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Ch.5 - Thermochemistry
Chapter 5, Problem 126

The methane molecule, CH4, has the geometry shown in Figure 2.17. Imagine a hypothetical process in which the methane molecule is 'expanded,' by simultaneously extending all four C—H bonds to infinity. We then have the process CH41g2¡C1g2 + 4 H1g2 (a) Compare this process with the reverse of the reaction that represents the standard enthalpy of formation of CH41g2.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Enthalpy of Formation

The standard enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. For methane (CH4), this involves the reaction of carbon (C) and hydrogen (H2) to produce CH4. Understanding this concept is crucial for comparing the enthalpy changes in the given process and its reverse.
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Enthalpy of Formation

Bond Energy

Bond energy is the amount of energy required to break a bond between two atoms. In the context of the methane molecule, extending the C—H bonds to infinity implies breaking these bonds, which requires energy input. This concept is essential for analyzing the energy changes associated with the hypothetical process described in the question.
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Thermodynamic Processes

Thermodynamic processes refer to the changes in energy and matter that occur during chemical reactions. The comparison of the hypothetical expansion of methane with the reverse reaction of its formation involves understanding how energy is absorbed or released in these processes. This concept helps in evaluating the overall energy balance and directionality of the reactions.
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Related Practice
Textbook Question
Consider the following acid-neutralization reactions involving the strong base NaOH(aq): HNO31aq2 + NaOH1aq2¡NaNO31aq2 + H2O1l2 HCl1aq2 + NaOH1aq2¡NaCl1aq2 + H2O1l2 NH4+1aq2 + NaOH1aq2¡NH31aq2 + Na+1aq2 + H2O1l2 (d) In the third equation NH4 +1aq2 is acting as an acid. Based on the value of H° for this reaction, do you think it is a strong or a weak acid? Explain.
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Textbook Question

Consider two solutions, the first being 50.0 mL of 1.00 M CuSO4 and the second 50.0 mL of 2.00 M KOH. When the two solutions are mixed in a constant-pressure calorimeter, a precipitate forms and the temperature of the mixture rises from 21.5 to 27.7 °C. (a) Before mixing, how many grams of Cu are present in the solution of CuSO4?

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Textbook Question
A sample of a hydrocarbon is combusted completely in O21g2 to produce 21.83 g CO21g2, 4.47 g H2O1g2, and 311 kJ of heat. (a) What is the mass of the hydrocarbon sample that was combusted?
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Textbook Question
One of the best-selling light, or low-calorie, beers is 4.2% alcohol by volume and a 355-mL serving contains 110 Calories; remember: 1 Calorie = 1000 cal = 1 kcal. To estimate the percentage of Calories that comes from the alcohol, consider the following questions. (a) Write a balanced chemical equation for the reaction of ethanol, C2H5OH, with oxygen to make carbon dioxide and water. (b) Use enthalpies of formation in Appendix C to determine ΔH for this reaction. (c) If 4.2% of the total volume is ethanol and the density of ethanol is 0.789 g/mL, what mass of ethanol does a 355-mL serving of light beer contain? (d) How many Calories are released by the metabolism of ethanol, the reaction from part (a)? (e) What percentage of the 110 Calories comes from the ethanol?
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