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 9.75P

(a)

Interpretation Introduction

Interpretation:

ΔHf° of MgCl 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.

(a)

Expert Solution
Check Mark

Answer to Problem 9.75P

ΔHf° of MgCl is 125kJ.

Explanation of Solution

The enthalpy change of the following reaction is ΔH1°.

Mg(s)Mg(g) (1)

The enthalpy change of the following reaction is ΔH2°.

Cl2(g)2Cl(g) (2)

The enthalpy change of the following reaction is ΔH3°.

Mg(g)Mg+(g)+e (3)

The enthalpy change of the following reaction is ΔH4°.

Cl(g)+eCl(g) (4)

The enthalpy change of the following reaction is ΔHlattice°.

MgCl(s)Mg+(g)+Cl(g) (5)

Reverse the equation (5).

Mg+(g)+Cl(g)MgCl(s) (6)

The enthalpy change for the reaction (6) is calculated as,

ΔH5°=ΔHlattice°

Multiply the equation (2) by 12.

12Cl2(g)Cl(g) (7)

The enthalpy change for the reaction (7) is calculated as,

ΔH6°=12ΔH2°

Add equation (1),(3), (4), (6) and (7).

Mg(s)+12Cl2(g)MgCl(s) (8)

The enthalpy change of the final reaction (8) is ΔHf°.

The expression to calculate ΔHf° is as follows:

ΔHf°=ΔH1°+ΔH3°+ΔH4°+ΔH5°+ΔH6° (9)

Substitute 148kJ for ΔH1° and 121.5kJ for ΔH3°, 768kJ for ΔH4°, 349kJ for ΔH5° and 783.5kJ for ΔH6° n the equation (9).

ΔHf°=148kJ+121.5kJ+768kJ+(349kJ)+(783.5kJ)=125kJ

Conclusion

ΔHf° of MgCl is 125kJ.

(b)

Interpretation Introduction

Interpretation:

Whether MgCl is favoured energetically relative to Mg and Cl2 is to be determined.

Concept introduction:

The standard enthalpy of formation (ΔHf°) is the enthalpy change of the formation of 1mol of compound from its elements and the reactants and products, in this case, is also present in their standard state.

Standard state includes 1atm pressure for gases, 1M concentration for substances in the aqueous phase and the most stable form of substance at 1atm pressure and 298K temperature. For example, metal like sodium exists in solid state and molecular elements like halogen exist in a gaseous state.

(b)

Expert Solution
Check Mark

Answer to Problem 9.75P

MgCl is favoured energetically relative to Mg and Cl2 due to the negative value of heat of formation.

Explanation of Solution

The chemical equation for the formation of MgCl is as follows:

Mg(s)+12Cl2(g)MgCl(s)

The enthalpy of formation (ΔHf°) of MgCl is 125kJ.

When the enthalpy of formation is negative then the compound formed is relatively more stable as compared to the elements. Therefore, MgCl is relatively more stable as compared to Mg and Cl2.

Conclusion

When the enthalpy of formation is negative then the compound formed is relatively more stable as compared to the elements.

(c)

Interpretation Introduction

Interpretation:

ΔH° for the conversion of MgCl to MgCl2 and Mg is to be calculated.

Concept introduction:

The standard enthalpy of reaction is calculated by the summation of standard enthalpy of formation of the product minus the summation of standard enthalpy of formation of product at the standard conditions. The formula to calculate the standard enthalpy of reaction (ΔHrxn°) is as follows:

ΔHrxn°=mΔHf (products)°mΔHf (reactants)°

Here, m and n are the stoichiometric coefficients of reactants and product in the balanced chemical equation.

(c)

Expert Solution
Check Mark

Answer to Problem 9.75P

ΔH° for the conversion of MgCl to MgCl2 and Mg is 392kJ.

Explanation of Solution

The chemical equation for the formation of MgCl2 is as follows:

2MgCl(s)MgCl2(s)+Mg(s)

The formula to calculate the standard enthalpy of reaction (ΔH°) is as follows:

ΔH°=[{ΔHf°[MgCl2(s)]+ΔHf°[Mg(s)]}{2ΔHf°[MgCl(s)]}] (10)

Substitute 641.6kJ/mol for ΔHf°[MgCl2(s)], 0 for ΔHf°[Mg(s)] and 125kJ for ΔHf°[MgCl(s)] in the equation (10).

ΔH°=[{(1mol)(641.6kJ/mol)+(1mol)(0)}{(2mol)(125kJ)}]=391.6kJ392kJ

Conclusion

ΔH° for the conversion of MgCl to MgCl2 and Mg is 392kJ.

(d)

Interpretation Introduction

Interpretation:

Whether MgCl is favoured energetically relative to MgCl2 is to be determined.

Concept introduction:

The standard enthalpy of formation (ΔHf°) is the enthalpy change of the formation of 1mol of compound from its elements and the reactants and products, in this case, is also present in their standard state.

Standard state includes 1atm pressure for gases, 1M concentration for substances in the aqueous phase and the most stable form of substance at 1atm pressure and 298K temperature. For example, metal like sodium exists in solid state and molecular elements like halogen exist in a gaseous state.

(d)

Expert Solution
Check Mark

Answer to Problem 9.75P

MgCl2 is favoured energetically relative to MgCl due to the more negative value of heat of formation of MgCl2 as compared to MgCl.

Explanation of Solution

The chemical equation for the formation of MgCl2 is as follows:

2MgCl(s)MgCl2(s)+Mg(s)

The enthalpy of formation (ΔHf°) of MgCl is 125kJ.

The enthalpy of formation (ΔHf°) of MgCl2 is 392kJ.

The compound with the more negative enthalpy of formation is relatively more stable than the compound with the less negative enthalpy of formation. Therefore, MgCl2 is relatively more stable as compared to MgCl.

Conclusion

The compound with the more negative enthalpy of formation is relatively more stable as compared to the compound with the less negative enthalpy of formation.

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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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