CHEMICAL RXNS AND ENERGY

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CHEMICAL RXNS AND ENERGY

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.

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

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.

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.

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.

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

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

CALORIMETERS 1) CONSTANT-PRESSURE CALORIMETER
A coffee-cup calorimeter Because the calorimeter isn’t sealed, the rxn happens under constant pressure of the atmosphere.

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.

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 solution In endothermic rxns: qgained by the rxn = - q lost by the solution - qsolution= -(specific heat of solution)x(grams of soln)xΔT=qrxn

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)

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

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.

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

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

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.

2)BOMB CALORIMETER(CONSTANT-VOLUME)
We calculate the heat evolved by the rxn with: Qrxn= - Ccal x ΔT

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.

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.

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

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)

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

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?