Calorimetry How is heat measured? It isn’t Temperature measured  T (K) C = heat capacity (J/K) = q (J) C = heat to raise T 1 o x.

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Presentation transcript:

Calorimetry How is heat measured? It isn’t Temperature measured  T (K) C = heat capacity (J/K) = q (J) C = heat to raise T 1 o x

heat capacity specific heat capacity q (J) = q system = C (J / K g) x  T (K) x mass (g) - molar heat capacity mol g q surroundings J / K (thermometer)(reaction)

heat capacity K.E. translational heat K.E. rotational compare: waterHelium He C He C H2OH2O a) > b) < c) = - raise T of system + K.E. vibrational + P.E.

heat capacities substance specific m.w.molar heat capacity (g/mol) heat capacity (J / K g) (J / K mol) Al (s) Fe (s) H 2 O(s) 36.5 H 2 O(l) CCl 4 (l)

Calorimetry 1. Measure  T (T final - T initial ) 2. Convert to q  T (K) x mass (g) =x C (J/K g) q (J)

50 mL 100 o 100 mL 25 o 150 mL (4.184 J/ o C g) q = C x  T x mass q 2 = (4.184 J/ o C g) x q 1 = - q 2 (T f - 100) x (50) = T f = 50 o C q 1 =x (T f - 100)x (50 g) T f Heat transfer experiments (T f - 25) x(100 g) -(T f - 25) x (100)

Calorimetry 1. Measure  T (T final - T initial ) 2. Convert to q  T (K) x mass (g) =x C (J/K g) q (J) q is a path function  E = q + w q = EE EE - w = + P ext  V

q =  E + P ext  V H q =  E + P  V At constant P,  H =  E + P  V q EnthalpyH  H =  E+ P  V+V  P  P = 0  H =  E +  PV  E + PV p = H =  H

Enthalpy H  E + PV Extensive State function  H rxn = q rxn = q system =- q surroundings (thermometer)(reaction) T f > T i  H rxn ><>< 0 endothermic exothermic

Enthalpy of reaction  H rxn = q rxn coffee cup calorimeter 10.5 g KBr T f = 21 o Calculate  H rxn KBr(s)  K + (aq) + Br - (aq) q system = =  H rxn in 125g waterat 24 o - q surroundings

 H rxn q system = - q surroundings =  H rxn q surroundings =C x q surroundings = (4.184 J/g o C)(21(10.5 g = J q system = - q surroundings = J =  H rxn H is extensive  H rxn = 1756 J = 167 J/g= J/mol TT x mass - 24 o C)+ 125 g) a) endothermic b) exothermic 10.5 g KBr

 E = q + w  E = q - P  V At constant V, Bomb calorimeter q rxn = q system = -q calorimeter q calorimeter = C (J / o C) x  E = q v  T ( o C)

Constant Volume calorimetry 2Fe (s) + 3/2 O 2 (g)  Fe 2 O 3 (s) 11.2 g Fe(s), 1 atm O 2 C calorimeter = 2.58 kJ/ o C  T calorimeter = o C q rxn = -q calorimeter =  E rxn = - (2.58 kJ/ o C) = kJ  E rxn = kJ/mol Fe 2 O 3 / 0.1 mol Fe 2 O 3 (31.9 o C)

q v v.s. q p q v =  E q p =  H H = E + PV  H =  E +  PV if  n = 0 2Fe (s) + 3/2 O 2 (g)  Fe 2 O 3 (s)  n =  H =  H = -826 kJ/mol =  E +  nRT  H =  E (0 - 3/2)= - 3/ kJ/mol + (- 3/2)(8.314 x kJ)(298)