Oxidation and Reduction (38)

In general, the relationship between conjugate oxidizing andreducing agents can be described as follows. Every strongreducing agent (such as Na) has a weak conjugate oxidizing agent(such as the Na+ ion). Every strongoxidizing agent (such as O2) has a weakconjugate reducing agent (such as the O2-ion).

The Relative Strengthof Metals as Reducing Agents

We can determine the relative strengths of a pair of metals asreducing agents by determining whether a reaction occurs when oneof these metals is mixed with a salt of the other. Consider therelative strength of iron and aluminum, for example. Nothinghappens when we mix powdered aluminum metal with iron(III) oxide.If we place this mixture in a crucible, however, and get thereaction started by applying a little heat, a vigorous reactiontakes place to give aluminum oxide and molten iron metal.

2 Al(s) + Fe2O3(s)Oxidation and Reduction (40) Al2O3(s) + 2 Fe(l)

By assigning oxidation numbers, we can pick out the oxidationand reduction halves of the reaction.

Oxidation and Reduction (41)

Aluminum is oxidized to Al2O3 in thisreaction, which means that Fe2O3 must bethe oxidizing agent. Conversely, Fe2O3 isreduced to iron metal, which means that aluminum must be thereducing agent. Because a reducing agent is always transformedinto its conjugate oxidizing agent in an oxidation-reductionreaction, the products of this reaction include a new oxidizingagent (Al2O3) and a new reducing agent(Fe).

Oxidation and Reduction (42)

Since the reaction proceeds in this direction, it seemsreasonable to assume that the starting materials contain thestronger reducing agent and the stronger oxidizing agent.

Oxidation and Reduction (43)

In other words, if aluminum reduces Fe2O3to form Al2O3 and iron metal, aluminum mustbe a stronger reducing agent than iron.

We can conclude from the fact that aluminum cannot reducesodium chloride to form sodium metal that the starting materialsin this reaction are the weaker oxidizing agent and the weakerreducing agent.

Oxidation and Reduction (44)

We can test this hypothesis by asking: What happens when wetry to run the reaction in the opposite direction? (Is sodiummetal strong enough to reduce a salt of aluminum to aluminummetal?) When this reaction is run, we find that sodium metal can,in fact, reduce aluminum chloride to aluminum metal and sodiumchloride when the reaction is run at temperatures hot enough tomelt the reactants.

3 Na(l) + AlCl3(l)Oxidation and Reduction (45) 3 NaCl(l) + Al(l)

If sodium is strong enough to reduce Al3+salts to aluminum metal and aluminum is strong enough to reduceFe3+ salts to iron metal, the relative strengths ofthese reducing agents can be summarized as follows.

Na > Al > Fe

Practice Problem 4:

Use the following equations to determine the relative strengths of sodium, magnesium, aluminum, and calcium metal as reducing agents.

2 Na + MgCl2Oxidation and Reduction (46)2 NaCl + Mg
Al + MgBr2 Oxidation and Reduction (47)
Ca + MgI2Oxidation and Reduction (48) CaI2 + Mg
Ca + 2 NaCl Oxidation and Reduction (49)

Click here to check your answer to Practice Problem 4

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