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April 30, 2014 Thermochemistry is the study of heat changes (energy!) that accompany chemical reactions and phase changes • Heat is exchanged between the system and surroundings system = part of the universe that contains the reaction or process being studied surroundings = rest of the universe that interacts with the system April 30, 2014 April 30, 2014 Enthalpy (H) is the heat content of a system at constant pressure. (Enthalpy is potential energy) Change in enthalpy for a reaction is called the enthalpy (or heat) of reaction ( Hrxn). Hrxn = Hfinal - Hinitial Hrxn = Hproducts - Hreactants The second equation shows you how you can think about enthalpy of a reaction as the difference in energy of the products and reactants. April 30, 2014 Energy required to break bonds < Energy released to form bonds is gy d er En ease l re Enthalpy Reactants Exothermic Reaction H<0, - H Products What happens to the surrounding? April 30, 2014 Energy required to break bonds > Energy released to form bonds Enthalpy Products Reactants is y d rg be e r En so ab Endothermic Reaction H>0, + H What happens to the surrounding? April 30, 2014 Is thi April 30, 2014 Thermochemical equations: balanced chemical equation that includes the physical states of all reactants and products and the energy change, usually expressed as the change in enthalpy ( H). Example: CH4(g) + 2O2 (g) CO2(g) + 2H2O(g) Hrxn= -802.3kJ April 30, 2014 Coffee cup calorimeter Bomb calorimeter Reaction happens in water -qsystem = qwater April 30, 2014 Calorimetry Lab Calculations Step 1: Finding how much heat was transferred from metal to water (remember...heat flows from higher temperature to lower temperature) Step 2: Using heat transferred, calculate specific heat of metal. April 30, 2014 Sample Calculations Mass of water = 75g Initial Temperature of water = 21.0ºC Final Temperature of water = 25.5ºC Mass of metal = 50.5g Initial temperature of metal = 100ºC Final Temperature of metal = 25.5ºC April 30, 2014 Hess's Law • Used when it is impractical to measure ΔH using a calorimeter • Hess's Law states if you can add two or more thermochemical equations to produce a final equation, the sum of the enthalpy changes of each individual reaction is the enthalpy for the final reaction ∆ Hoverall=)))H ∆ 3).).). ∆ 1)+)))H ∆ 2)+)))H Imagine that chemical reactions happen in small steps. You add all of the steps together along with their enthalpy changes April 30, 2014 April 30, 2014 Example 17: Calculate ΔH for the formation of sulfur trioxide 2S(s) + 3O2(g) ! 2SO3(g) ΔH =? Given: S(s) + O2(g) ! SO2(g) ΔH = -297kJ 2SO3(g) ! 2SO2(g) + O2(g) ΔH = 198 kJ April 30, 2014 Hess's Law Rules 1. If the reaction is reversed, the sign of ΔH is also reversed 2. If the coefficients are multiplied by a factor, then the ΔH is multiplied by the same factor 3. The state of matter (s, l, g, a q) of reactants and products affects whether they cancel out or not. April 30, 2014 Example 18: Calculate the enthalpy of reaction of 2C(s) + O2(g) + 2H2O(l) ! H2CO2(l) ΔH=? Given: C(s)+1/2O2(g) + H2O(l)!CO(g)+ H2O(l) ΔH=-110.5kJ CO(g)+ H2O(l)!H2CO2(g) ΔH=33.7 kJ H2CO2(g)! H2CO2(l) ΔH=-62.9 kJ April 30, 2014 Example 19: Calculate the enthalpy of reaction of 2H2O2(l) ! H2O(l) + O2 ΔH=? Given: 2H2(g) + O2(g)!2H2O(l) ΔH=-572 kJ H2(g) + O2 !H2O2(l) ΔH=-188 kJ April 30, 2014 Not practical to calculate every ΔH to use in Hess's Law. Use Standard Enthalpy of Formation! Standard Enthalpy of Formation: Change in enthalpy when one mole of a compound forms from it's constituent elements in their standard states. • standard state: normal physical state at 1 atm and 25ºC • For a pure elements in its standard state ΔH = 0 C(s) + 2H2(g) CH4(g) ∆ H= -74.6 kJ/mol April 30, 2014 Using Standard Enthalpies of formation • You can use standard enthalpies of formation to calculate enthalpy of reaction under standard conditions (ΔHºrxn) using Hess's Law Example 20: H2S(g) + 4F2(g) 2HF(g) + SF6(g) ΔHºrxn =? Given: • 1/2H2(g) + 1/2F2(g) HF(g) ΔHºf=-273kJ • S(s) + 3F2(g) SF6(g) ΔHºf= -1220kJ • H2(g) + S(s) H2S(g) ΔHºf= -21kJ ΔHºrxn= ΔHºf(product) - ΔHºf(reactants) April 30, 2014 ΔHºrxn= ΔHºf(product) - ΔHºf(reactants) April 30, 2014 Example 21: Using the chart given to you, calculate ΔHºrxn for the following reactions: a) CH4(g) + 2O2(g) b) CaCO3 CO2(g) + 2H2O(l) CaO(s) + CO2(g) April 30, 2014 Calculating ΔH using bond energy (BE) data • you can estimate ΔH for gaseous molecules • Bond formation is ____________ • Breaking bonds is ____________ • ΔH = BEbreaking - BEforming Example 21: Determine ΔH for the following reaction given the bond energies N2(g) + 3H2 2NH3(g) N=N 941 kJ/mol H-H 436 kJ/mol N-H 391 kJ/mol April 30, 2014