Top 5 Most-Missed Questions, Exam III, Spring 2000

Learn from your mistakes on these. Expect to see problems similar to these on the final exam.

1. Question 6: For which one of the following does G^o for the reaction correspond to the standard free energy of formation for the product?
   1. BF₃(g) + NH₃(g) BF₃NH₃
   2. H₂O(g) + 1/2 O₂(g) H₂O₂(l)
   3. 3 H₂(g) + N₂(g) 2 NH₃(g)
   4. P(s) + 1/2 Cl₂(g) PCl₃(l)

      This is a basic definition which is identical in form to the standard enthalpy of formation, something you had in 105 as well. A version of this question has appeared on almost every 105/107 exam since time began. Check out Question 22 from the Spring 1999 exam, Question 4 from the Spring 1998 exam or Question 2 from the Fall 1998 exam. In section 002 I warned the class that there was always a problem like this on the exam. Still, only 23% of you got this correct.

      If you know definition of standard free energy of formation is the free energy change that occurs when ONE mole of the compound is synthesized from its ELEMENTS in their STANDARD STATES the problem is easy.

      We can rule out answer C (the most chosen one) right away because it forms 2 moles of product, not one. We rule out answers A and B because the compounds are not formed from (only) elements. That only leaves answer D...elements, one mole...yep!

2. Question 18: Which one of the following statements is TRUE?
   1. The only metal to undergo corrosion is iron.
   2. Only water needs to be present for iron to rust.
   3. Attaching a piece of magnesium metal to iron prevents rusting of the iron.
   4. Zinc plated iron (galvinized iron) does not prevent the rusting of iron once the zinc coating is scratched and the iron underneath is exposed.

      Answers A is obviously false. Answer B is clearly false to anyone that read the book or came to class. That leaves answers C and D. The correct answer is C. This was done as a demonstration in class with a nail wrapped in Mg ribbon, an analog of this statement is shown in Figure 19.15 of Chang, and this statement itself is demonstrated in Figure 19.16 of Chang.

      Answer D is false. To quote your book as it discusses galvanized iron "So even if a scratch exposes the iron, the zinc is still attacked. In this case, the zinc metal serves as the anode and the iron is the cathode." Answer D would be true if the coating was not more easily oxidized than iron -- paint, for example.

3. Question 16: An electrochemical cell based on the following reaction had a cell voltage of 0.00 volts at 25 ^oC:

   Cd²⁺(aq) + Fe(s) Cd(s) + Fe²⁺

   If the initial [Cd²⁺] was 0.050 M, what was the initial [Fe²⁺]?

   A. 0.024 M

   B. 0.24 M

   C. 0.76 M

   D. 1.1 M

   1. We are clearly not in the standard state as the [Cd²⁺] was 0.050 M, not 1 M. The only equation we know that deals with non-standard state conditions is the Nernst equation, one that was given to you on the exam itself:

      

      What quantities do we know here? We are given E = 0.00 V. E^o and n can be determined from the balanced reaction and the table of reduction potentials. Q, the reaction quotient, is going to be products over reactants as always (and solids won't appear) so Q = [Fe²⁺] divided by [Cd²⁺].

      Solving for E^o simply requires knowing that E^o_cell = E^o_ox + E^o_red. The Cd is being reduced and the iron oxidized. Using Table I from the exam, E^o_cell = 0.44 V + (-0.40 V) = 0.04 V. As both half-reactions involve two electrons we don't need to multiply them by anything to cancel the electrons and so n = 2.

      Substituting into the Nernst Equation we get:

      

      This is clearly Answer D.

4. Question 19: Which of the following statements is FALSE?
   1. Electrolysis of molten NaCl is used commercially to make sodium metal and chlorine gas.
   2. Electrolysis is used to make non-spontaneous reactions occur.
   3. Electrolysis requires the presence of ions in solution to carry the electric current.
   4. The electrolysis of a 0.1 M H₂SO₄ solution produces hydrogen gas and SO₃.

      Statement A is clearly true as demonstrated by Figure 19.17 of your textbook and the accompanying (detailed) discussion...as well as emphasis given in class. Not to be mean, but how could 21% of you have picked this option?

      Statement B is the definition of electrolysis, so it better be true!

      Statement C is also true as is discussed in considerable detail in your book in the subsection titled "Electrolysis of Water".

      Statement D is false. This is also discussed extensively in the section of the chapter titled "Electrolysis of Water". But even if you forgot that, think about it. If this were true, you're saying that H⁺ must be reduced to hydrogen...so something else must be oxidized. But both SO₄^2- and SO₃ have the same oxidation state of 6+, so this will not be a product of our electrochemical (redox) reaction.

5. Question 14: Which of these reagents is capable of reducing Al³⁺(aq) to Al(s) under standard conditions?

   A. Mg^2+(aq)

   Mg(s)

   Fe(s)

   Ag^+(aq)

   For this to happen (be spontaneous), the sum of the oxidation and reduction reactions (E^o_cell) must be a positive number. If E is positive, then G is negative = spontaneous by the equation G = -nFE.

   The reduction potential for Al³⁺(aq) is -1.66 V according to Table 1. Therefore, anything with an oxidation potential greater than +1.66 V will be able to reduce Al³⁺(aq).

   Both answers A and D are clearly false as these species can't even be oxidized (remember that if we reduce Al³⁺(aq), we have to be oxidizing something else).

   That leaves B and C. Only answer B has an oxidation potential greater than +1.66 V.