Phase Changes and Heating Curves: Quiz

Phase Changes and Heating Curves: Quiz

7 questions

1. Question 1EasyWhich phase change is exothermic?

   AMeltingBSublimationCCondensationDVaporization
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   Answer: Condensation

   In condensation, gas particles come together and attractions form, releasing energy. Melting, sublimation and vaporization absorb energy.

2. Question 2EasyWhat is the name for a solid changing directly into a gas?

   ADepositionBSublimationCEvaporationDFusion
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   Answer: Sublimation

   Sublimation is solid → gas (e.g. dry ice). Deposition is the reverse, gas → solid.

3. Question 3EasyOn a heating curve, what is happening along a flat section?

   AThe particles are speeding upBA phase change: energy overcomes intermolecular forcesCNo heat is being addedDThe substance is decomposing
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   Answer: A phase change: energy overcomes intermolecular forces

   Heat is still being added, but it goes into changing state, so the temperature (average kinetic energy) does not rise.

4. Question 4MediumHow much heat is needed to vaporize 10.0 g of ethanol (46.07 g/mol, ΔH(vap) = 38.6 kJ/mol)?

   A8.38 kJB386 kJC0.217 kJD1778 kJ
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   Answer: 8.38 kJ

   n = 10.0 g ÷ 46.07 g/mol = 0.2171 mol; q = 0.2171 mol × 38.6 kJ/mol = 8.38 kJ.

5. Question 5MediumWhy does steam at 100 °C cause worse burns than water at 100 °C?

   ASteam is hotterBSteam releases its enthalpy of vaporization when it condenses on the skinCWater cannot transfer heatDSteam contains more oxygen
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   Answer: Steam releases its enthalpy of vaporization when it condenses on the skin

   Both are at 100 °C, but condensing steam first releases 40.7 kJ/mol before cooling, giving many times more heat than the water alone.

6. Question 6MediumA liquid boils when:

   Ait reaches 100 °CBits vapour pressure equals the external pressureCall its molecules have the same energyDits vapour pressure is zero
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   Answer: its vapour pressure equals the external pressure

   Boiling starts when vapour bubbles can form inside the liquid, which happens once the vapour pressure equals the pressure above the liquid. 100 °C is only water's boiling point at 1 atm.

7. Question 7HardHow much heat turns 10.0 g of ice at −20.0 °C into water at 50.0 °C? (c(ice) 2.09, c(water) 4.184 J/(g·°C); ΔH(fus) 6.01 kJ/mol)

   A2.51 kJB3.34 kJC5.85 kJD43.0 kJ
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   Answer: 5.85 kJ

   Warm ice 0.418 kJ + melt (10.0 g ÷ 18.02 g/mol × 6.01 kJ/mol) 3.34 kJ + warm water 2.09 kJ = 5.85 kJ. 2.51 kJ leaves out the melting step.