Ch.5 - Thermochemistry

Brown - Chemistry: The Central Science 14th Edition

Brown14th EditionChemistry: The Central ScienceISBN: 9780134414232당신이 사용하는 게 아니라요?교과서 변경

Brown 14th Edition

Ch.5 - Thermochemistry

문제 96

5장, 문제 96

It is interesting to compare the 'fuel value' of a hydrocarbon in a hypothetical world where oxygen is not the combustion agent. The enthalpy of formation of CF₄(g) is -679.9 kJ/mol. Which of the following two reactions is the more exothermic?
CH₄(g) + 2 O₂(g) → CO₂(g) + 2 H₂O(g)
CH₄(g) + 4 F₂(g) → CF₄(g) + 4 HF(g)

검증된 단계별 안내

Identify the two reactions given: (1) CH_4(g) + 2 O_2(g) \(\rightarrow\) CO_2(g) + 2 H_2O(g) and (2) CH_4(g) + 4 F_2(g) \(\rightarrow\) CF_4(g) + 4 HF(g).

For each reaction, calculate the enthalpy change (\(\Delta\) H) using the formula: \(\Delta\) H = \(\sum\) \(\Delta\) H_f(\(\text{products}\)) - \(\sum\) \(\Delta\) H_f(\(\text{reactants}\)).

For reaction (1), use standard enthalpies of formation: \(\Delta\) H_f(CO_2(g)) = -393.5 \(\text{ kJ/mol}\), \(\Delta\) H_f(H_2O(g)) = -241.8 \(\text{ kJ/mol}\), and \(\Delta\) H_f(CH_4(g)) = -74.8 \(\text{ kJ/mol}\). Assume \(\Delta\) H_f(O_2(g)) = 0.

For reaction (2), use the given \(\Delta\) H_f(CF_4(g)) = -679.9 \(\text{ kJ/mol}\) and \(\Delta\) H_f(HF(g)) = -268.6 \(\text{ kJ/mol}\). Assume \(\Delta\) H_f(F_2(g)) = 0 and \(\Delta\) H_f(CH_4(g)) = -74.8 \(\text{ kJ/mol}\).

Compare the calculated \(\Delta\) H values for both reactions to determine which is more exothermic (more negative \(\Delta\) H).

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주요 개념

질문에 올바르게 답하기 위해 반드시 이해해야 하는 핵심 개념들은 다음과 같습니다.

Enthalpy of Formation

The enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. It is a crucial concept in thermodynamics, as it allows for the calculation of the overall energy change in chemical reactions. In this question, the enthalpy of formation of CF4(g) is provided, which will help determine the energy changes in the reactions involving methane.

Exothermic Reactions

Exothermic reactions are chemical processes that release energy, usually in the form of heat, to the surroundings. This release of energy is often associated with the formation of stable products from less stable reactants. In the context of the question, comparing the two reactions will involve determining which one releases more energy, indicating a more exothermic nature.

Stoichiometry in Chemical Reactions

Stoichiometry is the calculation of reactants and products in chemical reactions based on the balanced chemical equation. It involves using mole ratios to relate the quantities of substances involved. In this question, stoichiometry will be essential for analyzing the two given reactions to determine the total energy changes and compare their exothermicity.

관련 실천

교과서 질문

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교과서 질문

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교과서 질문

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교과서 질문

The automobile fuel called E85 consists of 85% ethanol and 15% gasoline. E85 can be used in the so-called flex-fuel vehicles (FFVs), which can use gasoline, ethanol, or a mix as fuels. Assume that gasoline consists of a mixture of octanes (different isomers of C₈H₁₈), that the average heat of combustion of C₈H₁₈(l) is 5400 kJ/mol, and that gasoline has an average density of 0.70 g/mL. The density of ethanol is 0.79 g/mL. (a) By using the information given as well as data in Appendix C, compare the energy produced by combustion of 1.0 L of gasoline and of 1.0 L of ethanol.

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교과서 질문

The standard enthalpies of formation of gaseous propyne (C₃H₄), propylene (C₃H₆), and propane (C₃H₈) are +185.4, +20.4, and -103.8 kJ/mol, respectively. (a) Calculate the heat evolved per mole on combustion of each substance to yield CO₂(g) and H₂O(g). (b) Calculate the heat evolved on combustion of 1 kg of each substance.

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