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Presentation on theme: "CHEMICAL RXNS AND ENERGY"— Presentation transcript:


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

4 CALORIMETRY Heat 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 CALORIMETRY 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.

6 CALORIMETRY substance Specific 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 q=(grams of substance)x(specific heat)x ΔT
CALORIMETRY The amount of heat gained /lost by a substance: q=(grams of substance)x(specific heat)x ΔT Q=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.

A coffee-cup calorimeter Because the calorimeter isn’t sealed, the rxn happens under constant pressure of the atmosphere.

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.

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 solution In 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 CALORIMETER qrxn = - q solution ΔHrxn = qrxn/ (number of moles of the acid/base reacted)

For dilute aqueous solutions, the specific heat of solution will be approximately the same as that of water.

14 example 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.

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 =qrxn kJ =qrxn M= n/V=> n= MV => 1.0 x = 0.05 mol HCl 1.0 x = 0.05 mol NaOH NaOH(aq) + HCl(aq)  NaCl(aq) + H2O(l) 1: 1 ratio between NaOH and HCl in the balanced equation

16 Solution kJ =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

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.

We calculate the heat evolved by the rxn with: Qrxn= - Ccal x ΔT

19 exercise data 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.

21 solution ΔT= 23.78-22.01=1.77°C n=m/M
n= 1.389/ 180= mol=0.008mol C) Qrxn= -CΔT= x1.77= kJ ΔHcomb= kJ/ mol= kJ/mol ΔHcomb= kJ/mol

22 example Methyl 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 =qrxn Molar mass of CH6N2 = (1x x x14.01)= g/mol n=mass/molar mass=> n=4.00g / gmol-1 n= mol mol CH6N2 combusts kJ is released 1 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?

25 Answer: B) = 14 kJ/K.


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