Q: During the melting of ice at 0°C, which equation should be used to calculate the energy required for the phase change?

$$q = m \times \Delta H_{fus}$$

Q: Given the specific heat of a solid is $$1.6 \frac{J}{g \cdot ^\circ C}$$, how much energy is required to raise the temperature of 15.5 g of the solid from -110°C to -50°C?

$$q = 15.5 \times 1.6 \times (60)$$

Q: Which equation is used to calculate the energy required to heat a substance within a single phase?

$$q = m \times c \times \Delta T$$

Question 1

Which section of a heating curve represents a phase change, and what happens to the temperature during this section?

Accepted Answer

The flat sections; temperature remains constant.

Question 2

During the melting of ice at 0°C, which equation should be used to calculate the energy required for the phase change?

Accepted Answer

$$q = m 	imes \Delta H_{fus}$$

Question 3

A 76.4 g sample of a substance (molar mass = 58.08 g/mol) is converted from a liquid at -30°C to a solid at -110°C. Which steps must be considered in calculating the total energy required?

Accepted Answer

Energy for cooling the liquid, energy for phase change, and energy for cooling the solid.

Question 4

Given the specific heat of a solid is $$1.6 \frac{J}{g \cdot ^\circ C}$$, how much energy is required to raise the temperature of 15.5 g of the solid from -110°C to -50°C?

Accepted Answer

$$q = 15.5 	imes 1.6 	imes (60)$$

Question 5

What is the physical meaning of the enthalpy of fusion ($$\Delta H_{fus}) in $$the context of a heating curve?

Accepted Answer

It is the energy required to melt a solid into a liquid.

Question 6

If 51.2 J of heat is added to a 15.5 g ice cube at -10°C, what will be the resulting state and temperature of the substance?

Accepted Answer

The ice will remain solid but increase in temperature.

Question 7

Which equation is used to calculate the energy required to heat a substance within a single phase?

Accepted Answer

$$q = m 	imes c 	imes \Delta T$$

Question 8

On a cooling curve, what does a horizontal (flat) segment indicate?

Accepted Answer

A phase change is occurring.

Question 9

Why does temperature remain constant during a phase change, even though heat is being added or removed?

Accepted Answer

Because the heat is used to break or form intermolecular bonds.

Question 10

A student heats a sample and observes a plateau on the heating curve. What is happening at the molecular level?

Accepted Answer

Molecules are breaking intermolecular forces to change phase.

Question 11

Which of the following best describes the difference between specific heat and enthalpy of fusion?

Accepted Answer

Specific heat is the energy required to change temperature; enthalpy of fusion is the energy required for a phase change.

Question 12

If a liquid is cooled and reaches its freezing point, which equation should be used to calculate the energy released during solidification?

Accepted Answer

$$q = m 	imes \Delta H_{fus} ($$with a negative sign)

Question 13

A sample of a substance is heated from -110°C to -50°C, then melted at -50°C, and finally heated to 0°C as a liquid. Which calculation sequence is correct?

Accepted Answer

Calculate $$q$$ for heating solid, $$q$$ for melting, $$q$$ for heating liquid.

Question 14

Which of the following statements about phase changes is correct?

Accepted Answer

Phase changes occur at constant temperature.

Question 15

If the specific heat of a liquid is $$2.1 \frac{J}{g \cdot ^\circ C}$$, how much energy is required to heat 10 g of the liquid from -50°C to 0°C?

Accepted Answer

$$q = 10 	imes 2.1 	imes 50$$

Question 16

Which process requires more energy: heating a solid by 10°C or melting the same mass of solid at its melting point? Assume $$c_{solid} = 1.6 \frac{J}{g \cdot ^\circ C}$$ and $$\Delta H_{fus} = 7.23 \frac{J}{g}.$$

Accepted Answer

Melting the solid at its melting point.

Heating and Cooling Curves in Chemistry

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