During boiling at the boiling point

Answer: T constant; energy goes into breaking IMFs Phase change is isothermal at the transition; energy = ΔHvap.

For a given substance, generally

Answer: ΔHvap ΔHfus Vaporizing fully separates molecules; melting only loosens them — vap requires more energy.

Which segment represents melting (fusion)?

Answer: B (the first horizontal plateau) Plateau B at the lower temperature is where heat is absorbed without temperature change as the solid melts to liquid.

AP Chemistry · Topic 6.5

Energy of Phase Changes Practice

Part of Thermodynamics.(ENE-2.E)

Practice questions

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Sample questions

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Sample 1difficulty 2/5
$During boiling at the boiling point$
- A
  $T constant; energy goes into breaking IMFs$
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- B
  $ΔS = 0$
- C
  $ΔH = 0$
- D
  $T rises$
Why
$Phase change is isothermal at the transition; energy = ΔH_vap.$
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Sample 2difficulty 2/5
$For a given substance, generally$
- A
  $ΔH_vap < ΔH_fus$
- B
  $Both are zero$
- C
  $ΔH_vap > ΔH_fus$
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- D
  $ΔH_vap = ΔH_fus$
Why
$Vaporizing fully separates molecules; melting only loosens them — vap requires more energy.$
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Sample 3difficulty 2/5
$A heating curve for water shows two horizontal plateaus (B and D) and three sloped regions (A, C, E).$
$Which segment represents melting (fusion)?$
- A
  $E$
- B
  $C$
- C
  $A$
- D
  $B (the first horizontal plateau)$
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Why
$Plateau B at the lower temperature is where heat is absorbed without temperature change as the solid melts to liquid.$
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Sample 4difficulty 2/5
$The graph shows temperature versus heat added for a pure substance.$
$Along which segment of the heating curve does the substance undergo melting (solid to liquid)?$
- A
  $Segment C$
- B
  $Segment E$
- C
  $Segment B$
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- D
  $Segment A$
Why
$Flat segment B (first plateau) represents the solid-liquid phase change at constant T while heat is absorbed as ΔHfus.$
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Sample 5difficulty 2/5
$A student dissolves 5.00 g of NH4NO3 (M = 80.04 g/mol) in 100.0 g of water in a coffee-cup calorimeter. Initial water temperature = 22.0 C. After dissolution and complete mixing, the minimum temperature is 18.6 C. Specific heat of solution = 4.18 J/(g*C). Total mass of solution = 105.0 g. Calorimeter heat capacity is negligible.$
$What is the molar heat of solution of NH4NO3?$
- A
  $+23.9 kJ/mol$
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- B
  $+1.49 kJ/mol$
- C
  $-23.9 kJ/mol$
- D
  $-1.49 kJ/mol$
Why
$q_soln = m <em> c </em> deltaT = 105.0 * 4.18 * (-3.4) = -1492 J (lost by solution). Heat absorbed by reaction = +1492 J. mol = 5.00/80.04 = 0.0625. delta H_soln = 1492/0.0625 = 23,900 J/mol = +23.9 kJ/mol (endothermic).$
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