CHEM 124 Ch.21(1-3) Flashcards
Redox reactions, voltaic cells, standard cell and electrode potentials (exam 3+4)
An electrochemical ____ is the primary system for the study of electrochemical reactions
cell
Oxidation involves the ____ of electrons, while reduction involves the ____ of electrons
loss; gain
If the oxidation # of a substance becomes +, that substance has been ____; if the oxidation # of a substance becomes -, that substance has been____.
oxidized; reduced
Match the element with the most common O.N (assuming polar covalent or ionic bond)
a. Na
b. F
c. Ca
d. O
a. +2
b. -2 or -1
c.+1
d. -1
a. Na, c. +1
b. F, d. -1
c. Ca, a. +2
d. O, b. -2 or -1
O.N can be based on group number
After spectator ions are eliminated, which listed species should be used when balancing equation from former redox reaction?
KMnO4 (aq) + Na2C2O4 (aq) + H2SO4 (aq) → MnSO4 (aq) + K2SO4 (aq) + Na2SO4 (aq) + CO2 (g)+ H2O (l)
Multiple Choice:
a. SO42–
b. CO2
c. H2 (g)
d. MnO4– (aq)
e. H+
f. H2O (l)
g. Na2SO4 (aq)
h. K2SO4 (aq)
I. Mn2+ (aq)
j. C2O42–
b., d., e., f., i., and j.
Get rid of spectator ions and dissociate correctly before 1/2 reaction method
The branch of chemistry that deals with the interconversion of electrical and chemical energy is called ____.
electrochemistry
Based on the following reaction, identify ALL the species that should be included in the oxidation 1/2 reaction: Zn (s) + Cu2+ (aq) → Zn2+ (aq) + Cu (s)
a. Cu(s)
b. Cu ^2+ (aq)
c. Zn ^ 2+ (aq)
d. Zn(s)
c. and d
If a substance causes another substance to be oxidized then it’s a ____ agent. Likewise the substance that being oxidized in a electrochemical reaction is always the ____ agent.
oxidizing; reducing
Which of the following reactions result in the oxidation of sulfur? Select all that apply.
a. S8 + 8O2 → 8SO2
b. SO2 + H2O2 → H2SO4
c.SO2 + H2O → H2SO3
d. S8 + 8H2 → 8H2S
a and b
Prior to balancing O and H atoms, find missing coefficients from the 1/2 reaction: _NO2^ - -> _NH4^ +
1 and 1
Match the O.N of Cl w/ the appropriate chemical compound
a. NaClO4
b. HCl
c. KClO
d. RbClO3
a. +5
b. -1
c. +1
d. +7
NaClO4-+7
HCl- -1
KClO- +1
RbClO3- +5
Identify the number of water molecules needed to balance the following half-reaction. Also indicate whether the water will appear as a reactant or a product.
MnO4– → MnO2
a. 1H2O; product
b. 1H2O; reactant
c. 2H2O; reactant
d. 2H2O; product
d. 2H2O; product
After spectator ions are eliminated, which of the listed species should be used when balancing the equation for FeCl2 (aq) + NaOH (aq) + H2 (g) → Fe (s) + NaCl (aq) + H2O (l)?
a. H2O (l)
b. H2 (g)
c. OH– (aq)
d. NaOH (aq)
e. NaCl (aq)
f. Fe2+ (aq)
H2O(l), H2(g), OH-(aq), Fe2+ (aq)
For 2Na + 2H2O → 2NaOH + H2, which of the following (unbalanced) reactions should be used as the starting point for the reduction 1/2 reaction?
a. H2O-> H2
b. Na-> H2
c. H20-> NaOH
d. Na-> NaOH
H2O-> H2
When H and O atoms are balanced in the following 1/2 reaction under standard conditions, which of the following statements are correct TiO2 (s) → Ti (s)?
a. 2 mol of H+ as a reactant
b. 4 mols of H+ as a reactant
c. 2 mols of H2O as a product
d. 4 Mols of H2O as a product
4 mols of H+ as a reactant and 2 mols of H2O as a product
Identify the missing coefficient to balance Mn in Mn2O3 → _ MnO2
a. 1/2
b. 2
c. 3
d. 1
2
To which of the following 1/2 reactions must 2 electrons appear as products in order to balance the equation?
a. TiO2 + 4H+ → Ti2+ + 2H2O
b. H2 + 2OH– → 2H2O
c. H2O + PbO → PbO2 + 2H+
d. 2OH– → O2 + 2H+
H2 + 2OH– → 2H2O and H2O + PbO → PbO2 + 2H+.
In which of the following (unbalanced) 1/2 reaction will 1 mol of water appear as a reactant when O atoms are balanced?
a. SO2 → SO42–
Reason: SO2 + 2H2O → SO42–; this half-reaction contains 2 moles of water on the reactant side of the chemical equation.
a. As2O3 → 2AsO43–
b. N2O5 → 2NO3
c. Na → NaOH
d. SO2 → SO42–
N2O5 → 2NO3 and Na → NaOH
Which of the following reactions is Cl reduced?
a. 2LiClO3 + K2Cr2O7 → 2LiClO4 + 2CrO2 + K2O
b. 2NaClO + F2 → NaF + Cl2 + O2
c. Cl2 + 2Na → 2NaCl
d KCl + MnO2 → MnO + KClO
2NaClO + F2 → NaF + Cl2 + O2 and Cl2 + 2Na → 2NaCl
Which of the following are true when the reactions below are added together? Make sure you eliminate that species that appears on both sides of the equation.
Reaction 1: 2BiO+ + 4H+ + 6e– → 2Bi + 2H2O
Reaction 2: 6H2O → 3H2O2 + 6H+ + 6e–
a. 1mol H2O as reactant
b. 4 mol of H2O as a reactant
c. 6 mol of e- appear on each side of the equation
d. 2 mol of H+ appear as product
4 mol of H2O as a reactant and 2 mol of H+ appear as product
When H atoms are balanced in 2ClO3– → Cl2 + 6H2O, identify the amount of the substance that must be introduced, and where it will appear in the equation (the reaction is acidic).
a. 6 H2 as a reactant
b. 6 H2O as a reactant
c. 6 H+ as a reactant
d. 12 H+ as a reactant
12 H+ as a reactant
Which of the following species can be added to balance the H+ ions in a redox reaction occurring in basic medium?
a. OH-
b. H3O+
c. H+
d. H2O
OH-
Identify the # of e- required to balance the following 1/2 reaction. Also indicate whether theses e- must be reactants or products.
a. 1e-; product
b. 2e-; product
c. 1e-; reactant
d. 2e-; reactant
1e-; reactant
The reaction MnO2 + 4H+ + Zn → Mn2+ + 2H2O + Zn2+ is basic. Select all options that correctly describe the balanced equation obtained after all resulting species are combined and/or canceled correctly.
a. 4 mol OH- appear among the products
b. 4 mol of OH- reactant
c. 2 mol H2O reactants
d. 2 mol H2O products
4 mol OH- appear among the products and 2 mol H2O reactants
Identify the role played by the individual species in the redox reaction represented by the equation Fe(NO3)2 (s) + Al (s) → Fe (s) + Al(NO3)3 (aq)
Fe2+, Al, Reducing and oxidizing agents, and NO3-
a. species reduced
b. spectator ions
c. Al
d. Fe2+
e. species oxidized
Fe2+: species reduced
Al: species oxidized
Reducing Agents: Al
Oxidizing Agent: Fe 2+
NO-3: Spectator ion
Match each type of electrochemical cell w/ the appropriate description.
For a voltaic cell, For an electrolytic cell, Work is done on the system in a(n), and The system does work on the surroundings in a(n)
a. DeltaG>0
b. voltaic cell
c. electrolytic cell
d. DeltaG<0
Voltaic cell: DeltaG<0
Electrolytic cell: DeltaG>0
Work is done on the system in a(n): electrolytic cell
System does work on the surroundings: voltaic cell
When the 1/2 reactions (Equation 1: 2H2O → O2 + 4H+ + 4e– and Equation 2: Ni2+ + 2e– → Ni) are balanced to each other the 1st reaction should be multiplied by __ and the 2nd reaction should be multiplied by ___
a. 2;1
b. 4;2
c.1;2
d. 2;3
1;2
In an electrochemical cell, the electrochemical reactions occur at the surface of 2 ___ that are immersed in a(n)___ solution
electrodes; electrolyte, ion, or ionic
When the 2 1/2 reactions (Reaction 1: Cu2+ + 2e– → Cu and Reaction 2: Fe → Fe3+ + 3e–) are added together given all e- cancel, which choice is the net reaction?
a. 3Cu2+ + 2Fe → 3Cu + 2Fe3+
b. Cu2+ + 2e– + Fe → Cu + Fe3+ + 3e–
c. Cu2+ + Fe → Cu + Fe3+
d. 2Cu2+ + 3Fe → 2Cu + 3Fe3+
3Cu2+ + 2Fe → 3Cu + 2Fe3+
Reason: reactions 1 should be multiplied by 3 while reaction 2 should be multiplied by 2 to cancel out the electrons!
Which of the following is required in order for an electrochemical reaction to generate an electric current?
a. The electrochemical reaction must preform a redox reaction
b. A transfer of electrons from reactants to products must occur
c. The voltaic cell must be electrically dead
d. the half-reaction must be physically separated
the half-reaction must be physically separated
The approach to balance the redox reactions occurring in the basic solution is to 1st balance them as if they were in a acidic solution, then count the # of ____ and add this same # of ____ to each side of the 1/2 reaction
a. H2O molecules; OH- ions
b. H+ ions; OH- ions
c. OH- ions; H2O molecules
d. OH- ions; H+ ions
H+ ions; OH- ions
(Look up photo of Zn and Cu electrode cell) In the photo, electrons flow from the ___ electrode to the __ electrode. Cations flow from the ____ cell to the ____ cell.
a. Cu; Zn; anode; cathode
b. Cu; Zn; cathode; anode
c. Zn; Cu; anode; cathode
d. Zn; Cu; cathode; anode
Zn; Cu; anode; cathode
In a voltaic cell, electrons are generated due to the oxidation reaction at the ____ and flow through the external circuit towards the ____.
a. anode; cathode
b. cathode; anode
anode; cathode
The following redox reaction has been balanced in the acidic 2H+ + 3TeO32– + 4Sb → 3Te + 4SbO2– + H2O. Select the correct option that correctly describes the balanced equation in a basic solution after all resulting species are combined and/or canceled correctly.
a. 2 moles of OH– appear among the reactants.
b. 2 moles of OH– appear on the right side of the equation.
c. 1 mole of H2O appears on the left side of the equation.
d. 2 moles of H2O appear among the products.
2 moles of OH– appear on the right side of the equation and 1 mole of H2O appears on the left side of the equation.
A(n) ____ cell uses a spontaneous reaction to generate electrical energy. In this type of cell, the system does work on the ___.
voltaic; surroundings
A(n)___ electrode is composed of a material that doesn’t directly take part in an electrochemical reaction (other than transmitting electrons) while a(n) ___ electrodes does participate in a 1/2 reaction.
a. active; inactive
b. noble; reactive
c. anode; cathode
d. inactive; active
inactive; active
Reason: An inactive electrode may be the anode or cathode. An active electrode may be the anode or cathode
The electrode at which the oxidation reaction occurs is called the ____, while the electrode at which the reduction reaction occurs is called the ____.
a. anode; cathode
b. cathode; anode
anode; cathode
Match the electrode to it’s type.
- A disc of Pt used for: Pt2+ (aq) + 2e– → Pt (s)
- Pt wire used for: 2Cl– (aq) → Cl2 (g) + 2e–
a. Inactive Pt electrode
b. Active Pt electrode
A disc of Pt used for: Pt2+ (aq) + 2e– → Pt (s): Active
Reason: this electrode is active because the metals are components of the 1/2 reactions (cathode or anode)
Pt wire used for: 2Cl– (aq) → Cl2 (g) + 2e–: Inactive
Reason: acts as a no reaction component to carry on the reaction like rods (Pt and graphite)
When Zn metal is immersed in a 1M solution of copper(III) chloride at 25C, no electrochemical work may be extracted, even though a spontaneous reaction occurs. What’s wrong w/ this cell design?
a. Temp is low
b. Cu2+ conc. is high
c. The oxidation and reduction reactions must be physically separated
d. Cu2+ is too low
e. Zn metal doesn’t react w/ Cu 2+ in this solution
The oxidation and reduction reactions must be physically separated
Which of the following is the proper shorthand notation for the redox reaction 2H+ (aq) + Cu (s) → Cu2+ (aq) + H2 (g)? (The hydrogen half-cell uses a graphite inactive electrode.)
a. graphite | H+ (aq) | H2 (g) || Cu (s) | Cu2+ (aq)
b. Cu (s) | Cu2+ (aq) || H+ (aq) | H2 (g) | graphite
c. Cu (s) | H+ (aq) || Cu2+ (aq) | H2 (g)
d. Cu (s) | Cu2+ (aq) || H+ (aq) | H2 (g)
Cu (s) | Cu2+ (aq) || H+ (aq) | H2 (g) | graphite
Reason: inactive electrodes are included in reactions as well
An electrochemical cell operates because the electrodes in the 2 1/2 cells differ in their electrical potential. A metal electrode composed of ____ reducing agent will have a greater electrical potential to push electrons towards the electrode composed of a ____ reducing agent.
a. weaker; stronger
b. stronger; weaker
stronger; weaker
Match each symbol w/ the type of energy associated w/ it
q, V, H, E, S
a. entropy
b. enthalpy
c. Heat
d. internal energy
e. Electrochemical potenital
q-> heat
V-> electrochemical potential
H-> enthalpy
E-> internal energy
S->entropy
Select the active electrode:
a. A strip of zinc used for: Zn2+ (aq) + 2e– → Zn (s)
b. A sodium wire used for: Na (s) → Na+ (aq) + e–
c. A disc of platinum used for: Pt2+ (aq) + 2e– → Pt (s)
d. A strip of carbon used for: H2 (g) → 2H+ (aq) + 2e–
e. Platinum wire used for: O2 (g) + 4H+ (aq) + 4e– → 2H2O (l)
A strip of zinc used for: Zn2+ (aq) + 2e– → Zn (s)
A sodium wire used for: Na (s) → Na+ (aq) + e–
A disc of platinum used for: Pt2+ (aq) + 2e– → Pt (s)
Match each cell potential w/ its implication for the spontaneity of the reaction.
- Ecell > 0
- Ecell < 0
- Ecell = 0
a. Reaction is nonspontaneous
b. Reaction is a =m
c. Reaction is spontaneous
- Ecell > 0:Reaction is nonspontaneous
- Ecell < 0:Reaction is spontaneous
- Ecell = 0: Reaction is a =m
Given the shorthand notation of an electrochemical cell Ca (s) | Ca2+ (aq) || Pb2+ (aq) | Pb (s), match the component w/ the appropriate species.
- Ca (s)
- Ca2+ (aq)
- Pb (s)
- Pb2+ (aq)
a. Reactant in the cathode
b. Product in the cathode
c. Reactant in the anode
d. Product in the anode
- Ca (s): Reactant in the anode
- Ca2+ (aq): Product in the anode
- Pb (s): Product in the cathode
- Pb2+ (aq): Reactant in the anode
Ni (s) + 2Ag+ (aq) → Ni2+ (aq) + 2Ag (s) is spontaneous in the forward direction. Which of the statements below provides the best reason for this observation?
a. Ag (s) gives its electrons away more freely than Ni (s).
b. Ni2+ (aq) accepts electrons more readily than Ag+ (aq).
c.Ni (s) gives its electrons away more freely than Ag+ (aq).
d. Ni (s) gives its electrons away more freely than Ag (s).
Ni (s) gives its electrons away more freely than Ag (s).
Reason: Since Ag+ is reduced in the reaction, it must accept its electron from Ni. The product are more stable that the reactants
Which of the following options correctly describe the units used to express electrical potential, charge, and electrical energy? Select all that apply.
a. 1 joule = 1V1C
b. The unit for electrical charge has the symbol F.
c. The volt is the unit for electrical potential.
d. The coulomb is the unit for cell emf (electromotive force).
e Electrical energy can be measured in joules.
The volt is the unit for electrical potential and Electrical energy can be measured in joules.
The measure of energy used in electrochemistry is the cell ____ (Ecell), which is the difference in electrical potential between two electrodes. A common term for this difference in electrical potential is ____
force or emf.
potential; electromotive
Which compounds or solutions are in an appropriate state for the measurement of Ecell circle for Cu2+ (aq) + H2 (g) → Cu (s) + 2H+ (aq)?
a. H2 gas at 1 atm
b. H+ at 2.0 M
c. 1.0 M Cu2+
d. Cu coated with a film of Cu2O
H2 gas at 1 atm, 1M Cu2+
Which of the following electrochemical reactions are spontaneous under standard conditions?
a. 2Co (s) + 3Cl2 (g) → 2CoCl3 (aq); Ecello = 1.6 V
b. MgBr2 (aq) → Mg (s) + Br2 (g); Ecello = –3.4 V
c. 2Cr (s) + 3Sn4+ (aq) → 2Cr3+ (aq) + 3Sn2+ (aq); Ecello = 0.89 V
d. Sn (s) + ZnCl2 (aq) → SnCl2 (aq) + Zn (s); Ecello = –0.63 V
2Co (s) + 3Cl2 (g) → 2CoCl3 (aq); Ecello = 1.6 V and 2Cr (s) + 3Sn4+ (aq) → 2Cr3+ (aq) + 3Sn2+ (aq); Ecello = 0.89 V
(Remember Ecell >0 are spontaneous under standard conditions!)
Which of the following statements correctly describe standard electrode potentials? In what way must 1/2 reactions and/or electrodes potentials be manipulated when writing a balanced equation for a redox reaction?
a. By convention, standard electrode potentials are quoted as reduction potentials.
b. Both half-reactions must be reversed when writing the balanced equation for the overall reaction.
c. By convention, standard electrode potentials are quoted as oxidation potentials.
d. The sign for the anode potential must be reversed in order to to use the equation Ecell = Ecathode – Eanode.
e. The half-reaction for the anode must be reversed when writing the balanced equation for the overall reaction.
The half-reaction for the anode must be reversed when writing the balanced equation for the overall reaction
By convention, standard electrode potentials are quoted as reduction potentials.
Reason: Since standard half-reactions are generally written as reduction reactions, the reaction for the anode must be reversed when writing the balanced equation for the overall reaction.
The standard cell potential may be solved using the equation Ecircle____-Ecircle____.
a. cathode; anode
b. oxidation; reduction
c. reduction; oxidation
d. anode; cathode
cathode (reduction); anode (oxidation)
By definition, if one coulomb of charge passes between two electrodes that differ in potential by one volt, one ____ of energy is released. This relationship is correctly expressed as V = ____ / ____. (Fill in units only, without values.)
joule; joule; coulomb
Co3+ (aq) + Cu (s) → Co2+ (aq) + Cu+ (aq) Calculate the standard cell potential for the reaction above given the following information:
Co3+ (aq) + e– → Co2+ (aq); Eo = 1.842 V
Cu+ (aq) + e– → Cu (s); Eo = 0.521 V
a. –1.321 V
b. +1.321 V
c. +2.363 V
d. –2.363 V
+1.321 V
Reason: Eocell = Eocathode (red. larger #) – Eoanode (oxi. smaller #)
The cell potential for the standard hydrogen electrode (SHE) is by the definition equal to ____ and is used as the reference for other half-cells
a. +0.94 V
b. 0.00 V
c. +1.00 V
d. –1.00 V
0V
The ____ cell potential (Ecellcircle) is measured w/ gases at a pressure of 1atm, solutions at M and solids/liquids in their pure states
standard
Ca (s) + Mg2+ (aq) → Ca2+ (aq) + Mg (s); Eocell = 0.50 V
Mg2+ (aq) + 2e– → Mg (s); Eomagnesium = –2.37 V
Using the standard cell potential given above and the standard reduction potential listed above, calculate the reduction potential of the half-reaction Ca2+ (aq) + 2e– → Ca (s).
a. –1.87 V
b. +2.87 V
c. –2.87 V
d. +1.87 V
-2.87 V
If we were to solve for the cell potential of 2Ag+ (aq) + Zn (s) → 2Ag (s) + Zn2+ (aq), which 2 reactions would we first need to find in a standard reduction table?
a. Ag+ (aq) + e– → Ag (s)
b. Zn2+ (aq) + 2e– → Zn (s)
c. Zn (s) → Zn2+ (aq) + 2e–
d. Ag (s) → Ag+ (aq) + e–
Ag+ (aq) + e– → Ag (s) and Zn2+ (aq) + 2e– → Zn (s)
A standard electrochemical cell is constructed from an anode that has a standard reduction potential of –2.0 V and a cathode that has a standard reduction potential of +0.5 V. Which of the following is the correct expression to calculate the standard cell potential?
a. +0.5 + 2.0
b. –2.0 + 0.5
c. –2.0 - 0.5
d. 0.5 – 2.0
+0.5 + 2.0
Ag (s) + Li+ (aq) → Ag+ (aq) + Li (s)
Calculate the standard cell potential for the above reaction given the following information:
Li+ (aq) + e– → Li (s); Eo = –3.05 V
Ag+ (aq) + e– → Ag (s); Eo = 0.799 V
a. –2.251 V
b. +3.849 V
c. –3.849 V
d. +2.251 V
–3.849 V
Reason: .799-3.05
The cell potential for the standard hydrogen electrode (SHE) is by definition equal to ____
volts. If this half-cell is used as the anode and all of the substances in the cathode are under standard conditions, then the potential of the cell is equal to the standard ____ potential of the cathode.
0; reduction
Fe (s) + Hg22+ (aq) → Fe2+ (aq) + 2Hg (l); Eo = 1.229 V
Hg22+ (aq) + 2e– → 2Hg (l); Eo = +0.789 V
Using the standard cell potential given above and the standard reduction potential listed above, calculate the standard reduction potential for the half-reaction Fe2+ (aq) + 2e– → Fe (s).
a. +2.018 V
b. +0.440 V
c. –2.018 V
d. –0.440 V
–0.440 V
Reason: 1.229V=.789-x
