Oxidation and Reduction

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Question 1In the reaction

MnO2(s) + 4HCl(aq) Cl2(g) + MnCl2(aq) + 2H2O(l)

HCl is the oxidising agent Cl2 is the oxidation product

H2O is the reduction product

MnO2 is the reducing agent

Wrong Answer!The formation of Cl2 from HCl is an oxidation process. The oxidation number changes from 0 to -1.

Question 2

When MnO4-(aq) reacts in an acidic solution it

produces

MnO2 MnO42-

Mn2+Mn3+

Wrong Answer!The products of the reaction of manganate(VII) ion depend on pH. However, under acidic conditions, manganate(II) ions are produced.

MnO4- + 8H+ + 5e- Mn2+ + 4H2O

Question 3Bromide ions are oxidized to bromine by all of the

following except

K2Cr2O7 / H+ Cl2

KMnO4I2

Wrong Answer!I2 is below Br2 in group 7 and hence not sufficiently powerful to oxidise bromide ions to bromine molecules.

Question 4

All the following equations represent oxidation-reduction reactions except

2C3H7OH + 9O2 6CO2 +8H2O Ni + 4CO Ni(CO)4

Cl2 + 2NaBr 2NaCl + Br22Na + 2H2O 2NaOH + H2

Wrong Answer!Ni complex is neutral and consists of a Ni atom and 4 CO molecules.

SO3(g) + 2H2O(l)

HSO4-(aq) + H3O+(aq)

Question 5

Which of the following represents an oxidation-reduction reaction?

I2(s) + 2OH-(aq)

I-(aq) + OI-(aq) + 8H2O(l)

PO43- (aq) + H2O(l)

HPO42-(aq) + OH-(aq)

Cu2+ (aq) + H2S(aq)

CuS (s) + 2H+(aq)

Wrong Answer!The iodine undergoes a change in oxidation number from 0 to -1.

Question 6

Magnesium is a more reactive metal than copper. Which is the strongest oxidizing agent?

Mg Mg2+

Cu2+Cu

Wrong Answer!Metals that are weak reducing agents (electron donors) form ions that are strong oxidizing agents (electron acceptor).

Question 7The following information is given about reactions involving

the metals X, Y and Z and solutions of their sulfates.

X(s) + YSO4(aq) No reaction

Z(s) + YSO4(aq) Y(s) + ZSO4(aq)

When the metals are listed in decreasing order of reactivity (most reactive first), what is the correct order?

X > Y > Z Z > Y > X

Y > Z > XY > X > Z

Wrong Answer!X is less reactive than Y. Z is more reactive than Y.

Question 8

In which one of the following species does chlorine exhibit the highest oxidation number?

Cl2O Cl2

PCl5HClO3

Wrong Answer!The oxidation number is +5

Question 9

Which statement is correct for the electrolysis of molten sodium chloride?

Sodium ions move towards the positive electrode.

A gas is produced at the negative electrode.

Both sodium and chloride ions move toward the electrodes.

Only electrons move in the electrolyte.

Wrong Answer!Ions move to electrodes opposite in sign to their charges.

Question 10When a direct current of electricity is conducted by an

aqueous solution of an electrolyte, which one of the followig statements is false?

The movements of ions accounts for the current flow through the solution.

During electrolysis, the solution remains electrically neutral.

The number of positive ions moving toward one electrode is always equal to the number of negative ions moving toward the other electrode.

Electrons flow from the current source to the solution at one electrode, and an equal number of electrons flows away from the solution at the other electrode.

Wrong Answer!Eectrons consumed at the cathode by reduction are supplied by the anode by the action of oxidation.

Question 11Which of the following represents a redox

reaction?

Cu(NO3)2(aq) + Na2S(aq) CuS(s) +2NaNO3(aq)

2Na(s) + I2(s) 2NaI(s)

H2SO4(aq) + 2HBr(g) SO2(g) + 2H2O(l) + Br2(l)

KH(s) + H2O(l) KOH(aq) +H2(g)

Wrong Answer!Precipitation reactions do not involve change in oxidation number.

Question 12In which of the following does the metal undergo a change

in the oxidation state?

I. 2MnO42- + F2 2MnO4

- + 2F-

II. 2CrO42- + 2H+ Cr2O7

2- + H2O

III.[Fe(H2O)6]2+ + 6CN- [Fe(CN)6]3- + 6H2O

I only II only

I and III onlyI and II only

Wrong Answer!The manganese changes oxidation number from +6 to +7.

The chromium remains in oxidation number +6.

The iron changes from +2 to +3.

Question 13In acid solution, manganate(VII) ions, MnO4

-(aq), undergo reduction to manganate(II) ions, Mn2+

(aq). What amount of MnO4-(aq) is required to

convert 5.36 x 10-3 moles of the ion Y2+(aq) to YO3

-(aq)?

8.93 x 10-3 mol 5.36 x 10-3 mol

1.07 x 10-3 mol3.22 x 10-3 mol

Wrong Answer!MnO4

- + 8H+ + 5e- Mn2+ + 4H2O (multiply by 3)

3H2O + Y2+ YO3- + 6H+ + 3e- (multiply by 5)

3MnO4- + 24H++15e- + 15H2O + 5Y2+ 3Mn2+ +12H2O+ 5YO3

- + 30H+ + 15e-

3MnO4- +3H2O + 5Y2+ 3Mn2+ + 5YO3

- + 6H+

Amount of MnO4- = 3/5 x amount of Y2+ = 3.216 x 10-3 mol

Question 14

The following reaction occurs in acid solution:

__H+(aq) +__NO3-(aq) + I-(aq) IO3

-(aq)+ __NO2(g)+__H2O

The equation is not balanced. What is the coefficient of NO3-

in the balanced equation?

6 5

24

Wrong Answer!6H+(aq) +6NO3

-(aq) + I-(aq) IO3-(aq)+6NO2(g)+6H2O

Question 15

Which one of the following equations represents the half-equation that occurs at the anode during the electrolysis of potassium iodide?

K+ + e- K 2H2O O2 + 4H+ + 4e-

2I- I2+ 2e- 2H2O + 2e- H2+2OH-

Wrong Answer!The anode is positive and attracts negative ions. Oxidation occurs at the anode of an electrolytic cell.

Question 16

In the reaction

2MnO2 + 4KOH + O2 + Cl2 2KMnO4 + 4KCl + 2H2O the oxidizing agent is/are

KMnO4 only MnO2 only

O2 and Cl2MnO2 and O2

Wrong Answer!Oxidizing agents undergo reduction during a redox reaction. The oxidation number of oxygen changes frm 0 to -2, the oxidation number of chlorine changes from 0 to -1.

Congratulations!