Questions and Answers
What are the signs of ΔH and ΔS when heat is added and randomness increases?
ΔH is positive; ΔS is positive.
What does high or low entropy depend on?
Molecular mass.
How do you calculate ΔS_fus and ΔS_vap for a substance?
ΔH/Tm and ΔH/Tb.
What is the formula for calculating standard entropy change for a reaction?
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What is the Gibbs free energy equation?
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What do the signs of ΔG indicate about spontaneity?
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What is the possible combination of signs for ΔH and ΔS values in a spontaneous reaction at all temperatures?
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How is the boiling point of a compound related to ΔH_vap and ΔS_vap?
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How do you calculate the standard change in Gibbs free energy for a reaction?
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What do you need to do to the reaction A + B --> 2D and C --> D?
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What is the oxidation state of oxygen?
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What is the formula to calculate the standard cell potential for an electrochemical reaction?
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What occurs to X in electrolysis when it is more reactive than Y?
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A negative E°cell indicates a spontaneous reaction.
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What is the equation relating ΔG, K, and E_cell?
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Study Notes
Phase Changes and Entropy
- ΔH and ΔS: ΔH is positive when heat is added; ΔS is positive with increased randomness (decreased intermolecular forces).
- Entropy Factors: Entropy levels (high or low) are influenced by molecular mass.
Calculating Entropy Changes
- Fusion and Vaporization: ΔS fus and ΔS vap can be calculated using the formulas ΔH/Tm and ΔH/Tb, respectively.
- Standard Entropy Change: Calculated as ΔS rxn = Σ(mols x S° (products)) - Σ(mols x S° (reactants)).
Gibbs Free Energy
- Gibbs Equation: ΔG = ΔH - TΔS; critical for determining spontaneity.
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Spontaneity Indicators:
- ΔG negative indicates spontaneity in the forward reaction.
- ΔG positive indicates spontaneity in the reverse reaction.
Combinations of ΔH and ΔS
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Spontaneity by Temperature:
- Both ΔH negative and ΔS positive: spontaneous at all temperatures.
- ΔH positive and ΔS negative: spontaneous only in reverse at all temperatures.
- ΔH positive and ΔS positive: spontaneous above a certain temperature.
- Both ΔH negative and ΔS negative: spontaneous below a certain temperature.
Boiling Point and Reactions
- Boiling Point Calculation: ΔH vap is determined by TΔS vap.
- Standard Gibbs Free Energy Change: ΔG rxn can be found via ΔG rxn = ΔG prod - ΔG react.
Reaction Adjustments
- For the reaction A + B → 2D, C → D: Multiply C and D by two; reverse the signs of ΔH and ΔS.
Oxidation State Rules
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Common Oxidation States:
- Oxygen: -2
- Alkali metals: +1
- Alkaline earth metals: +2
- Halogens: -1
- Hydrogen: +1 or -1 based on context.
Electrochemistry
- Cell Potential Calculation: E°cell = E°cathode - E°anode, where oxidation occurs at anode and reduction at cathode.
- Electrolysis Process: In a reaction where X is more reactive than Y, X loses electrons (oxidation) and electrons flow toward the Y electrode through the circuit.
- Nonspontaneous Reactions: A negative E°cell indicates nonspontaneity as written.
Free Energy Change in Electrolysis
- Free Energy Calculation: Use ΔG = -nFE cell, where F = 96,500 C/mol (Faraday's constant) and n denotes moles exchanged.
- Relations: Equations connecting ΔG, equilibrium constant (K), and E cell highlight their interdependencies.
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Description
Prepare for your Chemistry Exam 3 with these flashcards that focus on phase changes, entropy, and calculations related to ΔH and ΔS. Each card provides essential definitions and formulas to help you grasp the concepts better. Test your knowledge and boost your confidence for the exam!
