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Ch.5 - Thermochemistry
Brown - Chemistry: The Central Science 14th Edition
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Capitolo 5, Problema 39b

The complete combustion of methane, CH4(g), to form H2O(l) and CO2(g) at constant pressure releases 890 kJ of heat per mole of CH4. (b) Draw an enthalpy diagram for the reaction.

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Draw a horizontal line to represent the enthalpy level of the reactants, which is methane (CH_4) and oxygen (O_2).
Draw another horizontal line below the reactants' line to represent the enthalpy level of the products, which are carbon dioxide (CO_2) and water (H_2O).
Label the vertical distance between the reactants and products as the change in enthalpy (ΔH), which is -890 kJ/mol, indicating an exothermic reaction.
Add an arrow pointing downward from the reactants' line to the products' line to show the release of energy.
Label the reactants' line with 'CH_4 + 2O_2' and the products' line with 'CO_2 + 2H_2O'.

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Enthalpy (H)

Enthalpy is a thermodynamic property that represents the total heat content of a system at constant pressure. It is defined as the sum of the internal energy and the product of pressure and volume (H = U + PV). In chemical reactions, changes in enthalpy (ΔH) indicate whether a reaction is exothermic (releases heat) or endothermic (absorbs heat), which is crucial for understanding energy changes during combustion.
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Enthalpy of Formation

Combustion Reaction

A combustion reaction is a chemical process in which a substance (typically a hydrocarbon) reacts with oxygen to produce carbon dioxide and water, releasing energy in the form of heat. In the case of methane (CH4), its complete combustion yields CO2 and H2O, and is characterized by a significant release of energy, as indicated by the 890 kJ of heat released per mole of CH4 in the question.
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Combustion Apparatus

Enthalpy Diagram

An enthalpy diagram visually represents the changes in enthalpy during a chemical reaction. It typically shows the enthalpy of the reactants and products, along with the enthalpy change (ΔH) for the reaction. For the combustion of methane, the diagram would illustrate the higher enthalpy of the reactants compared to the lower enthalpy of the products, indicating the exothermic nature of the reaction and the energy released.
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Enthalpy of Formation
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