Ch. 17 Thermochemistry

Transcription

1 Ch. 17 Thermochemistry

2 17.1 The Flow of Energy

3 Energy Transformations Thermochemistry: study of energy changes in chemical reactions and changes in state Chemical potential energy: energy stored in bonds Energy changes occur as heat transfer and/or work Heat (q): energy transfer because of difference in temperatures between objects Warm cool

4 Endothermic and Exothermic Processes System: part of the universe we are focusing on Surroundings: everything else in the universe Law of Conservation of Energy: energy is neither created nor destroyed in a chemical or physical process If energy of system increases, energy of surroundings must decrease by the same amount

5 Endothermic process: heat is absorbed by system Exothermic process: heat is released by system

6

7 Sample Problems A container of melted wax stands at room temperature. What is the direction of heat flow as the liquid wax solidifies? Is the process endothermic or exothermic? When barium hydroxide octahydrate is mixed in a beaker with ammonium thiocyanate, a reaction occurs. The beaker becomes very cold. Is the reaction endothermic or exothermic? 96EDE26A296139A782BD96EDE26A296139A782BD&FORM=VDFSRV&fsscr=0

8 Units for Measuring Heat Flow calorie (cal): quantity of heat needed to raise temperature of 1g of water by 1 C Dietary Calories = kilocalories Joule (J) SI unit of energy: 1 J of heat raises temperature of 1g of water C 1 J = cal J = 1 cal

9 Heat Capacity and Specific Heat Heat capacity: amount of heat needed to increase the temperature 1 C depends on mass and chemical composition Specific heat: amount of heat it takes to raise the temperature of 1g of the substance by 1 C

10 Calculating Specific Heat C=specific heat ΔT=T final - T initial

11 Sample Problems When 435 J of heat is added to 3.4 g of olive oil at 21 C, the temperature increases to 85 C. What is the specific heat of olive oil? How much heat is required to raise the temperature of g of mercury 52 C? The specific heat of mercury is 0.14 J/g C.

12 17.2 Measuring and Expressing Enthalpy Changes

13 Calorimetry Calorimetry: measurement of heat flow in or out of a system Calorimeter: insulated device used to measure absorption or release of heat

14 Constant pressure calorimeter Heat = enthalpy change

15 Constant volume calorimeter Measure the change in temperature of the water to calculate the heat released from burning the sample

16 Calculating ΔH

17 Sample Problems KNOWN C of water = 4.18 J/(g ⁰C) Final Volume= T initial= T final= Density= UNKNOWN ΔH=? kj

18 Sample Problems A copper coin is heated and placed in a foam cup calorimeter containing 32.0 ml of water at 22.0⁰C. The water reaches a temperature of 24.0⁰C. How many joules of heat were released by the coin?

19 Thermochemical Equations Enthalpy can be written as either a product or reactant Heat of reaction: enthalpy change for a reaction just as it is written (ΔH) Heat of combustion: is heat of reaction for the complete burning of one mol of a substance

20 17.3 Heat in Changes of State

21 Changing state involves a change in energy that does not affect the temperature Molar heat of fusion: heat absorbed by one mole of a substance as it melts to a liquid at constant temperature

22 Molar heat of solidification: heat lost when one mole of a liquid substance solidifies at constant temperature

23 Quantity of heat absorbed by melting is exactly the same as the heat lost in freezing

24 Molar heat of vaporization: heat required to vaporize one mole of a given liquid at constant temperature

25 Molar heat of condensation: heat released when one mol of substance condenses

26 During the formation of a solution, heat is either absorbed or released Molar heat of solution: enthalpy change with the dissolution of one mole of a substance

27 Sample Problems ΔH fus = 6.01 kj/mol (for water) 1. How many grams of ice at 0 o C could be melted by the addition of kj of heat? 2. How many kj of heat are required to melt a 50.0-g popsicle at 0 o C? Assume the popsicle has the same molar mass and heat of fusion of water.

28 Sample Problems ΔH vap = 40.7 kj/mol (for water) 1. How much heat is absorbed when 63.7 g H 2 O(l) at 100 o C and kpa is converted to H 2 O(g) at 100 o C? 2. How many kilojoules of heat are absorbed when 0.46 g of C 2 H 5 Cl (bp 12.3 o C) vaporizes at its normal boiling point? The molar heat of vaporization of chloroethane is 24.7 kj/mol.

29 Sample Problems 1. How much heat in kj is released when mol NaOH (s) is dissolved in water? ΔH soln = kj/mol ( for NaOH) 2. How many moles of NH 4 NO 3 (s) must be dissolved in water so that 88.0 kj of heat is absorbed from the water? ΔH soln = 25.7 kj/mol (for NH 4 NO 3 (s)).

30 17.4 Calculating Heats of Reaction

31 Hess s law of heat summation: If you add 2 or more thermochemical equations to give a final chemical equation, the heats of reaction can be added to give the final heat of reaction

32 Standard Heats of Formation Standard heat of formation (ΔH f o ) - change in enthalpy to form one mole of a compound from its elements with all substances in standard state free elements in their standard states have a ΔH f o = 0 kj/mol If a reaction occurs at standard conditions, you can calculate the heat of reaction ΔH o = ΔH f o (products) - ΔH f o (reactants)

33 Sample Problems Calculate ΔH o for the following reaction: Br 2 (g) Br 2 (l) What is the standard heat of reaction for the formation of NO 2 (g) from NO(g) and O 2 (g)? (Use table on page 580 to find ΔH f o values)

34

35

36