CHEMISTRY - TRO 4E CH.6 - THERMOCHEMISTRY.

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2 CONCEPT: ENERGY CHANGES AND ENERGY CONSERVATION is the branch of physical science concerned with heat and its transformations to and from other forms of energy. is the branch of chemistry that deals with the heat involved in chemical and physical changes. Energy Changes and Energy Conservation The is the specific part of the universe that we are focused on. The deals with everything outside of it. When talking about the movement of energy or heat between the & we use the terms: Endothermic & Exothermic. HEAT + 2 HgO (s) 2 Hg(l) + O 2 (g) 2 H 2 (g) + O 2 (g) 2 H 2 O (l) + HEAT EXAMPLE: Classify each of the following process as either exothermic or endothermic: a) Fusion of Ice. b) Sublimation of CO2. c) Vaporization of aqueous water. d) Deposition of chlorine gas. e) Condensation of water vapor. Page 2

3 CONCEPT: ENERGY FLOW TO AND FROM A SYSTEM The Law of Thermodynamics states that energy cannot be created nor destroyed, but only converted from one form to another. In chemistry, we are normally concerned with the energy changes associated with the system, not with its surroundings. E = q + w q = H (enthalpy) w = - P V E = q = * For q: (+) when system,,, heat or energy, (-) when system,,, heat or energy. w = * For w : (+) when work done system the surroundings. Key word: volume (-) when work done system the surroundings. Key word: volume EXAMPLE: Which of the following signs on q and w represent a system that is doing work on the surroundings, as well as losing heat to the surroundings? a) q = -, w = - b) q = +, w = + c) q = -, w = + d) q = +, w = - Page 3

4 PRACTICE: ENERGY FLOW TO AND FROM A SYSTEM EXAMPLE: An unknown gas expands in a container increasing the volume from 4.3 L to 8.2 L at a constant pressure of 931 mmhg. a. Calculate the work done (in k) by the gas as it expands. (1 L atm = ) b. Using part A, calculate the internal energy of the system if the system absorbs 2.3 k of energy. c. Using part B, calculate the internal energy of the system if the system does work against a vacuum. PRACTICE: The reaction of nitrogen with hydrogen to make ammonia has an enthalpy, H = k: N2 (g) + 3 H2 (g) 2 NH3 (g) What is in the internal energy of the system if the reaction is done at a constant pressure of 20.0 atm and the volume compresses from 10 L to 5 L? Page 4

5 CONCEPT: CONSTANT-VOLUME CALORIMETRY Every object has its own (C), the quantity of heat required to change its temperature by 1 K. C = q ΔT [in units of K ] (c), the quantity of heat required to change 1 gram of a substance by 1 degree K. c = [in units of g K ] If we know c of a substance, we can algebraically solve the amount of heat absorbed or released: q = EXAMPLE: At constant volume, the heat of combustion of a particular compound is k/mol. When grams of this compound (molar mass = g/mol) was burned in a bomb calorimeter, the temperature of the calorimeter (including its contents) rose by o C. What is the heat capacity of the calorimeter in /K? Page 5

6 PRACTICE: CONSTANT-VOLUME CALORIMETRY EXAMPLE: In an experiment a 9.87 carat (1 carat = 0.200g) diamond is heated to o C and immersed in g of water in a calorimeter. If the initial temperature of the water was 31.0 o C what is the final temperature of the water? (cdiamond = ) g o (cwater = C g o C ). PRACTICE 1: A sample of copper absorbs 35.3 k of heat, which increases the temperature by 25 o C, determine the mass (in kg) of the copper sample if the specific heat capacity of copper is g o C. PRACTICE 2: g of heated metal ore is placed into an insulated beaker containing g of water. Once the metal heats up the final temperature of the water is o C. If the metal gains k of energy, what is the initial temperature of the water? Page 6

7 CONCEPT: CONSTANT-PRESSURE CALORIMETRY The of a reaction can be calculated through the use of a coffee-cup calorimeter. EXAMPLE: You place 50.0 ml of M NaOH into a coffee-cup calorimeter at o C and carefully add 75.0 ml of M HCl, also at o C. After stirring, the final temperature is o C. (Heat capacity and density of water: g o C and 1.00 g ml ). HCl (aq) + NaOH (aq) NaCl (aq) + H2O (l) a) Calculate qsoln (in ) b) Calculate the enthalpy, Hrxn (in /mol), for the formation of water. Page 7

8 CONCEPT: HEAT SUMMATION Many reactions are difficult, even impossible, to carry out in a single chemical step. They may often times require multiple steps to get to the final products. Law states that the enthalpy change ( H) of an overall process is the sum of the enthalpy changes of its individual steps. EXAMPLE: For the following example calculate the unknown H from the given H values of the other equations. Calculate the Hrxn for S(s) O2 (g) SO3 (g) H =? Given the following set of reactions: 1 S (s) O2 (g) 1 2 SO2 (g) H1 = k 2 SO3 (g) 2 SO2(g) + O2 (g) H2 = k Page 8

9 PRACTICE: HEAT SUMMATION A. Calculate the Hrxn for CO(g) + NO (g) CO2 (g) N2 (g) H =? Given the following set of reactions: CO2 (g) CO (g) O2 (g) H1 = k N2 (g) + O2 (g) 2 NO (g) H2 = k B. Calculate the Hrxn for ClF (g) + F2 (g) ClF3 (g) Hrxn =? Given the following reactions: Cl2O (g) + F2O 2 ClF (g) + O2 (g) Hrxn = k 4 ClF3 (g) + 4 O2 (g) 2 Cl2O (g) + 6 F2O (g) Hrxn = k 2 F2 (g) + O2 (g) 2 F2O (g) Hrxn = k Page 9

10 CONCEPT: STANDARD HEATS OF FORMATION ( HRXN) In a equation, 1 mole of a compound forms from its elements. The ( H o f) is the enthalpy change for the chemical equation when all the substances are in their standard states. C (graphite) + 2 H2 (g) CH4 (g) ΔH f o = 74.9k When calculating H o f remember: 1) An element in its standard state (elemental state) is given an H o f of zero. Ex: Na (s) P4 (s) Cl2 (g) S8 (g) 2) Most compounds have a negative H o f. 3) To find the Hrxn use the following formula: o ΔH rxn o = ΔH f (products) o ΔH f (reactan ts) EXAMPLE: The oxidation of ammonia is given by the following reaction: 4 NH3 (g) + 5 O2 (g) 4 NO (g) + 6 H2O (g) Calculate the H o rxn if the ΔH f o value for NH3, NO and H2O are 45.9 k/mol, 90.3 k/mol and k/mol respectively. PRACTICE: Ibuprofen is used as an anti-inflammatory agent used to deal with pain and bring down fevers. If it has a molecular formula of C13H18O2 determine the balanced chemical equation that would give you directly the enthalpy of formation for ibuprofen. Page 10

11 PRACTICE: STANDARD HEATS OF FORMATION ( HRXN) EXAMPLE: Use the following bond strength values (k/mol): C H 412 C O 360 C=O 743 C C 348 H H 436 C=C 611 C C 837 C O 1072 O H 464 O=O 498 Calculate the enthalpy of the reaction shown in the formula below: O C H C C H H C C H + H H + C=O H H Page 11

12 4. An unknown gas expands in a container increasing the volume from 8.7 L to 18.9 L at a constant pressure of 1380 mmhg. (a) Calculate the work done (in ) by the gas as it expands. (1 L atm = ). (b) Calculate the internal energy of the system if the system absorbs of energy. (c) Calculate the internal energy of the system if work was done against a vacuum. (1 L atm = ). Page 12

13 8. Calculate the amount of heat absorbed when 12.0 g of water is heated from 20 o C to 100 o C. (c = /g o C). Page 13

14 g of an unknown metal has an initial temperature of 25 o C. If it absorbs of energy to obtain a final temperature of o C identify the unknown metal. Metal Specific Heat Capacity (/g oc) Au Fe Al Hg Page 14

15 10. Which substance has the highest molar heat capacity? a) Copper (specific heat Cu (s): 0.39 g o C ) b) Silver (specific heat Ag (s): 0.23 g o C ) c) Iron (specific heat Fe(s): 0.46 g o C ) d) Lead (specific heat Pb (s): 0.13 g o C ) Page 15

16 g of heated metal ore is placed into an insulated beaker containing g of water at o C. If the metal gains k of energy, what is the final temperature of the water? (cwater = /g o C). Page 16

17 12. If 53.2 g Al at 25.0 o C is placed in g H2O at 90 o C, what is the final temperature of the mixture? The specific heat capacities of water and aluminum are /g o C and /g o C, respectively. Page 17

18 13. A 20.0 g sample of iron (specific heat Fe (s) = 0.46 applied to the iron sample, calculate its final temperature. g o C ) has an initial temperature of 30.2 o C. If k are Page 18

19 14. A sample of H2O (l) containing 2.50 moles has a final temperature of 45.0 o C. If the sample absorbs 3.00 k of heat, what is the initial temperature of the H2O (l)? The specific heat of H2O (l) is g o C. Page 19

20 Determine the heat released when 80.0 g H2O (l) at 90 o C is cooled to ice at 10.0 o C. Specific Heat of H2O (l) = g o. Specific Heat of H2O (s) = 2.09 C g o C. Heat of Fusion of water = 333 g. Page 20

21 If 1050 g of aluminum metal with a specific heat capacity of g o C at 20 o C is placed in liquid water at 0.00 o C, how many grams of liquid water are frozen by the time that the aluminum metal has warmed to 10 o C? Heat of Fusion of water = 333 g. Page 21