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Ch.5 - Thermochemistry
Brown - Chemistry: The Central Science 15th Edition
Brown15th EditionChemistry: The Central ScienceISBN: 9780137542970Non è quello che usi tu?Cambia libro di testo
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Capitolo 5, Problema 82b

Methanol (CH3OH) is used as a fuel in race cars. (b) Calculate the standard enthalpy change for the reaction, assuming H2O(g) as a product.

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Determine the balanced chemical equation for the combustion of methanol (CH_3OH). The general form is: CH_3OH(l) + O_2(g) -> CO_2(g) + H_2O(g).
Balance the equation by ensuring the number of atoms of each element is the same on both sides. For methanol combustion, it becomes: 2 CH_3OH(l) + 3 O_2(g) -> 2 CO_2(g) + 4 H_2O(g).
Use standard enthalpy of formation values (ΔH_f^°) for each compound involved in the reaction. These values are typically found in a table of thermodynamic data.
Apply Hess's Law to calculate the standard enthalpy change (ΔH_rxn^°) for the reaction: ΔH_rxn^° = [Σ ΔH_f^°(products)] - [Σ ΔH_f^°(reactants)].
Substitute the ΔH_f^° values into the equation and perform the arithmetic to find the standard enthalpy change for the reaction.

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Standard Enthalpy Change

Standard enthalpy change (ΔH°) refers to the heat change that occurs at constant pressure when one mole of a substance is formed from its elements in their standard states. It is a crucial concept in thermodynamics, allowing chemists to predict the energy changes associated with chemical reactions. This value is typically measured in kilojoules per mole (kJ/mol) and is essential for calculating the energy released or absorbed during a reaction.
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Enthalpy of Formation

Combustion Reactions

Combustion reactions involve the reaction of a substance with oxygen, producing heat and light. In the case of methanol (CH₃OH), its combustion with oxygen yields carbon dioxide (CO₂) and water (H₂O) as products. Understanding the stoichiometry of combustion reactions is vital for calculating the enthalpy change, as it allows for the determination of the amount of reactants and products involved in the reaction.
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Combustion Apparatus

Hess's Law

Hess's Law states that the total enthalpy change for a reaction is the sum of the enthalpy changes for the individual steps of the reaction, regardless of the pathway taken. This principle is useful for calculating the standard enthalpy change of a reaction when direct measurement is difficult. By using known enthalpy values of related reactions, one can derive the enthalpy change for the desired reaction, such as the combustion of methanol.
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Hess's Law
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