The reaction between hydrogen gas and carbon to produce the gas known as ethylene is: 2 H2(g) + 2 C(s) → H2C=CH2(g), ∆G = +16.3 kcal/mol (+68 kJ/mol) 25 °C. a. Is this reaction spontaneous at 25 °C?

Ch.7 Chemical Reactions: Energy, Rate and Equilibrium

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McMurry 8th Edition

Ch.7 Chemical Reactions: Energy, Rate and Equilibrium

문제 48a

7장, 문제 48a

The reaction between hydrogen gas and carbon to produce the gas known as ethylene is:
2 H₂(g) + 2 C(s) → H₂C=CH₂(g), ∆G = +16.3 kcal/mol (+68 kJ/mol) 25 °C.
a. Is this reaction spontaneous at 25 °C?

검증된 단계별 안내

Step 1: Recall the concept of spontaneity in chemical reactions. A reaction is spontaneous if the Gibbs free energy change (∆G) is negative. If ∆G is positive, the reaction is non-spontaneous under the given conditions.

Step 2: Analyze the given information. The problem states that the Gibbs free energy change (∆G) for the reaction is +16.3 kcal/mol (+68 kJ/mol) at 25 °C. Since ∆G is positive, this suggests the reaction is not spontaneous under these conditions.

Step 3: Understand the implications of a positive ∆G. A positive ∆G means that the reaction requires an input of energy to proceed and will not occur on its own at the given temperature (25 °C).

Step 4: Consider the temperature dependence of ∆G. The spontaneity of a reaction can change with temperature, as ∆G is related to enthalpy (∆H), entropy (∆S), and temperature (T) by the equation:

Δ G = Δ H - T Δ S

. However, no additional information about ∆H or ∆S is provided in this problem.

Step 5: Conclude that the reaction is non-spontaneous at 25 °C based on the positive ∆G value provided. If conditions such as temperature or energy input change, the spontaneity of the reaction may need to be re-evaluated.

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주요 개념

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Gibbs Free Energy (∆G)

Gibbs Free Energy (∆G) is a thermodynamic potential that measures the maximum reversible work obtainable from a thermodynamic system at constant temperature and pressure. A negative ∆G indicates that a reaction is spontaneous, while a positive ∆G suggests that the reaction is non-spontaneous. In this case, the given ∆G of +16.3 kcal/mol indicates that the reaction is not spontaneous at 25 °C.

Spontaneity of Reactions

The spontaneity of a chemical reaction refers to whether it can occur without external intervention. This is determined by the change in Gibbs Free Energy (∆G). If ∆G is negative, the reaction is spontaneous; if positive, it is non-spontaneous. Understanding this concept is crucial for predicting the behavior of chemical reactions under specific conditions.

Temperature and Reaction Dynamics

Temperature plays a significant role in chemical reactions, influencing both the rate and spontaneity. The Gibbs Free Energy equation incorporates temperature, as it affects the enthalpy and entropy of the system. In this case, the reaction's non-spontaneity at 25 °C suggests that increasing the temperature may alter the reaction dynamics, potentially making it more favorable under different conditions.