CHM1045 Lecture 19 Outline of Last Lecture I Heat II Constant Volume Calorimetry III Constant Pressure Calorimetry Outline of Current Lecture I Example 1 II Standard Enthalpy III Example 2 IV Example 3 Current Lecture Example 1 A quantity of 1 00 102 mL of 0 500 M HCl was mixed with 1 00 102 mL of 0 500 M NaOH in a constant pressure calorimeter of negligible heat capacity The initial temperature of the HCl and NaOH solutions was the same 22 50 C and the final temperature of the mixed solution was 25 86 C Calculate the heat change for the neutralization reaction on a molar basis Assume that the densities and specific heats of the solutions are the same as for water 1 00 g mL and 4 184 J g C respectively Qsystem qsolution qreaction 0 qsolution ms T qsolution 200g 4 184 25 8 22 5 qsolution 2812J or 2 812 kJ qreaction r n 2 812kJ 500mol 1L 100L 0500 mol HCl 2 812kJ 56 12 kJ mol 0500 mol HCl Standard Enthalpy Because there is no way to measure the absolute value of the enthalpy of substance must I measure the enthalpy change for every reaction of interest standard enthalpy of formation H0f is as a reference point for all enthalpy expressions Standard enthalpy of formation H0f heat change that results when one mole of a compound is formed from its elements at a pressure of 1 atm The standard enthalpy of formation of any element in its most stable form is zero H0f O2 0 H0 O3 142 kJ mol H0 C graphite 0 H0 C diamond 1 90 kJ mol standard enthalpy of reaction H0f is the enthalpy of a reaction carried out at 1 atm Hess s 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 Example 2 Calculate the standard enthalpy of formation of acetylene C2H2 from its elements The equations for each step and the corresponding enthalpy changes are 2 Cgraphite O2 CO2 H2 1 2 O2 H2O flip and multiply by 1 2 2C2H2 5O2 4CO2 2H2O 2Cgraphite O2 2CO2 H2 1 2 O2 H2O H 787 H 285 8 H 393 5 H 285 8 H 2598 8 2CO2 H2O 2C2H2 5 2O2 H 1299 4 787 285 8 1299 4 226 6 kJ mol Example 3 The thermite reaction involves aluminum and iron III oxide This reaction is highly exothermic and the liquid iron formed is used to weld metals Calculate the heat released in kilojoules per gram of Al reacted with Fe 2O3 The for Fe l is 12 40 kJ mol H sum of products sum of reactants 1669 8 2 12 40 2 0 822 2 Hr n 822 8 kJ mol 822 8 kJ mol 1 mol Al 822 8 15 25kg g 2mol Al 26 98g Al 53 96
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