Without using thermochemical data, predict whether ΔG° for the reaction involving octane (1C8H182) is more negative or less negative than ΔH°.

Verified step by step guidance

Step 1: Understand the relationship between Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) using the equation ΔG° = ΔH° - TΔS°, where T is the temperature in Kelvin.

Step 2: Consider the nature of the reaction involving octane. Combustion reactions, such as the burning of octane, typically result in an increase in entropy (ΔS° > 0) because gases are produced from liquids and solids.

Step 3: Recognize that if ΔS° is positive, the term -TΔS° will be negative, which will make ΔG° more negative than ΔH° at higher temperatures.

Step 4: Analyze the implications: since combustion reactions are exothermic (ΔH° < 0) and increase entropy, the negative contribution of -TΔS° will make ΔG° more negative than ΔH°.

Step 5: Conclude that for the combustion of octane, ΔG° is expected to be more negative than ΔH° due to the positive entropy change and the exothermic nature of the reaction.

Key Concepts

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

Gibbs Free Energy (ΔG°)

Gibbs Free Energy (ΔG°) is a thermodynamic potential that measures the maximum reversible work obtainable from a thermodynamic system at constant temperature and pressure. It combines the system's enthalpy and entropy, providing insight into the spontaneity of a reaction. A negative ΔG° indicates a spontaneous process, while a positive value suggests non-spontaneity.

Enthalpy (ΔH°)

Enthalpy (ΔH°) is a measure of the total heat content of a system, reflecting the energy required to create a system at constant pressure. It is crucial in determining the heat absorbed or released during a chemical reaction. A negative ΔH° indicates an exothermic reaction, while a positive ΔH° signifies an endothermic reaction, influencing the overall energy balance of the process.

Entropy (ΔS°)

Entropy (ΔS°) is a measure of the disorder or randomness in a system. It plays a vital role in determining the spontaneity of a reaction alongside enthalpy. An increase in entropy (positive ΔS°) generally favors spontaneity, while a decrease (negative ΔS°) can hinder it. The relationship between ΔG°, ΔH°, and ΔS° is described by the equation ΔG° = ΔH° - TΔS°, where T is the temperature in Kelvin.

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Related Practice

Textbook Question

Use data in Appendix C to calculate ΔH°, ΔS°, and ΔG° at 25 °C for each of the following reactions.

a. 4 Cr(s) + 3 O₂(g) → 2 Cr₂O₃(s)

b. BaCO₃(s) → BaO(s) + CO₂(g)

c. 2 P(s) + 10 HF(g) → 2 PF₅(g) + 5 H₂(g)

d. K(s) + O₂(g) → KO₂(s)

Textbook Question

Using data from Appendix C, calculate ΔG° for the following reactions. Indicate whether each reaction is spontaneous at 298 K under standard conditions.

(a) 2 SO₂(g) + O₂(g) → 2 SO₃(g)

(b) NO₂(g) + N₂O(g) → 3 NO(g)

(d) SO₂(g) + 2 H₂(g) → S(s) + 2 H₂O(g)

Textbook Question

Using data from Appendix C, calculate the change in Gibbs free energy for each of the following reactions. In each case, indicate whether the reaction is spontaneous at 298 K under standard conditions.

(a) 2 Ag(s) + Cl2(g) → 2 AgCl(s)

(b) P₄O₁₀(s) + 16 H₂(g) → 4 PH₃(g) + 10 H₂O(g)

(d) 2 H₂O₂(l) → 2 H₂O(l) + O₂(g)

Textbook Question

Sulfur dioxide reacts with strontium oxide as follows: SO₂(g) + SrO(g) → SrSO₃(s) (a) Without using thermochemical data, predict whether ΔG° for this reaction is more negative or less negative than ΔH°.

Textbook Question

Classify each of the following reactions as one of the four possible types summarized in Table 19.3: (i) spontanous at all temperatures; (ii) not spontaneous at any temperature; (iii) spontaneous at low T but not spontaneous at high T; (iv) spontaneous at high T but not spontaneous at low T.

(a) N₂(g) + 3 F₂(g) → 2 NF₃₍g) ΔH° = -249 kJ; ΔS° = -278 J/K

(b) N₂(g) + 3 Cl₂(g) → 2 NCl₃(g) ΔH° = 460 kJ; ΔS° = -275 J/K

Textbook Question

Classify each of the following reactions as one of the four possible types summarized in Table 19.3: (i) spontaneous at all temperatures; (ii) not spontaneous at any temperature; (iii) spontaneous at low T but not spontaneous at high T; (iv) spontaneous at high T but not spontaneous at low T.