CHEM Thermodynamics. Heat calculations

Transcription

1 Thermodynamics Heat calculations l

2 Internal Energy, E The internal energy of other systems that are more complex than the ideal gas cannot be measured. But the internal energy of the system is still proportional to its temperature. E T As the temperature t of the system increases, the internal energy of the system also increases. 2

3 Internal Energy, E Because the internal energy of the system is proportional to its temperature, internal energy is also a state function. (E depends on the state of the system in terms of P, V and T.) Any change in the internal energy of the system is equal to the difference between its initial and final values. E system = E final -E initial 3

4 The First Law of Thermodynamics The total energy of the universe is constant. Energy can be transferred from the system to its surroundings and vice versa, but it cannot be created nor destroyed. The energy lost by the system is gained by the surroundings such that - E system = E surroundings E universe = E system + E surroundings =0 System Surroundings 4

5 There are two ways to change the internal energy of a system: 1. By flow of heat, q Heat is the transfer of thermal energy between the system and the surroundings 2. By doing work, w Work can be converted into heat and vice versa. q and w are process dependent, and are not state functions. 5

6 Units of Energy Calorie: One calorie is defined as the amount of heat required to raise the temperature of 1 g of water by 1 degree celsius. Joule: One joule is the work done when a force of one Newton is used to move an object one meter. 1 J = 1 N m Conversion: 1 calorie = joules 6

7 Sign Convention The internal energy and The internal energy and temperature of a system temperature of a system increase (E > 0) when the decrease(e < 0) when the system gains heat from system loses heat to its its surroundings. surroundings. q > 0 when heat is added to the substance q > 0 when T 2 > T 1 (i.e. T>0) q < 0 when heat is lost by the substance q < 0 when T 2 < T 1 (i.e. T<0) 7

8 Sign Convention The internal energy and temperature of a system decrease (E < 0) when the system loses heat. The internal energy and temperature increase (E > 0) when the system gains heat from its surroundings. HEAT System E < 0 E > 0 System loses heat System gains heat Surroundings 8

9 Example: Answer: g of Pb heated to 100 o C is dropped into g of water at 0 o C. What is the final temperature? Assume that t heat transfer is between the two substances. Specific heat, C p, of Pb is cal/g o C Specific heat, C p, of water is 1.00 cal/g o C Heat lost by the Pb = Heat gained by the water - q Pb = q water - m Pb c Pb (T 2 -T 1 )= m water c water (T 2 -T 1 ) T 2 =? T 2 =? T 1 = 100 o C T 1 = 0 o C T 2 = 23.6 o C 9

10 Calculation : Example 1: A 1.00 g sample of the rocket fuel hydrazine, N 2 H 4, is burned in a bomb calorimeter containing 1200 g of water. The temperature rises From o C to o C. Taking the heat capacity of the calorimeter to be 200 cal/ o C, calculate: (a) q for the combustion of the 1 g sample (b) The molar heat of combustion of hydrazine. Answer: (a) calories (b) -150 kcal/mole 10

11 Calculation: Example 2: When 5.00 g of sodium hydroxide is added to 100. g of water, the temperature rises from 25.0 o C to 37.5 o C. Calculate the molar heat of reaction for the process NaOH (s) Na + (aq) + OH - (aq) taking the specific heat of water to be 1.00 cal/g o C and that of NaOH to be 0.48 cal/g o C. Answer: kcal/mole 11

12 Calculation: Example 3: An experiment is designed to measure the heat of fusion of ice. 25 g of ice at 0 o C was dropped into 195 g of water at 30 o C. The water is contained in a copper calorimeter of mass 100g. The final temperature was 18 o C. Given the specific heat of copper is cal/g o C, find the heat of fusion of ice. Answer: Heat of fusion of ice = 80.1 cal/g 12

13 Calculation: Example 4: Steam at 100 o C is condensed in a large calorimeter. The heat capacity of the calorimeter is expressed as water equivalent to 272 g. The calorimeter contains 2.82 kg of water at 5 o C. The final temperature of 27.8 o C is reached after 115 g of steam has been condensed. Find the latent heat of vaporization of water as given by these data. Answer: Heat of vaporization of water = cal/g 13