Chemistry: The Molecular Nature of Matter and Change
Chemistry: The Molecular Nature of Matter and Change
8th Edition
ISBN: 9781259631757
Author: Martin Silberberg Dr., Patricia Amateis Professor
Publisher: McGraw-Hill Education
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Chapter 9, Problem 9.90P

(a)

Interpretation Introduction

Interpretation:

ΔHrxn° for the formation of dimethyl ether (CH3OCH3) and ethanol (CH3CH2OH) is to be calculated.

Concept introduction:

The heat of the reaction (ΔHrxn°) is defined as the heat released or absorbed during a chemical reaction as a result of the difference in the bond energies (BE) of reactant and product in the reaction. ΔHrxn° is negative for exothermic reaction and ΔHrxn° is positive for an endothermic reaction.

The formula to calculate ΔHrxn° of reaction is as follows:

ΔHrxn°=ΔHreactant bond broken°+ΔHproduct bond formed°

Or,

ΔHrxn°=BEreactant bond brokenBEproduct bond formed

The bond energy of reactants is positive and the bond energy of products is negative.

(a)

Expert Solution
Check Mark

Answer to Problem 9.90P

ΔHrxn° for the formation of dimethyl ether (CH3OCH3) and ethanol (CH3CH2OH) is 326kJ and 369kJ respectively.

Explanation of Solution

The given chemical equation for the formation of dimethyl ether (CH3OCH3) is as follows:

Chemistry: The Molecular Nature of Matter and Change, Chapter 9, Problem 9.90P , additional homework tip  1

The number of broken bonds is 8 CH bond and 1 O=O bonds.

The number of bonds formed is 6 CH bonds, 2 CO and 2 OH bonds.

The formula to the enthalpy of the given reaction is as follows:

ΔHrxn°=(8BECH+1BEO=O)(6BECH+2BECO+2BEOH) (1)

Substitute 413kJ/mol for BECH, 498kJ/mol for BEO=O, 358kJ/mol for BECO and 467kJ/mol for BEOH in the equation (1).

ΔHrxn°=[((8 mol)(413kJ/mol)+(1 mol)(498kJ/mol))(6 mol)(413kJ/mol)+(2 mol)(358kJ/mol)+(2 mol)(467kJ/mol)]=326kJ

The given chemical equation for the formation of ethanol (CH3CH2OH) is as follows:

Chemistry: The Molecular Nature of Matter and Change, Chapter 9, Problem 9.90P , additional homework tip  2

The number of broken bonds is 8 CH bond and 1 O=O bonds.

The number of bonds formed is 5 CH bonds, 1 CO, 1 CC and 3 OH bonds.

The formula to the enthalpy of the given reaction is as follows:

ΔHrxn°=(8BECH+1BEO=O)(5BECH+1BECO+1BECC+3BEOH) (2)

Substitute 413kJ/mol for BECH, 498kJ/mol for BEO=O, 358kJ/mol for BECO, 347kJ/mol for BECC and 467kJ/mol for BEOH in the equation (2).

ΔHrxn°=[((8 mol)(413kJ/mol)+(1 mol)(498kJ/mol))(5 mol)(413kJ/mol)+(1 mol)(358kJ/mol)+(1 mol)(347kJ/mol)+(2 mol)(467kJ/mol)]=369kJ

Conclusion

ΔHrxn° for the formation of dimethyl ether (CH3OCH3) and ethanol (CH3CH2OH) is 326kJ and 369kJ respectively.

(b)

Interpretation Introduction

Interpretation:

Among the formation reaction of dimethyl ether and ethanol, the more exothermic reaction is to be identified.

Concept introduction:

In the case of a reaction, the change in enthalpy (ΔH) is the difference in the energy of the product and reactant. The general expression to calculate ΔH is,

ΔH=HProductHReactant (1)

Here,

ΔH is the change in enthalpy of the system.

HProduct is the enthalpy of the products.

HReactant is the enthalpy of the reactants.

Endothermic reactions are the reactions in which energy in the form of the heat or light is absorbed by the reactant for the formation of the product. HProduct is greater than HReactant in the endothermic reactions.

Exothermic reactions are the reactions in which energy in the form of the heat or light is released with the product. HReactant is greater than HProduct in the exothermic reactions.

(b)

Expert Solution
Check Mark

Answer to Problem 9.90P

The formation reaction of ethanol is more exothermic as compared to dimethyl ether.

Explanation of Solution

ΔHrxn° for the formation of dimethyl ether (CH3OCH3) is 326kJ.

ΔHrxn° for the formation of ethanol (CH3CH2OH) is 369kJ.

The value of ΔHrxn° for the formation of ethanol (CH3CH2OH) is more negative as compared to dimethyl ether (CH3OCH3) so more energy is released in the case of ethanol (CH3CH2OH) than dimethyl ether. Therefore the formation reaction of ethanol (CH3CH2OH) is more exothermic.

Conclusion

Exothermic reactions are the reactions in which energy in the form of the heat or light is released with the product. HReactant is greater than HProduct in the exothermic reactions.

(c)

Interpretation Introduction

Interpretation:

ΔHrxn° for the conversion of ethanol to dimethyl ether is to be calculated.

Concept introduction:

Hess’s law is used to calculate the enthalpy change of an overall reaction that can be derived as a sum of two or more reaction. According to Hess’s law ΔH of an overall reaction is equal to the sum of the enthalpy change for each individual reaction. ΔHoverall rxn=ΔH1+ΔH2+.......+ΔHn

Enthalpy is a state function so the value depends upon the initial state and final state not on the path so ΔH of an overall reaction can be calculated by the addition or subtraction of the individual steps whose ΔH is known.

(c)

Expert Solution
Check Mark

Answer to Problem 9.90P

ΔHrxn° for the conversion of ethanol to dimethyl ether is 43kJ.

Explanation of Solution

The enthalpy change of the following reaction is ΔH1°.

2CH4(g)+O2(g)CH3OCH3(g)+H2O(g) (1)

The enthalpy change of the following reaction is ΔH2°.

2CH4(g)+O2(g)CH3CH2OH(g)+H2O(g) (2)

Reverse the equation (2).

CH3CH2OH(g)+H2O(g)2CH4(g)+O2(g) (3)

The enthalpy change for the reaction (3) is calculated as,

ΔH3°=ΔH2°

Add equation (1) and (3).

CH3CH2OH(g)CH3OCH3(g) (4)

The enthalpy change of the final reaction (4) is ΔHrxn°.

The expression to calculate ΔHrxn° is as follows:

ΔHrxn°=ΔH1°+ΔH3° (5)

Substitute 326kJ for ΔH1° and +369kJ for ΔH3° in the equation (5).

ΔHrxn°=326kJ+369kJ=43kJ

Conclusion

ΔHrxn° for the conversion of ethanol to dimethyl ether is 43kJ.

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Chapter 9 Solutions

Chemistry: The Molecular Nature of Matter and Change

Ch. 9.5 - Prob. 9.5AFPCh. 9.5 - Prob. 9.5BFPCh. 9 - Prob. 9.1PCh. 9 - Prob. 9.2PCh. 9 - What is the relationship between the tendency of a...Ch. 9 - Prob. 9.4PCh. 9 - Prob. 9.5PCh. 9 - State the type of bonding—ionic, covalent, or...Ch. 9 - State the type of bonding—ionic, covalent, or...Ch. 9 - State the type of bonding—ionic, covalent, or...Ch. 9 - Prob. 9.9PCh. 9 - Prob. 9.10PCh. 9 - Prob. 9.11PCh. 9 - Prob. 9.12PCh. 9 - Prob. 9.13PCh. 9 - Give the group number and condensed electron...Ch. 9 - Give the group number and condensed electron...Ch. 9 - Prob. 9.16PCh. 9 - Prob. 9.17PCh. 9 - Prob. 9.18PCh. 9 - Prob. 9.19PCh. 9 - Prob. 9.20PCh. 9 - Prob. 9.21PCh. 9 - Prob. 9.22PCh. 9 - Prob. 9.23PCh. 9 - Prob. 9.24PCh. 9 - Prob. 9.25PCh. 9 - For each pair, choose the compound with the larger...Ch. 9 - Prob. 9.27PCh. 9 - For each pair, choose the compound with the...Ch. 9 - Prob. 9.29PCh. 9 - Use the following to calculate of NaCl: Compared...Ch. 9 - Use the following to calculate of MgF2: Compared...Ch. 9 - Prob. 9.32PCh. 9 - Born-Haber cycles were used to obtain the first...Ch. 9 - Prob. 9.34PCh. 9 - Prob. 9.35PCh. 9 - Prob. 9.36PCh. 9 - How does the energy of the bond between a given...Ch. 9 - When liquid benzene (C6H6) boils, does the gas...Ch. 9 - Prob. 9.39PCh. 9 - Prob. 9.40PCh. 9 - Prob. 9.41PCh. 9 - Prob. 9.42PCh. 9 - The text points out that, for similar types of...Ch. 9 - Why is there a discrepancy between an enthalpy of...Ch. 9 - Which of the following gases would you expect to...Ch. 9 - Which of the following gases would you expect to...Ch. 9 - Use bond energies to calculate the enthalpy of...Ch. 9 - Prob. 9.48PCh. 9 - Prob. 9.49PCh. 9 - Prob. 9.50PCh. 9 - Prob. 9.51PCh. 9 - What is the general relationship between IE1 and...Ch. 9 - Is the H—O bond in water nonpolar covalent, polar...Ch. 9 - Prob. 9.54PCh. 9 - How is the partial ionic character of a bond in a...Ch. 9 - Using the periodic table only, arrange the...Ch. 9 - Prob. 9.57PCh. 9 - Prob. 9.58PCh. 9 - Prob. 9.59PCh. 9 - Prob. 9.60PCh. 9 - Use Figure 9.21 to indicate the polarity of each...Ch. 9 - Prob. 9.62PCh. 9 - Prob. 9.63PCh. 9 - Prob. 9.64PCh. 9 - Prob. 9.65PCh. 9 - Prob. 9.66PCh. 9 - Prob. 9.67PCh. 9 - Prob. 9.68PCh. 9 - Prob. 9.69PCh. 9 - Prob. 9.70PCh. 9 - Prob. 9.71PCh. 9 - Geologists have a rule of thumb: when molten rock...Ch. 9 - Prob. 9.73PCh. 9 - Use Lewis electron-dot symbols to represent the...Ch. 9 - Prob. 9.75PCh. 9 - Prob. 9.76PCh. 9 - By using photons of specific wavelengths, chemists...Ch. 9 - Prob. 9.78PCh. 9 - Prob. 9.79PCh. 9 - Prob. 9.80PCh. 9 - Prob. 9.81PCh. 9 - Prob. 9.82PCh. 9 - Prob. 9.83PCh. 9 - Find the longest wavelengths of light that can...Ch. 9 - The work function (ϕ) of a metal is the minimum...Ch. 9 - Prob. 9.86PCh. 9 - Prob. 9.87PCh. 9 - Prob. 9.88PCh. 9 - In a future hydrogen-fuel economy, the cheapest...Ch. 9 - Prob. 9.90PCh. 9 - Prob. 9.91P
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