c11 redox reactions

Define oxidation and reductionDefine the oxidation number from formulaeDescribe tests for oxidising and reducing agentsDistinguish between oxidising and reducing agents

Chapter 11

Redox Reactions

LEARNING OUTCOMES

For example, when magnesium is burned in oxygen, it changes into magnesium oxide. We say that the magnesium is oxidised into magnesium oxide.

Oxidation can be defined as the gain of oxygen by a substance.

The magnesium has gained oxygen to become magnesium oxide.

2Mg(s) + O2(g) 2MgO(s)

Oxygen added

Magnesium + Oxygen Magnesium oxide

Oxidation as the gain of oxygen

Chapter 11

Redox Reactions

For example, when copper(II) oxide is heated with hydrogen, it changes to copper. We say that the copper(II) oxide has been reduced to copper.

Reduction can be defined as the loss or removal of oxygen from a substance.

Reduction as the loss of oxygen

Chapter 11

Redox Reactions

Oxygen removed

The copper(II) oxide has changed into copper by its loss of oxygen.

CuO(s) + H2(g) Cu(s) + H2O(l)

Copper(II) oxide + Hydrogen Copper + water

Reduction as the loss of oxygen

Chapter 11

Redox Reactions

Hydrogen removed

Oxidation may also be defined as the loss or removal of hydrogen from a substance.

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

We say that the hydrogen sulphide is oxidised to sulphur, because it has lost hydrogen.

For example, hydrogen sulphide reacts with chlorine to form sulphur and hydrogen chloride:

Oxidation as the loss of hydrogen

Chapter 11

Redox Reactions

Conversely, reduction may be defined as the gain or addition of hydrogen to a substance.

N2(g) + 3H2(g) 2NH3(g)

Hydrogen added

In this reaction, nitrogen is reduced to ammonia, because it has gained hydrogen.

For example, nitrogen reacts with hydrogen to form ammonia in the Haber process:

Reduction as the gain of hydrogen

Chapter 11

Redox Reactions

Oxygen added

In a redox reaction, if one substance is oxidised, the other is being reduced.

E.g. The extraction of iron from iron(III) oxide in the blast furnace:

Fe2O3(s) + 3CO(g) 2Fe(l) + 3CO2(g)

Fe2O3 loses oxygen, and is thus reduced.

CO gains oxygen, and is thus oxidised.

We say that iron(III) oxide is reduced to iron, and carbon monoxide is oxidised to carbon dioxide.

Redox Reactions always occur together

Chapter 11

Redox Reactions

Hydrogen added

For example, in the reaction of hydrogen sulphide with chlorine:

H2S loses hydrogen, and is thus oxidised.

Cl2 gains hydrogen, and is thus reduced.

We say that hydrogen sulphide is oxidised to sulphur, and chlorine is reduced to hydrogen chloride.


Redox reactions always occur together

Chapter 11

Redox Reactions

Summary

Oxidation Reduction

Gain of oxygen Loss of oxygen

Loss of hydrogen Gain of hydrogen

Chapter 11

Redox Reactions

Quick check 1

1. State which substance is oxidised. What substance has it oxidised to? Give a reason for your answer.

(a) C + O2 CO2

(b) Mg + H2O MgO + H2

(c) 2CO + O2 2CO2

(d) H2I + Cl2 2HCl + I2

(e) CuO + H2 Cu + H2O(f) Cl2(g) + H2S(g) 2HCl(g) + S(s)

(g) 2NH3 + 3CuO 3Cu + N2 + 3H2OSolution

Chapter 11

Redox Reactions

Quick check 1 (cont’d)

2. State which substance is reduced. What substance has it been reduced to? Give a reason for your answer.

(a) ZnO + H2 Zn + H2O(b) CO2 + 2Mg 2MgO + C(c) Mg + H2O MgO + H2

(d) Fe2O3 + 3CO 2Fe + 3CO2

(e) H2 + Cl2 2HCl (f) CuO + Mg Cu + MgO(g) FeS + 2HCl FeCl2 + H2S

Solution

Chapter 11

Redox Reactions

We define: Oxidation is the loss of electrons from an

atom or ion. Reduction is the gain of electrons by an atom or ion.

Redox reactions can take place even if no oxygen or hydrogen is involved.

A redox reaction is deemed to occur if there is a transfer of electron(s) during the reaction.

Redox reactions in terms of electron transfer

Chapter 11

Redox Reactions

For example, when sodium and chlorine react to form sodium chloride:

The sodium atom has transferred its outermost electron to chlorine to form sodium chloride.

The sodium atom has lost an electron, hence it is oxidised.

The chlorine atom has gained an electron, hence it is reduced.

Chapter 11

Redox Reactions


2Na + Cl2 2Na+ + 2Cl-

Na loses electrons (oxidation)

Cl2 gains electrons (reduction)

We say that sodium is oxidised (loss of electron) and chlorine is

reduced (gain of electron) to form sodium chloride.

Example 1: Reaction of sodium with chlorine

Chapter 11

Redox Reactions


Example 2: Reaction of magnesium with hydrochloric acid

Mg + 2H+Cl- Mg2+Cl-2 + H2

H+ gains electrons (reduction) We say that magnesium is oxidised to magnesium chloride. (loss of electrons)

We say that hydrochloric acid is reduced to hydrogen. (gain of electron).

Mg loses electrons (oxidation)

Chapter 11

Redox Reactions

Redox Reactions In Terms of Electron Transfer

2Fe2+Cl-2 + Cl2 2Fe3+Cl-

3

Example 3: Reaction of iron(II) chloride with chlorine.Fe2+ loses electron to become Fe3+ (Oxidation)

Cl gains electron to become Cl- (Reduction)

Iron(II) chloride is oxidised to iron(III) chloride (loss of electrons)

Chlorine is reduced to iron(III) chloride (gain of electrons)

Chapter 11

Redox Reactions


To determine if an atom or ion has gained or lost electrons, we can look at its oxidation state (or oxidation number).

All free (uncombined) elements are assigned an oxidation state of zero: E.g. Na0, Mg0, Fe0, Cu0, H2

0, Cl20, O2

0

The oxidation state of an element in a compound is equal to the charge on the ion:

E.g. H+, Na+, K+ (oxidation state +1); Cl-, Br-, I- (oxidation state -1); Mg2+, Ca2+, Zn2+, Fe2+(oxidation state +2); O2-, S2-, (oxidation state -2); Fe3+, Al3+ (oxidation state +3)

Oxidation States

Chapter 11

Redox Reactions

When an atom or ion loses an electron, it is oxidised and its oxidation state increases:E.g. Na0 Na+ + e- (From 0 +1)E.g. Fe2+ Fe3+ + e- (From +2 +3)

When an atom or ion gains an electron, it is reduced and its oxidation state decreases:E.g. Cl0 + e- Cl- (From 0 -1)E.g. Mg2+ + 2e- Mg (From +2 0)

Redox reactions as changes in oxidation state

Chapter 11

Redox Reactions

Example 1: Reaction of magnesium with hydrochloric acid

Step 1: Write down the balanced chemical equation.

Step 2: Write down the oxidation number of each atom or ion in the equation.

Mg + 2H Cl Mg Cl2 + H2

0 + - 2+ - 0


Chapter 11

Redox Reactions

Step 3: Look for an atom or ion which has changed its oxidation number in going from left to right in the equation.

Mg + 2H Cl Mg Cl2 + H2

0 + - 2+ - 0

Step 4: Determine whether it is oxidation (increase in oxidation state) or reduction (decrease in oxidation state).

Oxidation (from 0 to +2)

Reduction (from + 1 to 0)


Chapter 11

Redox Reactions

2K+ I− + Cl20 2K+ Cl− + I20

Potassium iodide isPotassium iodide is oxidisedoxidised to iodine.to iodine.( ( increaseincrease in oxidation state)in oxidation state)

Chlorine isChlorine is reduced reduced to KClto KCl( ( decreasedecrease in oxidation state)in oxidation state)

Example 2: Reaction of potassium iodide with chlorine.

Notice that there is no change in K+ (in KI) to K+ (in KCl); hence the potassium ion has not been oxidised or reduced.


Chapter 11

Redox Reactions

KMnO4

KK++

(+1)(+1) xx4(O4(O2-2-))(-2)(-2)

Atoms in covalent and complex compounds can be given oxidation states, assuming they are ionic.

Oxidation states of all atoms in a compound must add up to zero

Example: Find the oxidation state of Mn in KMnO4.

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

x = +7

Determination of Oxidation States in a Compound

Chapter 11

Redox Reactions

Oxidation Reduction

Gain of oxygen Loss of oxygen

Loss of hydrogen Gain of hydrogen

Loss of electron(s)(Increase in oxidation state)

Gain of electron(s)(Decrease in oxidation state)

Summary

Chapter 11

Redox Reactions

1. State which substance is oxidised. What substance has it been oxidised to? State a reason for your answer.

(a) Zn + 2HCl ZnCl2 + H2

(b) Mg + H2SO4 MgSO4 + H2

(c) Fe + Cl2 FeCl2

(d) Zn + CuSO4 ZnSO4 + Cu(e) Fe + Pb(NO3)2 Fe(NO3)2 + Pb(f) 2KI + Br2 2KBr + I2

Solution

Quick check 2

Chapter 11

Redox Reactions

2. State which substance is reduced. What substance has it been reduced to? State a reason for your answer.

(a) CuO + Mg MgO + Cu(b) 2Fe3+ + 2Cl- 2Fe2+ + Cl2

(c) 2Na + Cl2 2NaCl(d) Zn + CuSO4 ZnSO4 + Cu(e) Mg + H2SO4 MgSO4 + H2

3. State the oxidation state of nitrogen in the following:(i) NO, (ii) N2O, (iii) NO2, (iv) NO3

- Solution

Quick check 2 (cont’d)

Chapter 11

Redox Reactions


In the above reaction, magnesium is oxidised into magnesium oxide by oxygen.

Consider the burning of magnesium in oxygen to form magnesium oxide:

Oxygen is called the oxidising agent.

Oxidising Agents and Reducing Agents

Chapter 11

Redox Reactions

An oxidising agent is a substance which causes oxidation. It acts as an acceptor of electrons.


In the above reaction, oxygen has received or accepted 2 electrons from magnesium to form magnesium oxide.

Hence oxygen is the oxidising agent. Hence oxygen is the oxidising agent.

Definition:

Oxidising Agents

Chapter 11

Redox Reactions

Other examples of oxidising agents are:

chlorine and bromine

potassium manganate(VII)

potassium dichromate(VI)

Oxidising Agents

Chapter 11

Redox Reactions

Consider the reaction between heated copper(II) oxide and hydrogen.

CuO(s) + H2(g) Cu(s) + H2O(g)

Copper(II) oxide is reduced to copper by hydrogen.

Hydrogen is called the reducing agentreducing agent..

Reducing Agents

Chapter 11

Redox Reactions

A reducing agent is a substance which causes reduction. It acts as a donor of electrons.

CuO(s) + H2(g) Cu(s) + H2O(g)

In the above reaction, hydrogen has given away (donated) 2 electrons to the copper(II) ion which then becomes copper.

Hence hydrogen is the reducing agent.Hence hydrogen is the reducing agent.

Definition:

Reducing Agents

Chapter 11

Redox Reactions

Other examples of reducing agents are:

carbon

carbon monoxide

reactive metals like potassium, sodium, magnesium and aluminium

potassium iodide

Reducing Agents

Chapter 11

Redox Reactions

Since redox reactions always occur together, an oxidising agent will be the substance reduced in the reaction.

Similarly, a reducing agent will be the substance oxidised in the reaction.


HH22S is oxidised to S is oxidised to sulphur bysulphur by chlorine.chlorine.

Chlorine is reduced to HCl Chlorine is reduced to HCl byby hydrogen sulphide.hydrogen sulphide.

HH22S is the reducing agent.S is the reducing agent.ClCl22 is the oxidising agent. is the oxidising agent.

Oxidising Agents and Reducing Agents

Chapter 11

Redox Reactions

(a) Which substance is oxidised?

Ans: ________________________________________(b) Which substance is reduced?

Ans: ________________________________________(c) Which is the oxidising agent?

Ans: ________________________________________(d) Which is the reducing agent?

Ans: ________________________________________

Worked Example

Fe2O3(s) + 3CO(g) 2Fe(l) + 3CO2(g)

Carbon monoxide is oxidised (gain of oxygen)

Iron(III) oxide is reduced (loss of oxygen)

Iron(III) oxide is the oxidising agent.

Carbon monoxide is the reducing agent.

Consider the following reaction:

Chapter 11

Redox Reactions

Test for oxidising agent

To test if an unknown substance is an oxidising agent, add a solution of potassium iodide to it.

If the mixture turns reddish brown due to the liberation of iodine from the potassium iodide, then the unknown substance is an oxidising agent.

Potassium iodide solution added

unknown solution

Mixture turns reddish brown

Chapter 11

Redox Reactions

To test if an unknown substance is a reducing agent, add an acidified solution of potassium dichromate(VI)solution of potassium dichromate(VI) to it.

If the mixture turns from yellow/orange to green,green, then the unknown substance is a reducing agentreducing agent.

Test for reducing agent

Chapter 11

Redox Reactions

1. In each of the following reactions, state (i) the substance oxidised, (ii) the substance reduced, (iii) the oxidising agent and (iv) the reducing agent.

(a) ZnO + CO Zn + CO2

(b) Al2O3 + 3Mg 2Al + 3MgO

(c) 2FeCl2 + Cl2 2FeCl3

2. (a) Define oxidation in terms of electron transfer.(b) Give an example of a redox reaction, including a chemical equation with state symbols. State clearly in your example, which substance is oxidised and which substance is reduced.

Solution

Quick check 3

Chapter 11

Redox Reactions

Solution to Quick check 11. (a) C + O2 CO2

Carbon is oxidised into carbon dioxide. (gain of oxygen)(b) Mg + H2O MgO + H2

Magnesium is oxidised into magnesium oxide. (gain of oxygen)(c) 2CO + O2 2CO2

Carbon monoxide is oxidised into carbon dioxide. (gain of oxygen)(d) H2I + Cl2 2HCl + I2

Hydrogen iodide is oxidised into iodine. (loss of hydrogen)(e) CuO + H2 Cu + H2O Hydrogen is oxidised into water. (gain of oxygen)(f) Cl2(g) + H2S(g) 2HCl(g) + S(s) Hydrogen sulphide is oxidised into sulphur. (loss of hydrogen) (g) 2NH3 + 3CuO 3Cu + N2 + 3H2O Ammonia is oxidised into nitrogen. (loss of hydrogen)

Return

Chapter 11

Redox Reactions

2.(a) ZnO + H2 Zn + H2O

Zinc oxide is reduced into zinc. (loss of oxygen)

(b) CO2 + 2Mg 2MgO + C Carbon dioxide is reduced into carbon. (loss of oxygen)(c) Mg + H2O MgO + H2

Water is reduced into hydrogen. (loss of oxygen)(d) Fe2O3 + 3CO 2Fe + 3CO2

Iron(III) oxide is reduced into iron. (loss of oxygen)(e) H2 + Cl2 2HCl Chlorine is reduced into hydrogen chloride. (gain of hydrogen)(f) CuO + Mg Cu + MgO Copper(II) oxide is reduced into copper.(loss of oxygen)(g) FeS + 2HCl FeCl2 + H2S Iron(II) sulphide is reduced to hydrogen sulphide. (gain of hydrogen)

Return

Solution to Quick check 1 (cont’d)

Chapter 11

Redox Reactions

1. (a) Zn + 2HCl ZnCl2 + H2

Zinc is oxidised into zinc chloride. (loss of electrons/increase in oxidation state)(b) Mg + H2SO4 MgSO4 + H2

Magnesium is oxidised to magnesium sulphate.(loss of electrons)(c) Fe + Cl2 FeCl2

Iron is oxidised to iron(II) chloride. (loss of electrons) (d) Zn + CuSO4 ZnSO4 + CuZinc is oxidised to zinc sulphate. (loss of electrons)(e) Fe + Pb(NO3)2 Fe(NO3)2 + PbIron is oxidised to iron(II) nitrate. (loss of electrons)(f) 2KI + Br2 2KBr + I2

Potassium iodide is oxidised to iodine (loss of electrons)

Return

Solution to Quick check 2

Chapter 11

Redox Reactions

2. (a) CuO + Mg MgO + CuCopper(II) oxide is reduced to copper. (loss of oxygen/decrease in oxidation state/gain of electrons)(b) 2Fe3+ + 2Cl- 2Fe2+ + Cl2

Iron(III) is reduced to iron(II). Decrease in oxidation state/gain of electron.(c) 2Na + Cl2 2NaClChlorine is reduced to sodium chloride. (gain of electron)(d) Zn + CuSO4 ZnSO4 + CuCopper(II) sulphate is reduced to copper (gain of electrons) (e) Mg + H2SO4 MgSO4 + H2

Sulphuric acid is reduced to hydrogen (gain of electron)

3. (i) +2, (ii) +1, (iii) +4, (iv) +5 Return

Solution to Quick check 2 (cont’d)

Chapter 11

Redox Reactions

1. (a) ZnO + CO Zn + CO2

(i) carbon monoxide, (ii) zinc oxide,(iii) zinc oxide, (iv) carbon monoxide

(b) Al2O3 + 3Mg 2Al + 3MgO(i) magnesium, (ii) aluminium oxide,(iii) aluminium oxide, (iv) magnesium

(c) 2FeCl2 + Cl2 2FeCl3

(i) iron(II) chloride, (ii) chlorine,(iii) chlorine, (iv) iron(II) chloride

2. (a) Oxidation occurs when there is a loss of electrons from an atom or ion.(b) 2KI(aq) + Cl2 (g) 2KCl(aq) + I2 (s)Potassium iodide is oxidised to iodine.Chlorine is reduced to potassium chloride.

Return

Solution to Quick check 3

Chapter 11

Redox Reactions

1. http://library.kcc.hawaii.edu/external/chemistry/redox_title.html2. http://www.chemistry.co.nz/redox_new.htm3. http://en.wikipedia.org/wiki/Redox

To learn more about Redox reactions, click on the links below!

Chapter 11

Redox Reactions

ReferencesReferences

Chemistry for CSEC Examinations by Mike Taylor and Tania Chung

Longman Chemistry for CSEC by Jim Clark and Ray Oliver

c11 redox reactions

Science

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