Sample Exercise Chap 6

Herbig Chapter 6 Sample Problems Sample Exercise 1: Energy Diagram The internal energy for Mg(s) and Cl 2 (g) is greater than that of MgCl 2 (s). Sketch an energy diagram that represents the reaction: Mg(s) + Cl 2 (g)  MgCl 2 (s) Sample Exercise 2: Relating Heat and Work to Changes of Internal Energy Two gases, A( g ) and B( g ), are confined in a cylinder-and-piston arrangement like that in the Figure. Substances A and B react to form a solid product: A( g ) + B( g ) → C( s ). As the reaction occurs, the system loses 1150 J of heat to the surroundings. The piston moves downward as the gases react to form a solid. As the volume of the gas decreases under the constant pressure of the atmosphere, the surroundings do 480 J of work on the system. What is the change in the internal energy of the system? 1

Herbig Chapter 6 Sample Problems Chemists Know These Things: Work = Pressure x Volume Sample Exercise 3: Relating Heat and Work to Changes of Internal Energy A sample of nitrogen gas expands in volume from 1.6 L to 5.4 L at constant temperature. What is the work done in joules if the gas expands: a) against a vacuum and b) against a constant pressure of 3.7 atm? Sample Exercise 4 : Why negative PΔV? → → 2

Herbig Chapter 6 Sample Problems Sample Exercise 6: Enthalpy Diagrams The complete combustion of acetic acid, CH 3 COOH(l) to form H 2 O(l) and CO 2 (g) at constant pressure releases 871.7 kJ per mole of CH 3 COOH. a) Write the balanced equation for the reaction. b) Draw the enthalpy diagram for the reaction. Sample Exercise 7: Relating ΔH to Quantities of Reactants and Products How much heat is released when 4.50 g of acetic acid is burned in a constant pressure system? 4

Herbig Chapter 6 Sample Problems Sample Exercise 8: Change in Internal Energy at Constant Temperature Calculate the change in internal energy when 2 moles of hydrogen peroxide decompose to water and oxygen at 25ºC: 2 H 2 O 2 ( l ) → 2 H 2 O( l ) + O 2 (g) Δ H = –196 kJ Sample Exercise 9: Relating Heat, Temperature Change, and Specific Heat Capacity How much heat, in joules, is needed to warm 250 g of water (about 1 cup) from 22 °C (about room temperature) to near its boiling point, 98 °C? Sample Exercise 10: More Fun With Specific Heat A 25.0 g piece of metal at 100.0 ºC is placed into 50.0 g of water at 20.0 ºC. The temperature of the water changes to 22.5 ºC. What is the specific heat of the metal in J/g·ºC? 5

Herbig Chapter 6 Sample Problems Sample Exercise 13: Measuring q rxn Using a Bomb Calorimeter A 0.5865-g sample of lactic acid (HC 3 H 5 O 3 ) is burned in a calorimeter whose heat capacity is 4.812 kJ/°C. The temperature increases from 23.10 °C to 24.95 °C. Calculate the heat of combustion of lactic acid (a) per gram and (b) per mole. Sample Exercise 14: Using Three Equations with Hess’s Law to Calculate Δ H Calculate Δ H for the reaction 2 C( s ) + H 2 ( g ) → C 2 H 2 ( g ) given the following chemical equations and their respective enthalpy changes: C 2 H 2 (g) + 5 / 2 O 2 (g)  2CO 2 (g) + H 2 O(l) ΔH = -1299.6 kJ C(s) + O 2 (g)  CO 2 (g) ΔH = -393.5 kJ H 2 (g) + ½ O 2 (g)  H 2 O(l) ΔH = -285.8 kJ 7

Herbig Chapter 6 Sample Problems Sample Exercise 15: Identifying Equations Associated with Enthalpies of Formation For which of the following reactions at 25°C would the enthalpy change represent a standard enthalpy of formation? For each that does not, what changes are needed to make it an equation whose Δ H is an enthalpy of formation? (a) 2Na(s) + ½O 2 (g) → Na 2 O(s) (b) 2K(l) + Cl 2 (g) → 2KCl(s) (c) C 6 H 6 O 6 (s) → 6C(diamond) + 6H 2 (g) +3O 2 (g) Sample Exercise 16: Calculating an Enthalpy of Reaction from Enthalpies of Formation Use Appendix 3 to calculate the standard enthalpy change for the combustion of 1 mol of benzene, C 6 H 6 ( l ), to form CO 2 ( g ) and H 2 O( l ). Sample Exercise 17: Calculating an Enthalpy of Formation Using an Enthalpy of Reaction The standard enthalpy change for the reaction CaCO 3 (s)  CaO(s) + CO 2 (g) is 178.1 kJ. From the values for the standard enthalpies of formation of CaO( s ) and CO 2 ( g ) given in Appendix 3, calculate the standard enthalpy of formation of CaCO 3 ( s ). 8