Chapter 18, Problem 97SCQ

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074

Chapter
Section

### Chemistry & Chemical Reactivity

10th Edition
John C. Kotz + 3 others
ISBN: 9781337399074
Textbook Problem

# Oxygen dissolved in water can cause corrosion in hot-water heating systems. To remove oxygen, hydrazine (N2H4) is often added. Hydrazine reacts with dissolved O2 to form water and N2. (a) Write a balanced chemical equation for the reaction of hydrazine and oxygen. Identify the oxidizing and reducing agents in this redox reaction. (b) Calculate ΔrH°, ΔrS°, and ΔrG° for this reaction involving 1 mol of N2H4 at 25 °C. (c) Because this is an exothermic reaction, energy is evolved as heat. What temperature change is expected in a heating system containing 5.5 × 104 L of water? (Assume no energy is lost to the surroundings.) (d) The mass of a hot-water heating system is 5.5 × 104 kg. What amount of O2 (in moles) would be present in this system if it is filled with water saturated with O2? (The solubility of O2 in water at 25 °C is 0.000434 g per 100 g of water.) (e) Assume hydrazine is available as a 5.0% solution in water. What mass of this solution should be added to totally consume the dissolved O2 [described in part (d)]? (f) Assuming the N2 escapes as a gas, calculate the volume of N2(g) (measured at 273 K and 1.00 atm) that will be produced.

(a)

Interpretation Introduction

Interpretation:

The balanced chemical equation for the given reaction between hydrazine and oxygen also the oxidizing and reducing agents in the reaction should be identified.

Concept introduction:

Redox reaction: It occurs when oxidation and reduction takes place at the same time in a chemical reaction.

Chemical equation is the representation of a chemical reaction, in which the reactants and products of the reactions are represented left and right side of an arrow respectively by using their respective chemical formulas.

Reactant of a chemical reaction is the substrate compounds or the compounds which undergo a chemical reaction.

Product of a chemical reaction is the produced compounds or the compounds formed after a chemical reaction.

Balanced chemical equation of a reaction is written according to law of conservation of mass.

Stoichiometry of a chemical reaction is the relation between reactants and products of the reaction and it is represented by the coefficients used for the reactants and products involved in the chemical equation.

The hot water system can corrode because of the presence of dissolved oxygen in water. This dissolved oxygen leads to corrosion. However, this oxygen can be removed from water with the use of hydrazine. The formula for hydrazine is N2H4. It reacts with oxygen to produce water and N2.

Explanation

The formula for hydrazine is N2H4. It reacts with oxygen to produce water and N2.

The balanced chemical equation for the reaction of hydrazine and oxygen is,

N2H4(l)+O

(b)

Interpretation Introduction

Interpretation:

The value of ΔrH°, ΔrS° and ΔrGo for the given reaction of 1molofN2H4at25oc should be identified.

Concept introduction:

The Gibbs free energy or the free energy change is a thermodynamic quantity represented by ΔGo. It is related to entropy and entropy by the following expression,

ΔGo= ΔHo- TΔSo

The sign of ΔGo should be positive for a product-favored reaction. Thus, spontaneous reactions are referred to those that have negative free energy formation.

The sign of ΔSo should be positive for an entropy-favoured reaction and the sign of ΔH should be negative for an enthalpy favoured-reaction.

ΔrHo is negative for an exothermic reaction and positive for endothermic reactions. The entropy change is positive whenever number of moles of gases is increasing in any reaction.

(c)

Interpretation Introduction

Interpretation:

The change in temperature expected in heating system that has 5.5×104L of water should be identified.

Concept introduction:

The Gibbs free energy or the free energy change is a thermodynamic quantity represented by ΔGo. It is related to entropy and entropy by the following expression,

ΔGo= ΔHo- TΔSo

The sign of ΔGo should be positive for a product-favored reaction. Thus, spontaneous reactions are referred to those that have negative free energy formation.

The sign of ΔSo should be positive for an entropy-favoured reaction and the sign of ΔHo should be negative for an enthalpy favoured-reaction.

ΔrHo is negative for an exothermic reaction and positive for endothermic reactions. The entropy change is positive whenever number of moles of gases is increasing in any reaction.

(d)

Interpretation Introduction

Interpretation:

The amount of O2 present in hot water heating system of 5.5×104kg should be identified.

Concept introduction:

Moles: One mole is equivalent to the mass of the substance consists same number of units equal to the atoms present in 12g of 12C.

From given mass of substance moles could be calculated by using the following formula,

Molesofsubstance GivenmassofsubstanceMolecularmass

(e)

Interpretation Introduction

Interpretation:

The mass of solution that has 5% dissolved hydrazine in water required to totally consume the dissolved amount of O2.

Concept introduction:

Moles: One mole is equivalent to the mass of the substance consists same number of units equal to the atoms present in 12g of 12C.

From given mass of substance moles could be calculated by using the following formula,

Molesofsubstance GivenmassofsubstanceMolecularmass

Mass: It is the quantitative measure of a substance. The amount of matter present in substance is expressed as mass. The S.I. unit of mass is kg.

(f)

Interpretation Introduction

Interpretation:

The volume of N2 produced under given conditions should be calculated.

Concept introduction:

Moles: One mole is equivalent to the mass of the substance consists same number of units equal to the atoms present in 12g of 12C.

From given mass of substance moles could be calculated by using the following formula,

Molesofsubstance GivenmassofsubstanceMolecularmass

Mass: It is the quantitative measure of a substance. The amount of matter present in substance is expressed as mass. The S.I. unit of mass is kg.

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