OXIDIZING AND REDUCING AGENTS
ELECTROLYTIC CELL
VOLTAIC CELL
STANDARD ELECTRODE POTENTIAL
REACTIVITY
100
Which species could be reduced to form NO2? a) N2O b) NO3– c) HNO2 d) NO
b) NO3–
100
Which processes occur during the electrolysis of molten sodium chloride?
I. Sodium and chloride ions move through the electrolyte.
II. Electrons move through the external circuit.
III. Oxidation takes place at the anode.
I, II and III
100
Which changes could take place at the positive electrode (cathode) in a voltaic cell?
I. Zn2+(aq) to Zn(s)
II. Cl2(g) to Cl–(aq)
III. Mg(s) to Mg2+(aq)
I and II only
100
Define standard electrode potential
The standard electrode potential of a half-cell is its electrode potential relative to a hydrogen half-cell, measured under standard conditions
100
Consider the following reactions of three unknown metals X, Y and Z.
2XNO3(aq) + Y(s) → 2X(s) + Y(NO3)2(aq)
Y(NO3)2(aq) + Z(s) → No reaction
2XNO3(aq) + Z(s) → 2X(s) + Z(NO3)2(aq)
What is the order of increasing reactivity of the metals (least reactive first)?
X < Z < Y
200
What happens to the manganese in the following reaction? 2MnO4–(aq) + 5H2O2(aq) + 6H+(aq) → 2Mn2+(aq) + 8H2O(l) + 5O2(g)
It is reduced and its oxidation number decreases.
200
What condition is necessary for the electroplating of silver, Ag, onto a steel spoon?
A. The spoon must be the positive electrode.
B. The silver electrode must be the negative electrode.
C. The spoon must be the negative electrode.
D. The electrolyte must be acidified.
C. The spoon must be the negative electrode.
200
What are the features of a standard hydrogen electrode?
I. A temperature of 298 K
II. A carbon electrode
III. Hydrogen gas at 1.01 × 105 Pa (1 atm) pressure
I and III only
200
The standard electrode potentials for two metals are given below.
Al3+(aq) + 3e– Al(s) EO = –1.66 V
Ni2+(aq) + 2e– Ni(s)EO = –0.23 V
What is the equation and cell potential for the spontaneous reaction that occurs?
2Al(s) + 3Ni2+(aq) → 2Al3+(aq) + 3Ni(s) EO = 1.43 V
200
State which of the species is the strongest oxidizing agent
Fe2+(aq) + 2e– Fe(s) –0.45 V
H+(aq) + e– H2(g) 0.00 V
Br2(l) + e– Br–(aq) +1.07 V
Bromine/Br2
300
Which is the strongest reducing agent according to the spontaneous reactions below?
2Cr(s) + 3Fe2+(aq) -> 2Cr3+(aq) + 3Fe(s)
Fe(s) + Pb2+(aq) -> Fe2+(aq) + Pb(s)
Cr(s)
300
Sodium metal can be obtained by the electrolysis of molten sodium chloride.
Tell why it is very difficult to obtain sodium from sodium chloride by any other method and why an aqueous solution of sodium chloride cannot be used to obtain sodium metal by electrolysis.
1) sodium is a very powerful reducing agent, its difficult to get sodium ions reduced
2) H+ ions gain electrons more readily than Na+, so hydrogen is produced
300
Explain the function of the salt bridge in an electrochemical cell.
to complete the electrical circuit by allowing the movement of ions
300
Consider the following standard electrode potentials.
Zn2+(aq) + 2e– Zn(s) EO = –0.76 V
Cl2(g) + 2e– 2Cl–(aq) EO = +1.36 V
Mg2+(aq) + 2e– Mg(s) EO = –2.37 V
What will happen when zinc powder is added to an aqueous solution of magnesium chloride?
No reaction will take place
300
Deduce the order of reactivity of the four metals, cadmium, nickel, silver and zinc in the below equations and list in order of decreasing reactivity
Cd(s) + Ni2+(aq) → Cd2+(aq) + Ni(s)
Ni(s) + 2Ag+(aq) → Ni2+(aq) + 2Ag(s)
Zn(s) + Cd2+(aq) → Zn2+(aq) + Cd(s)
Zn > Cd > Ni > Ag
400
Nitrogen monoxide may be removed from industrial emissions via a reaction with ammonia as shown by the equation below.
4NH3(g) + 6NO(g) → 5N2(g) + 6H2O(l)
Deduce the oxidation and reduction half-equations and identify the oxidizing agent for the reaction.
Oxidation: 2NH3 → N2 + 6H+ + 6e–
Reduction: 2NO + 4H+ + 4e– → N2 + 2H2O oxidizing agent: NO
400
Deduce the equations for the formation of the major product at the positive electrode (anode) when the following aqueous solutions are electrolysed.
• dilute sodium chloride
• concentrated sodium chloride
Dilute sodium chloride:
2H2O → O2 + 4H+ + 4e– / 4OH– → O2 + 2H2O + 4e–
Concentrated sodium chloride:
2Cl– → Cl2 + 2e–
400
Draw a labelled diagram of a voltaic cell made from an Fe(s) / Fe2+(aq) half-cell connected to a Cu(s) / Cu2+(aq) half-cell. In your diagram identify the positive electrode (cathode), the negative electrode (anode) and the direction of electron flow in the external circuit. And deduce the half-equations for the reactions taking place at the positive electrode (cathode) and negative electrode (anode) of this voltaic cell.
diagram has salt bridge, voltmeter
electrodes; positive electrode (Cu):cathode and negative electrode (Fe):anode
flow of electrons: from Fe to Cu
positive electrode: Cu2+ + 2e– → Cu;
negative electrode: Fe → Fe2+ + 2e–;
400
The amount of ethanol, C2H5OH(aq), in a person’s breath can be measured in an electrochemical cell. The relevant half-reaction equations are given below.
I CH3COOH(aq) + 4 H+(aq) + 4 e– → C2H5OH(aq) + H2O(l) E° = +0.58 V
II O2(g) + 4 H+(aq) + 4 e– → 2 H2O(l) E° = +1.23 V
O2(g) + 4 H+(aq) + 4 e– → 2 H2O(l)
400
The following equations indicate reactions that occur spontaneously.
Fe(s) + NiCl2(aq) → FeCl2(aq) + Ni(s)
Zn(s) + FeCl2(aq) → ZnCl2(aq) + Fe(s)
Ni(s) + PbCl2(aq) → NiCl2(aq) + Pb(s)
Which is the increasing order of the reactivity of the metals?
Pb < Ni < Fe < Zn
500
Which of the following metals will act spontaneously as reducing agents with silver iodide but not with silver sulfide?
A. Hg(l) and Cu(s)
B. Cr(s) and Al(s)
C. Ni(s) and Co(s)
D. Au(s) and Mg(s)
C. Ni(s) and Co(s)
500
Draw an electrolytic cell illustrating the electrolysis of molten nickel(II) bromide, NiBr2. Include in the diagram the direction of the electron flow, the polarity of electrodes and state the half-equations for the product formed at each electrode.
- Cathode/negative electrode equation:
Ni2+ + 2e– → Ni
- Anode/positive electrode equation:
2Br– → Br2 + 2e–
- power source present, electron flow from anode to cathode
- molten electrolyte/Ni2+(l) and Br–(l)/NiBr2(l)
500
Complete the sentence:
A zinc–carbon cell is an example of i cell. The moist paste of ammonium chloride allows for the flow of ii .
a voltaic and ions
500
If the reference half-cell were changed from the standard hydrogen half-cell to the standard cadmium half-cell, then the reduction potential for the following equation would become
CH3COOH(aq) + 4 H+(aq) + 4 e– → C2H5OH(aq) + H2O(l) E° = +0.58 V
+0.98 V
500
A 150 mL sample of SO2(aq) required 31.5 mL of 0.0100 mol/L KMnO4(aq) to completely react. The concentration of SO2(aq) in the sample was
5.25 mmol/L