Analysing Oxidants and Analysing Oxidants and ReductantsReductants

Chapter 5Chapter 5

Redox ReactionsRedox Reactions

Redox reactions involve complementary Redox reactions involve complementary processes of oxidation and reduction, and processes of oxidation and reduction, and can be identified on the basis of one or more can be identified on the basis of one or more of four definitions of oxidation and reduction.of four definitions of oxidation and reduction.

The most commonly used definition refers to The most commonly used definition refers to electron transfer. (OIL RIG)electron transfer. (OIL RIG)

Other definitions relate to oxygen transfer, Other definitions relate to oxygen transfer, hydrogen transfer and changes in oxidation hydrogen transfer and changes in oxidation numbers.numbers.

Oxygen TransferOxygen Transfer

Reduction is the loss of oxygenReduction is the loss of oxygen Oxidation is the gain of oxygenOxidation is the gain of oxygen

CuO(s) + H2(g) → Cu(s) + H2O(l)

REDUCTION

OXIDATION

Oxidant – CuOReductant - H2

Hydrogen TransferHydrogen Transfer

Reduction is the gain of hydrogenReduction is the gain of hydrogen Oxidation is the loss of hydrogenOxidation is the loss of hydrogen

Cl2(g) + 2HI(aq) → 2HCl(aq) + I2(s)

REDUCTION

OXIDATION

Oxidant – Cl2Reductant - HI

Electron TransferElectron Transfer

Oxidation is loss of electronsOxidation is loss of electrons Reduction is gain of electronsReduction is gain of electrons

Cu2+(aq) + Zn(s) → Cu(s) + Zn2+(aq)

REDUCTION

OXIDATION

Oxidant – Cu2+

Reductant - Zn

Oxidation NumbersOxidation Numbers

A key stage in the production of sulphuric acid is A key stage in the production of sulphuric acid is the conversion of SOthe conversion of SO22 to SO to SO33 according to the according to the

equationequation2SO2SO22(g) + O(g) + O22(g) (g) → 2SO→ 2SO33(g)(g)

This is a redox reaction but is not readily identified This is a redox reaction but is not readily identified as such by the previous definitions of oxidation as such by the previous definitions of oxidation and reduction. To overcome this difficulty numbers and reduction. To overcome this difficulty numbers called oxidation numbers can be used.called oxidation numbers can be used.

Oxidation Numbers or Oxidation Oxidation Numbers or Oxidation States – Rule 1States – Rule 1

Are assigned according to relatively simple Are assigned according to relatively simple rulesrules

The oxidation number of the atoms of free The oxidation number of the atoms of free (uncombined) elements is zero(uncombined) elements is zero– Eg. Fe in elemental iron and N in elemental Eg. Fe in elemental iron and N in elemental

nitrogen (Nnitrogen (N22) both have an oxidation number of ) both have an oxidation number of

zerozero

Rule 2Rule 2

The oxidation number of a monatomic ion is The oxidation number of a monatomic ion is the same as the charge on the ion.the same as the charge on the ion.– In MgFIn MgF22, which contains Mg, which contains Mg2+2+ and F and F-- ions the ions the

oxidation number of magnesium is +2 and the oxidation number of magnesium is +2 and the oxidation number of fluorine is -1oxidation number of fluorine is -1

Rule 3Rule 3

The oxidation number of oxygen in most of The oxidation number of oxygen in most of its compounds is -2its compounds is -2– Exceptions to this rule include hydrogen Exceptions to this rule include hydrogen

peroxide, Hperoxide, H22OO22 (where oxygen has an oxidation (where oxygen has an oxidation

number of -1) as well as the compound Fnumber of -1) as well as the compound F22O O

where oxygen is assigned an oxidation number where oxygen is assigned an oxidation number of +2 (and fluorine -1) because of fluorine’s of +2 (and fluorine -1) because of fluorine’s higher electronegativity.higher electronegativity.

Rule 4Rule 4

The oxidation number of hydrogen in the The oxidation number of hydrogen in the vast majority of its compounds is +1vast majority of its compounds is +1– Exceptions to this rule are metal hydrides such Exceptions to this rule are metal hydrides such

as KH and MgHas KH and MgH22 in which hydrogen has an in which hydrogen has an

oxidation number of -1oxidation number of -1

Rule 5Rule 5

The most electronegative element in a The most electronegative element in a compound has the negative oxidation compound has the negative oxidation number.number.– CFCF44 – fluorine is more electronegative than – fluorine is more electronegative than

carbon so it is assigned a negative oxidation carbon so it is assigned a negative oxidation number (-1)number (-1)

Rule 6 and 7Rule 6 and 7

The sum of the oxidation numbers in a The sum of the oxidation numbers in a neutral compound is zeroneutral compound is zero– NaNa22O, (Na = +1) (O = -2)O, (Na = +1) (O = -2)

– The sum is 2 x (+1) + -2 = 0The sum is 2 x (+1) + -2 = 0

The sum of the oxidation numbers in a The sum of the oxidation numbers in a polyatomic ion is equal to the charge on the polyatomic ion is equal to the charge on the ionion– COCO33

2-2- the sum is equal to -2 the sum is equal to -2

Worked ExamplesWorked Examples

Page 52Page 52 There are 3.There are 3. There are a few steps to follow when There are a few steps to follow when

assigning oxidation numbersassigning oxidation numbers– 1. Write down the formula1. Write down the formula– 2. Assign any known oxidation numbers, use x 2. Assign any known oxidation numbers, use x

for the unknown oxidation numberfor the unknown oxidation number– 3. Work out the total then use some 3. Work out the total then use some

mathematicsmathematics

Your TurnYour Turn

Page 54Page 54 Question 4 and 5Question 4 and 5

Oxidation NumbersOxidation Numbers

Oxidation numbers can change during a Oxidation numbers can change during a reaction. reaction.

This allows us to determine if a redox This allows us to determine if a redox reaction is occurring.reaction is occurring.

An increased oxidation number means the An increased oxidation number means the element has been oxidised (oxidation is element has been oxidised (oxidation is occurring)occurring)

A decrease in oxidation number means A decrease in oxidation number means reductionreduction

Your Turn Your Turn

Page 54Page 54 Question 6Question 6

Writing Half EquationsWriting Half Equations

All redox reactions involve some form of All redox reactions involve some form of electron transfer. electron transfer.

This is fairly evident in the equations for This is fairly evident in the equations for redox reactions such as the reaction of zinc redox reactions such as the reaction of zinc with dilute acids to produce hydrogen gas.with dilute acids to produce hydrogen gas.

Overall: Zn(s) + 2HOverall: Zn(s) + 2H++(aq) (aq) → Zn→ Zn2+2+(aq) + H(aq) + H22(g)(g)

Oxidation: Zn(s) → ZnOxidation: Zn(s) → Zn2+2+(aq) + 2e(aq) + 2e--

Reduction: 2Reduction: 2HH++(aq) + (aq) + 2e2e-- → H→ H22(g)(g)

Writing Half EquationsWriting Half Equations

For more complex half equations HFor more complex half equations H22O, HO, H++

and electrons (eand electrons (e--) may be utilised as part of ) may be utilised as part of the balancing procedure.the balancing procedure.

Remember: in any equationRemember: in any equation– The number of atoms of each element is equal The number of atoms of each element is equal

on both sideson both sides– The total charge is equal on both sidesThe total charge is equal on both sides

Steps for balancing half equationsSteps for balancing half equations

1.1. Balance all elements except hydrogen and Balance all elements except hydrogen and oxygen in the half equationoxygen in the half equation

2NO2NO33- - → N→ N22OO

2.2. Balance the oxygen atoms by adding Balance the oxygen atoms by adding waterwater

2NO2NO33- - → N→ N22O + 5HO + 5H22OO

3.3. Balance the hydrogen atoms by adding HBalance the hydrogen atoms by adding H++ ions ions

2NO2NO33- - + 10H+ 10H+ + → N→ N22O + 5HO + 5H22OO

4.4. Now balance the chargesNow balance the charges

2NO2NO33- - + 10H+ 10H+ + + 8e+ 8e- - → N→ N22O + 5HO + 5H22OO

5.5. Finally put in the statesFinally put in the states

2NO2NO33--(aq)(aq) + 10H + 10H++(aq) (aq) + 8e+ 8e- - → N→ N22O(g) + 5HO(g) + 5H22O(l)O(l)

Steps for balancing half equationsSteps for balancing half equations

Worked Example 5.3Worked Example 5.3

Page 55Page 55 Your Turn Your Turn Page 55Page 55 Question 7 and 10Question 7 and 10

Volumetric AnalysisVolumetric Analysis

Volumetric analysis is not only for acid-base Volumetric analysis is not only for acid-base reactions. We can use it for redox reaction too.reactions. We can use it for redox reaction too.

For redox titrations we react an oxidant with a For redox titrations we react an oxidant with a reductant.reductant.

Like acid-base titrations one solution is usually Like acid-base titrations one solution is usually pipetted into a conical flask and the other is pipetted into a conical flask and the other is dispensed from a burette.dispensed from a burette.

Some redox titrations will need an indicator but for Some redox titrations will need an indicator but for some a colour change will occur due to the some a colour change will occur due to the reacting solutionsreacting solutions

Volumetric AnalysisVolumetric Analysis

Worked Example 5.4 on age 56Worked Example 5.4 on age 56

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Page 59Page 59 Question 25 and 26Question 25 and 26

Volumetric AnalysisVolumetric Analysis