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Topic 5: Energetics

Heat & Calorimetry

Thursday, March 22, 2012

Heat is energy that is transferred from one object to another due to a difference in temperature

Temperature is a measure of the average kinetic energy of a body

Heat is always transferred from objects at a higher temperature to those at a lower temperature

2Thursday, March 22, 2012

The amount of heat contained by an object depends primarily on three factors:

–The mass of material

–The temperature

–The kind of material and its ability to absorb or retain heat.

3Thursday, March 22, 2012

The heat required to raise the temperature of 1.00 g of water 1 oC is known as a calorie

The SI unit for heat is the joule. It is based on the mechanical energy requirements.

1.00 calorie = 4.184 Joules

The energy required to raise 1 pound of water of 1 oF is called a British Thermal Unit or BTU

The BTU is widely used in the USA to

4Thursday, March 22, 2012

Calorimetry involves the measurement of heat changes that occur in chemical processes or reactions.

The heat change that occurs when a substance absorbs or releases energy is really a function of three quantities:

–The mass

–The temperature change

–The heat capacity of the material

5Thursday, March 22, 2012

The ability of a substance to absorb or retain heat varies widely.

The heat capacity depends on the nature of the material.

The specific heat of a material is the amount of heat required to raise the temperature of 1 gram of a substance 1 oC (or Kelvin)

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Substance CJ g-1 K-1 C J mol-1K-1

Water (liquid) 4.184 75.327

Water (steam) 2.080 37.47

Water (ice) 2.050 38.09

Copper 0.385 24.47

Aluminum 0.897 24.2

Ethanol 2.44 112

Lead 0.127 26.4

Specific Heat values for Some Common Substances

7Thursday, March 22, 2012

When two systems are put in contact with each other, there will be a net exchange of energy between them unless they are at thermal equilibrium, i.e. at the same temperature.

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Heat will flow from the substance at the higher temperature to that at a lower temperature

Thursday, March 22, 2012

The heat equation may be stated as

ΔQ = m C ΔT

where:

ΔQ = Change in heat

m = mass in grams

C = specific heat in J g-1 oC-19

Thursday, March 22, 2012

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Measuring the temperature change in a calorimetry experiment can be difficult since the system is losing heat to the surroundings even as it is generating heat.

By plotting a graph of time v temperature it is possible to extrapolate back to what the maximum temperature would have been had the system not been losing heat to the surroundings. A time v temperature graph

Thursday, March 22, 2012

Calculate the heat that would be required an aluminum cooking pan whose mass is 400 grams, from 20oC to 200oC. The specific heat of aluminum is 0.902 J g-1 oC-1.

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What is the final temperature when 50 grams of water at 20oC is added to 80 grams water at 60oC? Assume that the loss of heat to the surroundings is negligible. The specific heat of water is 4.184 J g-1 oC-1

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Thursday, March 22, 2012

Phase Changes & HeatEnergy is required to change the phase of a substance

The amount of heat necessary to melt a substance is called the Heat of fusion (ΔHfus).The heat of fusion is expressed in

terms of 1 mole or 1 gram

It takes 6.00 kJ of energy to melt 1 mole (18 grams) of ice into liquid water. This is equivalent to about 335 J per gramThe amount of heat necessary to boil a substance is

called the Heat of vaporization (ΔHvap)It may be expressed in terms of 1 mole or 1 gram

It takes 40.6 kJ of energy to boil away 1 mole (18 grams) of water. This is equivalent to about 2240 J

per gram.

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Substance ΔQfus ΔQvap

Mercury, Hg 2.29kJ/mol 59.1kJ/mol

Ethanol, C2H5OH 5.02kJ/mol 38.6kJ/mol

Water, H2O 6.00kJ/mol 40.6kJ/mol

Ammonia, NH3 5.65kJ/mol 23.4kJ/mol

Helium, He 0.02kJ/mol 0.08kJ/mol

Acetone 5.72kJ/mol 29.1kJ/mol

Methanol, CH3OH 3.16kJ/mol 35.3kJ/mol

Molar Heat Data for Some Common Substances

14Thursday, March 22, 2012

How much energy must be lost for 50.0 g of liquid wax at 85.0˚C to cool to room temperature at 25.0˚C?

(Csolid wax= 2.18 J/g˚C, m.p. of wax = 62.0 ˚C, Cliquid wax=2.31 J/g˚C; MM = 352.7 g/mol, ΔHfusion=70,500 J/mol)

Heat Transfer Problem 3

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Steam at 175°C that occupies a volume of 32.75 dm3 and a pressure of 2.60 atm. How much energy would it need to lose to

end as liquid water at 20 oC?

Heat Transfer Problem 4

Thursday, March 22, 2012

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1 The temperature of 335 g of water changed from 24.5oC to 26.4oC. How much heat

did this sample absorb? c for water = 4.18 J/goC (ans. 2.66 kJ)

 

2. How much heat in kilojoules has to be removed from 225g of water to lower its

temperature from 25.0oC to 10.0oC? (ans. –14.1 kJ)

3. How much heat does it take to bring 1.0kg of H2O from 25oC to 99oC? (309 kJ)

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Thursday, March 22, 2012

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PROBLEMS!

1 The temperature of 335 g of water changed from 24.5oC to 26.4oC. How much heat did this

sample absorb? c for water = 4.18 J/goC (ans. 2.66 kJ)

 2. How much heat in kilojoules has to be removed from 225g of water to lower its temperature from

25.0oC to 10.0oC? (ans. –14.1 kJ)

3. How much heat does it take to bring 1.0kg of H2O from 25oC to 99oC? (309 kJ)

4. An insulated cup contains 75.0g of water at 24.00oC. A 26.00g sample of metal at 82.25oC is

added. The final temperature of the water and metal is 28.34oC. What is the specific heat of the metal? (ans 0.971 J/goC )

5. A calorimeter has a heat capacity of 1265 J/oC. A reaction causes the temperature of the

calorimeter to change from 22.34oC to 25.12oC. How many joules of heat were released in this process? (ans. 3.52 kJ released )

Thursday, March 22, 2012