IMADUDDIN SCHOOL REDOX REACTION CHEMISTRY

Chemistry notes & work sheet/Grade 9 /First Term-2015/Chemistry Department Page 1

IMADUDDIN SCHOOL REDOX REACTION

CHEMISTRY/ GRADE-9 NOTES Redox Reactions

• A redox reaction is a reaction where both oxidation and reduction processes occur simultaneously.

• Oxidation reaction involves A gain of oxygen. A loss of hydrogen. A loss of electrons. An increase in oxidation number.

• Reduction reaction involves A loss of oxygen. A gain of hydrogen. A gain of electrons. A decrease in oxidation number.

• Oxidising agent A substance which brings about oxidation but itself gets reduced. An oxidising agent is the species that gives the oxygen or removes the electrons.

Examples: 1. Acidified potassium manganate(VII) 2. Acidified potassium chromate(VI) 3. Halogens. 4. Oxygen 5. Concentrated sulphuric acid. 6. Hydrogen peroxide

Common oxidixing agents Oxidixing agent Half equation Colour change when

added to reducing agents Acidified potassium permanganate, KMnO4

MnO4-(aq) + 8H+(aq) + 5e Mn2+ (aq) + 4H2O(l) Purple colourless

Purple to colourless.

Acidified potassium dichromate, K2Cr2O7

Cr2O7-2(aq) + 14H+(aq) + 6e 2Cr3+(aq) + 7H2O(l) Orange green

Orange to green

Chlorine Cl2 Cl2(g) + 2e 2Cl-(aq) Greenish yellow gas to colourless.


• Reducing agent A substance which brings about reduction but itself gets oxidized. A reducing agent is the species that removes the oxygen or acts as the electron donor.

Examples: 1. Sulfur dioxide 2. Carbon 3. Carbon monoxide 4. Potassium chloride, Potassium iodide etc’ 5. Hydrogen peroxide 6. Hydrogen sulphide.

Common reducing agents

Reducing agent Half equation Colour change when added to reducing agents

Iron(II)sulphate, solution FeSO4

Fe2+(aq) + e Fe3+ (aq) Pale green yellow brown

Pale green to yellow brown.

Hydrogen sulphide,H2S(aq)

S2-(aq) + 2e S(s) colourless yellow

milky at first, later yellow precipitate

Potassium iodide solution, KI(aq)

2I-(aq) + 2e I2(aq) Colourless brown

Colourles to brown.

OIL/ RIG Oxidation is losing electron and reduction is gaining electron

Redox reaction analysis based on the oxygen definitions

• (1) copper(II) oxide + hydrogen copper + water

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

copper oxide reduced to copper, hydrogen is oxidised to water

hydrogen is the reducing agent (removes O from CuO)

copper oxide is the oxidising agent (donates O to hydrogen)

• (2) iron(III) oxide + carbon monoxide iron + carbon dioxide

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

the iron(III) oxide is reduced to iron, the carbon monoxide is oxidised to carbon dioxide

CO is the reducing agent (O remover from Fe2O3)

the Fe2O3 is the oxidising agent (O donator to CO)]


• (3) nitrogen monoxide + carbon monoxide nitrogen + carbon dioxide

2NO(g) + 2CO(g) N2(g) + 2CO2(g)

nitrogen monoxide is reduced to nitrogen

carbon monoxide is oxidised to carbon dioxide

CO is the reducing agent and NO is the oxidising agent

• (4) iron(III) oxide + aluminium aluminium oxide + iron (the Thermit reaction)

Fe2O3(s) + 2Al(s) Al2O3(s) + 2Fe(s)

iron(III) oxide is reduced and is the oxidising agent

aluminium is oxidised and is the reducing agent.

Redox reaction analysis based on the electron definitions

• (1) magnesium + iron(II) sulphate magnesium sulphate + iron

Mg(s) + FeSO4(aq) MgSO4(aq) + Fe(s)

this is the 'ordinary molecular' equation for a typical metal displacement reaction, but this

does not really show what happens in terms of atoms, ions and electrons, so we use ionic

equations like the one shown below.

The sulphate ion SO42-

(aq) is called a spectator ion, because it doesn't change in the reaction

and can be omitted from the ionic equation. No electrons show up in the full equations because

electrons lost by magnesium = electrons gained by iron.

magnesium + iron(II) ion magnesium ion + iron

Mg(s) + Fe2+(aq) Mg2+

(aq) + Fe(s)

the magnesium atom loses 2 electrons (oxidation) to form the magnesium ion, the iron(II) ion

gains 2 electrons (reduced) to form iron atoms.

Mg is the reducing agent (electron donor) and the Fe2+ is the oxidising agent (electron remover

or acceptor)

Displacement reactions involving metals and metal ions are electron transfer reactions.


• (2) zinc + hydrochloric acid zinc chloride + hydrogen

Zn(s) + 2HCl(aq) ZnCl2(aq) + H2(g)

the chloride ion Cl- is the spectator ion

Zn(s) + 2H+ (aq) Zn2+

(aq) + H2(g)

Zinc atoms are oxidised to zinc ions by electron loss, so zinc is the reducing agent (electron

donor)

hydrogen ions are the oxidising agent (gaining the electrons) and are reduced to form hydrogen

molecules

• (3) copper + silver nitrate silver + copper(II) nitrate

Cu(s) + 2AgNO3(aq) 2Ag + Cu(NO3)2(aq)

the nitrate ion NO3- is the spectator ion

Cu(s) + 2Ag+(aq) 2Ag(s) + Cu2+

(aq)

copper atoms are oxidised by the silver ion by electron loss

electrons are transferred from the copper atoms to the silver ions, which are reduced

the silver ions are the oxidising agent and the copper atoms are the reducing agent

• (4) iron(II) chloride + chlorine iron(III) chloride

• (5) halogen (more reactive) + halide salt (of less reactive halogen) halide salt (of more reactive

halogen) + halogen (less reactive)

X2(aq) + 2KY(aq) 2KX(aq) + Y2(aq)

X2(aq) + 2Y-(aq) 2X-

(aq) + Y2(aq)

where halogen X is more reactive than halogen Y, F > Cl > Br > I

X is the oxidising agent (electron acceptor)

KY is the reducing agent (electron donor)

• (6) Electrode reactions in electrolysis are electron transfer redox changes

at the negative cathode positive ions are attracted:

metal ions are reduced to the metal by electron gain:

Mg2+ + 2e- Mg


or 2H+(aq) + 2e- H2(g) (for the discharge of hydrogen)

at the positive anode negative ions are attracted:

negative non-metal ions are oxidised by electron loss e.g.

for oxide ions: 2O2- - 4e- O2 or 2O2- O2 + 4e-

for hydroxide ion: 4OH- - 4e- O2 + 2H2O or 4OH- O2 + 2H2O + 4e-

for halide ions (X = F, Cl, Br, I): 2X- - 2e- X2 or 2X- X2 + 2e-

• Redox changes can often be observed as significant colour changes e.g.

iron + copper(II) sulphate iron(II) sulphate + copper

Fe(s) + CuSO4(aq) FeSO4(aq) + Cu(s)

iron + copper(II) ion iron(II) ion + copper

Fe(s) + Cu2+(aq) Fe2+

(aq) + Cu(s)

Sulphate, SO42-(aq), is colourless BUT a blue to pale green colour change is observed

in the solution as the blue copper(II) ion is replaced by the pale green iron(II) ion as

well as the pink-dark precipitate of copper metal.

Potassium manganate(VII) is a powerful oxidising agent and an intense purple colour in

water due to the MnO4- ion. In acidified solution it changes to an almost colourless*

manganese(II) ion, Mn2+ when it oxidises something (* which actually is a very pale pink

transition metal ion).

Potassium dichromate(VI) is another strong oxidising agent and is orange due to the

dichromate(VI) ion, Cr2O72- ion. When it oxidises something it changes to the green

chromium(III) ion, Cr3+.

Potassium iodide is a colourless salt dissolving in water to form a colourless solution. If it is

oxidised e.g. with chlorine a yellow orange brown colour develops as iodine is

formed from the colourless iodide ion.

• The use of Roman Numerals in names:

This indicates what is called the oxidation state of an atom in a molecule or ion.

It is easy to follow for simple metal ions because it equals the charge on the ion


1 In which reaction does the oxidation state of iron remain unchanged?

2 Which equation represents the neutralisation of dilute sulphuric acid by aqueous sodium hydroxide?

3 In which oxide does X have the same oxidation state as in the chloride, XCl3?

4 A colourless gas is passed into each of three different solutions. The results for each solution are shown in the table.

What is the colourless gas?

A an acid B an alkali C an oxidising agent D a reducing agent

5 The manufacture of sulphuric acid by the Contact process can be represented as follows.


6 In which pair is the underlined element in the same oxidation state in both compounds?

7 Which equation does not represent a redox reaction?

8 Acidified potassium dichromate(VI) can be used to detect the presence of ethanol vapour in the breath of a person who has consumed alcohol.

A colour change from orange to green is observed if ethanol is present. This shows that ethanol is A an acid B an alkali C an oxidising agent D a reducing agent

9 When acidified potassium manganate(VII) is reduced, which colour change occurs?

A from colourless to purple B from green to orange C from orange to green D from purple to colourless

10 Which series of changes includes both oxidation and reduction?

A C → CO → CO2 B PbO2 → PbO → Pb C N2 → NH3 → NO D C2H2 → C2H4 → C2H6

11 Separate samples of hydrogen peroxide are added to aqueous potassium iodide and to acidified potassium dichromate(VI). The iodide ions are oxidised and dichromate(VI) ions are reduced. What colour changes are seen?


12 Which of the reactions X, Y and Z involve oxidation?

A X only B X and Y C Y only D Y and Z

13 Which compound, when added to aqueous iron(II) sulphate, takes part in a redox reaction?

A ammonia B barium chloride C acidified potassium dichromate(VI) D sodium hydroxide

14 Which reaction does not involve either oxidation or reduction?

A CH4(g) + 2O2(g) → CO2(g) + 2H2O(g) B Cu2+(aq) + Zn(s) → Cu(s) + Zn2+(aq)

C CuO(s) + H2SO4(aq) → CuSO4(aq) + H2O(l) D Zn(s) + H2SO4(aq) → ZnSO4(aq) +

H2(g)

15 Iron is manufactured in the blast furnace from haematite.

(a) In the furnace, a redox reaction takes place between iron and carbon monoxide.

Fe2O3 + CO Fe + CO2 (i) Balance the equation by inserting numbers into the boxes. (ii) Explain how carbon monoxide is acting as a reducing agent.

(iii) State the change in oxidation state of iron during the reaction. from……………............................… to ...............................................................................

(iv) Explain why this is an example of reduction, in terms of electron transfer.

................................................................................................................................................. 16 Two naturally occurring ores of copper are cuprite, Cu2O, and tenorite, CuO.

(a) Give the oxidation state of copper in each ore.

oxidation state of copper in Cu2O …………………………………………………………

oxidation state of copper in CuO....................................................................................... (b) Copper can be extracted from tenorite by heating the ore with powdered carbon. (i) Write an equation for this reaction.

......................................................................................................................................................... (ii) Explain, in terms of electrons, why the copper in tenorite has been reduced.

.........................................................................................................................................................


(c) A sample of one of the ores was analysed and found to contain 4.48 g copper and 1.12 g oxygen. Calculate the empirical formula for the copper oxide in the ore, and deduce the name of the ore.

17 The reaction below is an example of a redox reaction.

F2(g) + H2(g) → 2HF(g)

(a) (i) Identify the oxidising agent in the reaction.

......................................................................................................................................................... (ii) Explain why this is a redox reaction

......................................................................................................................................................... .........................................................................................................................................................

(b) Some redox reactions can be used to propel rockets.

The following equations represent redox reactions used to propel rockets. Reaction A N2H4(g) + 2H2O2(g) → N2(g) + 4H2O(g) Reaction B 2H2(g) + O2(g) → 2H2O(g)

(i) Use these equations to complete the following table.

(ii) Reactions used to propel rockets need to produce large volumes of gas. Use the information in the table to suggest why reaction A is more likely to be used to propel rockets.

. 18 Carbon monoxide detectors can be used in the

home.

The orange spot turns black if there is a high concentration of carbon monoxide in the air. (a) Why is carbon monoxide hazardous?


....................................................................................................................................

(b) The spot turns black when palladium(II) chloride reacts with carbon monoxide to form palladium metal

…………………………………………………………………………………………………………..

(i) Complete the equation by writing the formula of the missing reactant in the box. (ii) Complete the table to show the oxidation states of palladium and carbon before and after the

reaction takes place.

(iii) Use information from the table to explain why this is a redox reaction.

19 In a catalytic converter, carbon monoxide and nitrogen dioxide undergo redox reactions.

These reactions reduce the amount of carbon monoxide and nitrogen dioxide in car exhausts.

(i) What is meant by the term redox reaction?

..................................................................................................................................................

.................................................................................................................................................. [1]

(ii) Explain how the redox reactions in the catalytic converter decrease the amounts of

carbon monoxide and nitrogen dioxide in car exhausts.

..............................................................................................................................................................

............................................................................................................................................................

.......................................................................................................................................................... [2]

20 The oxidation states of vanadium in its compounds are V(+5), V(+4), V(+3) and V(+2).

The vanadium(III) ion can behave as a reductant or an oxidant.

(i) Indicate on the following equation which reactant is the oxidant.

2V3+ + Zn → 2V2+ + Zn2+

(ii) Which change in the following equation is oxidation?

Explain your choice.

V3+ + Fe3+ → V4+ + Fe2+

.........................................................................................................................................................

..................................................................................................................................................... [2]

Documents

IMADUDDIN SCHOOL REDOX REACTION CHEMISTRY