Heating and Cooling Curves

In Heating and Cooling curves we have the representation of the amount of heat absorbed or released during phase changes. 

Heating & Cooling Curves

Concept: If a solid substance absorbs enough thermal energy it can undergo the phase changes of fusion and vaporization. 

10m

Concept: If a gaseous substance evolves enough thermal energy it can undergo the phase changes of condensation and freezing. 

6m

Example: How much energy (kJ) is required to convert a 76.4 g acetone (molar mass = 58.08 g/mol) as a liquid at -30 oC to a solid at -115.0 oC?

10m

Problem: If 53.2kJ of heat are added to a 15.5g ice cube at - 5.00  oC, what will be the resulting state and temperature of the substance?


13m

Heating and Cooling Curves Additional Practice Problems

After an afternoon party, a small cooler full of ice is dumped onto the hot ground and melts. If the cooler contained 5.60 kg of ice and the temperature of the ground was 43.0°C, calculate the energy that is required to melt all the ice at 0°C. The heat of fusion for water is 80.0 cal/g.

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The heat of vaporization of water is 40.66 kJ/mol. How much heat is absorbed when 2.96 g of water boils at atmospheric pressure?

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At 1 atm, how much energy is required to heat 81.0 g of H2O (s) at -24.0°C to H2O (g) at 135.0°C?

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Calculate the heat energy released when 23.6 g of liquid mercury at 25.00°C is converted to solid mercury at its melting point.

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An 1 atm, how much energy is required to heat 73.0 g of H 2O(s) at - 14.0 °C to H 2O(g) at 139.0 °C?

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At 1 atm, how much energy is required to heat 79.0 g of H2O(s) at-22.0 degree C to H2O(g) at 119.0 ° C? Helpful constants can be found here.

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After an afternoon party, a small cooler full of ice is dumped onto the hot ground and melts. If the cooler contained 7.80 kg of ice and the temperature of the ground was 38.0 ° C, calculate the energy that is required to melt all the ice at 0 ° C. The heat of fusion for water is 80.0 cal/g.

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The following information is given for cadmium at 1 atm:

boiling point = 765.0°C
ΔHvap(765.0°C) = 886.6 J/g
melting point = 321.0°C
Δ Hfus(321.40°C) = 54.40 J/g
specific heat solid = 0.2300 J/g° C
specific heat liquid = 0.2640 J/g°C


A 27.60 g sample of solid cadmium is initially at 303.0°C. If the sample is heated at constant pressure (P = 1 atm), kJ of heat are needed to raise the temperature of the sample to 398.0°C.


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The fluorocarbon compound C2Cl3F3 has a normal boiling point of 47.6- C. The specific heats of C2Cl3F3 (l) and C2Cl3F3(g) are 0.91 J/g-K and 0.67 J/g-K, respectively. The heat of vaporization for the compound is 27.49 kJ/mol. A. Calculate the heat required to convert 37.5 g of C2Cl3F3 from a liquid at 12.75 C to a gas at 75.10 C. Express your answer using two significant figures. 



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Calculate the heat released by cooling 54.0 g H2O from 57.0 C to -3.0 C. The transition described involves the following steps:

1. Cool water from 57  C to 0 C
2. Freeze water
3. Cool ice from 0 C to -3  C

The specific heat values and heat of fusion is provided below: c of water = 4.18 J/g degree C c of ice = 2.09 J/g degree C Delta Hfus = 6.01 kJ/mol


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From the data below, calculate the total heat (in J) needed to convert 0.333 mol of gaseous ethanol at 300°C and 1 atm to liquid ethanol at 25.0°C and 1 atm:

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From the data below, calculate the total heat (in J) needed to convert 22.00 g of ice at −6.00°C to liquid water at 0.500°C:

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Describe the changes (if any) in potential energy and in kinetic energy among the molecules when gaseous PCl3 condenses to a liquid at a fixed temperature.

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(a) Why is the heat of fusion (ΔHfus) of a substance smaller than its heat of vaporization (ΔHvap)?

(b) Why is the heat of sublimation (ΔHsubl) of a substance greater than its ΔHvap?

(c) At a given temperature and pressure, how does the magnitude of the heat of vaporization of a substance compare with that of its heat of condensation?

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The graph below shows the heating curve of water. One can plot heating curves by measuring the increase in temperature of a given amount of ice, water or steam as a function of heat that is added at a constant rate. The first slope (from A to B) refers to the change in temperature of ice as heat is added. At the melting point of ice (B) the temperature remains constant until all of the ice is melted. The graph between C and D represents the increase in the temperature of water, until the boiling point is reached (D). At the boiling point the temperature of the water remains constant until all of the water is converted into steam (from D to E). The last portion of the curve (from E) represents the heating of steam.

Determine if the following statements are True or False.


Select all that are True.
1. Evaporation is an endothermic process because the system is absorbing energy.
2. Cooling water is an exothermic process which decreases the internal energy of the system.
3. Hydrogen bonds between water molecules are broken when evaporation occurs. (From D to E)
4. As water is heated, the system is absorbing energy from the surroundings.
5. At the boiling point of water (1 atm and 373 K) water and steam are in equilibrium with each other.
6. Heating ice (going from A to B), increases the vibrations of the atoms.
7. Heating water (from C to D), decreases the kinetic energy of water.
8. The freezing of water is an endothermic process because the system absorbs energy.

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Label each letter by its phase diagram. 

**extra: what phase change is Solid to Gas and Gas to Solid** 

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You have a sample containing 15 grams of solid ethanol together with 15 g of liquid ethanol. The sample is at the freezing temperature of ethanol, -114C. What happens when you initially place this mixture in a deep freezer which is held at -180C and energy flows out of the sample in the form of heat? 

A. The temperature of the sample decreases while the liquid ethanol freezes.

B. The temperature of the sample remains constant while the liquid ethanol freezes.

C. The temperature of the sample remains constant while the solid ethanol melts.

D. The temperature of the sample increases while the liquid ethanol freezes.

E. The temperature of the sample remains constant and the relative amount of liquid and solid ethanol remains constant.

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You have a 100 g sample of water at standard pressure and 70 C. How much energy is required to heat this sample to steam at 120C?

 

A. 21 kJ

B. 239 kJ

C. 243 kJ

D. 17 kJ

E. 247 kJ

F. 185 kJ

 

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The molar heat capacity of some molecule X(l) is 10 cal/K · mol, its heat of vaporization is 5000 cal/mol and its boiling point is 75C. For the conversion of one mol of X(g) at 75C to one mol of X(l) at 60C,

1. 5150 cal of heat are released by X. 

2. 4850 cal of heat are released by X.

3. 150 cal of heat are released by X.

4. 4850 cal of heat are absorbed by X.

5. 5150 cal of heat are absorbed by X.

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How much energy is required to heat 36.0 g H 2O from a liquid at 65°C to a gas at 115°C?  The following physical data may be useful.

ΔHvap = 40.7 kJ/mol

Cliq = 4.18 J/g°C

Cgas = 2.01 J/g°C

Csol = 2.09 J/g°C

Tmelting = 0°C

Tboiling = 100°C

A) 52.7 kJ      B) 91.7 kJ      C) 87.7 kJ      D) 63.5 kJ      E) 10.9 kJ

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How much energy is required to vaporize 98.6 g of ethanol (C 2H5OH) at its boiling point, if its ΔH vap is 40.5 kJ/mol?

A) 39.9 kJ

B) 11.5 kJ

C) 52.8 kJ

 D) 86.7 kJ

E) 18.9 kJ

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The enthalpy of fusion of methanol (CH 3OH) is 3.16 kJ/mol. How much heat would be absorbed or released upon freezing 25.6 grams of methanol.

1. 0.253 kJ absorbed

2. 2.52 kJ released

3. 3.95 kJ absorbed

4. 2.52 absorbed

5. 3.95 kJ released

6. 0.253 kJ released

 

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 Calculate the heat (kJ) evolved when 100.0 g of steam at 130.0  C is converted to ice at – 5.00 C

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Describe the (physical/chemical) phenomena exhibited in the following diagram: Where would you place heat of fusion & heat of vaporization in the figure shown?

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Refer to the diagram below for the following questions:

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Refer to the diagram below for the following questions:

 

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Refer to the diagram below for the following questions:

 

 

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The heating curve shown was generated by measuring the heat flow and temperature for a solid as it was heated. The slope of the __________ segment corresponds to the heat capacity of the liquid of the substance.

 

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