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Ch.5 - Thermochemistry
Brown - Chemistry: The Central Science 15th Edition
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Capitolo 5, Problema 42b

Without referring to tables, predict which of the following has the higher enthalpy in each case: (b) 2 mol of hydrogen atoms or 1 mol of H2

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Understand that enthalpy is a measure of the total energy of a thermodynamic system, often related to the energy required to break or form chemical bonds.
Recognize that breaking chemical bonds requires energy, while forming bonds releases energy.
Consider the diatomic hydrogen molecule (H2), which consists of two hydrogen atoms bonded together. The formation of this bond releases energy, resulting in a lower enthalpy for the molecule compared to separate atoms.
Compare the two scenarios: 2 mol of hydrogen atoms are unbonded and thus have higher energy (enthalpy) compared to 1 mol of H2, where the atoms are bonded and have lower energy.
Conclude that 2 mol of hydrogen atoms have a higher enthalpy than 1 mol of H2 because the energy required to keep the atoms separate is greater than the energy of the bonded molecule.

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Enthalpy

Enthalpy is a thermodynamic property that represents the total heat content of a system. It is defined as the sum of the internal energy and the product of pressure and volume. In chemical reactions, changes in enthalpy (ΔH) indicate whether a reaction is exothermic (releases heat) or endothermic (absorbs heat). Understanding enthalpy is crucial for predicting the stability and energy changes associated with different molecular forms.
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Bond Energy

Bond energy is the amount of energy required to break a bond between two atoms in a molecule. In the case of hydrogen, the H-H bond in H2 is a covalent bond that holds two hydrogen atoms together. When comparing 2 moles of hydrogen atoms to 1 mole of H2, it is important to consider that breaking the H-H bond requires energy, which affects the overall enthalpy of the system.
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Bond Energy

Molecular Stability

Molecular stability refers to the tendency of a molecule to maintain its structure and resist decomposition. Molecules like H2 are generally more stable than individual atoms due to the presence of covalent bonds that lower the system's energy. In this context, the stability of H2 compared to free hydrogen atoms influences the enthalpy, as stable molecules typically have lower enthalpy than their constituent atoms.
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