Calculation for Calorimetry

CALCULATIONS Determining the amount Limiting Reagent used. nlimiting reagent = Molarity x Volume or Mass / Molar Mass instance Limiting reagent is 5mL of 1. 0 M HCl nlimiting reagent = Molarity x Volume nlimiting reagent = (1. 0 groyne/L) x 0. 005 L) = 0. 005 mol Determining the qrxn and qcal. qrxn + qcal = 0 -qrxn = qcal qrxn = ? Hrxn x nlimiting reagent qcal = Ccal ? T qrxn = Ccal ? T + mcsolid ? T (note only if there is a precipitate formed in the reaction) Examples 1) Calibration of the calorimeter given that ?Hrxn = -55. 8 kJ/mol and nLR = 0. 005 mol qrxn = ? Hrxn x nlimiting reagent qrxn = -55. 8 kJ/mol x 0. 005 mol = -279 J qcal = -(219 J) = 279 J (2) Determining the qrxn of a given chemical reaction NH3 (aq) + H+ (aq) ? NH4+ (aq) And given that ? T = 3. 5 C and Ccal=111. 6 J/C qrxn = Ccal ? T + mcsolid ? T qrxn = -( 111. 6 kJ/C x 3. 5 C) = -390. 6 J qcal = -(-390. 6 J) = 390. 6 Determining the Ccal. Ccal = qcal / ? T Example minded(p) qrxn = -279 J and ? T = 2. 5 C Ccal = -qrxn / ? T Ccal = -(-279 J) / (2. 5 C) = 111. 6 J/C Determining the experimental ? Hrxn. ?Hrxn = qrxn / nLR Example Given NH3 (aq) + H+ (aq) ? NH4+ (aq) With qrxn = -390. 6 J and nLR = 0. 005 mol ?Hrxn = qrxn / nLR ?Hrxn = -390. 6 J / 0. 005 mol = -78. 1 kJ/mol Determining the theoretical ? Hrxn. ?Hrxn = ? nproductHf product ? nreactantHf reactant Example Given that NH3 (aq) + H+ (aq) ? NH4+ (aq) Substance? Hf (kJ/mol) NH3 (aq)-80. 9 H+ (aq)0. 00 NH4+ (aq)-132. 51 ?Hrxn = ? nproductHf product ? nreactantHf reactant ? Hrxn = -132. 51 kJ/mol--80. 29 kJ/mol = ?Hrxn = -52. 2 kJ/mol Determining the % fracture. %error = (? Hexperimental ? Htheoretical) / (? Htheoretical) x 100% Example Given ? Hexperimental = -78. 1 kJ/mol and ? Htheoretical = -52. 2 kJ/mol %error = (? Hexperimental ? Htheoretical) / (? Htheoretical) x 100% %error = (-78. 1 kJ/mol) (-52. 2 kJ/mol) / -52. 2 kJ/mol x 100% = 49. 6 %