Name: bomb calorimeter heat measurement
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 2 min 41 s
Channel: JoVE

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.2

Constant Volume Calorimetry

The change in the internal energy of a reaction, &#916;E, is the sum of both heat, q, and work, w.

While it is simple to measure heat through temperature changes, measuring the pressure-volume type work through changes in volume is inconvenient.

For gaseous chemical reactions, if the volume is not allowed to change, then &#916;V equals zero, and consequently, w will be zero. Thus, the internal energy change of the reaction will exactly equal the heat transferred.

Under constant volume conditions, the heat transferred in a reaction is measured using a bomb calorimeter. 

This is different from a coffee cup calorimeter, which measures the heat of a reaction under constant pressure conditions.

In a bomb calorimeter, 0.512 grams of naphthalene is placed in a stainless steel bomb. Through an inlet, the ignition coil is placed in contact with the sample.

The bomb is then filled with oxygen and immersed in an insulated container filled with a known mass of water. Electrically igniting the reactant initiates the combustion reaction inside the bomb.

The heat released is absorbed by the water and the various components of the calorimeter assembly, which all together make up the surroundings. This causes the temperature to rise by 6.42 &#176;C.

The heat absorbed by the entire calorimeter assembly, qcal, equals its heat capacity, Ccal, multiplied by the temperature change, &#916;T.

If the experimentally determined value for Ccal is 3.20 kJ/&#176;C, then substituting this, along with the value for &#916;T gives the heat of the calorimeter as 20.5 kJ.

The heat gained by the calorimeter exactly equals the heat released by the reaction. The bomb being sealed, the reaction occurs at constant volume, and work done is zero. Thus, the heat of the reaction equals its internal energy change, &#8722;20.5 kJ.

Dividing this value by the number of moles in 0.512 grams of naphthalene (3.99 &#215; 10&#8722;3 moles) gives the change in internal energy per mole of naphthalene, which is &#8722;5140 kJ/mol.

constant volume calorimetry

Calorimeters are useful to determine the heat released or absorbed by a chemical reaction. Coffee cup calorimeters are designed to operate at constant (atmospheric) pressure and are convenient to measure heat flow (or enthalpy change) accompanying processes that occur in solution at constant pressure. A different type of calorimeter that operates at constant volume, colloquially known as a bomb calorimeter, is used to measure the energy produced by reactions that yield large amounts of heat and gaseous products, such as combustion reactions. (The term &#8220;bomb&#8221; comes from the observation that these reactions can be vigorous enough to resemble explosions that would damage other calorimeters.) 

The first law of thermodynamics suggests that the change in internal energy (&#916;E) of a reaction is the sum of heat (q) and work (w). 

<img alt="Thermodynamic equation &#916;Erxn=q+w; energy change in chemical reactions analysis." src="/files/core/CDNSource/lessons/797/797_Equations-01.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

In gaseous reactions, the work done is pressure-volume type, thereby resulting in changes in the volume of the reaction. 

<img alt="Chemical reaction energy equation &#916;Erxn=q+&#916;V, symbol diagram, thermodynamics concept." src="/files/core/CDNSource/lessons/797/797_Equations-02.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

Bomb calorimeters are designed to operate at constant volume, such that the volume of the reaction is not allowed to change (&#916;V = 0). 

<img alt="Thermodynamic equation &#916;E_rxn = q + 0, illustrating energy conservation principle." src="/files/core/CDNSource/lessons/797/797_Equations-03.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

Therefore, the work done is zero, and the heat (qv) measured using a bomb calorimeter is equivalent to the change in internal energy of the reaction.

<img alt="Chemical thermodynamics equation &#916;E_rxn=q_v, depicting energy change and heat at constant volume." src="/files/core/CDNSource/lessons/797/797_Equations-04.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

A bomb calorimeter consists of a robust steel container that contains the reactants and is itself submerged in water. The sample is placed in the bomb, which is then filled with oxygen at high pressure. A small electrical spark is used to ignite the sample. The energy produced by the reaction is absorbed by the steel bomb and the surrounding water. The temperature increase (&#916;T) is measured and, along with the known heat capacity of the calorimeter (Ccal), is used to calculate the heat absorbed by the entire calorimeter assembly (qcal). 

<img alt="Calorimetry equation q_cal = C_cal &#215; &#916;T for heat calculation in thermochemistry analysis." src="/files/core/CDNSource/lessons/797/797_Equations-05.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

Since the calorimeter is insulated and no heat is lost to the environment, heat gained by the calorimeter equals the heat released by the reaction.

<img alt="Calorimetry equation q_cal = -q_rxn; thermodynamic process formula." src="/files/core/CDNSource/lessons/797/797_Equations-06.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

Due to constant volume conditions, the heat evolved in the reaction corresponds to the internal energy change.

<img alt="Thermodynamics equation, \(q_{\text{rxn}} = -q_v = \Delta E_{\text{rxn}}\), energy change concept." src="/files/core/CDNSource/lessons/797/797_Equations-07.png" style="height: 40px;" data-altupdatedat="1761830439000" data-alt="Eq1" />

This is the internal energy change for the specific amount of reactant undergoing combustion. &#916;Erxn per mole of a particular reactant is obtained by dividing the value by the number of moles that actually reacted.

Bomb calorimeters require calibration to determine the heat capacity of the calorimeter and ensure accurate results. The calibration is accomplished using a reaction with a known q, such as a measured quantity of benzoic acid ignited by a spark from a nickel fuse wire that is weighed before and after the reaction. The temperature change produced by the known reaction is used to determine the heat capacity of the calorimeter. The calibration is generally performed each time before the calorimeter is used to gather research data.

This text is adapted from <a href="https://openstax.org/books/chemistry-2e/pages/5-2-calorimetry">Openstax, Chemistry 2e, Section 5.2: Calorimetry.</a>

Calorimeters are useful to determine the heat released or absorbed by a chemical reaction. Coffee cup calorimeters are designed to operate at constant ...

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KEY TERMS AND CONCEPTS

bomb calorimeter heat measurement

Bomb calorimetry measures the heat released by reactions that produce a lot of energy, especially combustion reactions. A coffee cup calorimeter works at constant pressure and is useful for reactions in solution. A bomb calorimeter works at constant volume and is used for reactions that make large amounts of heat and gaseous products.

The word &quot;bomb&quot; refers to the strong, explosive force some reactions can have. In a bomb calorimeter, the volume does not change, so 94V = 0. That means the pressure-volume work is zero. The heat measured at constant volume, qv, is equal to the change in internal energy for the reaction.

A bomb calorimeter has a tough steel container that holds the reactants. The steel bomb sits in water. The sample is placed inside the bomb, and the chamber is filled with oxygen at high pressure. A small electrical spark ignites the sample, and the heat from the reaction warms the steel bomb and the surrounding water.

The temperature increase, 94T, is measured and combined with the known heat capacity of the calorimeter, Ccal, to find the heat absorbed by the whole setup, qcal. Because the calorimeter is insulated, the heat gained by the calorimeter equals the heat released by the reaction. Under constant volume conditions, this heat corresponds to the internal energy change for the amount of reactant that burned.

To compare results fairly, the internal energy change per mole is found by dividing by the number of moles that reacted. Bomb calorimeters also need calibration to measure heat capacity accurately. A known reaction, such as a measured amount of benzoic acid ignited by a spark from nickel fuse wire, is used for this step. The temperature change from that reaction gives the heat capacity, and calibration is usually done each time before research data are collected.

Bomb Calorimeter Heat Measurement

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Bomb calorimetry measures the heat released by reactions that produce a lot of energy, especially combustion reactions. A coffee cup calorimeter works at constant pressure and is useful for reactions in solution. A bomb calorimeter works at constant volume and is used for reactions that make large amounts of heat and gaseous products. The word "bomb" refers to the strong, explosive force some reactions can have. In a bomb calorimeter, the volume does not change, so 94V = 0. That means the...

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key terms and concepts

Learn how a bomb calorimeter measures combustion heat at constant volume, uses temperature change to find internal energy, and is calibrated with benzoic acid.

Bomb Calorimeter Heat Measurement | JoVE K12

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Bomb Calorimeter Heat Measurement

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