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

is the capacity to do work ____________ 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 associated with the flow of electrons ____________ energy is the energy available by virtue of an object’s position ____________ energy is the energy of an object in motion 6.1

_______________________ is the transfer of thermal energy between two bodies that are at different temperatures. Energy Changes in Chemical Reactions _________________ is a measure of the thermal energy. Temperature = Thermal Energy 6.2

________________ is the study of heat changes 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 SYSTEM SURROUNDINGS 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 ________________

____________________ 6.7  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 p.178

________________ (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 ________________ H products < H reactants  H < 0 (negative) H products > H reactants  H > 0 (positive) 6.3

Enthalpy and the First Law of Thermodynamics 6.7  E = q + w At ________________________, q =  H and w = -P  V  E =  H - P  V  H =  E + P  V

Thermochemical Equations H 2 O (s) H 2 O (l)  H = 6.01 kJ Is  H negative or positive? System absorbs heat ___________________  H > kJ are absorbed for every 1 mole of ice that melts at 0 0 C and 1 atm. 6.3

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 ___________________  H < kJ are released for every 1 mole of methane that is burned at 25 0 C and 1 atm. 6.3

H 2 O (s) H 2 O (l)  H = 6.01 kJ The stoichiometric coefficients always refer to the number of _______________________________ Thermochemical Equations If you reverse a reaction, the sign of  H _____________ 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 __________________________________. 2H 2 O (s) 2H 2 O (l)  H = 2 x 6.01 = 12.0 kJ 6.3

H 2 O (s) H 2 O (l)  H = 6.01 kJ The ____________________ states of all reactants and products must be specified in thermochemical equations. Thermochemical Equations 6.3 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 ) burns in air? P 4 (s) + 5O 2 (g) P 4 O 10 (s)  H = kJ 266 g P 4 mol P 4 g P 4 x kJ mol P 4 x = ______________ kJ

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. (? K?) 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 = ms Heat (q) absorbed or released: q = ms  t q = C  t  t = t final - t initial 6.4

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 = ___ 0 C – ___ 0 C = ____ 0 C q = ms  t = ____ g x ____ J/g 0 C x ___ 0 C = _____ J 6.4

Constant- ________________ 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 = ms  t q bomb = C bomb  t 6.4 Reaction at Constant V  H ~ q rxn  H = q rxn

Constant- ________________ 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 = ms  t q cal = C cal  t 6.4 Reaction at Constant P  H = q rxn

6.4 Table 6.3 Heats of Some Reactions at Constant Pressure p.178

Because there is no way to measure the absolute value of the enthalpy of a substance, must we measure the enthalpy change for every reaction of interest? Establish an arbitrary scale with the standard __________ of formation (  H 0 ) as a reference point for all enthalpy expressions. f Standard ______________ of formation (  H 0 ) is the heat change that results when one mole of a compound is formed from its elements at a pressure of 1 atm. f The standard enthalpy of formation of any element in its most stable form (allotrope) is zero.  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 6.5

Table 6.4 | Standard Enthalpies of Formation at 25ºC p.180

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  ____________________: 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 __________________________. It doesn’t matter how you get there, only where you start and end.)

Thermodynamics _____________ 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 6.7

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. 6.5

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 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 6.5

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 [] 6.5

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 kJ 2 mol = ________ kJ/mol C 6 H 6 6.5

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 6.6 Which substance(s) could be used for melting ice? Which substance(s) could be used for a cold pack? Standard Enthalpies of Solution

The ___________________ for NaCl  H soln = Step 1 + Step 2 = 788 – 784 = 4 kJ/mol 6.6