Bomb Calorymetry

8/2/2019 Bomb Calorymetry

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INTRODUCTION TO

BOMB CALORIMETRY

Yosfi Rahmi SGz. MSc. Analisa Zat GiziJurusan Gizi FKUB Malang 2011


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Yosfi Analisa Zat Gizi - 2011

Definition

Bomb calorimetry:

a procedure which determines the heat of

combustion or calorific value of materialswhich are burned as fuels

a sealed container capable of holding severalatmospheres of gas pressure.


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Definition

Calorimetry: the science of measuring the amount ofheat.

calor(Latin) heat

metry(Greek) to measure

Heat, may be :

generated (exothermic process), consumed (endothermic process) or

simply dissipated by a sample.


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Calorimetry

Two types of calorimetry:

measurements based on constant pressure measurement based on constant volume.

Need to know Heat Capacity first


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Heat Capacity

Heat capacity:

the amount of heat required to raise the temperatureof the entire calorimeter by 1 K

it is usually determined experimentally before or

after the actual measurements of heat of reaction.

it is determined by transferring a known amount ofheat into it and measuring its temperature increase.

Because the temperature differences are very small,extreme sensitive thermometers are required for thesemeasurements.


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Heat Capacity

Example:

The temperature of a calorimeter increases 0.10

K when 7.52 J of electric energy is used to heat

it. What is the heat capacity of the calorimeter?

Solution:Dividing the amount of energy by the temperature

increase yields the heat capacity, C, C= 7.52 / 0.10 = 75.2 J/K.


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Calorimetry

Constant pressure calorimeterConstant volume

(bomb Calorimeter)


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Constant-pressure calorimeter

Example: a coffee-cup calorimeterThe inner cup holds the solution in which of the reaction occurs, andthe outer cup provides insulation.

Cp = W H/MT

whereCp= Specific heat at constant pressure

H= Enthalpy of solution

T= Change in temperature

W= mass of solute

M= molecular mass of solute


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Constant-volume calorimeter

Constant-volume calorimeter = Bomb Calorimeter

Older design:

Bomb calorimeters have to withstand the large pressure

within the calorimeter Electrical energy is used to ignite the fuel; as the fuel is

burning, it will heat up the surrounding air, whichexpands and escapes through a tube that leads the air out

of the calorimeter. When the air is escaping through thecopper tube it will also heat up the water outside the tube.The temperature of the water allows for calculatingcalorie content of the fuel.


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Constant-volume calorimeter

Recent calorimeter designs the whole bomb, pressurized with excess pure oxygen (typically at

30atm)

containing a known mass of sample (typically 1-1.5 g)

a small fixed amount of water (to absorb produced acid gases), issubmerged under a known volume of water (ca. 2000 ml) before thecharge is (again electrically) ignited.

The bomb, with sample and oxygen, form a closed system - no airescapes during the reaction.

The energy released by the combustion raises the temperature of thesteel bomb, its contents, and the surrounding water jacket.

The temperature change in the water is then accurately measured.

This temperature rise, along with a bomb factor (which is dependenton the heat capacity of the metal bomb parts) is used to calculate theenergy given out by the sample burn.


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Bomb Calorimeter

Basic Components:

a small cup to contain the sample,

oxygen,

a stainless steel bomb, water,

a stirrer,

a thermometer,

the dewar (to prevent heat flow from the calorimeter tothe surroundings) and

ignition circuit connected to the bomb


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Heat Calculations

the amount of heat a substance produces orabsorbs q = c m T

Where: q: heat in J, c: specific heat capacity in J/(gC), m: mass in g, T: temperature change in C


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Heat Calculations

Sample problem:

When 12 g of a food was burned in a calorimeter, the100 mL of water in the calorimeter changed from

20C to 33

C. Calculate the heat released (mustknow water = 4.18 J/gC)

Solution:

q= cmT= 4.18 J/(gC) x 12 g x 13C

= 0.65208 kJ


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Uses of Bomb Calorimeter

Measurements for:

1) Coal and coke, all varieties and types2) Fuel oil, both heavy and light varieties3) Gasoline, all motor fuels and aviation type jet fuels

4) Combustible wastes and refuse disposal5) Foodstuffs and supplements for human nutrition6) Forage crops and supplements for animal nutrition7) Building materials

8) Explosives and heat powders9) Rocket fuels and related propellants10) Thermodynamic studies of combustible materials11) Energy balance studies in ecology

Bomb Calorymetry

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