<p>Enthalpies of Reaction</p><p>Enthalpy change (H) for a chemical reaction is called </p><p>Hrxn and is given by</p><p>Hrxn = H(products) - H(reactants)</p><p>Notice: H(prod) and H(react) can’t be measured individually, but Hrxn is measured by measuring heat flow at constant P!!!</p><p>Notice:</p><p>Hrxn > 0: reaction endothermic; system absorbs heat</p><p>Hrxn < 0: reaction exothermic; system releases heat to surroundings</p><p>E.g., </p><p>N2(g) + 3H2(g)  2NH3(g) Hrxn = -46.19 kJ/mol</p><p>104 Important features of H</p><p>H is an extensive property - its magnitude is directly proportional to amount of reactant consumed, e.g.,</p><p>2SO2(g) + O2(g)  2SO3(g) Hrxn = -196 kJ</p><p>What is Hrxn for the reaction of 4 mol of SO2(g) with 2 mol of O2(g)?</p><p>What is Hrxn for the reaction of 2.4 g of SO2(g)?</p><p>105 Hrxn for a reaction is equal in magnitude but opposite in sign to Hrxn for the reverse reaction, e.g., </p><p>2SO3(g)  2SO2(g) + O2(g) Hrxn = 196 kJ H for a reaction depends on the state of the reactants and products (i.e., solid, liq., gas)</p><p>The states of the reacting species must be specified - we will assume that all are at 25o C</p><p>How do we determine H values experimentally?</p><p>Calorimetry Measurement of heat flow Heat capacity C: energy required to raise the temperature of an object by 1oC</p><p>C determines the temperature change a body experiences when it absorbs heat</p><p>The greater the heat capacity, the greater the heat required to produce a temperature increase</p><p>106 For pure substances, the heat capacity is usually given for a specified amount of substance</p><p>Heat capacity of 1 mol of a substance: molar heat capacity</p><p>Heat capacity of 1 g of a substance: specific heat</p><p>E.g., the specific heat of water is 4.184 J/g-K Calculate the molar heat capacity of water</p><p>Notice these units of the specific heat: if we multiply a specific heat x grams x T, we get units of heat</p><p>Therefore, heat flow q can be calculated from</p><p> q = (specific heat) x (mass) x T</p><p>107 E.g., How many kJ of heat are required to raise the temperature of 2.56 kg of water from 44.8oC to 92.0oC?</p><p>Calorimetric measurement of heats of reaction are usually done in one of two ways: constant pressure or constant volume Constant Pressure Calorimetry Works well for reactions in solution Easy to hold pressure constant – perform reaction in an insulated vessel open to atmosphere Assume that reaction vessel (calorimeter) neither absorbs nor loses heat – all heat produced (or absorbed) comes from the reaction being studied</p><p>How to calculate Hrxn from a constant-pressure calorimetric measurement?</p><p>108 E.g., When 4.25 g of NH4NO3 is dissolved in 60.0 g H2O in a 'coffee cup' calorimeter, the temperature drops from 22.0 oC to 16.9 oC. Find H for the solution process. Assume that the heat capacity of the solution is 4.184 J/g-K.</p><p>Hess's Law</p><p>H is defined in terms of state functions, so H is a state function H depends only on initial and final states...... </p><p>Suppose we want to calculate Hrxn for the reaction A(g) + B(g)  C(g)</p><p>The initial state is A(g, P, 25oC) + B(g, P, 25oC) The final state is C(g, P, 25oC)</p><p>109 The enthalpy change Hrxn depends only on the enthalpy difference between the final state and the initial state, and not how we get from one to the other...... </p><p>Suppose that we know Hrxn for the following reactions.....</p><p>A(g) + B(g)  E(g) Hrxn = 52 kJ/mol</p><p>C(g)  E(g) Hrxn = -105 kJ/mol</p><p>Can these two known reactions be combined to give the reaction that we want? (i.e., A(g) + B(g)  C(g)) (assume that all reactants and products are at pressure P and 25oC)</p><p>Question: are all reactants and products in the reaction we want contained in the reactions we know?</p><p>Question: are there any species in the known reactions that aren’t present in the reaction we want?</p><p>How to make these species go away?</p><p>What happens to the sign of Hrxn if we reverse a reaction?</p><p>110 Hess’s Law tell us that if a reaction can be carried out in a series of steps, then....</p><p>The sum of the H for the individual steps must = H for the overall process</p><p>In the above example, instead of going directly from A&B to C, we convert A&B to E, and then E to C</p><p>As long as we start with A&B and end up with C, it doesn’t matter how we get there!!!</p><p>E.g., from the following heats of reaction,</p><p>N2(g) + 2O2(g) 2NO2(g) Hrxn = 67.6 kJ</p><p>2NO(g) + O2(g) 2NO2(g) Hrxn = -56.6 kJ</p><p>Find the heat of reaction Hrxn for </p><p>N2(g) + O2(g)2NO(g)</p><p>111 Can we tabulate Hrxn values that would allow us to use Hess's Law to calculate enthalpy changes for common reactions?</p><p>We can’t tabulate Hrxn values for every single reaction known to science – some reactions are too dangerous or too difficult to attempt.....</p><p>Also: Hrxn depends on temperature and pressure – tabulating heats of reaction for every single reaction at different values of T and P would be rather nightmarish</p><p>How to get around these difficulties?</p><p>Heats of Formation</p><p>Define Hf : heat of formation</p><p>Hf is the enthalpy change associated with the formation of a compound from its constituent elements</p><p>REMEMBER: H depends on states of products and reactants</p><p>We define Ho as a standard enthalpy change - takes place with all elements in their standard states Standard state: most stable form of a substance at 1 atm pressure</p><p>112 So.... we can have </p><p> o H vap = standard heat of vaporization</p><p> o H fus = standard heat of fusion The “o” means ‘standard’, which means 1 atm pressure Standard enthalpies are generally tabulated at 25oC (298 K)</p><p> o E.g., What does H rxn mean?</p><p> o So... H f is a standard heat of formation Enthalpy change for the formation of 1 mol of substance from its elements, all substances in their standard states</p><p>KNOW the common standard states (e.g., O2(g), </p><p>C(graphite), H2(g))</p><p> o By definition, H f for the most stable form of an element = 0 kJ (think about this – write the formation reaction for C(graphite))</p><p>113 o H f values are tabulated in Appendix C, e.g.,</p><p>AlCl3(s) -705.6 kJ/mol</p><p>CO2(g) -393.5 kJ/mol</p><p>NH3(g) -46.19 kJ/mol</p><p>E.g. write the formation reactions for AlCl3(s), CO2(g), and NH3(g) </p><p> o Now: can we use Hess's Law and tabulated H f values to calculate enthalpy changes for reactions?</p><p> o H rxn : standard enthalpy change for a reaction is given by</p><p> o o o H rxn =  nH f(products) -  mH f(reactants)</p><p>Where n and m are the stoichiometric coefficients from the reaction...... </p><p>114 o E.g., given the following H f , </p><p>NH4NO3(s) -356.6 kJ/mol</p><p>N2O(g) 81.6 " "</p><p>H2O(g) -241.8 " " </p><p> o Calculate H rxn for</p><p>NH4NO3(s)  N2O(g) + 2H2O(g)</p><p>115 Problems du Jour</p><p>From the following enthalpies of reaction:</p><p>H2(g) + F2(g)  2HF(g) -537 kJ</p><p>C(s) + 2F2(g)  CF4(g) -680 kJ</p><p>2C(s) + 2H2(g)  C2H4(g) 52.3 kJ</p><p>Find Hrxn for the reaction</p><p>C2H4(g) + 6F2(g)  2CF4(g) + 4HF(g)</p><p>116 ProblemduJour </p><p>Complete combustion of 1 mol of acetone, C3H6O, results in the liberation of 1790 kJ of heat:</p><p> o C3H6O(g) + 4O2(g)  3CO2(g) + 3H2O(l) H rxn = -1790 kJ</p><p>Using this information, along with the following thermochemical data:</p><p> o Substance H f, kJ</p><p>CO2(g) -393.5</p><p>H2O(l) -285.83</p><p> o Calculate H f for acetone.</p><p>117 Problems du Jour</p><p> o Using data from Appendix C, calculate H rxn for the following reactions:</p><p>2KOH(s) + CO2(g)  K2CO3(s) + H2O(g)</p><p>Fe2O3(s) + 6HCl(g)  2FeCl3(s) + 3H2O(g)</p><p>118 Problem du Jour</p><p>When a 9.55 g sample of solid sodium hydroxide dissolves in 100.0 g of water in a coffee-cup calorimeter, the temperature rises from 23.6 oC to 47.4 oC. Calculate H (in kJ/mol NaOH) for the solution process NaOH(s)  Na+(aq) + Cl-(aq) Assume that the specific heat of the solution is the same as that of pure water.</p><p>119</p>