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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Convert the mass of each substance (1 kg) to moles using their respective molar masses.

Write the balanced chemical equation for the combustion of each substance: propyne, propylene, and propane.

Use the standard enthalpies of formation to calculate the enthalpy change (\( \Delta H_{\text{comb}} \)) for the combustion reaction of each substance.

Multiply the \( \Delta H_{\text{comb}} \) by the number of moles calculated in step 1 to find the total heat evolved for each substance.

Ensure the units are consistent and convert the total heat evolved to kilojoules if necessary.

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Key Concepts

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

Standard 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. It is a crucial concept in thermodynamics, as it allows for the calculation of the energy changes associated with chemical reactions, particularly combustion. The values provided for propyne, propylene, and propane indicate how much energy is absorbed or released during their formation.

Combustion Reaction

A combustion reaction is a chemical process in which a substance reacts rapidly with oxygen, releasing energy in the form of heat and light. For hydrocarbons like propyne, propylene, and propane, the combustion reaction typically produces carbon dioxide and water. Understanding the stoichiometry of these reactions is essential for calculating the heat evolved during combustion, as it relates to the enthalpy changes of the reactants and products.

Heat of Combustion

The heat of combustion is the amount of energy released when a substance undergoes complete combustion with oxygen. It is often expressed in kJ/mol and can be derived from the standard enthalpies of formation of the reactants and products. To calculate the heat evolved on combustion of 1 kg of each hydrocarbon, one must convert the mass to moles and apply the heat of combustion values, which can be determined from the enthalpy of formation data.

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Textbook Question

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. (c) Which is the most efficient fuel in terms of heat evolved per unit mass?

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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)

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