General Chemistry II Exam 2 Study Guide – Thermodynamics & Kinetics Guidance

Q1. Predict the sign of ΔS° for the following reaction: C2H2(g) + 2 H2(g) → C2H6(g)

Background

Topic: Entropy Change in Chemical Reactions

This question tests your ability to qualitatively predict the sign of the entropy change (ΔS°) for a chemical reaction by considering the number and physical states of reactants and products.

Key Terms and Concepts:

• Entropy (S): A measure of the disorder or randomness in a system.

• ΔS°: The standard entropy change for a reaction.

• General rule: Entropy increases when the number of gas molecules increases, or when a solid or liquid is converted to a gas.

Step-by-Step Guidance

1. Count the number of moles of gaseous reactants and products on each side of the equation.

2. Consider the physical states of all species (all are gases here).

3. Recall that a decrease in the number of gas molecules usually means a decrease in entropy (ΔS° < 0), and an increase means an increase in entropy (ΔS° > 0).

4. Compare the total number of gas molecules before and after the reaction to predict the sign of ΔS°.

Try solving on your own before revealing the answer!

Q2. Calculate ΔH, ΔS, and ΔG for the reaction: 2 Na(s) + Cl2(g) → 2 NaCl(s) (using standard values from the table)

Background

Topic: Thermodynamics – Calculating Enthalpy, Entropy, and Free Energy Changes

This question asks you to calculate the standard enthalpy change (ΔH°), entropy change (ΔS°), and Gibbs free energy change (ΔG°) for a reaction using tabulated standard values.

Key Formulas:

• $\Delta H^\circ = \sum n_p H^\circ_{f,\text{products}} - \sum n_r H^\circ_{f,\text{reactants}}$

• $\Delta S^\circ = \sum n_p S^\circ_{\text{products}} - \sum n_r S^\circ_{\text{reactants}}$

• $\Delta G^\circ = \sum n_p G^\circ_{f,\text{products}} - \sum n_r G^\circ_{f,\text{reactants}}$

• Alternatively, $\Delta G^\circ = \Delta H^\circ - T\Delta S^\circ$ (with T in Kelvin)

Step-by-Step Guidance

1. Write down the balanced chemical equation and identify the number of moles of each reactant and product.

2. List the standard enthalpy of formation ($H^\circ_f$), standard entropy ($S^\circ$), and standard free energy of formation ($G^\circ_f$) for each species from the provided table.

3. Calculate $\Delta H^\circ$ using the formula above, multiplying each value by its stoichiometric coefficient.

4. Calculate $\Delta S^\circ$ in the same way, using the standard entropy values.

5. Set up the calculation for $\Delta G^\circ$ using either the tabulated values or by using $\Delta H^\circ$ and $\Delta S^\circ$ at $T = 298$ K.

Try solving on your own before revealing the answer!

Q3. Acting as an ideal gas, 5 mol of Kr is compressed from 5.5 L to 4.0 L isothermally. What is the change in entropy for the gas?

Background

Topic: Entropy Change for Isothermal Expansion/Compression of an Ideal Gas

This question tests your ability to calculate the entropy change ($\Delta S$) for an ideal gas undergoing an isothermal (constant temperature) volume change.

Key Formula:

• $\Delta S = nR \ln \left( \frac{V_2}{V_1} \right)$

• Where:

  • $n$ = number of moles of gas

  • $R$ = gas constant ($8.314$ J/mol·K)

  • $V_1$ = initial volume

  • $V_2$ = final volume

Step-by-Step Guidance

1. Identify the values: $n = 5$ mol, $V_1 = 5.5$ L, $V_2 = 4.0$ L, $R = 8.314$ J/mol·K.

2. Recognize that the process is isothermal, so you can use the formula above.

3. Set up the calculation: $\Delta S = nR \ln \left( \frac{V_2}{V_1} \right)$.

4. Plug in the values for $n$, $R$, $V_1$, and $V_2$ (but do not calculate the final value yet).

Try solving on your own before revealing the answer!

Q4. What is the order with respect to each reactant and the overall order given Rate = k[H2O2][Fe2+]?

Background

Topic: Rate Laws and Reaction Order

This question tests your understanding of how to determine the reaction order with respect to each reactant and the overall order from a given rate law.

Key Terms:

• Rate Law: An equation that relates the reaction rate to the concentrations of reactants, each raised to a power (the order with respect to that reactant).

• Overall Order: The sum of the exponents in the rate law.

Step-by-Step Guidance

1. Write out the given rate law: $\text{Rate} = k[\text{H}_2\text{O}_2][\text{Fe}^{2+}]$.

2. Identify the exponent for each reactant in the rate law (the power to which each concentration is raised).

3. The order with respect to a reactant is the exponent of its concentration in the rate law.

4. The overall order is the sum of the individual orders.

Try solving on your own before revealing the answer!