Physical Chemistry Lecture 3

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Questions and Answers

What does the Enthalpy (H) of a system consist of?

  • The product of pressure and volume only
  • The internal energy plus the volume of the system
  • The sum of internal energy and the product of pressure and volume (correct)
  • Internal energy alone

Which statement is true concerning the change in enthalpy (ΔH) for reactions involving only solids and liquids?

  • ΔH can never be equal to ΔE
  • ΔH is significantly affected by PΔV
  • ΔH is the same as ΔE due to negligible PΔV (correct)
  • ΔH is always greater than ΔE

In which conditions are thermochemical measurements typically made?

  • At varying pressure and volume
  • At constant volume or constant pressure (correct)
  • Only at constant pressure
  • Only at constant temperature

What is the relationship between internal energy (ΔE) and enthalpy (ΔH) at constant pressure for gas reactions?

<p>ΔH is equal to ΔE plus the work done from pressure volume changes (C)</p> Signup and view all the answers

How is a change in enthalpy (ΔH) calculated?

<p>ΔH = HProducts - HReactants (A)</p> Signup and view all the answers

What does a negative value of ΔH indicate in a reaction?

<p>Heat is evolved, causing the temperature of the surroundings to decrease. (D)</p> Signup and view all the answers

In an endothermic reaction, which of the following occurs?

<p>The system absorbs heat from the surroundings. (A)</p> Signup and view all the answers

What is the relationship represented by the equation ΔH = ΔE + P ΔV?

<p>The total internal energy changes equals the enthalpy changes plus work done. (C)</p> Signup and view all the answers

Which of the following is an example of an exothermic process?

<p>Mixing water with strong acids. (A)</p> Signup and view all the answers

For a general reaction, how is Δn calculated?

<p>Total moles of products - total moles of reactants. (A)</p> Signup and view all the answers

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

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

What occurs in a reaction with ΔH = 0?

<p>Heat is neither evolved nor absorbed. (D)</p> Signup and view all the answers

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

<p>It denotes the enthalpy change when a mole of a substance is burnt in excess oxygen. (C)</p> Signup and view all the answers

What is the value of ΔH in an exothermic reaction?

<p>ΔH &lt; 0. (C)</p> Signup and view all the answers

What is the primary characteristic of the heat of combustion values?

<p>They are always negative. (D)</p> Signup and view all the answers

Which of the following statements is true regarding the equation ΔH = ΔE + Δn RT?

<p>The term Δn RT represents the influence of volume change on enthalpy. (A)</p> Signup and view all the answers

In the context of heat of solution (ΔHsolution), what does it refer to?

<p>The energy change associated with dissolving one mole of a solute in a solvent. (D)</p> Signup and view all the answers

If the heat of formation of hydrogen chloride (HCl) is -22.0 kcal, what does this indicate?

<p>It releases energy when one mole is formed. (D)</p> Signup and view all the answers

What is the enthalpy of formation of methane at constant pressure and 300 K?

<p>−75.83 kJ (A)</p> Signup and view all the answers

What does a negative ΔH indicate about a reaction?

<p>The reaction releases heat (C)</p> Signup and view all the answers

If the reaction 2H2O (s) → 2H2O (l) has ΔH = 2 x 6.01 kJ, what is ΔH for the reaction H2O (s) → H2O (l)?

<p>6.01 kJ (A)</p> Signup and view all the answers

What is the value of R given in the context?

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

Which of the following must be indicated in a thermochemical equation?

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

What happens to the sign of ΔH if a reaction is reversed?

<p>It becomes positive (B)</p> Signup and view all the answers

For the combustion of 1 mole of methane at 25°C and 1 atm, how much heat is released?

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

What is the heat of combustion of carbon monoxide at 17ºC when measured at constant volume?

<p>−283.3 kJ (D)</p> Signup and view all the answers

Flashcards

Exothermic Reaction

A chemical reaction that releases heat to the surroundings.

Endothermic Reaction

A chemical reaction that absorbs heat from the surroundings.

Enthalpy Change (ΔH)

The change in heat content of a system at constant pressure.

Internal Energy Change (ΔE)

The change in energy of a system.

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ΔH = ΔE + PΔV

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

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Change in Number of Moles (Δn)

Difference in the number of moles of gaseous products and reactants.

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Heat of Combustion

Heat released when a substance undergoes complete combustion.

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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 excess air/oxygen.

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Heat of Solution (ΔHsolution)

Enthalpy change when one mole of a substance dissolves in a specific solvent at a given temperature.

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ΔH value of H2O(l) to H2O(g)

44.0 kJ, the enthalpy change for one mole of liquid water vaporizing.

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Enthalpy (H)

The sum of internal energy (E) and the product of pressure (P) and volume (V) of a system.

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Enthalpy Change (ΔH)

The difference in enthalpy between products and reactants in a reaction.

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ΔH = ΔE + PΔV

An equation relating enthalpy change to internal energy change and pressure-volume work in a reaction.

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Constant Pressure

A process that occurs at a constant external pressure.

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Constant Volume

A process that occurs in a closed container, preventing any volume change.

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Internal Energy (ΔE)

The total energy contained within a system, excluding any work done by pressure-volume changes.

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State Function

A property of a system whose value only depends on the current conditions of the system(like pressure,temperature, volume) and is independent of its history.

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Solids and Liquids, ΔH and ΔE

For reactions involving solids and liquids, the pressure-volume change (PΔV) in ΔH is minimal and negligible. Therefore, ΔH and ΔE are nearly equal.

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Gases, PΔV and ΔH

For reactions involving gases, the pressure-volume change (PΔV) can be significant in calculating enthalpy change (ΔH), and is considerable.

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Thermochemical Equation

An equation showing a chemical reaction and the heat change (evolved or absorbed).

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Exothermic Reaction

A reaction releasing heat to the surroundings.

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Endothermic Reaction

A reaction absorbing heat from the surroundings.

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Constant Pressure

A process occurring at a constant external pressure.

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Constant Volume

A process occurring in a closed container, preventing volume change.

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Heat of Combustion

Heat released when a substance undergoes complete combustion.

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Enthalpy Change (ΔH)

The heat change at constant pressure.

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Internal Energy Change (ΔE)

The heat change at constant volume.

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ΔH = ΔE + PΔV

Relates enthalpy change, internal energy change, pressure, and volume changes.

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Physical States

Important for thermochemical equations; (s), (l), (g), (aq).

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ΔH sign change

Reversing a reaction changes the sign of ΔH.

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Stoichiometric coefficients

Refer to moles in thermochemical equations indicating the needed amounts.

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Multiplying equations, ΔH

If you multiply (or divide) both sides of an equation by a factor, ΔH changes by the same factor.

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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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