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Questions and Answers
What does a negative value of ΔH indicate about a chemical reaction?
What does a negative value of ΔH indicate about a chemical reaction?
Which of the following represents an endothermic reaction?
Which of the following represents an endothermic reaction?
In the formula ΔH = ΔE + PΔV, what does PΔV represent?
In the formula ΔH = ΔE + PΔV, what does PΔV represent?
When enthalpy of products is equal to enthalpy of reactants, what can be said about ΔH?
When enthalpy of products is equal to enthalpy of reactants, what can be said about ΔH?
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Which process would likely require heat to be absorbed from the surroundings?
Which process would likely require heat to be absorbed from the surroundings?
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If Δn for a reaction is positive, what can be inferred about the relationship between moles of products and reactants?
If Δn for a reaction is positive, what can be inferred about the relationship between moles of products and reactants?
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What is the relationship between ΔH and ΔE for a reaction where gas volume changes significantly?
What is the relationship between ΔH and ΔE for a reaction where gas volume changes significantly?
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Which condition must be true for calculating ΔH from ΔE at constant pressure?
Which condition must be true for calculating ΔH from ΔE at constant pressure?
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What is the primary condition under which most chemical reactions are conducted in the laboratory?
What is the primary condition under which most chemical reactions are conducted in the laboratory?
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Which equation correctly represents the definition of enthalpy?
Which equation correctly represents the definition of enthalpy?
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In a chemical reaction involving solids and liquids, how is the term PΔV considered?
In a chemical reaction involving solids and liquids, how is the term PΔV considered?
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What does the change in enthalpy (ΔH) for a process involving gases account for?
What does the change in enthalpy (ΔH) for a process involving gases account for?
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Which statement about enthalpy is true?
Which statement about enthalpy is true?
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What is the heat of formation (ΔHF) specifically defined as?
What is the heat of formation (ΔHF) specifically defined as?
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Which of the following statements about the heat of combustion (ΔHc) is correct?
Which of the following statements about the heat of combustion (ΔHc) is correct?
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Which aspect distinguishes the heat of solution (ΔHsolution) from the heat of formation (ΔHF)?
Which aspect distinguishes the heat of solution (ΔHsolution) from the heat of formation (ΔHF)?
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How is the heat of formation of hydrogen chloride (HCl) calculated if the overall reaction indicates -44.0 kJ for the formation of HCl?
How is the heat of formation of hydrogen chloride (HCl) calculated if the overall reaction indicates -44.0 kJ for the formation of HCl?
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In terms of enthalpy changes, which statement accurately describes the function of heat in chemical processes?
In terms of enthalpy changes, which statement accurately describes the function of heat in chemical processes?
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What is the value of ΔH for the combustion of one mole of methane at 25°C?
What is the value of ΔH for the combustion of one mole of methane at 25°C?
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What is the significance of the sign of ΔH in a thermochemical equation?
What is the significance of the sign of ΔH in a thermochemical equation?
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When a reaction is reversed, what happens to ΔH?
When a reaction is reversed, what happens to ΔH?
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If the stoichiometric coefficients in a thermochemical equation are doubled, how does ΔH change?
If the stoichiometric coefficients in a thermochemical equation are doubled, how does ΔH change?
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Which of the following represents the physical state of a solid in a thermochemical equation?
Which of the following represents the physical state of a solid in a thermochemical equation?
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What would be the heat of formation of methane at constant volume if its heat of formation at constant pressure is -75.83 kJ?
What would be the heat of formation of methane at constant volume if its heat of formation at constant pressure is -75.83 kJ?
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Which of the following must always be included in a thermochemical equation?
Which of the following must always be included in a thermochemical equation?
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What is the value of R used for calculating heat in this context?
What is the value of R used for calculating heat in this context?
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Study Notes
Physical Chemistry Lecture 3
- Thermochemical Measurements: Made at constant volume or constant pressure. The changes observed under these conditions differ.
- Internal Energy (ΔE): The heat change accompanying a chemical reaction at constant volume. No external work is performed.
- Enthalpy (H): A new term introduced to study heat changes at constant pressure and temperature. Most lab reactions occur at constant pressure.
- Definition of Enthalpy: H = E + PV (where E is internal energy, P is pressure, and V is volume).
- Enthalpy as a State Function: Like internal energy, enthalpy is a state function. It's not possible to measure its absolute value, only changes in enthalpy (ΔH).
- Calculating Change in Enthalpy (ΔH): ΔH = Hproducts – Hreactants = Hp - Hr
- ΔH and Volume Change (Solids and Liquids): For reactions involving solids or liquids only, the volume change (ΔV) is typically small, making the PAV term negligible, so ΔH ≈ ΔE.
- ΔH and Volume Change (Gases): For gas reactions, the volume change (ΔV) is appreciable, and the PΔV term matters. Therefore ΔH = ΔE + PΔV.
Exothermic and Endothermic Reactions
- General Reaction: A + B → C + D (at constant pressure)
- Enthalpy of Reactants and Products: HA, HB, HC, HD are the enthalpies of A, B, C, and D respectively.
- Change in Enthalpy (ΔH): ΔH = (HC + HD) – (HA + HB).
- ΔH = 0: No heat is evolved or absorbed (products and reactants have equal enthalpy).
- ΔH < 0 (Negative): Heat is evolved (exothermic). Products have lower enthalpy than reactants.
- ΔH > 0 (Positive): Heat is absorbed (endothermic). Products have higher enthalpy than reactants.
Calculating ΔH from ΔE
- General Reaction: aA + bB → cC + dD
- Change in Moles (Δn): Δn = (c + d) – (a + b).
- Change in Volume (ΔV): ΔV = Δn × V
- Relationship Between Pressure, Volume, and Moles (Ideal Gas Law): PΔV = ΔnRT
- Relationship between ΔH, ΔE, and Δn: ΔH = ΔE + ΔnRT
Solved Problem (Ethylene Combustion)
- Ethylene Combustion Equation: C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l)
- Given Heat of Combustion at Constant Volume (ΔE): -332.19 kcal (at 17°C)
- Calculate Heat of Combustion at Constant Pressure (ΔH): Using ΔH = ΔE + ΔnRT
- Find Constants: (R = 2 cals degree⁻¹ mol⁻¹, T = 290 K, Δn = -2)
Solved Problem (Carbon Monoxide Combustion)
- Given Heat of Combustion at Constant Volume (ΔE): -283.3 kJ (at 17°C)
- Calculate Heat of Combustion at Constant Pressure (ΔH): Using ΔH = ΔE + ΔnRT
- Find Constants: (R = 8.314 J degree⁻¹ mol⁻¹, T = 290 K, Δn = -1)
Thermochemical Equations
- Thermochemical Equation Definition: An equation that includes the amount of heat change (evolved or absorbed) in a reaction or process.
- Balancing: Equations must be balanced.
- ΔH or ΔE values: The numerical value of ΔH or ΔE corresponding to the given substance quantities must be clearly shown..
- Physical States: The physical states (s, l, g, aq) of reactants and products must be defined in the equation.
Different Types of Heat of Formation
- Heat of Formation (ΔHf): The change in enthalpy when 1 mole of a compound is formed from its constituent elements.
- Heat of Combustion (ΔHc): The enthalpy change when 1 mole of a substance is completely burned in excess oxygen. Heat of combustion is always negative.
- Heat of Solution (ΔHsolution): The enthalpy change when 1 mole of a substance is dissolved in a given quantity of solvent at a given temperature.
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Description
Explore the key concepts of thermochemical measurements, internal energy, and enthalpy as discussed in Physical Chemistry Lecture 3. Understand the differences between constant volume and constant pressure measurements, and learn how to calculate changes in enthalpy for chemical reactions.
