General Chemistry II Exam 2 Study Guide: Thermochemistry & Quantum Theory

Q1. Is the reaction endothermic or exothermic, and what is the value of ΔH for the reverse reaction?

Background

Topic: Thermochemistry – Enthalpy Changes

This question tests your understanding of exothermic and endothermic reactions, and how the sign of ΔH changes when a reaction is reversed.

Key Terms and Formulas

• Exothermic reaction: Releases heat (ΔH < 0)

• Endothermic reaction: Absorbs heat (ΔH > 0)

• Reverse reaction: The sign of ΔH is reversed

Step-by-Step Guidance

1. Look at the sign of ΔH given for the reaction. If ΔH is negative, the reaction is exothermic; if positive, it is endothermic.

2. For the reverse reaction, change the sign of ΔH (e.g., if the forward reaction is −20.2 kJ, the reverse is +20.2 kJ).

3. Match the correct combination of reaction type and ΔH value to the answer choices.

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Q2. What is ΔH when 6.0 g of NO decomposes to its elements?

Background

Topic: Enthalpy Calculations & Stoichiometry

This question tests your ability to use stoichiometry to relate the enthalpy change for a given amount of substance to a different amount.

Key Terms and Formulas

• ΔH: Enthalpy change for a reaction as written (per mole unless otherwise specified)

• Stoichiometry: Use molar mass to convert grams to moles

Step-by-Step Guidance

1. Determine the number of moles of NO in 6.0 g using its molar mass (30 g/mol).

2. Recall that the enthalpy change for forming 1.0 mol of NO is +90 kJ (endothermic).

3. For decomposition, reverse the reaction and change the sign of ΔH.

4. Calculate the enthalpy change for the number of moles you found in step 1.

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Q3. What are the signs of q and w for the sublimation of CO₂(s) to CO₂(g) at constant pressure?

Background

Topic: Thermodynamics – Heat (q) and Work (w)

This question tests your understanding of the direction of heat flow and work during a phase change at constant pressure.

Key Terms and Formulas

• q (heat): Positive if heat is absorbed by the system

• w (work): Negative if the system does work on the surroundings (expansion)

Step-by-Step Guidance

1. Consider whether sublimation (solid to gas) requires or releases heat.

2. Think about whether the gas occupies more volume than the solid, and whether the system does work on the surroundings.

3. Assign the correct signs to q and w based on your reasoning.

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Q4. Which process has a positive ΔH value?

Background

Topic: Enthalpy Changes in Physical and Chemical Processes

This question tests your ability to identify endothermic processes (ΔH > 0) among various physical and chemical changes.

Key Terms and Formulas

• ΔH > 0: Endothermic process (absorbs heat)

• ΔH < 0: Exothermic process (releases heat)

Step-by-Step Guidance

1. Review each process and decide if it absorbs or releases heat.

2. Recall that condensation and combustion are typically exothermic, while processes like atom formation from elements are often endothermic.

3. Identify the process that matches ΔH > 0.

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Q5. What is the value of work (w) for the reaction 2H₂(g) + O₂(g) → 2H₂O(g) at 298 K and 1.00 atm?

Background

Topic: Thermochemistry – Work of Gas Expansion/Compression

This question tests your ability to calculate the work done by a system during a chemical reaction involving gases at constant pressure.

Key Terms and Formulas

• Work at constant pressure: $w = -P\Delta V$

• Ideal gas law: $V = nRT/P$

• 1 mol of ideal gas at 298 K and 1 atm occupies 24.5 L

Step-by-Step Guidance

1. Calculate the change in moles of gas ($\Delta n$) during the reaction.

2. Find the change in volume ($\Delta V$) using $\Delta n$ and the molar volume (24.5 L/mol).

3. Plug $\Delta V$ and $P$ into the work formula to find $w$ (in L·atm).

4. Convert the result to kJ using the conversion factor (1 L·atm = 101.3 J).

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Q6. Calculate qp for the reaction between 20.0 mL of 1.00 M HCl and 20.0 mL of 1.00 M NaOH in a coffee-cup calorimeter.

Background

Topic: Calorimetry – Heat of Reaction at Constant Pressure

This question tests your ability to calculate the heat absorbed or released in a reaction using calorimetry data.

Key Terms and Formulas

• Heat (q): $q = mc\Delta T$

• Specific heat of water: $c = 4.184 \text{ J/g·K}$

• Density of water: $1.00 \text{ g/mL}$

Step-by-Step Guidance

1. Calculate the total mass of the solution (sum of volumes × density).

2. Find the temperature change ($\Delta T$) from the initial and final temperatures.

3. Plug the values into $q = mc\Delta T$ to find the heat absorbed or released.

4. Assign the correct sign to $q_p$ based on whether the temperature increased or decreased.

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Q7. How much heat is released when 35.0 g of Al reacts in the thermite reaction?

Background

Topic: Stoichiometry and Enthalpy Calculations

This question tests your ability to use stoichiometry to relate the enthalpy change for a reaction to a given mass of reactant.

Key Terms and Formulas

• ΔH: Enthalpy change for the reaction as written (per stoichiometric amount)

• Molar mass of Al: 27.0 g/mol

Step-by-Step Guidance

1. Calculate the number of moles of Al in 35.0 g.

2. Determine how many times the reaction occurs based on the stoichiometry (2 mol Al per reaction).

3. Multiply the number of reaction equivalents by the enthalpy change (−836 kJ per reaction as written).

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Q8. How much heat is released when 1.00 mol of C₂H₂ reacts with O₂?

Background

Topic: Enthalpy of Reaction from Standard Enthalpies of Formation

This question tests your ability to use standard enthalpies of formation to calculate the enthalpy change for a reaction.

Key Terms and Formulas

• ΔH°rxn: $\Delta H^\circ_{rxn} = \sum \Delta H^\circ_f (\text{products}) - \sum \Delta H^\circ_f (\text{reactants})$

• ΔH°f for elements in standard state is 0

Step-by-Step Guidance

1. Write the balanced equation and list the ΔH°f values for all reactants and products.

2. Multiply each ΔH°f by the stoichiometric coefficient in the equation.

3. Sum the ΔH°f values for products and reactants, then subtract reactants from products.

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Q9. What is the value of ΔH° for the formation of SO₃(g) from S(s) and O₂(g)?

Background

Topic: Hess's Law – Combining Thermochemical Equations

This question tests your ability to use Hess's Law to determine the enthalpy change for a reaction by combining given equations.

Key Terms and Formulas

• Hess's Law: The enthalpy change for a reaction is the sum of the enthalpy changes for the steps into which the reaction can be divided.

Step-by-Step Guidance

1. Write the target equation and the given equations.

2. Manipulate (reverse or multiply) the given equations so that, when added, they yield the target equation.

3. Add the enthalpy changes for the manipulated equations to find the overall ΔH°.

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Q10. Arrange photons A (360 nm), B (407 nm), and C (616 nm) in order of increasing energy.

Background

Topic: Quantum Theory – Energy of Photons

This question tests your understanding of the relationship between wavelength and energy for photons.

Key Terms and Formulas

• Photon energy: $E = \frac{hc}{\lambda}$

• Shorter wavelength = higher energy

Step-by-Step Guidance

1. Recall that energy is inversely proportional to wavelength.

2. Arrange the photons from longest to shortest wavelength to get lowest to highest energy.

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Q11. What is the wavelength of light for a photon with energy 7.50 × 10⁻¹⁹ J?

Background

Topic: Quantum Theory – Energy and Wavelength of Photons

This question tests your ability to relate the energy of a photon to its wavelength using Planck's equation.

Key Terms and Formulas

• Photon energy: $E = \frac{hc}{\lambda}$

• Planck's constant: $h = 6.63 \times 10^{-34} \text{ J·s}$

• Speed of light: $c = 3.00 \times 10^8 \text{ m/s}$

Step-by-Step Guidance

1. Rearrange the equation to solve for $\lambda$: $\lambda = \frac{hc}{E}$

2. Plug in the values for $h$, $c$, and $E$.

3. Calculate $\lambda$ in meters, then convert to nanometers (1 nm = $1 \times 10^{-9}$ m).

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Q12. What assumption did Planck make to fit the blackbody radiation data?

Background

Topic: Quantum Theory – Planck's Hypothesis

This question tests your understanding of Planck's revolutionary idea about quantization of energy.

Key Terms and Formulas

• Quantization: Energy can only be absorbed or emitted in discrete amounts (quanta)

• Planck's equation: $E = nh\nu$

Step-by-Step Guidance

1. Recall that Planck proposed energy changes occur in multiples of $h\nu$.

2. Identify the answer choice that matches this idea.

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Q13. What is true about the two-slit experiment with single electrons?

Background

Topic: Quantum Theory – Wave-Particle Duality

This question tests your understanding of the experimental evidence for the wave nature of electrons.

Key Terms and Formulas

• Wave-particle duality: Particles like electrons can exhibit both wave-like and particle-like behavior.

• Interference pattern: Evidence of wave behavior when many electrons are observed.

Step-by-Step Guidance

1. Recall what was observed when single electrons passed through the two-slit apparatus.

2. Identify which statement best describes the experimental results.

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Q14. Which electronic transitions in the H atom correspond to absorption or emission of a photon?

Background

Topic: Atomic Structure – Electronic Transitions

This question tests your understanding of how electrons absorb or emit photons when moving between energy levels.

Key Terms and Formulas

• Absorption: Electron moves to a higher energy level (n increases)

• Emission: Electron moves to a lower energy level (n decreases)

Step-by-Step Guidance

1. For each transition, determine if n increases (absorption) or decreases (emission).

2. Match each transition to the correct process and answer choice.

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Q15. What wavelength of light is emitted when a hydrogen atom relaxes from n = 4 to n = 1?

Background

Topic: Atomic Structure – Hydrogen Emission Spectrum

This question tests your ability to calculate the wavelength of light emitted during an electronic transition in hydrogen.

Key Terms and Formulas

• Rydberg equation: $\frac{1}{\lambda} = R_H \left( \frac{1}{n_1^2} - \frac{1}{n_2^2} \right)$

• Rydberg constant: $R_H = 1.097 \times 10^7 \text{ m}^{-1}$

• $n_1 = 1$, $n_2 = 4$ for this transition

Step-by-Step Guidance

1. Plug $n_1$ and $n_2$ into the Rydberg equation to solve for $\frac{1}{\lambda}$.

2. Calculate $\lambda$ in meters.

3. Compare your result to the answer choices.

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Q16. Which statement is true about Bohr's model of the emission spectrum of the H atom?

Background

Topic: Atomic Structure – Bohr Model

This question tests your understanding of the key features and limitations of Bohr's model for the hydrogen atom.

Key Terms and Formulas

• Bohr model: Electrons occupy quantized energy levels; emission/absorption corresponds to transitions between levels.

• Photon energy: $E = h\nu = \Delta E_{levels}$

Step-by-Step Guidance

1. Recall the main postulates of Bohr's model and what it successfully explained.

2. Identify which statement(s) accurately reflect Bohr's model and its predictions.

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