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Ch.18 - Free Energy and Thermodynamics
Tro - Chemistry: A Molecular Approach 4th Edition
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Tutti i libri di testoTro 4th EditionCh.18 - Free Energy and ThermodynamicsProblema 60
Capitolo 18, Problema 60

In photosynthesis, plants form glucose (C6H12O6) and oxygen from carbon dioxide and water. Write a balanced equation for photosynthesis and calculate ΔH°rxn, ΔS°rxn, and ΔG°rxn at 25 °C. Is photosynthesis spontaneous?

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Write the unbalanced chemical equation for photosynthesis: CO2 + H2O -> C6H12O6 + O2.
Balance the chemical equation by ensuring the number of atoms of each element is the same on both sides: 6 CO2 + 6 H2O -> C6H12O6 + 6 O2.
Calculate ΔH°rxn using standard enthalpies of formation: ΔH°rxn = ΣΔH°f(products) - ΣΔH°f(reactants).
Calculate ΔS°rxn using standard entropies: ΔS°rxn = ΣS°(products) - ΣS°(reactants).
Calculate ΔG°rxn using the Gibbs free energy equation: ΔG°rxn = ΔH°rxn - TΔS°rxn, where T is the temperature in Kelvin. Determine if the reaction is spontaneous by checking if ΔG°rxn is negative.

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Balanced Chemical Equation

A balanced chemical equation represents a chemical reaction with equal numbers of each type of atom on both sides of the equation. In photosynthesis, the equation is typically written as 6CO2 + 6H2O → C6H12O6 + 6O2. Balancing ensures the law of conservation of mass is upheld, meaning that matter is neither created nor destroyed during the reaction.
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Balancing Chemical Equations

Thermodynamics (ΔH°, ΔS°, ΔG°)

Thermodynamics involves the study of energy changes in chemical reactions. ΔH°rxn represents the change in enthalpy, indicating whether a reaction is exothermic or endothermic. ΔS°rxn is the change in entropy, reflecting the disorder of the system. ΔG°rxn, the Gibbs free energy change, determines the spontaneity of a reaction; a negative ΔG°rxn indicates a spontaneous process.
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First Law of Thermodynamics

Spontaneity of Reactions

A reaction is considered spontaneous if it occurs without external intervention. In thermodynamics, spontaneity is assessed using the Gibbs free energy change (ΔG°rxn). If ΔG°rxn is negative, the reaction is spontaneous under standard conditions. In the context of photosynthesis, despite being endothermic, it is driven by sunlight, making it effectively spontaneous in nature.
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Spontaneity of Processes
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For each reaction, calculate ΔH°rxn, ΔS°rxn, and ΔG°rxn at 25 °C and state whether or not the reaction is spontaneous. If the reaction is not spontaneous, would a change in temperature make it spontaneous? If so, should the temperature be raised or lowered from 25 °C? d. N2(g) + 3 H2(g) → 2 NH3(g)

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