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Ch.9 - Thermochemistry: Chemical Energy
McMurry - Chemistry 8th Edition
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Capitolo 9, Problema 96

Find ∆H° in kilojoules for the reaciton of nitric oxide with oxygen, 2 NO(g) + O2(g) → N2O4(g), given the following data: N2O4(g) → 2 NO2(g) ∆H° = 55.3 kJ NO(g) + 1/2 O2(g) → NO2(g) ∆H° = -58.1 kJ

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Identify the given reactions and their enthalpy changes: (1) N2O4(g) → 2 NO2(g), ∆H° = 55.3 kJ and (2) NO(g) + 1/2 O2(g) → NO2(g), ∆H° = -58.1 kJ.
Write the target reaction: 2 NO(g) + O2(g) → N2O4(g).
Reverse reaction (1) to align with the target reaction: 2 NO2(g) → N2O4(g), which changes the sign of ∆H° to -55.3 kJ.
Multiply reaction (2) by 2 to balance the number of NO2(g) molecules in the reversed reaction (1): 2 NO(g) + O2(g) → 2 NO2(g), and multiply ∆H° by 2, giving -116.2 kJ.
Add the modified reactions to get the target reaction: (2 NO(g) + O2(g) → 2 NO2(g)) + (2 NO2(g) → N2O4(g)) results in 2 NO(g) + O2(g) → N2O4(g). Sum the ∆H° values: -116.2 kJ + (-55.3 kJ).

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Hess's Law

Hess's Law states that the total enthalpy change for a chemical reaction is the sum of the enthalpy changes for the individual steps of the reaction, regardless of the pathway taken. This principle allows us to calculate the enthalpy change for a reaction by using known enthalpy changes of related reactions, making it essential for solving problems involving thermochemical equations.
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Hess's Law

Standard Enthalpy of Formation (∆H°f)

The standard enthalpy of formation (∆H°f) is the change in enthalpy when one mole of a compound is formed from its elements in their standard states. This concept is crucial for calculating the enthalpy change of a reaction, as it provides a reference point for the energy associated with the formation of reactants and products.
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Enthalpy of Formation

Enthalpy Change (∆H)

Enthalpy change (∆H) is a measure of the heat absorbed or released during a chemical reaction at constant pressure. It is a key concept in thermodynamics, allowing chemists to understand energy changes in reactions, predict reaction spontaneity, and calculate the energy required or released in various chemical processes.
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Enthalpy of Formation
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