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Ch.7 - Thermochemistry
Tro - Chemistry: A Molecular Approach 6th Edition
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Tutti i libri di testoTro 6th EditionCh.7 - ThermochemistryProblema 95
Capitolo 7, Problema 95

Top fuel dragsters and funny cars burn nitromethane as fuel according to the balanced combustion equation: 2 CH3NO2(l) + 3/2O2(g) → 2 CO2(g) + 3 H2O(l) + N2(g) ΔH°rxn = –1418 kJ The enthalpy of combustion for nitromethane is –709.2 kJ/mol. Calculate the standard enthalpy of formation (ΔH°f ) for nitromethane.

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Identify the given information: the balanced combustion equation, the enthalpy change for the reaction (\( \Delta H^\circ_{rxn} = -1418 \text{ kJ} \)), and the enthalpy of combustion for nitromethane (\( -709.2 \text{ kJ/mol} \)).
Recognize that the enthalpy of combustion given is for one mole of nitromethane, while the balanced equation involves two moles. Therefore, the enthalpy change for the combustion of two moles of nitromethane is \( 2 \times (-709.2 \text{ kJ/mol}) \).
Use Hess's Law, which states that the total enthalpy change for a reaction is the same, no matter how many steps the reaction is carried out in. This allows us to relate the enthalpy of formation of reactants and products to the overall reaction enthalpy.
Write the expression for the enthalpy change of the reaction in terms of the enthalpies of formation: \( \Delta H^\circ_{rxn} = [2 \Delta H^\circ_f (\text{CO}_2) + 3 \Delta H^\circ_f (\text{H}_2\text{O}) + \Delta H^\circ_f (\text{N}_2)] - [2 \Delta H^\circ_f (\text{CH}_3\text{NO}_2) + \frac{3}{2} \Delta H^\circ_f (\text{O}_2)] \).
Solve for \( \Delta H^\circ_f (\text{CH}_3\text{NO}_2) \) using the known values: \( \Delta H^\circ_f (\text{CO}_2) = -393.5 \text{ kJ/mol} \), \( \Delta H^\circ_f (\text{H}_2\text{O}) = -285.8 \text{ kJ/mol} \), and \( \Delta H^\circ_f (\text{O}_2) = 0 \text{ kJ/mol} \) (since it is an element in its standard state).

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Enthalpy of Formation

The standard enthalpy of formation (ΔH°f) 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. The values of ΔH°f are typically tabulated and can be used to determine the enthalpy changes for reactions using Hess's law.
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Enthalpy of Formation

Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the same, regardless of the number of steps taken to achieve the reaction. This principle allows chemists to calculate the enthalpy change of a reaction by using known enthalpy changes of other reactions. In the context of the combustion of nitromethane, Hess's Law can be applied to relate the enthalpy of combustion to the enthalpy of formation.
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Hess's Law

Combustion Reactions

Combustion reactions are exothermic reactions that occur when a substance reacts with oxygen, producing heat and light. In the case of nitromethane, the balanced combustion equation shows the reactants and products involved, allowing for the calculation of energy changes. Understanding the stoichiometry of combustion reactions is essential for determining the enthalpy changes associated with the formation and breakdown of fuels.
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Combustion Apparatus
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