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Questions and Answers
What does ΔH represent in a chemical reaction?
What does ΔH represent in a chemical reaction?
In what situation is the change in internal energy (ΔE) equal to the change in enthalpy (ΔH)?
In what situation is the change in internal energy (ΔE) equal to the change in enthalpy (ΔH)?
What is the relationship between change in enthalpy (ΔH) and pressure-volume work for gaseous reactions?
What is the relationship between change in enthalpy (ΔH) and pressure-volume work for gaseous reactions?
Which statement about enthalpy is true?
Which statement about enthalpy is true?
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If a chemical reaction occurs at constant pressure, what can you infer about the thermochemical measurements?
If a chemical reaction occurs at constant pressure, what can you infer about the thermochemical measurements?
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What is the heat of formation when one mole of hydrogen chloride (HCl) is produced from its elements?
What is the heat of formation when one mole of hydrogen chloride (HCl) is produced from its elements?
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What does the heat of combustion (ΔHc) represent?
What does the heat of combustion (ΔHc) represent?
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Which of the following statements about heat of combustion is true?
Which of the following statements about heat of combustion is true?
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What is the heat of solution (ΔHsolution)?
What is the heat of solution (ΔHsolution)?
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If the heat of formation for a compound is positive, what can be inferred about the reaction?
If the heat of formation for a compound is positive, what can be inferred about the reaction?
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What is the heat of combustion of carbon monoxide at constant volume?
What is the heat of combustion of carbon monoxide at constant volume?
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What is the correct value of the heat of formation of methane at constant pressure and 300 K?
What is the correct value of the heat of formation of methane at constant pressure and 300 K?
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Which of the following describes a thermochemical equation?
Which of the following describes a thermochemical equation?
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What is the sign of ΔH for an exothermic reaction?
What is the sign of ΔH for an exothermic reaction?
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If you reverse a thermochemical reaction, what happens to the sign of ΔH?
If you reverse a thermochemical reaction, what happens to the sign of ΔH?
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What must be specified in thermochemical equations?
What must be specified in thermochemical equations?
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When the stoichiometric coefficients are multiplied by a factor n, what must happen to ΔH?
When the stoichiometric coefficients are multiplied by a factor n, what must happen to ΔH?
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How much energy is released when one mole of methane is combusted at standard conditions?
How much energy is released when one mole of methane is combusted at standard conditions?
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What does a negative value of ΔH indicate about a reaction?
What does a negative value of ΔH indicate about a reaction?
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Which of the following processes is considered exothermic?
Which of the following processes is considered exothermic?
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In the equation ΔH = ΔE + PΔV, what does PΔV represent?
In the equation ΔH = ΔE + PΔV, what does PΔV represent?
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What condition corresponds to ΔH being equal to zero?
What condition corresponds to ΔH being equal to zero?
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What happens to the temperature of the surroundings in an endothermic reaction?
What happens to the temperature of the surroundings in an endothermic reaction?
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When calculating ΔH using ΔE, how is ΔV determined?
When calculating ΔH using ΔE, how is ΔV determined?
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Which of the following is a key characteristic of an endothermic reaction?
Which of the following is a key characteristic of an endothermic reaction?
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In which scenario would ΔH be positive?
In which scenario would ΔH be positive?
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Study Notes
Physical Chemistry Lecture 3 Notes
- Thermochemical Measurements: Measurements are made at either constant volume or constant pressure.
- Internal Energy (ΔE): The heat change accompanying a reaction at constant volume. No external work is done.
- Enthalpy (H): A new term introduced to study heat changes at constant pressure and temperature.
- Enthalpy Definition: The sum of internal energy and the product of pressure and volume (H = E + PV)
- Enthalpy as a State Function: Like internal energy, enthalpy can't be measured absolutely, only changes in enthalpy can be determined.
- Change in Enthalpy (ΔH): Accurately measured using the formula: ΔH = Hproducts - Hreactants = Hp - Hr
- ΔH for Solids and Liquids: For solids and liquids, the change in volume (ΔV) is small, and the term PΔV is often negligible. Therefore, ΔH ≈ ΔE.
- ΔH for Gases: For gases, ΔV can be appreciable, and the term PΔV must be accounted for. ΔH = ΔE + PΔV
- Exothermic Reactions: Reactions where heat is released by the system to the surroundings (ΔH < 0)
- Endothermic Reactions: Reactions where heat is absorbed by the system from the surroundings (ΔH > 0)
- Constant Pressure: The value of most chemical reactions is studied at constant pressure.
- Important points: Values of ΔH, ΔE and Δn must be taken into consideration in chemical equations. The physical state of reactants/products in equations must be identified.
Calculation of ΔH from ΔE
- General Reaction: A + B → C + D
- Change in moles (Δn): The difference in the number of moles between products and reactants ((c + d) – (a + b))
- PΔV Calculation: PΔV = RT Δn
Solved Problem (Ethylene Combustion)
- Chemical Equation: C₂H₄(g) + 3O₂(g) → 2CO₂(g) + 2H₂O(l)
- ΔE: Heat of combustion at constant volume: -332.19 kcal at 17°C
- Given Values: Δn = -2, R = 2 cal deg⁻¹ mol⁻¹, T = 290 K
- Calculate ΔH: ΔH = ΔE + ΔnRT = -332.19 kcal + (-2 * 2 cal deg⁻¹ mol⁻¹ * 290 K) = -333.3 kcal
Solved Problem (Carbon Monoxide Combustion)
- Chemical Equation: CO(g) + ½O₂(g) → CO₂(g)
- ΔE: Heat of combustion at constant volume: -283.3 kJ at 17°C
- Given values: Δn = -1, R = 8.314 J deg⁻¹ mol⁻¹, T = 290 K
- Calculate ΔH: ΔΗ = ΔΕ + ΔnRT = -283.3 kJ + (-1 * 8.314 J deg⁻¹ mol⁻¹ * 290 K) = -284.5 kJ
Thermochemical Equations
- Thermochemical equations: Equations that indicate heat changes, either released or absorbed, during a chemical process.
- Key Characteristics: Equations must be balanced; the quantities of substances must correspond to heat changes; Physical states of reactants and products must be specified in parentheses (e.g. (g) for gas, (l) for liquid, (s) for solid).
Is ΔH negative or positive?
- Exothermic: Heat is released by the system, ΔH < 0
- Endothermic: Heat is absorbed by the system, ΔH > 0
Different Types of Heat (Enthalpy) of Reaction
- Heat of Formation (ΔHf): The enthalpy change when one mole of a compound is formed from its elements.
- Heat of Combustion (ΔHc): The enthalpy change when one mole of a substance burns completely in excess air or oxygen.
- Heat of Solution (ΔHsolution): The enthalpy change when one mole of a substance dissolves in a specific amount of solvent at a given temperature.
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Description
Explore the key concepts from Lecture 3 of Physical Chemistry, focusing on thermochemical measurements, internal energy, and enthalpy. Understand how these components interact, particularly in constant pressure and volume settings. Grasp the crucial formulas needed to assess changes in enthalpy and internal energy for various states of matter.
