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Types of Chemical Reactions. Chemical Reactions. Neutralisation reaction Precipitation reaction Synthesis reaction Displacement reaction Decomposition reaction. Oxidation and Reduction. Rusting. Combustion. Batteries. Restoring Old Paintings.

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chemical reactions
Chemical Reactions
  • Neutralisation reaction
  • Precipitation reaction
  • Synthesis reaction
  • Displacement reaction
  • Decomposition reaction
restoring old paintings
Restoring Old Paintings
  • Done by oxidation process. Most old paintings are lead-based paints.
  • Become discoloured due to reactions with pollutant gases in the air, esp. with hydrogen sulphide gas, H2S.
  • This reacts with the lead ions to form black lead sulphide:

Pb2+ (aq) + S2- (aq) PbS (s)

slide9
The original white colour can be restored by repainting with oxidising agent such as hydrogen peroxide, H2O2.
  • PbS(s) + 4H2O2 (aq) PbSO4(aq) + 4H2O (l)
topics to be learnt
Topics to be learnt:
  • Oxidation and Reduction as Gain or Loss of Oxygen
  • Oxidation and Reduction as Loss or Gain of Hydrogen
  • Oxidation and Reduction as Loss or Gain of Electrons
  • Oxidation State
  • Oxidation and Reduction as Change of Oxidation State
  • Oxidising and reducing agents
oxidation and reduction as gain or loss of oxygen
Oxidation and Reduction as Gain or Loss of Oxygen
  • Oxidation involves the gain of oxygen
  • A substance is oxidised if it gains oxygen
  • An oxidising agent is a substance which causes oxidation and itself is reducedin the process
oxidation and reduction as gain or loss of oxygen1
Oxidation and Reduction as Gain or Loss of Oxygen
  • Reduction involves the loss of oxygen
  • A substance is reduced if it loses oxygen
  • A reducing agent is a substance which causes reduction and itself is oxidised in the process
oxidation and reduction as gain or loss of oxygen2

gain of oxygen

reducing agent

loss of oxygen

oxidising agent

Oxidation and Reduction as Gain or Loss of Oxygen

For example

Zn(s) + CuO(s)  ZnO(s) + Cu(s)

Zn oxidised

CuO reduced

oxidation and reduction as gain or loss of oxygen3
Oxidation and Reduction as Gain or Loss of Oxygen

Combustion

Combustion reactions are oxidation reactions - gaining of oxygen

e.g.1 burning of C

C + O2 CO2

e.g.2 burning of Mg

2Mg + O2 2MgO

oxidation and reduction as gain or loss of oxygen4
Oxidation and Reduction as Gain or Loss of Oxygen

Extraction of metals

metals can extracted from their ores by heating with oxides of C

e.g. 1 Fe2O3 + 3CO  2Fe + 3CO2

e.g.2 ZnO + CO  Zn + CO2

Which are the oxidised and reduced species?

oxidation and reduction as gain or loss of oxygen5

reduction

oxidation

Oxidation and Reduction as Gain or Loss of Oxygen

Oxidation and Reduction usually takes place together in the same reaction,hence such reactions are also known as

REDOX

oxidation and reduction as gain or loss of hydrogen
Oxidation and Reduction as Gain or Loss of Hydrogen
  • Oxidation involves the loss of hydrogen
  • A substance is oxidised if it loses hydrogen
  • Reduction involves the gain of hydrogen
  • A substance is reduced if it gains hydrogen
oxidation and reduction as gain or loss of hydrogen1

loss of hydrogen

gain of hydrogen

reducing agent

oxidising agent

Oxidation and Reduction as Gain or Loss of Hydrogen

H2S oxidised

For example

H2S(g) + Cl2(g) -> 2HCl(g) + S(s)

Cl2 reduced

oxidation and reduction as gain or loss of electrons
Oxidation and Reduction as Gain or Loss of Electrons
  • Oxidation involves the loss of electrons
  • A substance is oxidised if it loses electrons
  • Reduction involves the gain of electrons
  • A substance is reduced if it gains electrons
oxidation and reduction as gain or loss of electrons2

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Oxidation and Reduction as Gain or Loss of Electrons
oxidation and reduction as gain or loss of electrons3

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Oxidation and Reduction as Gain or Loss of Electrons
oxidation and reduction as gain or loss of electrons4

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Na

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Cl

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Na

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Oxidation and Reduction as Gain or Loss of Electrons

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Cl

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oxidation and reduction as gain or loss of electrons5

+

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Na

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Cl

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+

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Na oxidised

Na

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loss of electrons

e

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Cl2 reduced

e

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gain of electrons

Oxidation and Reduction as Gain or Loss of Electrons

e

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Cl

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oxidation and reduction as gain or loss of electrons6

+

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-

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Na

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Cl

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Na oxidised

Na

e

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Reducing agent

e

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Cl2 reduced

e

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Oxidising agent

Oxidation and Reduction as Gain or Loss of Electrons

e

e

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Cl

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oxidation and reduction as gain or loss of electrons8

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

Mg

Mg

Mg

H+

Mg

Mg

Mg

H+

Oxidation and Reduction as Gain or Loss of Electrons
oxidation and reduction as gain or loss of electrons9
Oxidation and Reduction as Gain or Loss of Electrons

H+

H+

H+

H+

H+

H+

H+

H+

H+

H+

Mg

Mg

Mg

H+

H+

H+

Mg

Mg

Mg

H+

oxidation and reduction as gain or loss of electrons17

gain of electrons

H+ reduced

H

H

Mg oxidised

loss of electrons

Oxidation and Reduction as Gain or Loss of Electrons

H+

Mg

Mg2+

Mg

oxidation and reduction as gain or loss of electrons18

Oxidising agent

H+ reduced

H

H

Mg oxidised

Reducing agent

Oxidation and Reduction as Gain or Loss of Electrons

H+

Mg

Mg2+

Mg

historical development
Historical Development
  • Ancient times, 7 metals (Au, Ag, Cu, Fe, Pb, Sn, Zn)
  • Some metals occur free in nature.
  • Other metals occurred as oxides in ores
  • Man used charcoal to reduce the ore – lead the metal in the ore back to its original native state.
  • ‘Re’ = back; ‘ducere’ = to lead
historical development1
Historical Development
  • Thru centuries, the meaning of the term ‘reduce’ changed into ‘to diminish’ or ‘to decrease’.
  • Large portions of iron ores, haematite and magnetite, were diminished to smaller lumps of iron.
  • A reduction took place.
historical development2
Historical Development
  • Man burn fuels to obtain fire.
  • Combustion was followed by visible flames and release of energy.
  • 1779 CW Scheele called the newly discovered air component that sustained combustion, ildluft (fire air).
  • Antoine Lavoisier, let different substances burn in air and discovered that the products gave acidic solutions with water.
historical development3
Historical Development
  • He suggested that the substances had reacted with something in the air that was acidic. – principe oxygene (the acidifying principle, from oxys (Greek) = sharp).
  • Lavoisier coined the term oxidation to any reaction with oxygen and reserved the expression reduction to the removal of oxygen.
historical development4
Historical Development
  • In 1837, J Liebig proposed another definition.
  • He had noticed that hydrogen gas was effective in removing oxygen from compounds.
  • Organic compounds were oxidised by releasing hydrogen (atoms) in reactions as when alcohols were dehydrogenated.
historical development5
Historical Development
  • After the discovery of the electron in 1897, GA Lewis explained the formation of sodium chloride by the complete transfer of electrons from sodium atoms to chlorine atoms.
  • His electronic theory of valence of 1916 lay the ground for defining redox reactions as reactions with transfer of electrons.
historical development6
Historical Development
  • The terms oxidation and reduction are today connected to changes in oxidation numbers, a term introduced by W Latimer.
  • Different redox models have evolved through years and four models are still in use in schools.
  • Incompatibility of various models when applied to the same type of reaction and to the overlaps and anomalies of models.
oxidation state
Oxidation State

Definition:

The charge an atom would have if it existed as an ion.

oxidation states
Oxidation States

As a rule of thumb:

1. Oxidation state of hydrogen is always +1, except - in metal hydrides where it is -1 e.g. in LiH ( lithium hydride)

oxidation states1
Oxidation States

As a rule of thumb:

2. Oxidation state of oxygen is always -2 except- in peroxides, where it is -1, e.g. in hydrogen peroxide, H2O2

- when H or F are present, the oxidation state of H or F must remain the same , while oxidation state of oxygen is adjusted e.g. in OF2 , oxidation state of oxygen is +2

oxidation states2
Oxidation States

Basic rules to writing oxidation state:

1. A free, uncombined element has 0 oxidation state

E.g. Carbon Magnesium Iron

o.s. 0

oxidation states3
Oxidation States

Basic rules to writing oxidation state:

2. The sum of the oxidation numbers of the atoms in a molecule or ions in an ionic compound is equal to 0

E.g. Hydrogen gas Hydrogen sulphate Water (H2O) Sodium chloride

o.s. 0

oxidation states4

o.s. +2

o.s. -1

o.s. +1

Oxidation States

Basic rules to writing oxidation state:

3. Charge on a simple ion is its oxidation state

E.g. Zinc ion, Zn2+

Chloride ion, Cl-

Copper (I) ion , Cu+

oxidation states5
Oxidation States

Basic rules to writing oxidation state:

4. The sum of the oxidation state of atoms in a polyatomic (complex) ion is equal to the charge on the ion

E.g. 1 hydroxide ion, OH -

o.s. of O = -2

o.s. of H = +1

Therefore, o.s. of OH- = -2 + 1 = -1

oxidation states6
Oxidation States

Basic rules to writing oxidation state:

4. The sum of the oxidation state of atoms in a polyatomic (complex) ion is equal to the charge on the ion

E.g. 2 nitrate ion, NO3-

o.s. of N = +5 o.s. of O = 3 x (-2) = -6

Therefore, o.s. of NO3- = +5 + (-6) = -1

oxidation states7
Oxidation States

Basic rules to writing oxidation state:

5. Covalent and/or complicated compounds are allocated oxidation states by assuming that they are ionic.

oxidation states8
Oxidation States

Basic rules to writing oxidation state:

E.g. 1 NO2

assuming it is ionic - it is made up of “nitrogen ion” and “oxide ion”,

- sum of “nitrogen ion” and “oxide ion is 0 (Rule No. 2)

- let oxidation state of “nitrogen” be x

oxidation states9
Oxidation States

Basic rules to writing oxidation state:

E.g. 1

NO2

x

oxidation states10
Oxidation States

Basic rules to writing oxidation state:

E.g. 1

NO2

x + 2

oxidation states11
Oxidation States

Basic rules to writing oxidation state:

E.g. 1

NO2

x + 2(-2)

oxidation states12

Rule No. 2

Oxidation States

Basic rules to writing oxidation state:

E.g. 1

NO2

x + 2(-2)=

0

0

oxidation states13

Thus, o.s. of nitrogen is +4

Oxidation States

Basic rules to writing oxidation state:

E.g. 1

NO2

x + 2(-2)=

x = +4

0

oxidation states14

K+MnO4-

and covalent bonds

Oxidation States

Basic rules to writing oxidation state:

E.g. 2 KMnO4

K+MnO4-

- a complex compound that has a mixture of ionic bonds

oxidation states15
Oxidation States

Basic rules to writing oxidation state:

E.g. 2 KMnO4

assuming it is ionic

- made up of “potassium ion”, “manganese ion” and “oxide ion”

- sum of “potassium ion”, “manganese ion” and “oxide ion” is 0(Rule No. 2)

- let oxidation state of “manganese” be x

oxidation states16
Oxidation States

Basic rules to writing oxidation state:

E.g. 2 KMnO4

X +

KMnO4

oxidation states17
Oxidation States

Basic rules to writing oxidation state:

E.g. 2 KMnO4

X + (+1)

KMnO4

oxidation states18
Oxidation States

Basic rules to writing oxidation state:

E.g. 2 KMnO4

X +(+1) + 4

KMnO4

oxidation states19
Oxidation States

Basic rules to writing oxidation state:

E.g. 2 KMnO4

X +(+1) + 4(-2)

KMnO4

oxidation states20
Oxidation States

Basic rules to writing oxidation state:

E.g. 2 KMnO4

X +(+1) + 4(-2) = 0

0

KMnO4

oxidation states21
Oxidation States

Basic rules to writing oxidation state:

E.g. 2 KMnO4

X +(+1) + 4(-2) = 0

X + (+1) - 8 = 0

KMnO4

oxidation states22

Thus, o.s. of Mn is +7

Oxidation States

Basic rules to writing oxidation state:

E.g. 2 KMnO4

X +(+1) + 4(-2) = 0

X + (+1) - 8 = 0

X = +7

KMnO4

oxidation states23

E.g. 3 H2SO4

assuming it is ionic

- it is made up “hydrogen ion”, “sulphur ion” and “oxide ion”

- sum of “hydrogen ion”, “sulphur ion” and “oxide ion” is 0 (Rule No. 2)

- let oxidation state of “sulphur” be x

Oxidation States

Basic rules to writing oxidation state:

oxidation states24

Thus, o.s. of S is +6

Oxidation States

Basic rules to writing oxidation state:

E.g. 3 H2SO4

X +2(+1) + 4(-2) = 0

X + (+2) - 8 = 0

X = +6

H2SO4

oxidation states25
Oxidation States

Recall:

Name of compounds

- some compounds include a Roman numeral (I, II, III)

=> oxidation state of that element

- some elements, e.g. the transition elements have more than one o.s.

oxidation states26
Oxidation States

E.g. 1 Copper

Copper (I) oxide => Cu+, o.s. +1

Copper (II) oxide => Cu2+ , o.s. +2

E.g. 2 Iron

iron (II) sulphate => Fe2+, o.s. +2

iron (III) suphate => Fe3+, o.s. +3

oxidation and reduction as change of oxidation states
Oxidation and reduction as change of oxidation states
  • Oxidationoccurs when there is an increase in the oxidation state
  • A substance is oxidised if its oxidation state increases
  • Reductionoccurs when there is a decrease in the oxidation state
  • A substance is reduced if its oxidation state decreases
oxidation and reduction as change of oxidation states1
Oxidation and reduction as change of oxidation states

E.g. 1

Cl2 + 2KI -> 2KCl + I2

What is the oxidation state of Cl and I found in the reactants and products in the above reaction?

oxidation and reduction as change of oxidation states2

Decrease in o.s.

o.s. of Cl

0

-1

0

o.s. of I

-1

Increase in o.s.

Oxidation and reduction as change of oxidation states

Cl2 reduced

E.g. 1

Cl2 + 2KI -> 2KCl + I2

reducing agent - KI

KI oxidised

oxidising agent - Cl2

oxidation and reduction as change of oxidation states3
Oxidation and reduction as change of oxidation states

E.g. 2

Mg + CuO -> MgO + Cu

What is the oxidation state of Mg and Cu found in the reactants and products in the above reaction?

oxidation and reduction as change of oxidation states4

Increase in o.s.

o.s. of Mg

0

+2

0

o.s. of Cu

+2

Decrease in o.s.

Oxidation and reduction as change of oxidation states

Mg oxidised

E.g. 2

Mg + CuO -> MgO + Cu

reducing agent - Mg

CuO reduced

oxidising agent - CuO

advantages disadvantages of the oxidation number concept
Advantages & disadvantages of the oxidation number concept
  • It becomes clear whether or not redox
  • is involved in a particular reaction.
  • Reactions such as neutralization &
  • precipitation are shown to be non-redox
  • reactions even though they involve
  • ions.
advantages disadvantages of the oxidation number concept1
Advantages & disadvantages of the oxidation number concept
  • Using oxidation number concept, we
  • can see exactly which part of a molecule
  • or complex ion is reduced or oxidised.
advantages disadvantages of the oxidation number concept2
Advantages & disadvantages of the oxidation number concept
  • Main disadvantage:
  • Can cause a misunderstanding concerning
  • the structure of molecular substances.
  • it is important to realize that no physical
  • or structural significance can be attached
  • to oxidation numbers.
  • The assignment of +4 as the ox. state of
  • the carbon atom in CO2 is arbitrary; there
  • is no such charge on the carbon atom.
advantages disadvantages of the oxidation number concept3
Advantages & disadvantages of the oxidation number concept
  • 2 other problems concern the use with
  • organic compounds.
  • What is the ox. no of carbon in
  • (i) CH4 (ii) C2H6 (iii) C3H8
advantages disadvantages of the oxidation number concept4
Advantages & disadvantages of the oxidation number concept
  • (i) CH4 (ii) C2H6 (iii) C3H8
  • The bonding in carbon & its oxidation state
  • in each of these cpds is essentially the
  • same, yet the C atoms have different
  • oxidation numbers.
  • Also, in some cpds, atoms have oxidation
  • numbers which are not whole numbers.
oxidising and reducing agents
Oxidising and reducing agents
  • List of some common oxidising agent:
          • oxygen
          • hydrogen
          • chlorine
          • bromine
          • metal ions
          • potassium dichromate
          • potassium manganate
          • concentrated sulphuric acid
oxidising and reducing agents1
Oxidising and reducing agents
  • List of some common reducing agent:
          • sulphur dioxide
          • metals
          • hydrogen
          • carbon monoxide
          • iron (II) ions
          • hydrogen peroxide
          • potassium bromide
          • potassium iodide
oxidising and reducing agents2
Oxidising and reducing agents
  • Certain substances may be an oxidising agent in one reaction and a reducing agent in another reaction
  • E.g. SO2
  • 2SO2 (g) + O2 (g)  2SO3 (g)
  • SO2 is the reducing agent
  • SO2 (g) + 2H2S (g)  3S (s) + 2H2O (l) SO2 is the oxidising agent

Conclusion: the reaction involved must be considered

oxidising and reducing agents3

Oxidising agent

Reducing agent

Oxidising and reducing agents

Known oxidising agents or reducing agents are used to test whether an unknown substance is oxidising or reducing

Commonly used reagentsPotassium manganate (VII),KMnO4Potassium dichromate (VI),K2Cr207Potassium iodide, KI

oxidising and reducing agents4
Oxidising and reducing agents

Potassium manganate (VII), KMnO4, potassium dichromate (VI), K2Cr207 being oxidising agents are used to test for reducing agents

Potassium iodide, KI, being a reducing agent is used to test for oxidising agents

oxidising and reducing agents5
Oxidising and reducing agents

Testing for reducing agent

Potassium manganate (VII) KMnO4

(purple solution)

Mn 2+

(colourless)

oxidising and reducing agents6

decrease of o.s.

reduced

Oxidising and reducing agents

What is the change of o.s. of Mn from KMnO4 to Mn2+?

o.s. of Mn in MnO4-= +7

o.s. of Mn in Mn2+ = +2

Therefore, a substance that causes MnO4- to change to Mn2+ is a reducing agent

oxidising and reducing agents7
Oxidising and reducing agents

Testing for reducing agent

Potassium dichromate (VI) K2Cr207

(orange solution)

Cr 3+

(green)

oxidising and reducing agents8

decrease of o.s.

reduced

Oxidising and reducing agents

What is the change of o.s. of Cr from K2Cr207to Cr 3+?

o.s. of Cr in K2Cr207= +6

o.s. of Cr in Cr3+ = +3

Therefore, a substance that causes Cr2O72- to change to Cr3+ is a reducing agent

oxidising and reducing agents9
Oxidising and reducing agents

Testing for oxidising agent

Potassium iodide KI (aq)

(colourless solution)

I2

(brown solution)

oxidising and reducing agents10

increase of o.s.

oxidised

Oxidising and reducing agents

What is the change of o.s. of I from KI to I2 ?

o.s. of I in KI = -1

o.s. of I in I2 = 0

Therefore, a substance that causes I- to change to I2 is an oxidising agent