2 CALORIMETRYΔH of a chemical rxn can experimentally be determined by measuring the heat flow accompanying the rxn at constant pressure.When heat flows into/out of a substance, its temperature changes. The heat flow is experimentally determined by using the temperature change produced.
3 CALORIMETRYThe measurement of heat flow is called “calorimetry” and the apparatus used to measure the heat flow is called “a calorimeter.”
4 CALORIMETRYHeat capacity (C) of an object is the amount of heat required to raise its temperature by 1 K or 1 °C.The greater the heat capacity, the greater the heat required to produce a certain rise in temperature.
5 CALORIMETRYSpecific heat capacity or specific heat (s or c) is the heat capacity of 1 g of a substance.Specific heat of H2O(l) is the amount of energy required to change temperature of 1 g of water by 1°C. Therefore,it is4.184 J/g-K or 1 cal/g –K.
6 CALORIMETRY substance Specific heat ( J/g-K) N2 (g) 1.04 Al(s) .90 Fe(s).45H2O(l)4.18Specific heat of water is quite higher than those of other substances. It’s very important for Earth’s climate since it makes oceans resistant to temperature changes.
7 q=(grams of substance)x(specific heat)x ΔT CALORIMETRYThe amount of heat gained /lost by a substance:q=(grams of substance)x(specific heat)x ΔTQ=mcΔT!!!ΔT in K = ΔT in °C
8 CALORIMETRY When a substance gains heat - its temperature rises. When a substance loses heat,- Its temperature lowers.
9 CALORIMETERS 1) CONSTANT-PRESSURE CALORIMETER A coffee-cup calorimeterBecause the calorimeter isn’t sealed, the rxn happens under constant pressure of the atmosphere.
10 CALORIMETERS 1) CONSTANT-PRESSURE CALORIMETER Since the calorimeter has a very low thermal conductivity & heat capacity, we assume that;The heat absorbed/gained during the rxn doesn’t escape the coffe cup.The calorimeter itself doesn’t absorb/release heat.
11 CALORIMETERS 1) CONSTANT-PRESSURE CALORIMETER Heat exchange happens only between the solution and the chemicals reacting in the calorimeter. Therefore;In exothermic rxns:qlost by the rxn = - q gained by the solutionIn endothermic rxns:qgained by the rxn = - q lost by the solution- qsolution= -(specific heat of solution)x(grams of soln)xΔT=qrxn
12 ΔHrxn = qrxn/ (number of moles of the acid/base reacted) CALORIMETERS 1) CONSTANT-PRESSURE CALORIMETERqrxn = - q solutionΔHrxn = qrxn/ (number of moles of the acid/base reacted)
13 CALORIMETERS 1) CONSTANT-PRESSURE CALORIMETER For dilute aqueous solutions, the specific heat of solution will be approximately the same as that of water.
14 exampleWhen a student mixes 50. mL of 1.0 M HCl and 50. mL of 1.0 M NaOH in a coffee-cup calorimeter, the temperature of the resultant solution increases from 21.0 °C to 27.5 °C. Calculate the enthalpy change for the rxn , assuming that the calorimeter loses only a negligible quantity of heat, that the total volume of the solution is 100 mL, that its density is 1.0 g/mL, and that its specific heat is 4.18 J/ g-K.
15 Solution-qsolution= -(specific heat of solution)x(grams of soln)xΔT=qrxn-[( 4.18 J/ g-K) x (50 g+50 g)x ( ) ] =qrxn-2717 J =qrxnkJ =qrxnM= n/V=> n= MV => 1.0 x = 0.05 mol HCl1.0 x = 0.05 mol NaOHNaOH(aq) + HCl(aq) NaCl(aq) + H2O(l)1: 1 ratio between NaOH and HCl in the balanced equation
16 SolutionkJ =qrxn NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) 1: 1 ratio between NaOH and HCl in the balanced equation 0.05 mol HCl reacted w/ 0.05 mol NaOH ΔHrxn = qrxn/ number of moles of the acid/base reacted ΔHrxn = kJ / 0.05 mol ΔHrxn = kJ/mol
17 2)BOMB CALORIMETER(CONSTANT-VOLUME) It’s usually used to determine “molar heat of combustion (ΔH°comb )” of substances.molar heat of combustion is the enthalpy change when 1 mole of the substance undergoes a complete combustion in excess oxygen under standard conditions. It’s always negative in sign.
18 2)BOMB CALORIMETER(CONSTANT-VOLUME) We calculate the heat evolved by the rxn with:Qrxn= - Ccal x ΔT
19 exercisedata above is from an experiment used to measure the enthalpy change for the combustion of 1 mole of glucose (C6H12O6(s)).The time-temperature data was taken from a data-logging software programme.
20 Mass of sample of glucose, m = 1.389 g Heat capacity of the system, Csystem = kJ K–1( C : 12 ; H: 1 ; O : 16 )Calculate ΔT, for the water, surrounding the chamber in the calorimeter.Determine the amount, in moles, of glucose.Calculate the enthalpy change for the combustion of 1 mole of glucose.
22 exampleMethyl hydrazine (CH6N2) is commonly used as a liquid rocket fuel. The combustion of methyl hydrazine w/ oxygen produces N2(g), CO2(g), and H2O(l).When 4.00 g of methyl hydrazine is combusted in a bomb calorimeter, the temperature of the calorimeter increases from °C to 39.50°C. In a separate experiment the heat capacity of the calorimeter is measured to be kJ/°C. What is heat of reaction for the combustion of a mole of methyl hydrazine in this calorimeter? (N: g/mol, H: 1.01g/mol, C: g/mol)
23 Solution- (heat capacity of the calorimeter)xΔT=qrxn- (7.794 kJ/°C) x (39.50 °C °C)kJ =qrxnMolar mass of CH6N2 = (1x x x14.01)= g/moln=mass/molar mass=> n=4.00g / gmol-1n= molmol CH6N2 combusts kJ is released1 mol CH6N2 combusts ?? = kJ/mol
24 HW Exercise: Under constant-volume conditions the heat of combustion of glucose (C6H12O6) is kJ/g. A g sample of glucose is burned in a bomb calorimeter. The temperature of the calorimeter increased from °C to °C. (O: g/mol)a) Write the balanced chemical equation of the combustion rxn.b) What is the total heat capacity of the calorimeter?