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Calorimetry

Key Concepts

Type of Calorimeter	Uses and Calculations
Bomb Calorimeter	Reaction is always exothermic since it involves a combustion reaction. Used to measure heat content of foods and fuels. Calibration Calculations Electrical energy is supplied for known time and the temperature change is recorded. E = V x I x t E = energy (J, joules) V = voltage (V, volts) I = current (A, amps) t = time (s, seconds) Calibration factor, C.F. = E ÷ T T = T_f - T_i where T_f is the final temperature reached during calibration and T_i is the initial temperature. H Calculations Energy released = C.F. x T(reaction) T(reaction) = final temperature of reaction mixture - initial temperature reaction mixture H = Energy released ÷ moles of substance H is negative since energy is released in the combustion reaction.

Solution Calorimeter	Reaction may be exothermic or endothermic. Used to measure heat of solution, heat of neutralisation or other heats of reaction in solution. Calibration Calculations Electrical energy is supplied for known time and the temperature change is recorded. E = V x I x t E = energy (J, joules) V = voltage (V, volts) I = current (A, amps) t = time (s, seconds) Calibration factor, C.F. = E ÷ T T = T_f - T_i where T_f is the final temperature reached during calibration and T_i is the initial temperature. H Calculations Energy released = C.F. x T(reaction) T(reaction) = highest temperature of reaction mixture - lowest temperature reaction mixture H = Energy released ÷ moles of substance H is positive for an endothermic reaction (heat absorbed, temperature decreases) H is negative for an exothermic reaction (heat released, temperature increases)

Example

In an experiment to determine the energy content of naphthlane, C₁₀H₈, a bomb calorimeter was used.

Initially a constant current of 1.80 A was passed through the electric heater for 75.00 seconds. The potential difference was 4.95 V. The temperature increased from 18.25^oC to 18.32^oC.

A 1.19g sample of naphthalene was then burnt in the bomb calorimeter and the temperature increased from 18.32^oC to 23.74^oC.

Calculate the heat of combustion for naphthalene.

Calibration Factor (C.F.) Calculations
E = energy supplied by the electric heater in Joules, J
V = potential difference (voltage) in Volts, V = 4.95 V
I = current in Amps (A) = 1.80 A
t = time in seconds = 75.00 sec
E = 4.95 x 1.80 x 75.00
= 668.25 J
Calibration Factor (C.F.) = E ÷ (T_{f calibration} - T_{i calibration})
E = 668.25 J
T_{f calibration} = 18.32^oC
T_{i calibration} = 18.25^oC
C.F. = 668.25 ÷ (18.32 - 18.25) = 668.25 ÷ 0.07
= 9546.43 J^oC^-1
Heat Released by Naphthalene Calculations
Energy released = C.F. x (T_{f reaction} - T_{i reaction})
C.F. = 9546.43 J^oC^-1
T_{f reaction} = 23.74^oC
T_{i reaction} = 18.32^oC
Energy released = 9546.43 x (23.74 - 18.32) = 9546.43 x 5.42
= 51741.65 J (per 1.19g naphthalene)
Heat of Combustion of Naphthalene Calculations
moles naphthalene = mass ÷ molecular mass
mass = 1.19g
molecular mass = (10 x 12.01) + (8 x 1.008) = 120.1 + 8.064 = 128.164 g mol^-1
moles (n) = 1.19 ÷ 128.164 = 0.00928 mol (9.28 x 10^-3 mol)
Energy released per mole = Energy released ÷ moles
Energy released = 51741.65 J
n = 0.00928 mol
Energy released per mole = 51741.65 ÷ 0.00928
= 5575608.8 J mol^-1 = 5575.6 kJ mol^-1 = 5.58 x 10³ kJ mol^-1
H = - 5.58 x 10³ kJ mol^-1
H is negative since the reaction is exothermic, heat is released.