2. CALORIMETRY ΔH 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. The measurement of heat flow is called “calorimetry” and the apparatus used to measure the heat flow is called “a calorimeter.” CALORIMETRY

4. Heat capacity (C or m.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. CALORIMETRY

5. Specific 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 is 4.184 J/g-K or 1 cal/g –K. CALORIMETRY

6. substanceSpecific heat ( J/g-K) N2 (g)1.04 Al(s).90 Fe(s).45 H2O(l)4.18 Specific 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. The amount of heat gained /lost by a substance: CALORIMETRY q=(grams of substance)x(specific heat)x ΔT Q=mcΔT !!!ΔT in K = ΔT in °C

8. When a substance gains heat - its temperature rises. When a substance loses heat, - Its temperature lowers. CALORIMETRY

9. CALORIMETERS 1) CONSTANT-PRESSURE CALORIMETER - A coffee-cup calorimeter - Because 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; 1. The heat absorbed/gained during the rxn doesn’t escape the coffee cup. 2. The calorimeter itself doesn’t absorb/release heat.

11. Coffee cup calorimeter is usually used to determine the “heat of solution” and “heat of neutralization.” CALORIMETERS 1) CONSTANT-PRESSURE CALORIMETER

12. - Heat exchange happens only between the solution and the chemicals reacting in the calorimeter. Therefore; In exothermic rxns: q lost by the rxn = - q gained by the solution In endothermic rxns: q gained by the rxn = - q lost by the solution - q solution = -(specific heat of solution)x(grams of soln)xΔT=q rxn

13. CALORIMETERS 1) CONSTANT-PRESSURE CALORIMETER q rxn = - q solution ΔH rxn = q rxn / (number of moles of the acid/base reacted ) ΔH rxn = q rxn / (number of moles of the solid dissolved )

14. For dilute aqueous solutions, the specific heat of solution will be approximately the same as that of water. CALORIMETERS 1) CONSTANT-PRESSURE CALORIMETER

15. Example 1 When 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.

16. Solution -q solution = -(specific heat of solution)x(grams of soln)xΔT=q rxn -[( 4.18 J/ g-K) x (50 g+50 g)x (27.5-21.0) ] =q rxn -2717 J =q rxn - 2.717 kJ =q rxn M= n/V=> n= MV => 1.0 x 0.050 = 0.05 mol HCl 1.0 x 0.050 = 0.05 mol NaOH NaOH(aq) + HCl(aq)  NaCl(aq) + H2O(l) 1: 1 ratio between NaOH and HCl in the balanced equation

17. Solution - 2.717 kJ =q rxn 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 ΔH rxn = q rxn / number of moles of the acid/base reacted ΔH rxn = - 2.717 kJ / 0.05 mol ΔH rxn = - 54.34 kJ/mol

18. 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.

19. Qrxn= - [Qwater + Qmetal] Qrxn = - [(m w.c w.ΔT) + (m metal. c metal. ΔT)] OR Qrxn = - [(m w.c w ) + (m metal. c metal )]. ΔT If the calorimeter cup is made of a different material (e.g. a metal): Q rxn = - C cal x ΔT

20. 2)BOMB CALORIMETER(CONSTANT- VOLUME) We calculate the heat evolved by the rxn with: Q rxn = - C cal x ΔT

21. Example 2 data above is from an experiment used to measure the enthalpy change for the combustion of 1 mole of glucose (C 6 H 12 O 6(s) ). The time-temperature data was taken from a data-logging software programme.

22. Mass of sample of glucose, m = 1.389 g Heat capacity of the system, Csystem = 12.224 kJ K–1 ( C : 12 ; H: 1 ; O : 16 ) (a)Calculate ΔT, for the water, surrounding the chamber in the calorimeter. (b)Determine the amount, in moles, of glucose. (c)Calculate the enthalpy change for the combustion of 1 mole of glucose.

23. solution A) ΔT= 23.78-22.01=1.77°C B) n=m/M n= 1.389/ 180=0.007717mol=0.008mol C) Qrxn= -C ΔT= -12.224x1.77= -21.63648 kJ ΔH comb =-21.63648 kJ/0.007717mol= - 2803.7 kJ/mol ΔH comb = -3000 kJ/mol

24. Example 3 Methyl hydrazine (CH 6 N 2 ) is commonly used as a liquid rocket fuel. The combustion of methyl hydrazine w/ oxygen produces N 2 (g), CO 2 (g), and H 2 O(l). When 4.00 g of methyl hydrazine is combusted in a bomb calorimeter, the temperature of the calorimeter increases from 25.00 °C to 39.50°C. In a separate experiment the heat capacity of the calorimeter is measured to be 7.794 kJ/°C. What is heat of reaction for the combustion of a mole of methyl hydrazine in this calorimeter? (N: 14.01 g/mol, H: 1.01g/mol, C: 12.01 g/mol)

25. Solution - (heat capacity of the calorimeter)xΔT=q rxn - (7.794 kJ/°C) x (39.50 °C-25.00 °C) - 113.013 kJ =q rxn Molar mass of CH 6 N 2 = (1x12.01+ 6x1.01+ 2x14.01)= 46.09 g/mol n=mass/molar mass=> n=4.00g / 46.09 gmol -1 n= 0.0868 mol 0.0868 mol CH 6 N 2 combusts - 113.013 kJ is released 1 mol CH 6 N 2 combusts ? ? = -1302.19 kJ/mol