Thermochemistry Chapter 6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Energy is the capacity to do work __________ energy comes from the sun and is earth’s primary energy source __________ energy is the energy associated with the random motion of atoms and molecules __________ energy is the energy stored within the bonds of chemical substances __________ energy is the energy stored within the collection of neutrons and protons in the atom __________ energy is the energy available by virtue of an object’s position __________ energy is the energy available by virtue of an object’s motion 6.1

______ is the transfer of ____________ energy between two bodies that are at different temperatures. Energy Changes in Chemical Reactions ____________ is a measure of the thermal energy C 40 0 C greater thermal energy 6.2 Temperature = Thermal Energy

_______________________ is the study of heat change in chemical reactions. The ____________ is the specific part of the universe that is of interest in the study. open mass & energyExchange: closed energy isolated nothing 6.2

_______________ process is any process that gives off heat – transfers thermal energy from the system to the surroundings. _______________ process is any process in which heat has to be supplied to the system from the surroundings. 2H 2 (g) + O 2 (g) 2H 2 O (l) + energy H 2 O (g) H 2 O (l) + energy energy + 2HgO (s) 2Hg (l) + O 2 (g) 6.2 energy + H 2 O (s) H 2 O (l)

_______________ 6.2

_______________ is the scientific study of the interconversion of heat and other kinds of energy. _______________ _______________ are properties that are determined by the state of the system, regardless of how that condition was achieved. Potential energy of hiker 1 and hiker 2 is the same even though they took different paths. energy, pressure, volume, temperature 6.3  E = E final - E initial  P = P final - P initial  V = V final - V initial  T = T final - T initial

First law of _____________ – energy can be converted from one form to another, but cannot be created or destroyed.  E system +  E surroundings = 0 or  E system = -  E surroundings C 3 H 8 + 5O 2 3CO 2 + 4H 2 O Exothermic chemical reaction! 6.3 Chemical energy ____ by combustion = Energy _______ by the surroundings system surroundings

Another form of the first law for  E system 6.3  E = q + w  E is the change in internal energy of a system q is the heat exchange between the system and the surroundings w is the work done on (or by) the system w = -P  V when a gas expands against a constant external pressure

_______________ Done On the System 6.3 w = F x d w = -P  V P x V = x d 3 = F x d = w F d2d2  V > 0 -P  V < 0 w sys < 0 ____ is not a state function!  w = w final - w initial initialfinal

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? w = -P  V (a)  V = 5.4 L – 1.6 L = 3.8 L P = 0 atm W = -0 atm x 3.8 L = 0 Latm = ____ joules (b)  V = 5.4 L – 1.6 L = 3.8 L P = 3.7 atm w = -3.7 atm x 3.8 L = Latm w = Latm x J 1Latm = - _____ J 6.3

Chemistry in Action: Making Snow  E = q + w q = 0 w < 0,  E < 0  E = C  T  T < 0, SNOW!

Enthalpy and the First Law of Thermodynamics 6.4  E = q + w  E =  H - P  V  H =  E + P  V q =  H and w = -P  V At _________ pressure:

______________ (H) is used to quantify the heat flow into or out of a system in a process that occurs at constant pressure.  H = H (______) – H (_______)  H = heat given off or absorbed during a reaction at constant pressure H products < H reactants  H > 0 H products > H reactants  H < 0 6.4

Thermochemical Equations H 2 O (s) H 2 O (l)  H = 6.01 kJ Is  H negative or positive? System absorbs heat Endothermic  H > kJ are absorbed for every 1 mole of ice that melts at 0 0 C and 1 atm. 6.4

Thermochemical Equations CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O (l)  H = kJ Is  H negative or positive? System gives off heat Exothermic  H < kJ are released for every 1 mole of methane that is burned at 25 0 C and 1 atm. 6.4

H 2 O (s) H 2 O (l)  H = 6.01 kJ The stoichiometric coefficients always refer to the number of moles of a substance Thermochemical Equations If you reverse a reaction, the sign of  H changes H 2 O (l) H 2 O (s)  H = kJ If you multiply both sides of the equation by a factor n, then  H must change by the same factor n. 2H 2 O (s) 2H 2 O (l)  H = 2 x 6.01 = 12.0 kJ 6.4

H 2 O (s) H 2 O (l)  H = 6.01 kJ The physical states of all reactants and products must be specified in thermochemical equations. Thermochemical Equations 6.4 H 2 O (l) H 2 O (g)  H = 44.0 kJ How much heat is evolved when 266 g of white phosphorus (P 4 ) burn in air? P 4 (s) + 5O 2 (g) P 4 O 10 (s)  H = kJ 266 g P 4 1 mol P g P 4 x 3013 kJ 1 mol P 4 x = ______ kJ

A Comparison of  H and  E 2Na (s) + 2H 2 O (l) 2NaOH (aq) + H 2 (g)  H = kJ/mol  E =  H - P  V At 25 0 C, 1 mole H 2 = 24.5 L at 1 atm P  V = 1 atm x 24.5 L = 2.5 kJ  E = kJ/mol – 2.5 kJ/mol = ________ kJ/mol 6.4

The ______ __________ (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 ______ __________ (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 = m x s Heat (q) absorbed or released: q = m x s x  t q = C x  t  t = t final - t initial 6.5

How much heat is given off when an 869 g iron bar cools from 94 0 C to 5 0 C? s of Fe = J/g 0 C  t = t final – t initial = 5 0 C – 94 0 C = ____ 0 C q = ms  t = 869 g x J/g 0 C x –89 0 C= ________ J 6.5

Constant-Volume Calorimetry No heat enters or leaves! q sys = q water + q bomb + q rxn q sys = 0 q rxn = - (q water + q bomb ) q water = m x s x  t q bomb = C bomb x  t 6.5 Reaction at Constant V  H ~ q rxn  H = q rxn

Constant-Pressure Calorimetry No heat enters or leaves! q sys = q water + q cal + q rxn q sys = 0 q rxn = - (q water + q cal ) q water = m x s x  t q cal = C cal x  t 6.5 Reaction at Constant P  H = q rxn

6.5

Chemistry in Action: Fuel Values of Foods and Other Substances C 6 H 12 O 6 (s) + 6O 2 (g) 6CO 2 (g) + 6H 2 O (l)  H = kJ/mol 1 cal = J 1 Cal = 1000 cal = 4184 J Substance  H combustion (kJ/g) Apple-2 Beef-8 Beer-1.5 Gasoline-34

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 __________________ (  H 0 ) as a reference point for all enthalpy expressions. f __________________________ (  H 0 ) is the heat change that results when one _____ of a compound is formed from its _______________ at a pressure of __________. f The standard enthalpy of formation of any element in its most stable form is ____________.  H 0 (O 2 ) = 0 f  H 0 (O 3 ) = 142 kJ/mol f  H 0 (C, graphite) = 0 f  H 0 (C, diamond) = 1.90 kJ/mol f 6.6

The __________________________(  H 0 ) is the enthalpy of a reaction carried out at 1 atm. rxn aA + bB cC + dD H0H0 rxn d  H 0 (D) f c  H 0 (C) f = [+] - b  H 0 (B) f a  H 0 (A) f [+] H0H0 rxn n  H 0 (products) f =  m  H 0 (reactants) f  ________ 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 _________________________.)

C (graphite) + 1/2O 2 (g) CO (g) CO (g) + 1/2O 2 (g) CO 2 (g) C (graphite) + O 2 (g) CO 2 (g) 6.6

Calculate the standard enthalpy of formation of CS 2 (l) given that: C (graphite) + O 2 (g) CO 2 (g)  H 0 = kJ rxn S (rhombic) + O 2 (g) SO 2 (g)  H 0 = kJ rxn CS 2 (l) + 3O 2 (g) CO 2 (g) + 2SO 2 (g)  H 0 = kJ rxn 1. Write the enthalpy of formation reaction for CS 2 C (graphite) + 2S (rhombic) CS 2 (l) 2. Add the given rxns so that the result is the desired rxn. rxn C (graphite) + O 2 (g) CO 2 (g)  H 0 = kJ 2S (rhombic) + 2O 2 (g) 2SO 2 (g)  H 0 = x2 kJ rxn CO 2 (g) + 2SO 2 (g) CS 2 (l) + 3O 2 (g)  H 0 = kJ rxn + C (graphite) + 2S (rhombic) CS 2 (l)  H 0 = (2x-296.1) = _____ kJ rxn 6.6

Benzene (C 6 H 6 ) burns in air to produce carbon dioxide and liquid water. How much heat is released per mole of benzene burned? The standard enthalpy of formation of benzene is kJ/mol. 2C 6 H 6 (l) + 15O 2 (g) 12CO 2 (g) + 6H 2 O (l) H0H0 rxn n  H 0 (products) f =  m  H 0 (reactants) f  - H0H0 rxn 6  H 0 (H 2 O) f 12  H 0 (CO 2 ) f = [+] - 2  H 0 (C 6 H 6 ) f [] H0H0 rxn = [ 12x– x–187.6 ] – [ 2x49.04 ] = ______ kJ 2 mol = __________ kJ/mol C 6 H 6 6.6

Chemistry in Action: Bombardier Beetle Defense C 6 H 4 (OH) 2 (aq) + H 2 O 2 (aq) C 6 H 4 O 2 (aq) + 2H 2 O (l)  H 0 = ? C 6 H 4 (OH) 2 (aq) C 6 H 4 O 2 (aq) + H 2 (g)  H 0 = 177 kJ/mol H 2 O 2 (aq) H 2 O (l) + ½O 2 (g)  H 0 = kJ/mol H 2 (g) + ½ O 2 (g) H 2 O (l)  H 0 = -286 kJ/mol  H 0 = – 286 = -204 kJ/mol Exothermic!

The ______________________ (  H soln ) is the heat generated or absorbed when a certain amount of solute dissolves in a certain amount of solvent.  H soln = H soln - H components 6.7 Which substance(s) could be used for melting ice? Which substance(s) could be used for a cold pack?

The Solution Process for NaCl  H soln = Step 1 + Step 2 = 788 – 784 = 4 kJ/mol 6.7