Chapter 10- Causes of Change Heat and Chemical change 9/20/2018

Chapter 6.1 The flow of energy -heat: Energy and heat: Thermochemistry- concerned w/ the _____ _____ that occur during _______ ___. Energy- capacity for _____ ___ or ________ _____ Heat ( )- energy that transfers from one object to another due to temperature difference only changes in heat can be detected! flows from warmer  cooler object until objects have the same temperature this energy that transfers is thermal energy Joule (J) the SI unit of heat and energy heat change chemical rxn doing work supplying heat q 9/20/2018

Exothermic and Endothermic Processes Essentially all chemical reactions, and changes in physical state, involve either: release of heat (exothermic), or absorption of heat (endothermic) 9/20/2018

Ex. water freezing, raining, & Ex. ice melting, sweating, & Exothermic - heat flowing out of a system into its surroundings: defined as negative q has a negative value system loses heat as the surroundings heat up Ex. water freezing, raining, & CH4 + 2O2 ® CO2 + 2H2O + Heat Endothermic - heat flowing into a system from its surroundings: defined as positive q has a positive value system gains heat as the surroundings cool down Ex. ice melting, sweating, & CaCO3 + heat ® CaO + CO2 9/20/2018

Heat Capacity Ans:Fe Factor 1: Chemical Structure Ans: Cup A Heat Capacity: the amount of heat needed to increase the temperature of an object exactly 1 oC Heat Capacity of a substance depends on two factors: 1) A 20 g iron bar and 20 g of water (both at room temp) are set outside on a sunny day for 20 minutes. After the 20 minutes elapse which will have a higher temperature? 2) Two cups of room temp water are set outside on a sunny day for 1 hour. Cup A: 20 g of water Cup B: 200 g of water Which will be warmer after the one hour? Ans:Fe Factor 1: Chemical Structure Ans: Cup A Factor 2: Mass 9/20/2018

Heat Capacity and Specific Heat A calorie is defined as the quantity of heat needed to raise the temperature of 1 g of pure water 1 oC a Calorie, written with a capital C, always refers to the energy in food 1 Calorie = 1 kilocalorie = 1000 cal. 1 cal = 4.184 J 9/20/2018

also called “Specific Heat” cal/g°C J/g°C (or J/gK) water 1.00 4.18 specific heat capacity (Cp )- the amount of heat it takes to raise the temperature of 1 gram of the substance by 1 oC at constant pressure also called “Specific Heat” cal/g°C J/g°C (or J/gK) water 1.00 4.18 aluminum 0.22 0.90 copper 0.093 0.39 silver 0.057 0.24 gold 0.031 0.13 9/20/2018

Heat Capacity and Specific Heat For water, C = 4.18 J/(g oC), and also C = 1.00 cal/(g oC) Thus, for water: it takes a long time to heat up, and it takes a long time to cool off! 9/20/2018

Chapter 10 Visual Concepts Specific Heat 9/20/2018

To solve for heat capacity (q)… q = mCp T To calculate, use the formula: “q” is heat, unit- J or cal “m” is mass, unit- g “Cp”or “C”= Specific Heat Capacity units: J/(g oC) or cal/(g oC) J/(gK) or cal/(gK) “T” = change in temperature, (T = Tf -Ti ) unit- oC or K 9/20/2018

Same eq’n, different look! q = m · Cp · T Cp = T = (T = Tf -Ti ) m = q m ·T q m · Cp q Cp·T 9/20/2018

Example Problems 0.14 cal/(g oC) It takes 24.3 calories to heat 15.4 g of a metal from 22 ºC to 33ºC. What is the specific heat of the metal? given: q = 24.3 cal T = Tf -Ti =33 – 22 = 11ºC m = 15.4 g Cp = ?? q = m Cp T Cp = = = q m T (24.3 cal) (15.4 g)(11 ºC) 0.14 cal/(g oC) 9/20/2018

Example Problems q m Cp T = T = 28 ºC 2. Iron has a specific heat of 0.39 J/gºC. How much will the temperature change when 520 J of energy is absorbed by 48.3 g of Fe? given: q= 520 J T =?? m = 48.3 g Cp = 0.39 J/gºC q = mCp T T = q m Cp (520 J) (48.3 g)(0.39 J/gºC) T = T = 28 ºC 9/20/2018

Practice Problem ex. 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? 2.0 J/g·°C 9/20/2018

Enthalpy Enthalpy (H) Is the total energy content of a sample 9/20/2018

Chapter 10 Visual Concepts Enthalpy Change 9/20/2018

10.3 Measuring and Expressing Heat changes: -         Calorimetry- accurate and precise measurement of heat change for chemical and physical processes enthalpy ( )- -         enthalpy change ( ): ∆H = Hproducts – Hreactants H is negative for exothermic rxns H is positive for endothermic rxns H heat content at constant P ∆ H change in enthalpy 9/20/2018

Calorimeter Foam cups are excellent heat insulators, and are commonly used as simple calorimeters The heat absorbed by the water is equal to the heat released by the substance 9/20/2018

Calorimeter Chapter 10 Sample is ignited in high pressured O2 Visual Concepts Calorimeter Sample is ignited in high pressured O2 Energy from combustion is absorbed by water bath Csp of water is known With temperature increase, q can be measured 9/20/2018

Change in enthalpy of a reaction (Using Q to determine H) B. Units of ____: C. Exothermic reactions: ___ is ________. D. Endothermic reactions: ___ is _______. E. Heat transfer between reaction and water: Q released by reaction = Q absorbed by water in calorimeter q = mCpT J/mol; cal/mol; H H negative Temp. of H20 ↑ H positive Temp. of H20 ↓ 9/20/2018

q = mcT q = (500.0 g)(4.184J/goC)(20.3oC) q = 42467.6 J = 42500 J 1. When 5.00 g of liquid toluene, C7H8 is burned, it causes 500.0 mL (500g) of water at 20.0C to rise to 40.3C. What is the heat of combustion (H c) of liquid toluene in kJ/mol? mwater = 500ml = 500g T = 40.3 – 20.0 = 20.3 oC Cp water =4.184 J/(goC) q = ? q = mcT q = (500.0 g)(4.184J/goC)(20.3oC) q = 42467.6 J = 42500 J = 42.5 kJ 5.00 g C7H8 1 mol C7H8 = 0.0543 mol C7H8 92.15 g C7H8 = - 783 kJ/mol of C7H8 exothermic 9/20/2018

+ Q = mcT Q = (1000.0 g)(4.184J/goC)(7.50 oC) Q = 31380 J = 31.4 kJ 2. What is the H of a reaction that lowers the temperature of 1.00 L (1000g) of water by 7.50C when 5.00 g of the substance (molar mass = 120.0 g/mol) is reacted. Give your answer in kJ/mol mwater = 1.00L = 1000g T = 7.50 oC Cp water =4.184 J/(goC) q = ? Q = mcT Q = (1000.0 g)(4.184J/goC)(7.50 oC) Q = 31380 J = 31.4 kJ 5.00 g 1 mol = 0.0417 mol 120.0 g = 753 kJ/mol + endothermic 9/20/2018

Chemistry Happens in MOLES An equation that includes energy is called a thermochemical equation CH4 + 2O2 ® CO2 + 2H2O + 802.2 kJ 1 mole of CH4 releases 802.2 kJ of energy. When you make 802.2 kJ you also make 2 moles of water 9/20/2018

Thermochemical Equations A heat of reaction (H°rxn) is the heat change for the equation, exactly as written The physical state of reactants and products must also be given. Standard conditions for the reaction is 1 atm and 25 oC 9/20/2018

1 mol CH4 802.2 kJ 10. 3 g CH4 16.05 g CH4 1 mol CH4 = 515 kJ CH4 + 2 O2 ® CO2 + 2 H2O + 802.2 kJ If 10. 3 grams of CH4 are burned completely, how much heat will be produced? (just like stoichiometry) 1 mol CH4 802.2 kJ 10. 3 g CH4 16.05 g CH4 1 mol CH4 = 515 kJ 9/20/2018

How many grams of water would be produced with 506 kJ of heat? CH4 + 2 O2 ® CO2 + 2 H2O + 802.2 kJ How many liters of O2 at STP would be required to produce 23 kJ of heat?(22.4L/mol) 23 kJ 2 mol oxygen 22.4 L oxygen 802.2 kJ 1 mol oxygen = 1.3 L oxygen How many grams of water would be produced with 506 kJ of heat? 506 kJ 2 mol water 18.01 g water 802.2 kJ 1 mol water = 22.7 g water 9/20/2018

***Note states of matter*** IV. Change in enthalpy of a reaction (Using Heats of Formation) – 2nd Technique released A. Standard heat of formation (Hf)= The energy ________ or __________ when one mole of a substance is formed from its _________ at standard conditions (______and ______). ***Note difference from STP*** Example:C(s) + O2(g)  CO2(g) Hf = -393.5 kJ/mol of CO2 B. Standard heats of formation are given in Appendix ___ in your book and attached tables. C. Hf = ___ for elements at standard state. It takes ____ energy to go from an element to an element. Example:   Br2(l) = _________ Br2(g) = __________ ***Note states of matter*** @ 25C, bromine is a ________. absorbed elements 1 atm 25 oC A-11 no Heat + Br2(l) Br2(g) 0 kJ/mol 30.71 kJ/mol liquid 9/20/2018

Horxn = Hof products - Hof reactants sum of D. Calculation of Hrxn  (sigma) = _________   Units of H: Hf (Heat of formation) = ________________ Hc (Heat of combustion) = ________________ Hrxn (Heat of reaction) = ________________ Horxn = Hof products - Hof reactants kJ/mol (A compound is formed) kJ/mol (Lit on fire; reacts w/ O2) kJ per rxn 9/20/2018

F. Sample problems: CH4(g) + O2(g) 2 CO2(g) + H2O(l) 2 1. Calculate the heat of combustion of methane gas. (a) Balanced equation (include states):   (b) Sum products and reactants and subtract. (c) Exothermic or endothermic reaction? ____________ CH4(g) + O2(g) 2 CO2(g) + H2O(l) 2 Horxn = Hof products - Hof reactants Horxn = [(CO2) + 2(H2O)] – [(CH4) + 2 (O2)] Horxn = [1 mol(-393.5 kJ/mol) + 2 mol(-285.83 kJ/mol)] – [1 mol(-74.8 kJ/mol) + 2 mol(0 kJ/mol)] Horxn = -890.5 kJ Exothermic = –890.5 kJ/mol CH4 9/20/2018

Horxn = Hof products - Hof reactants 2. Find Hrxn: Cl2(g) + 2 HBr(g)  2 HCl(g) + Br2(g) Horxn = Hof products - Hof reactants Horxn = [2(HCl) + (Br2)] – [(Cl2) + 2(HBr)] Horxn = [2 mol(-92.3 kJ/mol) + 1 mol(30.9 kJ/mol)] – [1 mol( 0 kJ/mol) + 2 mol(-36.2 kJ/mol)] Horxn = -81.3 kJ 9/20/2018

Horxn = Hof products - Hof reactants 3. Find Hrxn and Hc of acetylene gas (C2H2). (a) Balanced equation:   (b) Calculation: 2 C2H2(g) + O2(g) 5 4 CO2(g) + H2O(l) 2 Horxn = Hof products - Hof reactants Horxn = [4(CO2) + 2(H2O)] – [2(C2H2) + 5(O2)] Horxn = [4 mol(-393.5 kJ/mol) + 2 mol(-285.83 kJ/mol)] – [2 mol(226.6 kJ/mol) + 5 mol(0)] a) Horxn = -2599.1 kJ = -1299.5 kJ/mol of C2H2 9/20/2018

Horxn = Hof products - Hof reactants 4. Find Hrxn and Hc of propane gas (C3H8). (a) Balanced equation: (b) Calculation: C3H8(g) + O2(g) 5 3 CO2(g) + H2O(l) 4 Horxn = Hof products - Hof reactants Horxn = [3(CO2) + 4(H2O)] – [(C3H8) + 5(O2)] Horxn = [3 (-393.5) + 4 (-285.83)] – [(-103.85) + 5 (0)] a) Horxn = -2220.0 kJ = -2220.0 kJ/mol of C3H8 9/20/2018