Q: A 5.00 g sample of water is heated from 0.0°C to 100.0°C. The specific heat of liquid water is $$4.184\ \mathrm{J/g\ ^\circ C}. $$How much heat is required for this step?

$$2.09 \times 10^3$\ \mathrm{J}$$

Question 1

Which phase change is represented by the transition from the solid to the liquid state, and what is the sign of the enthalpy change for this process?

Accepted Answer

Fusion (melting); $$\Delta H is $$positive

Question 2

A 150.0 g ice cube melts at 0°C. If the molar heat of fusion for water is $$6.01\ \mathrm{kJ/mol}$$, how much heat is absorbed?

Accepted Answer

$$50.0\ \mathrm{kJ}$$

Question 3

Which of the following substances would you expect to have the highest boiling point: $$\mathrm{H_2O}$$, $$\mathrm{H_2S}$$, or $$\mathrm{H_2Se}$$? Explain based on intermolecular forces.

Accepted Answer

$$\mathrm{H_2O}$$, due to hydrogen bonding

Question 4

Which of the following best explains why vapor pressure increases with temperature?

Accepted Answer

More molecules have enough kinetic energy to escape the liquid phase

Question 5

Given the Clausius-Clapeyron equation $$\ln \frac{P_2$}{P_1$} = -\frac{\Delta H_{vap}}{R} \left( \frac{1}{T_2$} - \frac{1}{T_1$} \right)$$, what does $$\Delta H_{vap}$$ represent?

Accepted Answer

The enthalpy of vaporization

Question 6

Which of the following statements about phase changes is correct?

Accepted Answer

During a phase change, the temperature of the substance remains constant

Question 7

A sample of water is heated from $$-30.0\ ^\circ\mathrm{C} to$$ $$0.0\ ^\circ\mathrm{C}. If $$the mass is $$5.00\ \mathrm{g}$$ and the specific heat of ice is $$2.09\ \mathrm{J/g\ ^\circ C}$$, how much heat is required for this step?

Accepted Answer

$$313.5\ \mathrm{J}$$

Question 8

Which of the following best describes the process of condensation?

Accepted Answer

Gas to liquid, exothermic

Question 9

Why does water have a higher boiling point than ethyl alcohol, even though ethyl alcohol has a lower molecular mass?

Accepted Answer

Water forms stronger hydrogen bonds

Question 10

Which of the following is true about the heat of vaporization ($$\Delta H_{vap}) $$for a given substance?

Accepted Answer

It is always positive because vaporization is endothermic

Question 11

A beaker contains both liquid and vapor phases of a substance at equilibrium. What happens to the vapor pressure if the temperature is increased?

Accepted Answer

Vapor pressure increases

Question 12

Which of the following best explains why ethylene glycol has a much higher boiling point than ethyl ether?

Accepted Answer

Ethylene glycol can form more hydrogen bonds

Question 13

If the enthalpy of vaporization for acetone is $$31.0\ \mathrm{kJ/mol}$$ and the normal boiling point is $$56.1\ ^\circ\mathrm{C}$$, what is the vapor pressure at $$25.0\ ^\circ\mathrm{C}$$? (Use $$R = 8.3145\ \mathrm{J/(mol\ K)}$$ and $$T in $$Kelvin.)

Accepted Answer

$$0.31\ \mathrm{atm}$$

Question 14

Which of the following statements about heating curves is correct?

Accepted Answer

Temperature remains constant during a phase change

Question 15

Which of the following best describes the process of sublimation?

Accepted Answer

Solid to gas, endothermic

Question 16

Which of the following is NOT a correct statement about intermolecular forces (IMFs)?

Accepted Answer

All molecules have hydrogen bonding

Question 17

A 5.00 g sample of water is heated from 0.0°C to 100.0°C. The specific heat of liquid water is $$4.184\ \mathrm{J/g\ ^\circ C}. $$How much heat is required for this step?

Accepted Answer

$$2.09 	imes 10^3$\ \mathrm{J}$$

Question 18

Which of the following best explains why you cannot heat an ice-water mixture above 0°C until all the ice has melted?

Accepted Answer

All added energy goes into breaking intermolecular forces during melting

Question 19

Which of the following is the correct relationship between vapor pressure and intermolecular forces for different compounds at the same temperature?

Accepted Answer

Stronger intermolecular forces result in lower vapor pressure

L12: Phase Changes and Thermodynamics of Phase Transitions

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