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? c. N2(g) + O2(g) → 2 NO(g)
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Identify the standard enthalpy of formation (ΔH°f) for each reactant and product from a reliable source, such as a chemistry textbook or database.
Calculate the standard enthalpy change of the reaction (ΔH°rxn) using the formula: ΔH°rxn = ΣΔH°f(products) - ΣΔH°f(reactants).
Identify the standard entropy (S°) values for each reactant and product.
Calculate the standard entropy change of the reaction (ΔS°rxn) using the formula: ΔS°rxn = ΣS°(products) - ΣS°(reactants).
Calculate the standard Gibbs free energy change (ΔG°rxn) using the formula: ΔG°rxn = ΔH°rxn - TΔS°rxn, where T is the temperature in Kelvin (298 K for 25 °C). Determine spontaneity by checking if ΔG°rxn is negative. If not spontaneous, consider how temperature affects ΔG°rxn: if ΔH°rxn > 0 and ΔS°rxn > 0, increasing temperature may make the reaction spontaneous.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Enthalpy (ΔH°rxn)
Enthalpy change (ΔH°rxn) is the heat absorbed or released during a chemical reaction at constant pressure. It indicates whether a reaction is exothermic (releases heat, ΔH° < 0) or endothermic (absorbs heat, ΔH° > 0). This value is crucial for determining the energy dynamics of the reaction and influences its spontaneity.
Entropy change (ΔS°rxn) measures the disorder or randomness of a system during a reaction. A positive ΔS° indicates an increase in disorder, while a negative ΔS° suggests a decrease. Entropy is a key factor in assessing the spontaneity of a reaction, as reactions tend to favor states of higher entropy.
Gibbs Free Energy change (ΔG°rxn) combines enthalpy and entropy to determine the spontaneity of a reaction at constant temperature and pressure. It is calculated using the equation ΔG° = ΔH° - TΔS°. A negative ΔG° indicates a spontaneous reaction, while a positive ΔG° suggests non-spontaneity. The temperature's effect on spontaneity can be analyzed through this relationship.
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