REDOX

OXIDIZING AND REDUCING AGENTS

100

Which species could be reduced to form NO2? a) N2O b) NO3– c) HNO2 d) NO

b) NO3–

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.

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)

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

ELECTROLYTIC CELL

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

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.

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

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–

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)

VOLTAIC CELL

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

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

300

Explain the function of the salt bridge in an electrochemical cell.

to complete the electrical circuit by allowing the movement of ions

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–;

STANDARD ELECTRODE POTENTIAL

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

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

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

REACTIVITY

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

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

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

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