Thermochemistry Chapter 6 Copyright 2000 by Houghton Mifflin

Overview ü Introduce the nature of energy and the general topics related to energy problems. ü Familiarize with the experimental procedures for measuring heats of reactions. ü Hess’s law and its applications based on enthalpies. ü Read the present and new sources of energy. Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 2

Energy is the capacity to do work or produce heat • Thermal energy is the energy associated with the random motion of atoms and molecules • Chemical energy is the energy stored within the bonds of chemical substances • Nuclear energy is the energy stored within the collection of neutrons and protons in the atom • Electrical energy is the energy associated with the flow of electrons • Potential energy is the energy available by virtue of an object’s position Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 3

Law of Conservation of Energy can be converted from one form to another but can neither be created nor destroyed. (Euniverse is constant) Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 4

The Two Types of Energy Potential: due to position or composition - can be converted to work (water before falling from dam, gasoline, etc. ) Kinetic: due to motion of the object (water falling and doing work , gasoline burning and driving engine, etc. ) KE = 1/2 mv 2 (m = mass, v = velocity) Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 5

Figure 6. 1: (a) In the initial positions, ball A has a higher potential energy than ball B. (b) After A has rolled down the hill, the potential energy lost by A has been converted to random motions of the components of the hill (frictional heating) and to the increase in the potential energy of B. Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 6

Total Energy = (PE)A + (KE)B + frictional heat Copyright© 2000 by Houghton Mifflin

Energy is a State Function Depends only on the present state of the system - not how it arrived there. It is independent of pathway. Internal Energy or Total Energy E, Enthalpy H, V, P, T are State Functions Heat and Work are not state functions Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 8

Thermodynamics State functions are properties that are determined by the state of the system, regardless of how that condition was achieved. energy, pressure, volume, temperature Potential energy of hiker 1 and hiker 2 is the same even though they took different paths. Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 9

Temperature v. Heat Temperature reflects random motions of particles, therefore related to kinetic energy of the system. Heat involves a transfer of energy between 2 objects due to a temperature difference Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 10

Energy Changes in Chemical Reactions Heat is the transfer of thermal energy between two bodies that are at different temperatures. Temperature is a measure of thermal energy. Temperature = Thermal Energy 900 C 400 C greater thermal energy Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 11

Thermochemistry is the study of heat change in chemical reactions. The system is the specific part of the universe that is of interest in the study. SURROUNDINGS SYSTEM open closed Exchange: mass & energy Copyright© 2000 by Houghton Mifflin Company. All rights reserved. isolated nothing 12

System and Surroundings System: That on which we focus attention Surroundings: Everything else in the universe Universe = System + Surroundings Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 13

Exo and Endothermic Heat exchange accompanies chemical reactions. Exothermic: Heat flows out of the system (to the surroundings). Endothermic: Heat flows into the system (from the surroundings). Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 14

Exothermic process is any process that gives off heat – transfers thermal energy from the system to the surroundings. PE stored in chemical bonds are the source of chemical energy. 2 H 2 (g) + O 2 (g) H 2 O (g) 2 H 2 O (l) + energy Energy lost by system = Energy gained by surrounding Endothermic process is any process in which heat has to be supplied to the system from the surroundings. energy + 2 Hg. O (s) energy + H 2 O (s) 2 Hg (l) + O 2 (g) H 2 O (l) Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 15

Exothermic versus endothermic. Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 16

The study of energy and its conversion is called thermodynamics. The law of conservation of energy is called First Law of Thermodynamics. Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 17

First Law of Thermodynamics: The energy of the universe is constant. Copyright© 2000 by

Mathematical Definition of the First Law E = q + w E = change

Figure 6. 4: (a) The piston, moving a distance ∆h against a pressure P, does work on the surroundings. (b) Since the volume of a cylinder is the area of the base times its height, the change in volume of the gas is given by ∆h x A = V. Expansion Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 20

Work work = force distance since pressure = force / area, work = pressure

Enthalpy = H = E + PV E = H P V H = E + P V At constant pressure, q. P = E + P V, where q. P = H at constant pressure H = energy flow as heat (at constant pressure) E, H, P, V are state functions Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 22

At constant pressure where PV work is allowed the change in enthalpy H is

Signs Convention ü Heat Flowing to the system from surrounding (heating) : E > 0, q or H >0 ü Heat Flowing from the system to the surrounding (cooling) : E < 0, q or H <0 ü Work done by the system on the surrounding : W < 0 (-) ü Work done on the system by the surrounding : W > 0 (+) Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 24

Thermochemical Equations Is H negative or positive? System gives off heat Exothermic H < 0 890. 4 k. J are released for every 1 mole of methane that is combusted at 250 C and 1 atm. CH 4 (g) + 2 O 2 (g) CO 2 (g) + 2 H 2 O (l) H = -890. 4 k. J Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 25

Enthalpy (H) is used to quantify the heat flow into or out of a system in a process that occurs at constant pressure. H = H (products) – H (reactants) H = heat given off or absorbed during a reaction at constant pressure Hproducts < Hreactants Hproducts > Hreactants H < 0 H > 0 Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 26

Heat Capacity Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 27

The specific heat (s) of a substance is the amount of heat (q) required to raise the temperature of one gram of the substance by one degree Celsius. The heat capacity (C) of a substance is the amount of heat (q) required to raise the temperature of a given quantity (m) of the substance by one degree Celsius. C = ms Heat (q) absorbed or released: q = ms t q = C t t = tfinal - tinitial Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 28

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How much heat is given off when an 869 g iron bar cools from 940 C to 50 C? s of Fe = 0. 444 J/g • 0 C t = tfinal – tinitial = 50 C – 940 C = -890 C q = ms t = 869 g x 0. 444 J/g • 0 C x – 890 C = -34, 000 J Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 30

Some Heat Exchange Terms specific heat capacity per gram = J/°C g or J/K

Calorimetry ü The device used experimentally to determine the heat from chemical reaction is called a calorimeter. ü Calorimetry is the science to measure heat changes and it is based on the temperature change. ü Heat capacity is a measure of the body or system ability to absorb heat. Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 32

Two Types of Calorimeters 1. Constant Volume Calorimeter 2. Constant Pressure Calorimeter Copyright© 2000

Constant-Pressure or Coffee Cup Calorimetry May be used to determine many Heats such as heat of solution, neutralization (acid-base), etc. qlost = qgained qrxn = - (qwater + qcalorimeter) qwater = ms t qcal = Ccal t Reaction at Constant P H = qrxn Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 34

Constant-Volume Calorimetry E = q + W W = -p V = 0 E = q. V qsys = qwater + qbomb + qrxn qsys = 0 qrxn = - (qwater + qbomb) qwater = ms t qbomb = Cbomb t Reaction at Constant V H = qrxn No heat enters or leaves! H ~ qrxn Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 35

The enthalpy of solution (DHsoln) is the heat generated or absorbed when a certain amount of solute dissolves in a certain amount of solvent. Hsoln = Hsoln - Hcomponents Which substance(s) could be used for melting ice? Which substance(s) could be used for a cold pack? Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 36

The Solution Process for Na. Cl DHsoln = Step 1 + Step 2 =

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Sample Exercise Calculate the energy per gram for burning 1. 50 g of CH 4 in a constant-volume calorimeter (S=11. 3 KJ/0 C) where t = 7. 30 C. . Energy released = m x s x t = 1. 50 x 11. 3 x 103 x 7. 3 = 83 KJ per 1. 50 g of CH 4 83 Energy released per g = = 55 KJ/g 1. 50 Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 39

Thermochemical Equations • The stoichiometric coefficients always refer to the number of moles of a substance H 2 O (s) • H = 6. 01 k. J If you reverse a reaction, the sign of H changes H 2 O (l) • H 2 O (l) H 2 O (s) H = -6. 01 k. J If you multiply both sides of the equation by a factor n, then H must change by the same factor n. 2 H 2 O (s) 2 H 2 O (l) H = 2 x 6. 01 = 12. 0 k. J Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 40

Thermochemical Equations • The physical states of all reactants and products must be specified in thermochemical equations. H 2 O (s) H 2 O (l) H = 6. 01 k. J H 2 O (l) H 2 O (g) H = 44. 0 k. J How much heat is evolved when 266 g of white phosphorus (P 4) burn in air? P 4 (s) + 5 O 2 (g) 1 mol P 4 O 10 (s) 123. 9 g 266 g H = -3013 k. J Q Copyright© 2000 by Houghton Mifflin Company. All rights reserved. Q = -6470 k. J 41

Hess’s Law Reactants Products The change in enthalpy is the same whether the reaction

Because there is no way to measure the absolute value of the enthalpy of a substance, must I measure the enthalpy change for every reaction of interest? Establish an arbitrary scale with the standard enthalpy of 0 formation ( H f ) as a reference point for all enthalpy expressions. Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 43

Standard enthalpy of formation ( H 0 f) is the heat change that results when one mole of a compound is formed from its elements at a pressure of 1 atm. Example CO 2(g) is formed from C(s) and O 2(g) C(s) + O 2(g) Copyright© 2000 by Houghton Mifflin Company. All rights reserved. H 0 f 44

The standard enthalpy of formation of any element in its most stable form is zero. H 0 f (O 2) = 0 H 0 f (C, graphite) = 0 H 0 f (O 3) = 142 k. J/mol H 0 f (C, diamond) = 1. 90 k. J/mol Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 45

Then Apply Hess’s Law Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 46

Standard States Compound 4 For a gas, pressure is exactly 1 atmosphere. 4 For a solution, concentration is exactly 1 molar. 4 Pure substance (liquid or solid), it is the pure liquid or solid. Element 4 The form [N 2(g), K(s)] in which it exists at 25°C. Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 1 atm and 47

Keep in mind the following concepts. Copyright© 2000 by Houghton Mifflin Company. All rights

Change in Enthalpy Can be calculated from enthalpies of formation of reactants and products. Hrxn° = np Hf (products) nr Hf (reactants) Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 49

0 ) is the enthalpy of The standard enthalpy of reaction ( Hrxn a reaction carried out at 1 atm. a. A + b. B c. C + d. D H 0 rxn = [ c H 0 f (C) + d H 0 f (D) ] - [ a H 0 f (A) + b H 0 f (B) ] H 0 rxn = n H 0 f (products) - m Hf 0 (reactants) Hess’s Law: When reactants are converted to products, the change in enthalpy is the same whether the reaction takes place in one step or in a series of steps. (Enthalpy is a state function. It doesn’t matter how you get there, only where you start and end. ) Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 50

Calculations via Hess’s Law 1. If a reaction is reversed, H is also reversed. N 2(g) + O 2(g) 2 NO(g) N 2(g) + O 2(g) 2. H = 180 k. J If the coefficients of a reaction are multiplied by an integer, H is multiplied by that same integer. 6 NO(g) 3 N 2(g) + 3 O 2(g) Copyright© 2000 by Houghton Mifflin Company. All rights reserved. H = 540 k. J 51

Calculate the standard enthalpy of formation of CS 2 (l) given that: 0 = -393. 5 k. J C(graphite) + O 2 (g) CO 2 (g) Hrxn S(rhombic) + O 2 (g) CS 2(l) + 3 O 2 (g) SO 2 (g) 0 = -296. 1 k. J Hrxn CO 2 (g) + 2 SO 2 (g) 0 = -1072 k. J Hrxn 1. Write the enthalpy of formation reaction for CS 2 C(graphite) + 2 S(rhombic) CS 2 (l) 2. Add the given rxns so that the result is the desired rxn. C(graphite) + O 2 (g) 2 S(rhombic) + 2 O 2 (g) + CO 2(g) + 2 SO 2 (g) CS 2 (l) + 3 O 2 (g) C(graphite) + 2 S(rhombic) H 0 rxn= 0 = -393. 5 k. J CO 2 (g) Hrxn 0 = -296. 1 x 2 k. J 2 SO 2 (g) Hrxn 0 = +1072 k. J Hrxn CS 2 (l) Copyright© 2000 by Houghton Mifflin Company. All rights reserved. -393. 5 + (2 x-296. 1) + 1072 = 86. 3 k. J 52

Benzene (C 6 H 6) burns in air to produce carbon dioxide and liquid water. How much heat is released per mole of benzene combusted? The standard enthalpy of formation of benzene is 49. 04 k. J/mol. 2 C 6 H 6 (l) + 15 O 2 (g) 12 CO 2 (g) + 6 H 2 O (l) H 0 rxn = n H 0 f (products) - m Hf 0 (reactants) H 0 rxn = [ 12 H 0 f (CO 2) + 6 H 0 f (H 2 O)] - [ 2 H 0 f (C 6 H 6)] H 0 rxn = [ 12 x– 393. 5 + 6 x– 187. 6 ] – [ 2 x 49. 04 ] = -5946 k. J = - 2973 k. J/mol C 6 H 6 2 mol Copyright© 2000 by Houghton Mifflin Company. All rights reserved. 53

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Sample Exercise Calculate the work done when 50. 0 g of magnesium (Mg) dissolve

Sample Exercise Determine the heat of reaction at 298 K for the reaction which occurs in a welder's acetylene torch: 2 C 2 H 2(g) + 5 O 2(g) 4 CO 2(g) + 2 H 2 O(l) Given the following equations and Ho values H 2(g) + 1/2 O 2(g) 2 C(s) + H 2(g) C(s) + O 2(g) H 2 O(l) C 2 H 2(g) CO 2(g) Copyright© 2000 by Houghton Mifflin Company. All rights reserved. Ho/k. J = -285. 8 Ho/k. J = +226. 7 Ho/k. J = -393. 5 57

Sample Exercise A 15. 0 g nickel is heated to 100. 00 C and dropped into 55. 0 g of water initially at 230 C. Assuming an isolated system, calculate the final temperature of the nickel and water. Snickel = 0. 444 J/0 Cg. Q lost by nickel = - Q gained by water (ms t) nickel = - t = t f - t i (ms t)water tf is the same. Copyright© 2000 by Houghton Mifflin Company. All rights reserved. ti is different. 58