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Thermochemical Stoichiometry Problems Thermochemical Stoichiometry In a balanced thermochemical equation… You can make conversion factors involving energy, as if they were part of the equation. Example: 2P + 5Cl2 2PCl5 ∆Ho = -886 kJ ∆Ho = standard enthalpy of reaction 2 mol P = -886 kJ 5 mol Cl2 = -886 kJ 2 mol PCl5 = -886 kJ You can use these as unit conversions in stoichiometric calcualtions. Problem Solving Strategies MAKE YOUR ROAD MAPS. In Honors Chemistry, you can convert from moles into… Moles of another substance Concentration Molarity, molality, % mass Numbers of particles Mass Liters of gas at STP P, V, T, NOT at STP Enthalpy of reaction molar ratio equations Avogadro’s number molar mass molar volume (22.4) ideal gas law standard enthalpy (kJ/mol) You can be asked to convert between all sorts of different units! Practice!!!! (do HW and review!) Problem 1 When a 4.25g sample of solid NH4NO3 dissolves in 60.0g of water the temperature drops from 21.0°C to 16.9°C. Calculate the ∆H for this process. The specific heat of water (solution) is 4.18 J/g • °C. In these problems, assume the specific heat of the solution is the same as water. Also assume no volume change from the addition of solute. Problem 2 When 50.0mL of 1.0 M HCl and 50.0mL of 1.0M NaOH are mixed in a calorimeter, the temperature of the mixture increases from 23.0 oC to 29.8 oC. Neglecting the effects of the calorimeter and assuming the solution to have the same density and specific heat of water (4.184 J/gK) calculate the ΔHrxn in kJ/mole.