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Questions and Answers
What does the Enthalpy (H) of a system consist of?
What does the Enthalpy (H) of a system consist of?
Which statement is true concerning the change in enthalpy (ΔH) for reactions involving only solids and liquids?
Which statement is true concerning the change in enthalpy (ΔH) for reactions involving only solids and liquids?
In which conditions are thermochemical measurements typically made?
In which conditions are thermochemical measurements typically made?
What is the relationship between internal energy (ΔE) and enthalpy (ΔH) at constant pressure for gas reactions?
What is the relationship between internal energy (ΔE) and enthalpy (ΔH) at constant pressure for gas reactions?
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How is a change in enthalpy (ΔH) calculated?
How is a change in enthalpy (ΔH) calculated?
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What does a negative value of ΔH indicate in a reaction?
What does a negative value of ΔH indicate in a reaction?
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In an endothermic reaction, which of the following occurs?
In an endothermic reaction, which of the following occurs?
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What is the relationship represented by the equation ΔH = ΔE + P ΔV?
What is the relationship represented by the equation ΔH = ΔE + P ΔV?
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Which of the following is an example of an exothermic process?
Which of the following is an example of an exothermic process?
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For a general reaction, how is Δn calculated?
For a general reaction, how is Δn calculated?
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What is the heat of formation (ΔHF) defined as?
What is the heat of formation (ΔHF) defined as?
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What occurs in a reaction with ΔH = 0?
What occurs in a reaction with ΔH = 0?
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Which of the following statements about heat of combustion (ΔHc) is true?
Which of the following statements about heat of combustion (ΔHc) is true?
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What is the value of ΔH in an exothermic reaction?
What is the value of ΔH in an exothermic reaction?
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What is the primary characteristic of the heat of combustion values?
What is the primary characteristic of the heat of combustion values?
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Which of the following statements is true regarding the equation ΔH = ΔE + Δn RT?
Which of the following statements is true regarding the equation ΔH = ΔE + Δn RT?
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In the context of heat of solution (ΔHsolution), what does it refer to?
In the context of heat of solution (ΔHsolution), what does it refer to?
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If the heat of formation of hydrogen chloride (HCl) is -22.0 kcal, what does this indicate?
If the heat of formation of hydrogen chloride (HCl) is -22.0 kcal, what does this indicate?
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What is the enthalpy of formation of methane at constant pressure and 300 K?
What is the enthalpy of formation of methane at constant pressure and 300 K?
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What does a negative ΔH indicate about a reaction?
What does a negative ΔH indicate about a reaction?
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If the reaction 2H2O (s) → 2H2O (l) has ΔH = 2 x 6.01 kJ, what is ΔH for the reaction H2O (s) → H2O (l)?
If the reaction 2H2O (s) → 2H2O (l) has ΔH = 2 x 6.01 kJ, what is ΔH for the reaction H2O (s) → H2O (l)?
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What is the value of R given in the context?
What is the value of R given in the context?
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Which of the following must be indicated in a thermochemical equation?
Which of the following must be indicated in a thermochemical equation?
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What happens to the sign of ΔH if a reaction is reversed?
What happens to the sign of ΔH if a reaction is reversed?
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For the combustion of 1 mole of methane at 25°C and 1 atm, how much heat is released?
For the combustion of 1 mole of methane at 25°C and 1 atm, how much heat is released?
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What is the heat of combustion of carbon monoxide at 17ºC when measured at constant volume?
What is the heat of combustion of carbon monoxide at 17ºC when measured at constant volume?
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Study Notes
Physical Chemistry Lecture 3
- This lecture covers thermochemistry, specifically enthalpy changes in reactions.
- Thermochemical measurements are taken at constant volume or constant pressure.
Enthalpy of a Reaction
- Internal energy (ΔE) is the heat change accompanying a reaction at constant volume (no external work).
- Enthalpy (H) is a state function, defined as E + PV (internal energy + pressure x volume).
- Enthalpy change (ΔH) in a reaction at constant pressure is a useful measure = Hproducts - Hreactants.
- ΔH can be determined accurately, unlike absolute enthalpy values.
Solids, Liquids, and Gases
- For reactions involving solids and liquids, the volume change (ΔV) is small, and the term PΔV is negligible, thus ΔH ≈ ΔE.
- Reactions involving gases have a noticeable volume change, and PΔV is significant, so ΔH = ΔE + PΔV.
Exothermic and Endothermic Reactions:
- A reaction is exothermic if ΔH < 0 (heat released to surroundings), the system's enthalpy decreases.
- A reaction is endothermic if ΔH > 0 (heat absorbed from surroundings), the system's enthalpy increases.
- If ΔH = 0, there is no change in enthalpy (no heat evolved or absorbed).
Calculating ΔH from ΔE
- ΔH = ΔE + ΔnRT (where Δn = change in moles of gas).
- This equation allows calculating the enthalpy change at constant pressure from the enthalpy change at constant volume.
Solved Problems
- Examples provided demonstrating the calculation of ΔH from ΔE.
- The process involves establishing the balanced chemical equation, noting moles and volume changes.
Thermochemical Equations
- Equations showing heat changes in reactions.
- They should be balanced to reflect the mole ratios.
- The equation must include the physical states (s, l, g, aq).
- Sign (+ or -) shows whether heat is evolved (exothermic) or absorbed (endothermic).
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Description
This lecture focuses on thermochemistry, specifically the concept of enthalpy changes in chemical reactions. It covers internal energy, enthalpy as a state function, and characteristics of exothermic and endothermic reactions. Understanding these principles is essential for interpreting thermochemical measurements at constant volume and pressure.
