Physical Chemistry Lecture 3
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Questions and Answers

What does a negative value of ΔH indicate about a chemical reaction?

  • Heat is evolved and the temperature of the surroundings increases.
  • The enthalpy of products is greater than that of reactants.
  • Heat is absorbed by the system.
  • The process is exothermic. (correct)

Which of the following represents an endothermic reaction?

  • Sublimation of solid iodine. (correct)
  • Rusting of iron.
  • Freezing water.
  • Mixing water with calcium chloride.

In the formula ΔH = ΔE + PΔV, what does PΔV represent?

  • The heat absorbed by the surroundings.
  • The change in internal energy.
  • The work done by the system on the surroundings. (correct)
  • The energy released during the reaction.

When enthalpy of products is equal to enthalpy of reactants, what can be said about ΔH?

<p>ΔH is zero. (C)</p> Signup and view all the answers

Which process would likely require heat to be absorbed from the surroundings?

<p>Melting of solid salts. (B)</p> Signup and view all the answers

If Δn for a reaction is positive, what can be inferred about the relationship between moles of products and reactants?

<p>There are more moles of products than reactants. (B)</p> Signup and view all the answers

What is the relationship between ΔH and ΔE for a reaction where gas volume changes significantly?

<p>ΔH can be calculated using ΔH = ΔE + ΔnRT. (D)</p> Signup and view all the answers

Which condition must be true for calculating ΔH from ΔE at constant pressure?

<p>The pressure must remain constant. (D)</p> Signup and view all the answers

What is the primary condition under which most chemical reactions are conducted in the laboratory?

<p>Constant pressure (A)</p> Signup and view all the answers

Which equation correctly represents the definition of enthalpy?

<p>H = E + PV (B)</p> Signup and view all the answers

In a chemical reaction involving solids and liquids, how is the term PΔV considered?

<p>Negligible and inconsequential (B)</p> Signup and view all the answers

What does the change in enthalpy (ΔH) for a process involving gases account for?

<p>Both B and C (D)</p> Signup and view all the answers

Which statement about enthalpy is true?

<p>ΔH can be measured accurately (C)</p> Signup and view all the answers

What is the heat of formation (ΔHF) specifically defined as?

<p>The change in enthalpy when one mole of a compound is formed from its elements. (B)</p> Signup and view all the answers

Which of the following statements about the heat of combustion (ΔHc) is correct?

<p>ΔHc is defined for the burning of one mole of a substance in excess air or oxygen. (B)</p> Signup and view all the answers

Which aspect distinguishes the heat of solution (ΔHsolution) from the heat of formation (ΔHF)?

<p>ΔHsolution measures the enthalpy change when a compound is dissolved in a solvent. (C)</p> Signup and view all the answers

How is the heat of formation of hydrogen chloride (HCl) calculated if the overall reaction indicates -44.0 kJ for the formation of HCl?

<p>It is half of -44.0 kJ, hence -22.0 kJ. (A)</p> Signup and view all the answers

In terms of enthalpy changes, which statement accurately describes the function of heat in chemical processes?

<p>Enthalpy changes depend only on the reactants and products, not on the pathway taken. (A)</p> Signup and view all the answers

What is the value of ΔH for the combustion of one mole of methane at 25°C?

<p>-890.4 kJ (B)</p> Signup and view all the answers

What is the significance of the sign of ΔH in a thermochemical equation?

<p>It indicates whether the reaction involves heat absorption or release. (B)</p> Signup and view all the answers

When a reaction is reversed, what happens to ΔH?

<p>Its sign changes. (B)</p> Signup and view all the answers

If the stoichiometric coefficients in a thermochemical equation are doubled, how does ΔH change?

<p>It is multiplied by two. (B)</p> Signup and view all the answers

Which of the following represents the physical state of a solid in a thermochemical equation?

<p>(s) (B)</p> Signup and view all the answers

What would be the heat of formation of methane at constant volume if its heat of formation at constant pressure is -75.83 kJ?

<p>-73.34 kJ (A)</p> Signup and view all the answers

Which of the following must always be included in a thermochemical equation?

<p>The physical states of the reactants and products (C)</p> Signup and view all the answers

What is the value of R used for calculating heat in this context?

<p>8.314 J degree–1 mol–1 (D)</p> Signup and view all the answers

Flashcards

Exothermic Reaction

A reaction that releases heat to the surroundings, resulting in a decrease in the system's internal energy.

Endothermic Reaction

A reaction that absorbs heat from the surroundings, resulting in an increase in the system's internal energy.

Enthalpy Change (ΔH)

The heat absorbed or released by a reaction at constant pressure.

Internal Energy Change (ΔE)

The heat absorbed or released by a reaction at constant volume.

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ΔH = ΔE + PΔV

Equation relating enthalpy change to internal energy change, pressure, and volume change in a reaction.

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Δn

Change in the number of moles of gas in a reaction.

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Heat of Combustion

The amount of heat released when a substance is completely burned in oxygen.

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Heat of Formation (ΔHf)

Enthalpy change when one mole of a compound forms from its elements.

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Heat of Combustion (ΔHc)

Enthalpy change when one mole of a substance burns completely in air/oxygen.

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Heat of Solution (ΔHsolution)

Enthalpy change when one mole of a substance dissolves in a specific amount of solvent.

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ΔH

Change in enthalpy

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Enthalpy

A thermodynamic function that represents the total heat content of a system at constant pressure.

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ΔH

Change in enthalpy; Difference in enthalpy between final state (products) and initial state (reactants).

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Internal Energy (ΔE)

The total energy of a system; Measured at constant volume.

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H=E+PV

Equation describing enthalpy; Sum of internal energy, pressure and volume.

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Constant Pressure Reactions

Reactions where the pressure remains constant during the process.

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Constant Volume Reactions

Reactions where the volume stays constant.

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State Function

Property that only depends on the current state of the system, not the path taken to reach that state.

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Solids and Liquids

For reactions involving these, change in volume (ΔV) is negligible, so ΔH ≈ ΔE.

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Gases

Reactions involving gases often occur at constant pressure, and (PΔV) term is significant.

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Heat of Combustion

The amount of heat released when a substance is burned completely in oxygen.

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Thermochemical Equation

Equation showing heat change in a reaction. Equation must be balanced, give heat change (ΔE or ΔH) for the species, and include physical states.

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Exothermic Reaction

A reaction releasing heat to surroundings.

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Endothermic Reaction

Reaction absorbing heat from surroundings.

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Constant Pressure

Reaction where pressure remains constant.

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Constant Volume

Reaction where volume remains constant.

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ΔH

Change in enthalpy; Difference in enthalpy between final state (products) and initial state (reactants).

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ΔH > 0

Process has absorbed heat; Endothermic reaction, or system absorbs heat.

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ΔH < 0

Process released heat; Exothermic Reaction

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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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Physical Chemistry Lecture Notes (F/R-010) PDF

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.

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