Redox Chemistry and Redox Chemistry and CorrosionCorrosion

Chapter 16Chapter 16

Oxidation and ReductionOxidation and Reduction

►So far we have looked at precipitation So far we have looked at precipitation reactions and acid-base reactions.reactions and acid-base reactions.

►Now we shall look at a third group of Now we shall look at a third group of chemical reactions.chemical reactions.

►They are called oxidation-reduction They are called oxidation-reduction reactions. reactions.

►These reactions are commonly These reactions are commonly referred to as redox reactions.referred to as redox reactions.

Redox ReactionsRedox Reactions

►Many of the chemical reactions that play a Many of the chemical reactions that play a significant role in maintaining our significant role in maintaining our environment are redox reactions.environment are redox reactions.

► Corrosion and the deterioration of metals Corrosion and the deterioration of metals are redox reactions. are redox reactions.

► Iron which is used as a structural base for Iron which is used as a structural base for buildings and bridges is particularly prone to buildings and bridges is particularly prone to corrosion. corrosion.

► Australia spends about 3 billion dollars a Australia spends about 3 billion dollars a year in an effort to prevent corrosion and year in an effort to prevent corrosion and replacing structures that have corroded.replacing structures that have corroded.

Redox ReactionsRedox Reactions

►These reactions are also These reactions are also used in the processing of used in the processing of mineral ores to extract mineral ores to extract from then the metals our from then the metals our society requires.society requires.

►One of Australia’s biggest One of Australia’s biggest exports is the mining of exports is the mining of these mineral ores.these mineral ores.

Redox ReactionsRedox Reactions

►Other redox reactions include:Other redox reactions include: The respiration reaction that is the source of The respiration reaction that is the source of

energy in almost all living things.energy in almost all living things. Photosynthesis in green plantsPhotosynthesis in green plants Burning of fuels to propel our cars.Burning of fuels to propel our cars. Combustion of coal in electricity power stations.Combustion of coal in electricity power stations. Use of chemicals such as chlorine to disinfect Use of chemicals such as chlorine to disinfect

swimming pools.swimming pools. Manufacture and use of explosives.Manufacture and use of explosives. Use of electrolysis to produce many chemicals.Use of electrolysis to produce many chemicals. Production and use of fertilisers.Production and use of fertilisers.

Redox ReactionsRedox Reactions►Many chemicals react with oxygen. Many chemicals react with oxygen. ►Reactions such as these were Reactions such as these were

described as oxidation reactions.described as oxidation reactions.► In air, the combustion of carbon, sulfur, In air, the combustion of carbon, sulfur,

iron or even octane always produced iron or even octane always produced at least one oxide:at least one oxide: C(s) + OC(s) + O22(g) (g) ―› CO―› CO22(g)(g)

S(s) + S(s) + OO22(g) (g) ―› SO―› SO22(g)(g)

4Fe(s) + 34Fe(s) + 3OO22(g) (g) ―› 2Fe―› 2Fe22OO33(s)(s)

2C2C88HH1818(l) + 25O(l) + 25O22(g) (g) ―›16CO―›16CO22(g) + 18H(g) + 18H22O(l)O(l)

OxidationOxidation

►Oxidation means the addition of oxygen.Oxidation means the addition of oxygen.►When oxygen reacts with an element, When oxygen reacts with an element,

the element is said to be oxidised. the element is said to be oxidised. ►Because elemental iron reacts with Because elemental iron reacts with

oxygen, there are no large deposits of oxygen, there are no large deposits of elemental iron found on earth. elemental iron found on earth.

► Iron is generally found as a compound of Iron is generally found as a compound of mineral oxide ores (haematite (Femineral oxide ores (haematite (Fe22OO33) ) and magnetite (Feand magnetite (Fe33OO44))))

ReductionReduction

► Iron used in modern society has been Iron used in modern society has been extracted from iron ores.extracted from iron ores.

►This extraction process involves This extraction process involves reduction of the iron oxide to iron. reduction of the iron oxide to iron.

► It involves the removal or oxygen.It involves the removal or oxygen.►When oxygen is removed from a When oxygen is removed from a

substance, that material has been substance, that material has been reduced.reduced.

ReductionReduction

► The production of iron from haematite can be The production of iron from haematite can be represented by the reduction equation:represented by the reduction equation:

FeFe22OO33(s) + 3CO(g) (s) + 3CO(g) ―› 2Fe(s) + ―› 2Fe(s) + 3CO3CO22(g)(g)

The iron(III) oxide has lost an oxygen – it has been The iron(III) oxide has lost an oxygen – it has been reduced.reduced.

Reduction cannot occur without oxidation occurring at Reduction cannot occur without oxidation occurring at the same time.the same time.

In this reaction the carbon monoxide has gained an In this reaction the carbon monoxide has gained an oxygen – it has been oxidised.oxygen – it has been oxidised.

Reduction – loss of oxygen

Oxidation – gain of oxygen

A Better DefinitionA Better Definition

►There are many oxidation and There are many oxidation and reduction reactions that don’t involve reduction reactions that don’t involve oxygen.oxygen.

► Instead we define oxidation as the loss Instead we define oxidation as the loss of electrons.of electrons.

►Similarly, reduction is the gain of Similarly, reduction is the gain of electrons rather than the loss of electrons rather than the loss of oxygen.oxygen.

OIL RIGOIL RIG

►OOxidation xidation iis the s the lloss of electronsoss of electrons►RReduction eduction iis the s the ggain of electrons.ain of electrons.

Magnesium OxideMagnesium Oxide

►You have used magnesium in class You have used magnesium in class before, remember how it has a coating before, remember how it has a coating on it that sometimes we have had to on it that sometimes we have had to scrape off.scrape off.

►That is magnesium oxide which results That is magnesium oxide which results in corrosion of magnesium in air.in corrosion of magnesium in air.

Magnesium OxideMagnesium Oxide

►The magnesium has reacted with The magnesium has reacted with atmospheric oxygen to form atmospheric oxygen to form magnesium oxide.magnesium oxide.

►The magnesium has been oxidised.The magnesium has been oxidised.

2Mg(s) + O2Mg(s) + O22(g) (g) ――> 2MgO(s)> 2MgO(s)

2Mg(s) + O2Mg(s) + O22(g) (g) ――> 2MgO(s)> 2MgO(s)

►Magnesium oxide is an ionic compound Magnesium oxide is an ionic compound and consists of Mgand consists of Mg2+2+ ions and O ions and O2-2- ions. ions.

►Each magnesium ion, therefore must Each magnesium ion, therefore must have lost two electrons to form an Mghave lost two electrons to form an Mg2+2+ ion. Each oxygen atom in the oxygen ion. Each oxygen atom in the oxygen molecule must have gained two molecule must have gained two electrons to form an oxide ion Oelectrons to form an oxide ion O2-2-..

►The reaction can now be represented The reaction can now be represented by two half equations.by two half equations.

►The first half equation show the gain of The first half equation show the gain of two electrons by each oxygen atom in two electrons by each oxygen atom in the oxygen molecule:the oxygen molecule:

Mg(s) Mg(s) ――> Mg> Mg2+2+(s) + 2e(s) + 2e--

►The second show the gain of two The second show the gain of two electrons by electrons by eacheach oxygen atom in the oxygen atom in the oxygen molecule.oxygen molecule.

OO22(g) + 4e(g) + 4e-- ― ―> 2O> 2O2-2-(s)(s)

2Mg(s) + O2Mg(s) + O22(g) ―(g) ―> 2MgO(s)> 2MgO(s)

Mg(s) Mg(s) ――> Mg> Mg2+2+(s) + 2e(s) + 2e--

OO22(g) + 4e(g) + 4e-- ― ―> 2O> 2O2-2-(s)(s)► So the oxidation of magnesium involves the So the oxidation of magnesium involves the

transfer of electrons from magnesium atoms transfer of electrons from magnesium atoms to oxygen atoms. to oxygen atoms.

►Note that there is no real ‘loss of electrons’ Note that there is no real ‘loss of electrons’ but rather a transfer of electrons from the but rather a transfer of electrons from the magnesium to the oxygen.magnesium to the oxygen.

► If an atom loses electrons, there must be If an atom loses electrons, there must be another atom that can gain electrons.another atom that can gain electrons.

► Therefore oxidation and reduction occur Therefore oxidation and reduction occur simultaneously.simultaneously.

Writing Redox Half EquationsWriting Redox Half Equations

►Worked Example 16.2a page 275Worked Example 16.2a page 275►16.2b16.2b

Your TurnYour Turn

►Page 278Page 278►Question 1Question 1►Question 2Question 2

Writing an Overall Redox Writing an Overall Redox EquationEquation

►When we write equation for redox When we write equation for redox reactions, we normally write the two reactions, we normally write the two half equations first.half equations first.

►We then follow this with the overall We then follow this with the overall equation.equation.

► In the overall equation we do not show In the overall equation we do not show any electrons transferred as:any electrons transferred as: The electrons lost in the oxidation The electrons lost in the oxidation

reaction are gained in the reduction reaction are gained in the reduction reaction.reaction.

Copper and the solution of silver Copper and the solution of silver ionsions

► In the previous example:In the previous example: Each copper atom that is oxidised loses Each copper atom that is oxidised loses

two electronstwo electrons Each AgEach Ag++ ion that is reduced gains one ion that is reduced gains one

electron.electron.►When writing full equations we must When writing full equations we must

balance the electrons first.balance the electrons first.►Therefore two AgTherefore two Ag++ ion must be ion must be

reduced to take up the electrons lost reduced to take up the electrons lost by each copper atom that is oxidised. by each copper atom that is oxidised.

Cu(s) Cu(s) ――> Cu> Cu2+2+(aq) + 2e(aq) + 2e--

AgAg++(aq) + e(aq) + e-- ――> Ag(s) > Ag(s)

So we need to times the silver ions by 2So we need to times the silver ions by 2

The overall equation is:The overall equation is:

Cu(s) + 2AgCu(s) + 2Ag++(aq) (aq) ――> Cu> Cu2+2+(aq) + 2Ag(s)(aq) + 2Ag(s)

Copper and the solution of silver Copper and the solution of silver ionsions

( ) x 2

RememberRemember

► In both half and overall equations.In both half and overall equations. The number of atoms of each element The number of atoms of each element

present in the products is equal to the present in the products is equal to the number present in the reactants.number present in the reactants.

Atoms are conserved in all chemical Atoms are conserved in all chemical equations.equations.

The total charge on the product side of the The total charge on the product side of the equation is equal to the total charge on the equation is equal to the total charge on the reactant side of the equation.reactant side of the equation.

Charge is conserved in chemical reactions.Charge is conserved in chemical reactions.

Worked Example 16.2cWorked Example 16.2c

When sodium is oxidised by atmospheric When sodium is oxidised by atmospheric oxygen, the reaction can be represented oxygen, the reaction can be represented by the following half equations:by the following half equations:

Na(s) Na(s) ――> Na> Na++(s) + e(s) + e--

OO22(g) + 4e(g) + 4e-- ――> 2O> 2O2-2-(s)(s)

Identify the half equation representing the Identify the half equation representing the oxidation reaction and write the oxidation reaction and write the balanced overall equation.balanced overall equation.

Oxidants and ReductantsOxidants and Reductants

►An oxidant (or oxidising agent) is a An oxidant (or oxidising agent) is a species that causes another to be species that causes another to be oxidised.oxidised.

►A reductant (or reducing agent) is a A reductant (or reducing agent) is a species that causes another to be species that causes another to be reduced.reduced.

►The oxidant itself is reduced.The oxidant itself is reduced.►The reductant is oxidised.The reductant is oxidised.

Your TurnYour Turn

►Page 278Page 278►Question 3 and 4Question 3 and 4

Predicting electron transferPredicting electron transfer

►Read pages 283 – 285Read pages 283 – 285►What is a galvanic cell?What is a galvanic cell?

Galvanic Cell Galvanic Cell

►All galvanic cells are composed of two All galvanic cells are composed of two half cells.half cells.

►Oxidation occurs in one half cell.Oxidation occurs in one half cell.►Reduction occurs in the other.Reduction occurs in the other.►A half cell must contain an electrode and A half cell must contain an electrode and

an electrolyte.an electrolyte.►An electrode is an electronic conductor – An electrode is an electronic conductor –

a material that has delocalised electrons a material that has delocalised electrons that can move through the circuit.that can move through the circuit.

Galvanic CellsGalvanic Cells

►The electrode at which oxidation takes The electrode at which oxidation takes place is called the anode.place is called the anode.

►The electrode at which reduction takes The electrode at which reduction takes place is called the cathode.place is called the cathode.

Galvanic CellsGalvanic Cells

► Zinc is the anode.Zinc is the anode.► Copper is the cathode.Copper is the cathode.► In galvanic cells the anode is negatively In galvanic cells the anode is negatively

charged and the cathode is positively charged and the cathode is positively charged.charged.

Galvanic CellsGalvanic Cells

►CuCu2+2+ ions are reduced to Cu atoms at ions are reduced to Cu atoms at the cathode.the cathode.

►Cations will migrate from the salt Cations will migrate from the salt bridge into the beaker containing that bridge into the beaker containing that cathode to compensate for the loss of cathode to compensate for the loss of the Cuthe Cu2+2+ ions. ions.

►At the anode, zinc metal is oxidised and At the anode, zinc metal is oxidised and so more Znso more Zn2+2+ ions are added to the ions are added to the solution in that beaker.solution in that beaker.

The salt bridgeThe salt bridge

►To avoid the build up of a positive To avoid the build up of a positive charge, anions (negatively charged charge, anions (negatively charged ions) will migrate from the salt bridge ions) will migrate from the salt bridge into the beaker and so maintain into the beaker and so maintain electrical neutrality.electrical neutrality.

ElectrolyteElectrolyte

►An electrolyte contains ions that are An electrolyte contains ions that are free to move through the solution.free to move through the solution.

► In the example the electrolyte in In the example the electrolyte in beaker A was the zinc chloride.beaker A was the zinc chloride.

►The electrolyte in beaker B was the The electrolyte in beaker B was the copper sulfate solution.copper sulfate solution.

Galvanic Cells Comprise Of:Galvanic Cells Comprise Of:

►Two half cells, which are separate and do Two half cells, which are separate and do not mix.not mix.

►A length of wire connecting the electrodes A length of wire connecting the electrodes of the half cells. This is the external of the half cells. This is the external current.current.

►A salt bridge to connect the solutions in the A salt bridge to connect the solutions in the half cells. This is the electrical conductor.half cells. This is the electrical conductor.

►The salt bridge balances the overall charge The salt bridge balances the overall charge during the circuit.during the circuit.

The electrochemical seriesThe electrochemical series

►Sodium, magnesium and iron are all Sodium, magnesium and iron are all metals that corrode easily because they metals that corrode easily because they are easily oxidised.are easily oxidised.

►Sodium is oxidised so easily that it is Sodium is oxidised so easily that it is stored under paraffin oil.stored under paraffin oil.

►Other metals, however, do not corrode Other metals, however, do not corrode readily. Platinum and gold are readily. Platinum and gold are sufficiently inert to be found free in sufficiently inert to be found free in nature.nature.

The Electrochemical SeriesThe Electrochemical Series

►Table 16.2 on page 287 represents the Table 16.2 on page 287 represents the electrochemical series.electrochemical series.

►What can you tell me about the What can you tell me about the electrochemical series?electrochemical series?

The electrochemical seriesThe electrochemical series

► Each half equation represents the reduction Each half equation represents the reduction reactions.reactions.

► The top equation is the strongest oxidant so The top equation is the strongest oxidant so it is most easily reduced.it is most easily reduced.

► The strongest reductants are at the bottom The strongest reductants are at the bottom and are oxidised quite easily. What kind of and are oxidised quite easily. What kind of metals do these mainly consist of?metals do these mainly consist of?

► In general the smaller amount of energy In general the smaller amount of energy required to remove a valance electron the required to remove a valance electron the more readily the metal will act as a more readily the metal will act as a reductant and itself be oxidised.reductant and itself be oxidised.

Electrochemical SeriesElectrochemical Series

►Non-metals tend to gain electrons and Non-metals tend to gain electrons and therefore act as oxidants.therefore act as oxidants.

►Reactive metals tend to be stronger Reactive metals tend to be stronger reductants.reductants.

►Transition metals are less readily Transition metals are less readily oxidised.oxidised.

Predicting Redox ReactionsPredicting Redox Reactions

►We use the electrochemical series to We use the electrochemical series to predict redox reactions. predict redox reactions.

►More reactive metals tend to be found More reactive metals tend to be found on the lower right of the on the lower right of the electrochemical series.electrochemical series.

►A more reactive metal will be oxidised A more reactive metal will be oxidised by, and donate its electrons to the by, and donate its electrons to the cation of a less reactive metal. cation of a less reactive metal.

►The cation receives the electrons and is The cation receives the electrons and is reduced.reduced.

Predicting Redox ReactionsPredicting Redox Reactions

►A spontaneous redox reaction can be A spontaneous redox reaction can be expected to occur when a relatively expected to occur when a relatively strong oxidant is mixed with a relatively strong oxidant is mixed with a relatively strong reductant.strong reductant.

►The oxidant is reduced and the half The oxidant is reduced and the half equation occurs in the forward direction.equation occurs in the forward direction.

►The reductant is reduced and the half The reductant is reduced and the half equation occurs in the reverse direction equation occurs in the reverse direction of the that on the electrochemical series.of the that on the electrochemical series.

Predicting Redox ReactionsPredicting Redox Reactions

►We can predict that zinc metal with We can predict that zinc metal with react with Cureact with Cu2+2+ ions because zinc is ions because zinc is more reactive than copper.more reactive than copper.

Cu2+(aq)

Is reduced

+ 2e- ―> Cu(s)

Zn(s)

Is oxidised

Zn2+ + 2e- <―

Reacts with

What is the overall Equation????

AN OIL RIG CATAN OIL RIG CAT

►AAnode + node + OOxidation xidation iis s lloss of electrons:oss of electrons:►RReduction eduction iis s ggain of electrons + ain of electrons +

CatCathodehode

►A way to remember oxidation occurs A way to remember oxidation occurs at the anode. Reduction occurs at the at the anode. Reduction occurs at the cathode.cathode.

Predicting ReactionsPredicting Reactions

►For reactions to occur spontaneously, For reactions to occur spontaneously, the aqueous cation in the solution the aqueous cation in the solution must be a stronger oxidant than the must be a stronger oxidant than the cation of the metal added.cation of the metal added.

►Your TurnYour Turn►Try Question 13 on page 291Try Question 13 on page 291►Try Question 15 as wellTry Question 15 as well

Your TurnYour Turn

►Finish reading this chapter yourself Finish reading this chapter yourself about corrosion.about corrosion.