A)How many moles of electrons are transferred in the following balanced reaction

Question

A)How many moles of electrons are transferred in the following balanced reaction?

3 As2O3 + 4 NO3? + 7 H2O + 4 H+ ? 6 H3AsO4 + 2 N2O2

B) Which of the following statements are true for a voltaic cell in which the following reaction occurs:

              2 Fe(s) + Cr2O72?(aq)+ 14H+(aq) à 2Cr3+(aq) + 7H2O(l) + 2 Fe3+(aq)

I: The anode electrode is inactive

II: The cathode electrode decreases in mass

III: Without a salt bridge, the anode would have a buildup of cations

A) Only I          B) Only II         C) Only III                     D) More than one are true                        E) None are true

C) What is the balanced reaction that occurs in a cell designated by the following notation?

Pt(s)?NO(g)?NO3?(aq), H+(aq)??MnO42?(aq), MnO4?(aq)?Pt(s)

D)Using the half-reaction method, balance the following reaction in basic solution so that the coefficients are the lowest whole numbers: H2O2(aq) + Cl2O7(aq) ? ClO2?(aq) + O2(g). How many water molecules are in the overall balanced reaction?

Explanation / Answer

A) Write the skeletons of the oxidation and reduction half-reactions. (The skeleton reactions contain the formulas of the compounds oxidized and reduced, but the atoms and electrons have not yet been balanced.) See Example.

As2O3 ? H3AsO4
NO3- ? NO

Balance all elements other than H and O.
Balance As by making 2 H3AsO4 on the right side.
As2O3 ? 2 H3AsO4

Balance the oxygen atoms by adding H2O molecules where needed.
5 H2O + As2O3 ? 2 H3AsO4
NO3- ? NO + 2 H2O

Balance the hydrogen atoms by adding H+ ions where needed.
5 H2O + As2O3 ? 2 H3AsO4 + 4 H+
4 H+ + NO3- ? NO + 2 H2O

Balance the charge by adding electrons, e-.
5 H2O + As2O3 ? 2 H3AsO4 + 4 H+ + 4 e-
3 e- + 4 H+ + NO3- ? NO + 2 H2O

To balance the electrons, multiply the top equation by 3 and the bottom equation by 4.
15 H2O + 3 As2O3 ? 6 H3AsO4 + 12 H+ + 12 e-
This is the oxidation half reaction.
12 e- + 16 H+ + 4 NO3- ? 4 NO + 8 H2O
This is the reduction half reaction.

So total 12 electrons are transferred.

and number of moles of electron transferred = 12* 6.023*10-23

= 72.27 *10-23 moles of electrons

B) Answer is C- only III

C)

Oxidation : NO -----------> NO3-

Reduction: MnO4-   --------------> MnO42-

balancing oxidation half reaction :

NO   ------------> NO3-

+2                         +5    ---------------> oxidation state. so 3 electrons transfer

NO + 2 H2O   --------------> NO3-

NO + 2 H2O -------------> NO3- + 4 H+

NO + 2 H2O -------------> NO3- + 4 H+ + 3e- ---------------> eq 1

balancing reduction half reaction :

MnO4-   --------------> MnO42-

+7                                 +6   --------> oxidation state ,

(MnO4- + e- ------------> MnO42- ) x 3

3 MnO4- + 3 e- ------------> 3 MnO42-   -----------------> eq 2

now add both the equations :

NO + 2 H2O -------------> NO3- + 4 H+ + 3e-

3 MnO4- + 3 e- ------------> 3 MnO42-

_________________________________________________

NO + 2 H2O + 3 MnO4? -------------> NO3? + 3 MnO42? + 4 H+

balanced equation :

NO + 2 H2O + 3 MnO4? -------------> NO3? + 3 MnO42? + 4 H+

D) There are 2 water molecules in overall reaction.

H2O2 + ClO4- ? O2 + ClO2-

2H2O2 + ClO4- ? 2O2 + ClO2- + 2H2O

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