Electrochemistry
Word
/ DefinitionOxidation number / The charge of an ion or the apparent charge of an atom in a covalent bond.
Oxidation / Losing electrons
Reduction / Gaining electrons
Half-reaction / A reaction that describes the change in oxidation number and subsequent gain or loss of electrons that occurs during oxidation or reduction.
Oxidizing agent / The species that is reduced and therefore removes the electrons from the species that was oxidized.
Reducing agent / The species that is oxidized and therefore gives electrons to the species that was reduced.
Voltaic cell / An electrochemical cell that produces electric current as a result of a spontaneous redox reaction, used to make batteries. Consists of two half-cells connected by a salt bridge and two electrodes.
Anode / The electrode at which oxidation occurs.
Cathode / The electrode at which reduction occurs.
Salt bridge / Allows the flow of ions from one half-cell to each other, but prevents the direct mixing of those ions.
Electrolytic cell / A cell that uses electricity from an outside source to force a nonspontaneous redox reaction to occur.
Species / A substance in a reaction.
Oxidation-Reduction
A. Oxidation-Reduction Reactions (REDOX)
1. Oxidation numbers can be assigned to atoms and ions. Changes in oxidation numbers indicate that oxidation and reduction have occurred.
(Hint: if asked to identify a redox reaction, look for a reaction that has an element by itself or a diatomic molecule)
- an element or compound has a total charge of 0
- An ion has a total charge = to the charge of the ion
- Possible oxidation numbers are found on the periodic table in the upper right hand corner of each element.
2. An oxidation-reduction reaction involves the transfer of electrons.
3. Oxidation is the loss of electrons and Reduction is the gain of electrons.
Remember LEO the lion says GER or OIL RIG
4. The particle oxidized is the reducing agent, and the particle reduced is the oxidizing agent.
Remember agents are opposite
5. Half reactions can be written to represent both oxidation and reduction.)
a.) Write and balance half reactions for oxidation and reduction of free elements and their monatomic ions.
6. In redox reaction the number of electrons lost must equal the number of electrons gained.
7. Balance redox reactions by identifying changes in oxidation number writing half reactions.
B. Electrochemistry
1. An electrochemical cell can be either voltaic or electrolytic.
a.) Compare and contrast voltaic and electrolytic cells.
2. Describe the assembly and operation of voltaic cell.
a.) Cathode, anode, salt bridge, and direction of electron flow given the reaction equation.
Know the purpose of the salt bridge
b.) Use an activity series (Reference Table J) to determine whether a redox reaction is spontaneous. If the unbonded element is higher than the bonded element (ion), then the reaction will spontaneously occur.
- Cu + 2H+ Cu2+ + H2 Not spontaneous b/c Cu is lower than H
- Fe + PbCl2 FeCl2 + Pb Spontaneous b/c Fe is higher than Pb
3. A voltaic cell spontaneously converts chemical energy into electrical energy.
4. In a voltaic cell, oxidation occurs at the anode and reduction occurs at the cathode.
AN OX sat on a Fat RED CAT
5. An electrolytic cell requires electrical energy to produce a chemical change. This process is electrolysis.
6. Describe the assembly and operation of an electrolytic cell.
a.) Cathode, anode, and direction of electron flow given the reaction.
1) Half-Reactions Homework
1) Determine the oxidation numbers of each element in the following substances. Report only the individual oxidation number, not the total contribution.
1) N2 / N:02) ZnCl2 / Zn:2+ / Cl:1-
3) K2SO3 / K:1+ / S:4+ / O:2-
4) S8 / S:0
5) Fe / Fe:0
6) PbO / Pb:2+ / O:2-
7) PbO2 / Pb:4+ / O:2-
8) CuCO3 / Cu: 2+ / C: 4+ / O: 2-
9) Na2CrO4 / Na:1+ / Cr:6+ / O:2-
10) Cu(ClO3)2 / Cu:2+ / Cl:5+ / O:2-
11) Cu(ClO2)2 / Cu: 2+ / Cl:3+ / O:2-
12) Cu(ClO)2 / Cu: 2+ / Cl:1+ / O:2-
13) K / K:0
14) NO2 / N:4+ / O:2-
15) SO2 / S:4+ / O:2-
16) Pb(Cr2O7)2 / Pb:4+ / Cr:6+ / O:2-
2) Write all of the charges for each species above their symbols, then write the oxidation half-reaction and reduction half-reaction, identify the oxidizing and reducing agents and any spectator ions.
2 Li + Zn(NO3)2 2 LiNO3 + Zn
Oxidation Half-Reaction:___Li Li+ + 1e-______
Reduction Half-Reaction:___Zn2+ + 2e- Zn______
Oxidizing Agent:___Zinc 2+______Reducing Agent:___Lithium______Spectator Ion:__Nitrate (NO3-)__
2 K + Cl2 2 KCl
Oxidation Half-Reaction:___K K+ + 1e-______
Reduction Half-Reaction:___Cl2 +2e- 2Cl-______
Oxidizing Agent:__Chlorine gas______Reducing Agent:__Potassium_____ Spectator Ion:__none______
2 Na2O 4 Na + O2
Oxidation Half-Reaction:____2O2- O2 + 4e-______
Reduction Half-Reaction:____Na+ + 1e- Na______
Oxidizing Agent:__Sodium 1+______Reducing Agent:___Oxygen 2-______Spectator Ion:___none______
SnCl2 + 2 FeCl3 SnCl4 + 2FeCl2
Oxidation Half-Reaction:___Sn2+ Sn4+ + 2e-______
Reduction Half-Reaction:___Fe3+ + 1e- Fe2+______
Oxidizing Agent:__Iron 3+______Reducing Agent:___Tin 2+______Spectator Ion:__Chloride (Cl-)____
2 H2 + O2 2 H2O
Oxidation Half-Reaction:____H2 2H+ + 2e-______
Reduction Half-Reaction:____O2 + 4e- 2O2-______
Oxidizing Agent:___Oxygen______Reducing Agent:___Hydrogen______Spectator Ion:___none______
2) Electrochemistry Homework
1) When the following pairs of elements are reacted, indicate which element will undergo oxidation and which element will undergo reduction based on Reference Table J.
Pair / Oxidation / Reduction / Pair / Oxidation / ReductionMg and Cu / Mg / Cu / Ni and Pb / Ni / Pb
Zn and Li / Li / Zn / Na and Cu / Na / Cu
Na and Ca / Ca / Na / Cr and Sr / Sr / Cr
Mn and Ba / Ba / Mn / Au and Ag / Au / Ag
2) Four pairs of ions are listed below. Either 1) or 2) will react. For each pair, use Reference Table J to determine which will spontaneously react and circle that pair (either 1) or 2)).
A) 1) Ni0 + Pb+22) Pb0 + Ni+2B) 1) Ca+2 + Li02) Ca0 + Li+1
C) 1) Zn0 + Co+22) Zn+2 + Co0D) 1) Cu+2 + Na02) Na+1 + Cu0
3) Given the following voltaic cell diagram, label the anode, cathode, + electrode, - electrode, direction of electron flow, direction of anion flow, salt bridge and load Base your decisions on Reference Table J.
According to Table J, _Barium (Ba)__ will undergo oxidation and __Nickel 2+ (Ni2+)__ will undergo reduction.
Base your answers on the following redox reaction, which occurs spontaneously in an electrochemical cell.
Zn + Cr3+Zn2+ + Cr
Write the half-reaction for the reduction that occurs. [1]Cr3+ + 3e- Cr
Write the half-reaction for the oxidation that occurs. [1] Zn Zn2+ + 2e-
Balance the equation using the smallest whole-number coefficients. [1]
_3_ Zn + _2_Cr3+ + _3_Zn2+ + _2_ Cr
73 Which species loses electrons and which species gains electrons? [1]
Zinc loses electrons and the chromium 3+ ion gains electrons.
74 Which half-reaction occurs at the cathode? [1]
Cr3+ + 3e- Cr
75 State what happens to the number of protons in a Zn atom when it changes to Zn2+ as the redox reaction occurs. [1]
The number of protons in the zinc atom does not change when the zinc 2+ ion is formed, only the number of electrons changes. Never change the number of protons in redox reaction!
Base your answers to questions on the diagram of a voltaic cell provided and on your knowledge of chemistry.
Indicate with one or more arrows the direction of electron flow through the wire. [1]
Write an equation for the half-reaction that occurs at the zinc electrode. [1] Zn Zn2+ + 2e-
Explain the function of the salt bridge. [1]
The salt bridge allows for the exchange of cations and anion between the two half cells in order to maintain charge balance.
Base your answers on the unbalanced redox reaction below.
Write the reduction half-reaction. [1]
Ag+ + 1e- Ag
Balance the redox equation using the smallest whole-numbercoefficients. [1]
Base your answers on the diagram and balanced equation below, which represent the electrolysis of molten NaCl.
When the switch is closed, which electrode will attract the sodium ions? [1]
The negative electrode or cathode, will attract the sodium ions.
What is the purpose of the battery in this electrolytic cell? [1]
The battery serves as a “pump” and pushes the electrons in the non-spontaneous direction in order to achieve electrolysis of the molten salt.
Write the balanced half-reaction for the reduction that occurs in this electrolytic cell. [1]
Na+ + 1e- Na