Unit 5 Thermochemistry Thermochemistry looks at relationships between

Unit 5: Thermochemistry

�Thermochemistry looks at relationships between chemical reactions and energy changes involving heat. �The SI unit of energy is the __joule___ (J). �Because a joule is not a large amount of energy, we will often use the unit ____kilojoule_____ (k. J).

Types of Energy – Kinetic energy – energy of motion; increases as the speed of an object increases – Potential energy – energy related to the position of an object; happens when a force is operating on (or acting upon) an object

�Work (w) – energy used to cause an object with mass to move against a force �Force (F) – any kind of push or pull exerted on an object

�Heat – energy transferred from a hotter object to a colder one � A combustion reaction (e. g. burning of natural gas) releases chemical energy stored in molecules of gas in the form of heat.

Energy �The capacity to do work or transfer heat • First Law of Thermodynamics can be summed up in one statement: ENERGY IS CONSERVED Any energy lost by the system must be gained by the surroundings or vice versa.

• Internal energy – a sum of all the ___kinetic_____ and __potential___ energy of all the components in the system • The change in internal energy (ΔE) is the difference in energy before and after work is done or heat is transferred ΔE = Efinal – Einitial ØIf ΔE > 0 (positive), then Efinal > Einitial ØIf ΔE < 0 (negative), then Efinal < Einitial

ΔE = q + w System +q +w −q Surroundings −w

�Enthalpy (H) is a function of thermodynamics. �Accounts for __heat_ _flow__ in chemical changes occurring at: � Constant pressure � No forms of work are performed other than pressure-volume (P- V) work � ΔH represents the __change___ in enthalpy

ΔH = qp System ΔH > 0 HEAT Endothermic ΔH < 0 HEAT Exothermic Surroundings

�Endothermic –the system absorbs heat; heat flows into the system from the surroundings �Exothermic – the system evolves heat; heat flows out of the system into the surroundings

Heat and Work � Identify each energy exchange as heat or work and determine whether the sign of heat or work (relative to the system) is positive or negative: �An ice cube melts and cools the surrounding beverage. (The ice cube is the system. )

Heat and Work � Identify each energy exchange as heat or work and determine whether the sign of heat or work (relative to the system) is positive or negative: �A metal cylinder is rolled up a ramp. (The metal cylinder is the system. )

Heat and Work � Identify each energy exchange as heat or work and determine whether the sign of heat or work (relative to the system) is positive or negative: �Steam condenses on skin, causing a burn. (The condensing steam is the system. )

Internal Energy Problems (ΔE) �Calculate ∆E for a system undergoing an endothermic process in which 15. 6 k. J of heat flows and where 1. 4 k. J of work is done on the system. �A system absorbs 196 k. J of heat and the surroundings do 117 k. J of work on the system. What is the change in internal energy of the system?

Internal Energy Problems (ΔE) �A system releases 622 k. J of heat and does 105 k. J of work on the surroundings. What is the change in internal energy of the system? �The gas in a piston (defined as the system) warms and absorbs 655 J of heat. The expansion performs 344 J of work on the surroundings. What is the change in internal energy for the system? �The air in an inflated balloon (defined as the system) warms over a toaster and absorbs 115 J of heat. As it expands, it does 77 k. J of work. What is the change in internal energy for the system?

Pressure-Volume Work �When work is done involving a gas, work is a function of pressure. �Work done by the expansion of a gas �Pressure is defined as force per unit of area, so when the volume of a gas is changed work was either done on the gas or by the gas. work = −P∆V � 101. 3 J = 1 L·atm

work = −P∆V �To inflate a balloon you must do pressure-volume work on the surroundings. If you inflate a balloon from a volume of 0. 100 L to 1. 85 L against an external pressure of 1. 00 atm, how much work is done (in Joules)?

work = −P∆V �A cylinder equipped with a piston expands against an external pressure of 1. 58 atm. If the initial volume is 0. 485 L and the final volume is 1. 245 L, how much work (in J) is done?

work = −P∆V �When fuel is burned in a cylinder equipped with a piston, the volume expands from 0. 255 L to 1. 45 L against an external pressure of 1. 02 atm. In addition, 875 J is emitted as heat. What is ΔE for the burning of the fuel?

work = −P∆V �The average human lunch expands by 0. 50 L during each breath. If this expansion occurs against an external pressure of 1. 0 atm, how much work (in J) is done during the expansion?

Quantifying Heat Capacity (Calculating with Heat Capacity) �Temperature is a measure of thermal energy within a sample of matter. �Heat is the transfer of thermal energy �Thermal energy always flows from higher to lower temperatures. �Heat transfer stops when the system and surroundings reach the same temperature, a condition called thermal equilibrium.

Quantifying Heat Capacity (Calculating with Heat Capacity) �When a system absorbs heat (q), its temperature changes by ΔT: SYSTEM HEAT (q) �Heat absorbed by the system is directly proportional to its corresponding temperature change:

Quantifying Heat Capacity (Calculating with Heat Capacity) �

Quantifying Heat Capacity (Calculating with Heat Capacity) �Specific heat capacity (Cs)– the amount of heat required to raise the temperature of 1 gram of the substance by 1 °C. �Each substance has its own specific heat capacity (or “specific heat”) �Water’s specific heat = 4. 184 J/°C �Molar heat capacity – the amount of heat required to raise the temperature of 1 mole of a substance by 1 °C.

Temperature Changes and Heat Capacity Heat (J) Mass (grams) Specific Heat (J/g·°C) Change in temperature (°C)

�Suppose you find a penny (minted before 1982, when pennies were almost entirely copper) in the snow. How much heat is absorbed by the penny as it warms from the temperature of the snow, which is 8. 0 °C, to the temperature of your body, 37. 0 °C? Assume the penny is pure copper and has a mass of 3. 10 g.

�How much heat is required to warm 150 g of sand from 25. 0°C to 100. 0°C? (The specific heat of sand is 0. 84 J/°C. ) �How much heat is required to warm 1. 50 L of water from 25. 0°C to 100. 0°C? (Assume the density of 1. 0 g/m. L for water. )

�Suppose that 25 grams of each substance is initially at 27. 0°C. What is the final temperature of each substance upon absorbing 2. 35 k. J of heat? REMEMBER: ΔT = Tfinal - Tinitial �GOLD �SILVER

Thermal Energy Transfer �Heat transfers from the hotter object to the colder object. �If we assume that the two objects are thermally isolated from everything else, then the heat lost by one substance exactly equals the heat gained by the other. �This is according to the law of conservation of energy. �If we define one substance as the system, and one as the surroundings, we can say:

�Suppose a block of metal initially at 55°C is submerged into water initially at 25°C. Thermal energy transfers as heat from the metal to the water. qmetal = - qwater

�A 32. 5 g cube of aluminum initially at 45. 8°C is submerged into 105. 3 g of water at 15. 4°C. What is the final temperature of both substances at thermal equilibrium? Metal Water X X m C Tf Ti ΔT

�A block of copper of unknown mass has an initial temperature of 65. 4°C. The copper is immersed in a beaker containing 95. 7 g of water at 22. 7°C. When the two substances reach thermal equilibrium, the final temperature is 24. 2°C. What is the mass of the copper block? Metal m C Tf Ti ΔT Water

�A 31. 1 g wafer of pure gold, initially at 69. 3°C, is submerged into an unknown mass of water at 64. 2 g of water at 27. 8°C in an insulated container. What is the final temperature of both substances at thermal equilibrium?

Q: Substances A and B, initially at different temperatures, come in contact with each other and reach thermal equilibrium. The mass of substance A is twice the mass of substance B. The specific heat capacity of substance B is twice the specific heat capacity of substance A. Which statement is true about the final temperature of the two substances once thermal equilibrium is reached? a) The final temperature will be closer to the initial temperature of substance A than substance B. b) The final temperature will be closer to the initial temperature of substance B than substance A. c) The final temperature will be exactly midway between the initial temperature of substances A and B.

�Bomb calorimetry – occurs at constant volume and measures ΔE for a reaction. �Performed in a bomb calorimeter, a piece of equipment designed to measure ΔE for combustion reactions. �Reaction occurs in a sealed container.

�Burn a sample inside the bomb and measure the temperature. ΔT is rlated to the heat absorbed by the calorimeter (qcal) by the equation: �If no heat escapes from the calorimeter, then the amount of heat gained by the calorimeter exactly equals the heat released by the reaction:

�When 1. 010 g of sucrose (C 12 H 22 O 11) undergoes combustion in a bomb calorimeter, the temperature rises from 24. 92°C to 28. 33°C. Find ΔErxn for the combustion of sucrose in k. J/mol sucrose. The heat capacity of the bomb calorimeter, determined in a separate experiment, is 4. 90 k. J/°C.

�When 1. 550 g of liquid hexane (C 6 H 14) undergoes combustion in a bomb calorimeter, the temperature rises from 25. 87°C to 38. 13°C. Find ΔErxn for the reaction in k. J/mol hexane. The heat capacity of the bomb calorimeter, determined in a separate experiment, is 5. 73 k. J/°C.

Thermochemical Equations �Balanced chemical equation that also includes ΔHrxn, or heat of reaction �Also known as “enthalpy of reaction” �Extensive property – means that it depends on the amount of material undergoing the reaction. �The ΔHrxn is for the stoichiometry amounts of reactants and products for the reaction AS WRITTEN.

C 3 H 8(g) + 5 O 2(g) 3 CO 2(g) + 4 H 2 O(g) ΔHrxn = -2044 k. J �The equation tells us that when 1 mol of C 3 H 8 reacts with 5 mol of O 2 to form 3 mol of CO 2 and 4 mol of H 2 O, 2044 k. J of heat is released. �We can use the coefficients with the ΔHrxn as a ratio of quantities: 1 mol C 3 H 8 : -2044 k. J 5 mol O 2 : -2044 k. J 1 mol C 3 H 8 -2044 k. J 5 mol O 2

�To determine the amount of released when a certain mass of C 3 H 8 is combusted, use the following plan: g C 3 H 8 k. J mol C 3 H 8 1 mol C 3 H 8 44. 09 g C 3 H 8 -2044 k. J 1 mol C 3 H 8

Examples: � An LP gas tank in a home barbeque contains 13. 2 kg of propane, C 3 H 8. Calculate the heat (in k. J) associated with the complete combustion of all the propane in the tank. C 3 H 8(g) + 5 O 2(g) 3 CO 2(g) + 4 H 2 O(g) ΔHrxn = -2044 k. J

Examples: �

Examples: �

Examples: �

Examples: �

Examples: �

�Coffee-cup calorimetry – occurs at constant pressure and measures ΔH for a reaction. �Used for aqueous reactions, or reactions taking place in water. �Coffee cup calorimeter – equipment that insulates the reaction from the external environment �The reaction occurs in a specifically measured quantity of solution within the calorimeter. �Thermometer measures heat change of the solution. We assume the heat gained by the solution equals the heat lost by the reaction (or vice versa).

�Thermometer measures heat change of the solution. We assume the heat gained by the solution equals the heat lost by the reaction (or vice versa). �Since the reaction happens under constant pressure (open to the atmosphere), qrxn = ΔHrxn. �To find the ΔHrxn per mole, then divide by the number of moles actually reacted.

�Often times, you’ll make the following assumptions: �The total volume of combined solutions is the sum of the two solutions. �The specific heat of the aqueous solution is the specific heat of water.

Game Plan: A. Find qsoln … All values related to SOLUTION B. Change sign for qsoln … Put in k. J if not already C. To find ΔHrxn in k. J/mol …

� When a student mixes 50 m. L of 1. 0 M HCl and 50 m. L of 1. 0 M Na. OH in a coffeecup calorimeter, the temperature of the resultant solution incrases from 21. 0°C to 27. 5°C. Calculate the enthalpy change for the reaction in KJ/mol HCl. Assume the following: the total volume of the solution is 100. 0 m. L, the density is 1. 0 g/m. L, and the specific heat of the solution is that of water.

� When 50. 0 m. L of 0. 100 M Ag. NO 3 and 50. 0 m. L of 0. 100 M HCl are mixed in a constant-pressure calorimeter, the temperature of the mixture increases from 22. 3°C to 23. 11°C. The temperature increase is caused by the following reaction: Ag. NO 3(aq) + HCl(aq) Ag. Cl(s) + HNO 3(aq) ΔH = ? Calculate ΔH for this reaction in k. J/mol Ag. NO 3, assuming that the combined solution has a mass of 100. 0 g and a specific heat of 4. 18 J/g·°C.

� When a 6. 50 -g sample of solid sodium hydroxide dissolves in 100. 0 g of water in a coffee cup calorimeter, the temperature rises from 21. 6°C to 37. 8°C. Calculate ΔH (in k. J/mol Na. OH) for the solution process: Na. OH(s) Na+(aq) + OH-(aq) ΔH = ? Assume the specific heat of the solution is the same as pure water.

� When a 4. 25 -g sample of solid ammonium nitrate dissolves in 60. 0 g of water in a coffee-cup calorimeter, the temperature drops from 22. 0°C to 16. 9°C. Calculate the ΔH (in k. J/mol NH 4 NO 3) for the solution process: NH 4 NO 3 (s) NH 4+ (aq) + NO 3 -(aq) ΔH = ? Assume that the specific heat of the solution is the same as that of pure water.

Potential Energy Diagrams �Diagrams relative energies of reactants and products in a reaction. �The difference between the levels represents the change in enthalpy (ΔH).

Exothermic �energy is released �products have a lower energy �ΔH arrow points down

Endothermic �energy is absorbed �products have a higher energy �ΔH arrow points up

I. �If a chemical equation is multiplied by some factor, then ΔHrxn is also multiplied by the same factor. A + 2 B C 2 A + 4 B 2 C ΔH 1 ΔH 2 = 2 x ΔH 1

II. �If a chemical equation is reversed, then ΔHrxn changes sign. A + 2 B C ΔH 1 C A + 2 B ΔH 2 = - ΔH 1

A + 2 B - ΔH 1 C

�Use thermochemical equation to determine the ΔH for the following: 2 H 2 (g) + O 2 (g) 2 H 2 O (g) ΔH = -483. 6 k. J � 6 H 2 + 3 O 2 6 H 2 O ΔH = __________ � 2 H 2 O 2 H 2 + O 2 ΔH = __________ �H 2 O H 2 + ½ O 2 ΔH = __________ �H 2 + ½ O 2 H 2 O ΔH = __________

�Use thermochemical equation to determine the ΔH for the following: C 3 H 8 (g) + 5 O 2 (g) 3 CO 2(g) + 4 H 2 O(g) ΔH = -2043 k. J � 2 C 3 H 8 (g) + 10 O 2 (g) 6 CO 2(g) + 8 H 2 O(g) ΔH = � 6 CO 2(g) + 8 H 2 O(g) 2 C 3 H 8 (g) + 10 O 2 (g) ΔH = � 3 C 3 H 8 (g) + 15 O 2 (g) 9 CO 2(g) + 12 H 2 O(g) ΔH =

III. �If a chemical equation can be expressed as the sum of a series of steps, then ΔHrxn for the overall equation is the sum of the heats of reactions for each step. �Hess’s Law – the change in enthalpy for a stepwise process is the sum of the enthalpy changes of the steps.

Summing the steps in a chemical reaction to determine ΔHrxn: A + 2 B C 2 D A + 2 B 2 D ΔH 1 ΔH 2 ΔH 3 = ΔH 1 + ΔH 2

Find ΔHrxn for the reaction: C (graphite) C (diamond) Use these reactions with known ΔH‘s: C (graphite) + O 2 CO 2 (g) C (diamond) + O 2 (g) ΔH = ? ΔH = -393. 5 k. J ΔH = +395. 4 k. J

Find ΔHrxn for the reaction: CO (g) + 2 H 2 (g) CH 3 OH (g) ΔH = ? Use these reactions with known ΔH‘s: 2 CO (g) 2 C (s) + O 2 (g) ΔH = + 221. 0 k. J 2 C (s) + O 2 (g) + 4 H 2 (g) 2 CH 3 OH (g) ΔH = - 402. 4 k. J

TYPO ALERT! No subscript of 2 here! Hess’s Law Calculations �C (s) + H 2 O(g) CO (g) + H 2(g) ΔHrxn = ? �Find ΔHrxn for the reaction between C(s) and H 2 O(g) using the listed reactions with known ΔHrxn: CO 2 (g) � 2 CO (g) + O 2 (g) 2 CO 2 (g) � 2 H 2 (g) + O 2 (g) 2 H 2 O (g) � C (s) + O 2 (g) ΔH = – 393. 5 k. J ΔH = – 566. 0 k. J ΔH = – 483. 6 k. J TYPO ALERT! Should be 2 H 2 O.

�C (s) + H 2 O(g) CO 2 (g) + H 2(g) ΔHrxn = ? C (s) + O 2 (g) CO 2 (g) ΔH = – 393. 5 k. J 2 CO (g) + O 2 (g) 2 CO 2 (g) ΔH = – 566. 0 k. J 2 H 2 (g) + O 2 (g) H 2 O (g) ΔH = – 483. 6 k. J

� 4 NH 3 (g) + 5 O 2 (g) 4 NO (g) + 6 H 2 O (g) N 2 (g) + O 2 (g) 2 NO (g) N 2 (g) + 3 H 2 (g) 2 NH 3 (g) 2 H 2 (g) + O 2 (g) 2 H 2 O (g) ΔH = -180. 5 k. J ΔH = -91. 8 k. J ΔH = -483. 6 k. J

�CH 4 (g) + NH 3 (g) HCN (g) + 3 H 2 (g) N 2 (g) + 3 H 2 (g) 2 NH 3 (g) C (s, graphite) + 2 H 2 (g) CH 4 (g) H 2 (g) + 2 C (s, graphite) + N 2 (g) 2 HCN (g) ΔH = -91. 8 k. J ΔH = -74. 9 k. J/mole ΔH = +270. 3 k. J

� 2 Al (s) + 3 Cl 2 (g) 2 Al. Cl 3 (s) 2 Al (s) + 6 HCl (aq) 2 Al. Cl 3 (aq) + 3 H 2 (g) HCl (g) HCl (aq) H 2 (g) + Cl 2 (g) 2 HCl (g) Al. Cl 3 (s) Al. Cl 3 (aq) ΔH = -1049. k. J ΔH = -74. 8 k. J/mole ΔH = -1845. k. J ΔH = -323. k. J/mole

�C 2 H 4 (g) + H 2 (g) C 2 H 6 (g) C 2 H 4 (g) + 3 O 2 (g) 2 CO 2 (g) + 2 H 2 O (l) C 2 H 6 (g) + 7/2 O 2 (g) 2 CO 2 (g) + 3 H 2 O (l) H 2 (g) + 1/2 O 2 (g) H 2 O (l) ΔH = -1411. k. J ΔH = -1560. k. J ΔH = -285. 8 k. J

Enthalpies of Formation

Enthalpies of Formation Standard States (for pure substances) �For a gas: a pressure of exactly 1 atm �For a liquid or solid: Substance in its most stable form, pressure of 1 atm, temperature of interest (usually 25°C) �For a substance in solution: concentration of exactly 1 M

Enthalpies of Formation Standard Enthalpy Change (ΔH°) �The change in enthalpy for a process when all reactants and products are in their standard states. �The degree sign (°) indicates standard states.

Enthalpies of Formation �

Calculating the standard enthalpy change for a reaction Sum np = # of moles (coefficient) for each product nr = # of moles (coefficient) for each reactant

Using Standard Enthalpies of Formation to determine stoichiometric amounts �

Your turn to practice… � 12 th edition book, chapter 10: � 72, 73, 75

Wagon Wheel You have 15 minutes to prepare your topic! For your topic: �Important equations & information �Create a word problem related to your topic �Solve the word problem (and be able to explain the answer. ) TOPICS: 1) Internal energy (combination of heat and work) 2) Calorimetry (using heat capacity to determine q) 3) Hess’s Law (using the Big Mamma equation)