
Concept explainers
13.106 Hydrazine, N2H4 , has been proposed as the fuel in a fuel cell in which oxygen is the oxidizing agent. The reactions are
N2H4(aq)+4 OH-(aq)→N2(g)+4 H2O(l)+4 e−O2(g)+2 H2O(l)+4 e−→4 OH-(aq)
(a) Which reaction occurs at the anode and which at the cathode?
(b) What is the net cell reaction?
(C) If the cell is to produce 0.50 A of current for 50.0 h, what mass in grams of hydrazine must be present?
(d) What mass in grams of O2 must be available to react with the mass of N2H4 determined in part (c)?
(b)

Interpretation:
To identify the net reaction.
Concept introduction:
- The net reaction is the total reaction including all reactants and products.
- It is obtained via addition of the half reactions.
Answer to Problem 13.106PAE
Solution:
Explanation of Solution
In order to find out the reaction, we must include the reduction-oxidation reactions. They always take place together, since one donates electrons and the other receives them.
In this case, the oxidation reaction: hydrazine oxidation to nitrogen gas. The reduction is oxygen forming hydroxide ion via the addition of electron
Adding both reactions, getting rid of repeating/same units on the left/right side of the equation:
Therefore, the reaction taking place is:
(c)

Interpretation:
To calculate the mass of hydrazine present in given amount of time and current.
Concept introduction:
- A stands for ampere, the amount of current.
- Time and current can be used to calculate total charge.
- The total charge relates mol of e- and Faraday constant.
Answer to Problem 13.106PAE
Solution:
7.46 g
Explanation of Solution
This consist of the following steps:
1st Step: Calculate the total amount of charge.
t = 50.0 h = 50 h ×3600 s1 h = 180,000 sI = 0.50 A = 0.50 C/sq = I×t = 0.50 C/s × 180,000 s = 90,000 C1 mol e- =96500 Cx mol e- = 90000 Cx = 0.9326 mol e-
2nd Step: Calculate the total amount of moles of hydrazine
4 mol e- = 1 mol N2H40.9326 mol e- = y mol N2H4y =0.2332 mol N2H4
3rd Step: Calculate the mass of hydrazine
mass N2H4 = mol × Molar Mass = 0.2332 mol × 32.0 g/mol = 7.46 g
(d)

Interpretation:
To calculate the mass oxygen required
Concept introduction:
- Use stoichiometric ratios.
Answer to Problem 13.106PAE
Solution:
mass O2 = 7.46 g
Explanation of Solution
Relate the moles of oxygen and hydrazine:
1st Step: Calculate total moles of oxygen required
The ratio is 1:1 since 1 mol of hydrazine requires 1 mol of Oxygen gas to react.
2nd Step: Calculate the total mass of oxygen gas
mass O2 = mol × Molar Mass = 0.2332 mol × 32.0 g/mol = 7.46 g
The mass is exactly the same as hydrazine since they have 1:1 ratio and similar molar masses.
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Chapter 13 Solutions
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