Textbook Question
The decomposition of Ca(OH)2 into CaO(s) and H2O at constant pressure requires the addition of 109 kJ of heat per mole of Ca(OH)2 . b. Draw an enthalpy diagram for the reaction.
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Ch.5 - Thermochemistry
Chapter 5, Problem 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 (H<sub>2</sub>), 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 H<sub>2</sub>, where the atoms are bonded and have lower energy.
Conclude that 2 mol of hydrogen atoms have a higher enthalpy than 1 mol of H<sub>2</sub> because the energy required to keep the atoms separate is greater than the energy of the bonded molecule.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
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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Enthalpy of Formation
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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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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Related Practice
Textbook Question
Ozone, O3(g), is a form of elemental oxygen that plays an important role in the absorption of ultraviolet radiation in the stratosphere. It decomposes to O2(g) at room temperature and pressure according to the following reaction: 2 O3(g) → 3 O2(g) ΔH= -284.6 kJ b. Which has the higher enthalpy under these conditions, 2 O3(g) or 3 O2(g)?
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Textbook Question
Without referring to tables, predict which of the following has the higher enthalpy in each case: (a) 1 mol CO2(s) or 1 mol CO2(g) at the same temperature
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Textbook Question
Without referring to tables, predict which of the following has the higher enthalpy in each case: (c) 1 mol H2(g) and 0.5 mol O2(g) at 25 °C or 1 mol H2O(g) at 25 °C
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Textbook Question
Consider the following reaction: 2 CH3OH(g) → 2 CH4(g) + O2(g) ΔH = +252.8 kJ (b) Calculate the amount of heat transferred when 24.0 g of CH3OH(g) is decomposed by this reaction at constant pressure.
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Textbook Question
Consider the following reaction: 2 CH3OH(g) → 2 CH4(g) + O2(g) ΔH = +252.8 kJ (d) How many kilojoules of heat are released when 38.5 g of CH4(g) reacts completely with O2(g) to form CH3OH(g) at constant pressure?
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