Reduced or oxidized?
What is the oxidation number of chromium, Cr, in Cr2O72-?
+6
The oxidation of lead metal by oxygen causes corrosion of lead pipes and the leaching of dangerous amounts of Pb2+(aq) into tap water. Calculate E°cell for the oxidation of Pb(s) by O2(g).
Pb2+ (aq) + 2e– → Pb(s) Ered = –0.13
O2 (g) + 2H2O(l) + 4e– → 4OH– (aq) Ered = +0.40
True or false: for a spontaneous process, ΔSsystem and ΔSsurroundings must both be positive values
FALSE
For a reaction to be spontaneous at all temperatures, ∆Hrxn must be ________ and ∆Srxn must be __________.
negative, positive
How many coulombs are delivered if 5 amps are applied for 30 minutes in an electrolysis reaction?
9000 C
Balance the reaction below in acidic media. How many protons are there?
(Hint: balance the entire reaction.)
___ Cu(s) + ___ NO3– (aq) + ___ H+(aq) →
___ Cu2+ (aq) + ___ NO2(aq) + ___ H2O(l)
4 H+ (aq)
A concentration cell is built by using the Zn/Zn2+ system. In one half cell, a zinc metal strip is immersed in a solution of 0.010 M Zn2+ (aq), in the other half-cell, a zinc metal strip is immersed in a solution of 0.10 M Zn2+ (aq). What is the Ecell?
Zn2+ (aq) + 2e− → Zn(s) Ered = −0.76 V
0.0296 V
Which gaseous substance has the highest standard entropy?
CH4, C2H6, or C3H6
C3H6
The normal boiling point of C2Cl3F3 is 47.6°C and its molar enthalpy of vaporization is 27.49 kJ/mol. What is the change in entropy in the system in J/K when 24.1 grams of C2Cl3F3 vaporizes to a gas at the normal boiling point? C2Cl3F3, molar mass: 187.5 g/mol
11.0 J/K
How long does it take to deposit 1.00 g of copper metal from the electrolysis of molten copper (II) chloride, CuCl2, with an electrical current of 12.0 A?
253 seconds
When a strip of zinc metal, Zn(s), is placed in a solution of copper sulfate, the redox reaction,
Zn(s) + Cu2+ (aq) → Zn2+ (aq) + Cu(s), takes place spontaneously.
Which species in this redox reaction is the strongest oxidizing agent?
Cu2+ (aq)
A galvanic cell is created based on the following half reactions (written as reduction reactions). In this galvanic cell, what reaction will occur at the cathode?
Ni2+ + 2e− → Ni(s), E°red = −0.28V
Mg2+ (aq) + 2e− → Mg(s), E°red = −2.37 V
Reduction of nickel
Which of the following physical processes causes the largest increase of the entropy of the system?
a. Melting
b. Freezing
c. Vaporization
d. Condensation
Vaporization
Consider the reaction exothermic reaction,
SF4 (g) + F2 (g) ⟶ SF6 (g).
At what relative temperatures is this reaction spontaneous?
only at low temps
Consider the reaction, N2 (g) + 3F2 (g) ⇌ 2NF3 (g), at 800 K. At equilibrium, PN2 = 0.021 atm, PF2 = 0.063 atm, and PNF3 = 0.45 atm. What is ΔG° of the reaction?
-70.2 kJ
A galvanic cell is created based on the following half reactions (written as reduction reactions). In this galvanic cell, what reaction will occur at the anode?
Ag 2+ + 2e− → Ag(s), E°red = +0.80 V
Pb2+ + 2e− → Pb(s), E°red = −0.13 V
Oxidation of lead
Consider the melting of some solid metal, M [M(s) → M(l)]. If the enthalpy of fusion is 45 kJ and the entropy of the surroundings for this phase change is +13 J/K, what is the melting point for this metal?
3460 K
For the combustion of methane, ΔH°rxn = –891 kJ and ΔS°rxn = –242 J/K.
Calculate the value of ΔG°rxn (recall that ° indicates standard conditions)
-819 kJ
Some of the species present in tap water are: molecular oxygen, O2, NaClO (aka bleach), and fluoride, F–. Which is the strongest oxidizing agent?
Fe2+ (aq) + 2e– → Fe(s) Ered = –0.44 V
O2 (g) + 2H2O(l) + 4e– → 4OH– (aq) Ered = +0.40
F2(g) + 2e– → 2F–(aq) Ered = +2.87
O2 (g)
Which of the following reactions will be spontaneous at all temperatures in the direction it is written? Select the best answer.
a. Ag+ (aq) + Cl− (aq) → AgCl(s), ∆H° = −66 kJ
b. N2 (g) + 3H2 (g) → 2NH3 (g), ∆H° = −81 kJ
c. 2NO2 (g) → 2NO(g) + O2 (g), ∆H° = +112 kJ
d. 2C(s) + O2 (g) → 2CO(g), ∆H° = −221 kJ
2C(s) + O2 (g) → 2CO(g), ∆H° = −221 kJ