Skip to main content
Ch.18 - Free Energy and Thermodynamics
Tro - Chemistry: A Molecular Approach 4th Edition
Tro4th EditionChemistry: A Molecular ApproachISBN: 9780134112831Not the one you use?Change textbook
All textbooksTro 4th EditionCh.18 - Free Energy and ThermodynamicsProblem 60
Chapter 18, Problem 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?

Verified step by step guidance
1
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.

Verified video answer for a similar problem:

This video solution was recommended by our tutors as helpful for the problem above.
Was this helpful?

Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

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.
Recommended video:
Guided course
01:32
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.
Recommended video:
Guided course
01:18
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.
Related Practice
Textbook Question

Methanol (CH3OH) burns in oxygen to form carbon dioxide and water. Write a balanced equation for the combustion of liquid methanol and calculate ΔH°rxn, ΔS°rxn, and ΔG°rxn at 25 °C. Is the combustion of methanol spontaneous?

7361
views
2
rank
Textbook Question

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? a. N2O4(g) → 2 NO2(g)

731
views
Textbook Question

Find ΔS° for the formation of CH2Cl2(g) from its gaseous elements in their standard states. Rationalize the sign of ΔS°.

1957
views
1
comments
Textbook Question

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)

1011
views