Q1. Consider a reaction that has a positive ΔH and a negative ΔS. Which of the following statements is TRUE?

Background

Topic: Thermodynamics – Spontaneity of Reactions

This question tests your understanding of how enthalpy (ΔH) and entropy (ΔS) changes affect the spontaneity of a reaction, using the Gibbs free energy equation.

Key Terms and Formulas

• ΔH: Change in enthalpy (heat content)

• ΔS: Change in entropy (disorder)

• ΔG: Change in Gibbs free energy

Key formula:

Step-by-Step Guidance

1. Recall that a reaction is spontaneous when .

2. Analyze the signs: Here, (endothermic) and (decrease in disorder).

3. Plug these signs into the Gibbs free energy equation: .

4. Consider how temperature affects with these signs.

Try solving on your own before revealing the answer!

Q2. Calculate ΔS°rxn for the following reaction: N2H4(l) + H2(g) → 2 NH3(g). S° (J/mol·K): N2H4(l) = 121.2, H2(g) = 130.7, NH3(g) = 192.8

Background

Topic: Entropy Change in Chemical Reactions

This question tests your ability to calculate the standard entropy change for a reaction using standard molar entropy values.

Key Terms and Formulas

• ΔS°rxn: Standard entropy change of reaction

• S°: Standard molar entropy

Key formula:

Step-by-Step Guidance

1. Write the balanced equation: N2H4(l) + H2(g) → 2 NH3(g).

2. List the S° values for each species and their coefficients.

3. Calculate the total S° for products: J/mol·K.

4. Calculate the total S° for reactants: J/mol·K.

5. Set up the formula: .

Try solving on your own before revealing the answer!

Q3. The ________ Law of Thermodynamics states that for any spontaneous reaction, the entropy of the universe increases.

Background

Topic: Laws of Thermodynamics

This question tests your knowledge of the Second Law of Thermodynamics and its statement about entropy and spontaneity.

Key Terms

• Entropy (S): A measure of disorder or randomness.

• Second Law of Thermodynamics: The entropy of the universe increases for any spontaneous process.

Step-by-Step Guidance

1. Recall the statements of the four main laws of thermodynamics.

2. Identify which law specifically addresses entropy and spontaneity.

3. Match the law to the statement in the question.

Try solving on your own before revealing the answer!

Q4. Calculate the ΔG°rxn using the following information: 2 HNO3(aq) + NO(g) → 3 NO2(g) + H2O(l). ΔG°f (kJ/mol): HNO3(aq) = -110.9, NO(g) = 87.6, NO2(g) = 51.3, H2O(l) = -237.1

Background

Topic: Standard Gibbs Free Energy Change of Reaction

This question tests your ability to calculate the standard Gibbs free energy change for a reaction using standard free energies of formation.

Key Terms and Formulas

• ΔG°rxn: Standard Gibbs free energy change of reaction

• ΔG°f: Standard Gibbs free energy of formation

Key formula:

Step-by-Step Guidance

1. Write the balanced equation and identify the coefficients for each species.

2. Multiply each ΔG°f value by its coefficient in the balanced equation.

3. Sum the ΔG°f values for the products and for the reactants separately.

4. Set up the formula: .

Try solving on your own before revealing the answer!

Q5. Determine the equilibrium constant for the following reaction at 298 K: SO3(g) + H2O(g) → H2SO4(l), ΔG° = -90.5 kJ

Background

Topic: Relationship Between ΔG° and Equilibrium Constant (K)

This question tests your ability to relate the standard Gibbs free energy change to the equilibrium constant using the appropriate thermodynamic equation.

Key Terms and Formulas

• ΔG°: Standard Gibbs free energy change

• K: Equilibrium constant

• R: Gas constant (8.314 J/mol·K)

• T: Temperature in Kelvin

Key formula:

Step-by-Step Guidance

1. Convert ΔG° from kJ to J (multiply by 1000).

2. Plug in the values for ΔG°, R, and T into the equation .

3. Rearrange the equation to solve for K: .

4. Set up the calculation for the exponent: .

Try solving on your own before revealing the answer!

Q6. Use Hess's law to calculate ΔG°rxn for CO(g) → C(s) + 1/2 O2(g) given: CO2(g) → C(s) + O2(g), ΔG°rxn = +394.4 kJ; CO(g) + 1/2 O2(g) → CO2(g), ΔG°rxn = -257.2 kJ

Background

Topic: Hess's Law for Gibbs Free Energy

This question tests your ability to use Hess's Law to combine reactions and their ΔG° values to find the ΔG° for a target reaction.

Key Terms and Formulas

• Hess's Law: The total ΔG° for a reaction is the sum of ΔG° values for individual steps that add up to the overall reaction.

Step-by-Step Guidance

1. Write out all given reactions and their ΔG° values.

2. Manipulate (reverse or multiply) the given reactions as needed so that when added, they yield the target reaction.

3. Remember to reverse the sign of ΔG° if you reverse a reaction.

4. Add the ΔG° values for the manipulated reactions to get the total ΔG° for the target reaction.

Try solving on your own before revealing the answer!