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What is the definition of Enthalpy in a system?
What is the definition of Enthalpy in a system?
Under what conditions are most chemical reactions in the laboratory performed?
Under what conditions are most chemical reactions in the laboratory performed?
What is the correct expression for the change in enthalpy (ΔH) during a reaction?
What is the correct expression for the change in enthalpy (ΔH) during a reaction?
In reactions involving solids and liquids, how does the change in enthalpy (ΔH) compare to the change in internal energy (ΔE)?
In reactions involving solids and liquids, how does the change in enthalpy (ΔH) compare to the change in internal energy (ΔE)?
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What is the relationship between change in enthalpy (ΔH), change in internal energy (ΔE), and the work done (PΔV) for reactions studied at constant temperature and pressure?
What is the relationship between change in enthalpy (ΔH), change in internal energy (ΔE), and the work done (PΔV) for reactions studied at constant temperature and pressure?
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What is the heat of formation (ΔHf)?
What is the heat of formation (ΔHf)?
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What characterizes the heat of combustion (ΔHc) of a substance?
What characterizes the heat of combustion (ΔHc) of a substance?
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Which process does not involve a change in enthalpy as described in the content?
Which process does not involve a change in enthalpy as described in the content?
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During which enthalpy change is one mole of a substance dissolved in a specified quantity of solvent?
During which enthalpy change is one mole of a substance dissolved in a specified quantity of solvent?
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If the heat of formation of hydrogen chloride (HCl) is -44.0 kcal, what would be the value for one mole?
If the heat of formation of hydrogen chloride (HCl) is -44.0 kcal, what would be the value for one mole?
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What is the heat of combustion of carbon monoxide at constant pressure if the heat of combustion at constant volume is -283.3 kJ?
What is the heat of combustion of carbon monoxide at constant pressure if the heat of combustion at constant volume is -283.3 kJ?
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Which of the following statements is true about thermochemical equations?
Which of the following statements is true about thermochemical equations?
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In an exothermic reaction, how is ΔH characterized?
In an exothermic reaction, how is ΔH characterized?
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What is the sign of ΔH when the direction of a reaction is reversed?
What is the sign of ΔH when the direction of a reaction is reversed?
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What is the relationship between ΔH and ΔE at constant pressure?
What is the relationship between ΔH and ΔE at constant pressure?
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Which statement correctly describes an endothermic reaction?
Which statement correctly describes an endothermic reaction?
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If you multiply the coefficients of a thermochemical equation by 3, how should ΔH change?
If you multiply the coefficients of a thermochemical equation by 3, how should ΔH change?
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How is ΔH calculated for a general reaction?
How is ΔH calculated for a general reaction?
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What is the heat of formation of methane at constant volume if its heat of formation at constant pressure is -75.83 kJ?
What is 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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How is the enthalpy change represented in thermochemical equations?
How is the enthalpy change represented in thermochemical equations?
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What indicates that a reaction is exothermic?
What indicates that a reaction is exothermic?
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Which symbol represents a liquid state in a thermochemical equation?
Which symbol represents a liquid state in a thermochemical equation?
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In the equation ΔH = ΔE + PΔV, what does P represent?
In the equation ΔH = ΔE + PΔV, what does P represent?
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What happens to the surroundings' temperature during an exothermic reaction?
What happens to the surroundings' temperature during an exothermic reaction?
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What is represented by Δn in the context of a chemical reaction?
What is represented by Δn in the context of a chemical reaction?
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Which of the following processes is an example of an endothermic reaction?
Which of the following processes is an example of an endothermic reaction?
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Study Notes
Physical Chemistry Lecture 3
- Thermochemical measurements are performed at constant volume or constant pressure.
- Changes observed under these conditions have different magnitudes.
- Internal energy (ΔE) is the heat change during a reaction at constant volume, as no external work is done.
- Most chemical reactions occur at constant pressure.
- Enthalpy (H) is a state function, representing the sum of internal energy and the product of pressure and volume (H = E + PV).
- Enthalpy change (ΔH) is measurable and calculated as ΔH = Hproducts - Hreactants (ΔH = Hp - Hr)
- For reactions involving solids and liquids, the change in volume (ΔV) is small, making the PΔV term negligible. ΔH ≈ ΔE.
- In gas reactions, the PΔV term is significant, and ΔH = ΔE + PΔV.
Enthalpy Changes and Reactions
- A general chemical reaction at constant pressure is A + B → C + D
- If HA and HB are enthalpy of reactants (A&B), and HC and HD are enthalpy of products (C&D), then ΔH = (HC + HD) − (HA + HB).
- ΔH can be zero, negative (exothermic), or positive (endothermic).
- Exothermic reactions release heat to the surroundings (ΔH < 0; products have less enthalpy).
- Endothermic reactions absorb heat from the surroundings (ΔH > 0; products have higher enthalpy).
Calculation of ΔH from ΔE
- ΔH = ΔE + PΔV.
- Change in moles (Δn) = (moles of products) - (moles of reactants).
- For reactions involving gases, ΔH = ΔE + ΔnRT. (where R is the gas constant and T is the temperature.)
- For reactions involving solids and liquids, ΔH = ΔE (PΔV is negligible).
Solved Examples
- Examples of heat of combustion calculations at constant volume and pressure. Calculations using the equation ΔH = ΔE + ΔnRT demonstrate different enthalpy calculation methods.
Thermochemical Equations
- Thermochemical equations describe reactions, including the heat change.
- Equations should be balanced and include the physical states of reactants and products (e.g., (s) for solid, (l) for liquid, (g) for gas, (aq) for aqueous).
ΔH and Exothermic/Endothermic
- Exothermic reactions have negative ΔH. Heat flows out of the system.
- Endothermic reactions have positive ΔH. Heat flows into the system.
Other Kinds of Heat Change
- Heat of Formation (ΔHf): Enthalpy change when one mole of a compound forms from its elements.
- Heat of Combustion (ΔHc): Enthalpy change when one mole of a substance completely burns in oxygen. ΔHc is always negative.
- Heat of Solution (ΔHsol): Enthalpy change when one mole of a substance is dissolved in a specific amount of solvent.
Additional Notes on Thermochemical Equations
- Stoichiometric coefficients represent the number of moles, not just atoms.
- When a reaction is reversed, the sign of ΔH is reversed as well.
- If you multiply the equation by a factor, the resulting ΔH is multiplied by the same factor.
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Description
Explore key concepts in physical chemistry related to thermochemical measurements, internal energy, and enthalpy changes. This quiz focuses on the differences between constant volume and constant pressure scenarios in chemical reactions. Test your understanding of these important thermodynamic principles.
