Unit 3 Redox reactions

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How many moles of I2 are reduced by 1 mole of Cr2O7 2- ions?

Which of the following is not a redox reaction?

In order to balance this ion-equation I – to IO 3 – . You need to …….

The balanced redox equation for the two ion-equations below is (you will need to look a you data book, page 11)

What colour change takes place during this reaction?

25 cm 3 of 0.200 mol –1 potassium permanganate was titrated against a solution of Fe 2+ ions. The concentration of Fe 2+ ions was?

The production of Aluminium during the electrolysis of aluminium oxide can be represented by ……….

A solution of HCl is electrolysed. What current is needed to produce 4.8 l of H2 gas in 3 min 13 sec? 1 mole of gas occupies 24 l .

a. 4

b. 3

c. 2

d. 1

Cr2O7 2- (aq) + 14H+ (aq) + 6e 2Cr 3+ (aq) + 7H2O (l)

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

How many moles of I2 are reduced by 1 mole of Cr2O7 2- ions?

1st hintThe number of electrons on both sides of the redox equation must balance.

a hint!!!!

2nd hintHow many more moles of iodine are now needed?

Ans: b. 3

How many moles of I2 are reduced by 1 mole of Cr2O7 2- ions?

Correct because…..

The number of electrons lost and gained must be equal.

Cr2O7 2- (aq) + 14H+ (aq) + 6e 2Cr 3+ (aq) + 7H2O (l)

I2 (aq) + 2e- 2I- (aq) X 3

Cr2O7 2- (aq) + 14H+ (aq) + 3I2 (aq) 2Cr 3+ (aq) + 7H2O (l) + 6I- (aq)

Which of the following is not a redox reaction.

a. Mg (s) + 2HCl (aq) MgCl2 (aq) + H2 (g)

b. CuCO3 (s) + H2SO4 (aq) CuSO4(aq) + CO2 (g) + H2O (l)

c. Zn(NO3)2 (aq) + 2Ag (s) 2AgNO3 (aq) + Zn (s)

d. H2 (g) + Cl2 (g) 2HCl (g)

Which reaction does not involve the formation of ions.?

a hint!!!!

Which of the following is not a redox reaction.

Correct because……

A redox reaction involves the transfer of electronsbetween reactants.

a. Mg (s) + 2HCl (aq) MgCl2 (aq) + H2 (g)Mg + 2H+ Mg 2+ + H2

b. CuCO3 (s) + H2SO4 (aq) CuSO4(aq) + CO2 (g) + H2O (l)

CO32- + 2H+ CO2 + H2O

c. Zn(NO3)2 (aq) + 2Ag(s) 2AgNO3 (aq) + Zn (s) Zn 2+ + 2Ag 2Ag + + Zn

Ans: d. H2 (g) + Cl2 (g) HCl (g)

In order to balance this ion-equation

I – to IO 3 –

You need to

a.

b.

c.

d.

Number H + Number of e-

6 3

3 6

3 3

6 6

1st hintBalance the oxygen by adding water molecules.

a hint!!!!

2nd hintBalance the hydrogen by adding H+ ions.

3rd hintBalance the charges by adding electrons

In order to balance this ion-equation

I – to IO 3 –

Correct because………

1st balance the O using water moleculesI – + 3H2O IO 3 –

2nd balance the H using H+ ions.I – + 3H2O IO 3 – + 6H+

3rd balance the charges by adding electronsI – + 3H2O IO 3 – + 6H+ + 6e-

The balanced redox equation for the two ion-equationsbelow is (you will need to look a your data book, page 11)

a. 2Fe (s) + 3SO4 2- (aq) 2Fe 3+ (aq) + 3SO3 2- (aq)

b. 2Fe (s) + 3SO4 2- (aq) + 6H+ (aq) 2Fe 3+ (aq) + 3SO3 2- (aq) + 3H2 (g)

c. 2Fe (s) + 3SO4 2- (aq) + 6H+ (aq) 2Fe 3+ (aq) + 3SO3 2- (aq) + 3H2O (l)

d. 3Fe (s) + SO4 2- (aq) + 6H+ (aq) Fe 3+ (aq) + SO3 2- (aq) + 3H2O (l)

SO4 2- (aq) SO3 2- (aq)

Fe (s) Fe 3+ (aq)

a hint!!!!

1st hintSO4 2- (aq) + 2H + (aq) + 2e- SO3 2- (aq) + H2O (l)

2nd hintFe (s) Fe 3+ (aq) + 3e-

The balanced redox equation for the two ion-equationsbelow is (you will need to look a your data book, page 11)

SO4 2- (aq) SO3 2- (aq)

Fe (s) Fe 3+ (aq)

SO4 2- (aq) + 2H + (aq) + 2e- SO3 2- (aq) + H2O (l)

Correct because……

Fe (s) Fe 3+ (aq) + 3e- X 2

3SO4 2-(aq) + 6H +(aq) + 2Fe(s) 3SO3 2-

(aq) + 3H2O(l) + 2Fe 3+ (aq)

X 3

Ans: C

What colour change takes place during this reaction?

a. Blue to colourless

b. Colourless to purple

c. Colourless to blue

d. Purple to colourless

5 Fe 2+ (aq) + 8H+ (aq) + MnO4- (aq) 5 Fe 3+ (aq) + Mn 2+ (aq) + 4H2O(l)

a hint!!!!

The permanganate ion is purple.

What colour change takes place during this reaction?

Correct because………..

The MnO4- (aq) ion is purple,

while the Mn 2+ (aq) ion is colourless.

5 Fe 2+ (aq) + 8H+ (aq) + MnO4- (aq) 5 Fe 3+ (aq) + Mn 2+ (aq) + 4H2O(l)

25 cm 3 of 0.200 mol l –1 potassium permanganate was titrated against a solution of Fe 2+ ions. What was the concentration of Fe 2+ ions?

5 Fe 2+ (aq) + 8H+ (aq) + MnO4- (aq) 5 Fe 3+ (aq) + Mn 2+ (aq) + 4H2O(l)

Titration results, 20.5 cm3, 20.1 cm3 and 20.0 cm 3

a. 1.25 mol l -1

b. 1.24 mol l -1

c. 0.250 mol l -1

d. 1.00 mol l -1

Have you used concordant titration results?

a hint!!!!

n = V x C and a 5:1 ratio of moles of Fe 2+ to MnO4 -

a hint!!!!

a hint!!!!

V x x C x

n x

= V y x C y

n y

25 cm 3 of 0.200 mol l –1 potassium permanganate was titrated against a solution of Fe 2+ ions. The concentration of Fe 2+ ions was?

5 Fe 2+ (aq) + 8H+ (aq) + MnO4- (aq) 5 Fe 3+ (aq) + Mn 2+ (aq) + 4H2O(l)

Titration results, 20.5 cm3, 20.1 cm3 and 20.0 cm 3

V x x C x

n x

= V y x C y

n y

X = MnO4- Y = Fe 3+

(20.1+20.0)/2 + Y 25 x 0.200

1 5=

Ans : b. 1.24 mol l -1

The production of aluminium during the electrolysisof aluminium oxide can be represented by

a. 96,500 C

b. 19300 C

c. 57,900 C

d. 193,000 C

What is the quantity of electricity needed to produce 0.2 moles of Al?

Al 3+ + 3e Al

1st hintHow many moles of electrons are needed to deposit 1 molof Al?

a hint!!!!

2nd hintThe charge on a mole of electrons is 96,500 C?

The production of aluminium during the electrolysisof aluminium oxide can be represented by

What is the quantity of electricity needed to produce 0.2 moles of Al?

Correct because……..

To produce 1 mole of aluminium, 3 moles of electrons are needed. i.e. 96,500 C x 3 = 289,500 C needed.

Al 3+ + 3e Al

1 mole Al = 289,500 C0.2 mole Al = 289,500 x 0.2 = 57,900C

A solution of HCl is electrolysed. What current is neededto produce 4.8 litres of H2 gas in 3 min 13 sec? 1 mole of gas occupies 24 litres .

a. 200 A

b. 50 A

c. 100 A

d. 20 A

1st hintHow many moles of hydrogen does 4.8 litres represent?

a hint!!!!

2nd hintHow many moles of electrons do you need to deposit 1 moleof hydrogen gas?

Q = I x t

A solution of HCl is electrolysed. What current is neededto produce 4.8 l of H2 gas in 3 min 13 sec? 1 mole of gasoccupies 24 l ?

Correct because……..

2H+ (aq) + 2e- H2 (g)

So to produce a mole of hydrogen gas, 2 moles of electrons are needed.No of moles of H2 gas porduced = 4.8/24 mol = 0.2 mol of gas produced.1 mol of gas would need 96500 x 2 C0.2 mol would need (96500 x 2) x 0.2 C = 38,600 CQuantity of charge = current x time

Q = I x t

So I = Q/t = 38600 / 193 = 200 A