Topic 5: Energetics. Heat & Calorimetry. Thursday, March 22, 2012

Transcription

1 Topic 5: Energetics Heat & Calorimetry 1

2 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 2

3 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. 3

4 The heat required to raise the temperature of 1.00 g of water 1 o C is known as a calorie The SI unit for heat is the joule. It is based on the mechanical energy requirements calorie = Joules The energy required to raise 1 pound of water of 1 o F is called a British Thermal Unit or BTU The BTU is widely used in the USA to 4

5 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 5

6 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 o C (or Kelvin) 6

7 Specific Heat values for Some Common Substances Substance CJ g -1 K -1 C J mol -1 K -1 Water (liquid) Water (steam) Water (ice) Copper Aluminum Ethanol Lead

8 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. Heat will flow from the substance at the higher temperature to that at a lower temperature 8

9 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 o C -1 9

10 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 10

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

12 What is the final temperature when 50 grams of water at 20 o C is added to 80 grams water at 60 o C? Assume that the loss of heat to the surroundings is negligible. The specific heat of water is J g-1 o C -1 12

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14 Phase Changes & Heat Energy is required to change the phase of a substance The amount of heat necessary to melt a substance is called the Heat of fusion (ΔH fus ).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 gram The amount of heat necessary to boil a substance is called the Heat of vaporization (ΔH vap ) 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. 13

15 Molar Heat Data for Some Common Substances Substance ΔQ fus ΔQ vap Mercury, Hg 2.29kJ/mol 59.1kJ/mol Ethanol, C 2 H 5 OH 5.02kJ/mol 38.6kJ/mol Water, H 2 O 6.00kJ/mol 40.6kJ/mol Ammonia, NH kJ/mol 23.4kJ/mol Helium, He 0.02kJ/mol 0.08kJ/mol Acetone 5.72kJ/mol 29.1kJ/mol Methanol, CH 3 OH 3.16kJ/mol 35.3kJ/mol 14

16 Heat Transfer Problem 3 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? (C solid wax = 2.18 J/g C, m.p. of wax = 62.0 C, C liquid wax =2.31 J/g C; MM = g/mol, ΔH fusion =70,500 J/mol) 15

17 Heat Transfer Problem 4 Steam at 175 C that occupies a volume of dm 3 and a pressure of 2.60 atm. How much energy would it need to lose to end as liquid water at 20 o C?

18 1 The temperature of 335 g of water changed from 24.5 o C to 26.4 o C. How much heat did this sample absorb? c for water = 4.18 J/g o C (ans kj) 2. How much heat in kilojoules has to be removed from 225g of water to lower its temperature from 25.0 o C to 10.0 o C? (ans kj) 3. How much heat does it take to bring 1.0kg of H2O from 25 o C to 99 o C? (309 kj) 18 18

19 PROBLEMS! 1 The temperature of 335 g of water changed from 24.5 o C to 26.4 o C. How much heat did this sample absorb? c for water = 4.18 J/g o C (ans kj) 2. How much heat in kilojoules has to be removed from 225g of water to lower its temperature from 25.0 o C to 10.0 o C? (ans kj) 3. How much heat does it take to bring 1.0kg of H2O from 25 o C to 99 o C? (309 kj) 4. An insulated cup contains 75.0g of water at o C. A 26.00g sample of metal at o C is added. The final temperature of the water and metal is o C. What is the specific heat of the metal? (ans J/g o C ) 5. A calorimeter has a heat capacity of 1265 J/ o C. A reaction causes the temperature of the calorimeter to change from o C to o C. How many joules of heat were released in this process? (ans kj released ) 1