For each reaction, calculate ΔH°rxn, ΔS°rxn, and ΔG°rxn at 25 °C and state whether 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? b. NH4Cl(s) → HCl(g) + NH3(g) c. 3 H2(g) + Fe2O3(s) → 2 Fe(s) + 3 H2O(g)

Verified step by step guidance

1. insert step 1: Identify the standard enthalpy change (ΔH°), entropy change (ΔS°), and Gibbs free energy change (ΔG°) for each reaction.
2. insert step 2: Use standard enthalpies of formation (ΔH°f) to calculate ΔH°rxn for each reaction. The formula is ΔH°rxn = ΣΔH°f(products) - ΣΔH°f(reactants).
3. insert step 3: Use standard molar entropies (S°) to calculate ΔS°rxn for each reaction. The formula is ΔS°rxn = ΣS°(products) - ΣS°(reactants).
4. insert step 4: Calculate ΔG°rxn using the equation ΔG°rxn = ΔH°rxn - TΔS°rxn, where T is the temperature in Kelvin (298 K for 25 °C).
5. insert step 5: Determine spontaneity by checking the sign of ΔG°rxn. If ΔG°rxn is negative, the reaction is spontaneous. If not, consider how temperature affects spontaneity: if ΔH° and ΔS° are both positive, increasing temperature may make the reaction spontaneous; if both are negative, decreasing temperature may help.

Key Concepts

Thermodynamic Functions (ΔH, ΔS, ΔG)

Thermodynamic functions such as enthalpy change (ΔH), entropy change (ΔS), and Gibbs free energy change (ΔG) are essential for understanding chemical reactions. ΔH indicates the heat absorbed or released during a reaction, ΔS measures the disorder or randomness of the system, and ΔG determines the spontaneity of the reaction. A negative ΔG suggests a spontaneous reaction, while a positive ΔG indicates non-spontaneity.

Spontaneity of Reactions

The spontaneity of a reaction is determined by the sign of ΔG. A reaction is spontaneous at a given temperature if ΔG is negative, meaning it can occur without external energy. The relationship between ΔG, ΔH, and ΔS is given by the equation ΔG = ΔH - TΔS, where T is the temperature in Kelvin. Understanding this relationship helps predict whether changing the temperature can make a non-spontaneous reaction spontaneous.

Temperature Effects on Spontaneity

Temperature can significantly influence the spontaneity of a reaction, particularly when ΔH and ΔS have opposite signs. If ΔH is positive and ΔS is positive, increasing the temperature can make ΔG negative, thus making the reaction spontaneous. Conversely, if ΔH is negative and ΔS is negative, lowering the temperature may be necessary to achieve spontaneity. Analyzing these conditions is crucial for determining the appropriate temperature adjustments.