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

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NC K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.2

Constant Volume Calorimetry

Zmiana energii wewnętrznej reakcji, ΔE, jest sumą zarówno ciepła, q, jak i pracy, w.

Podczas gdy pomiar ciepła poprzez zmiany temperatury jest prosty, pomiar pracy typu ciśnienie-objętość poprzez zmiany objętości jest niewygodny.

W przypadku gazowych reakcji chemicznych, jeśli objętość nie może się zmienić, to ΔV jest równe zero, a w konsekwencji w wyniesie zero. Zatem wewnętrzna zmiana energii reakcji będzie dokładnie równa przenoszonemu ciepłu.

W warunkach stałej objętości ciepło przenoszone w reakcji jest mierzone za pomocą kalorymetru bombowego. 

Różni się to od kalorymetru do kawy kawy, który mierzy ciepło reakcji w warunkach stałego ciśnienia.

W kalorymetrze bombowym 0,512 grama naftalenu umieszcza się w bombie ze stali nierdzewnej. Poprzez wlot cewka zapłonowa styka się z próbką.

Bomba jest następnie napełniana tlenem i zanurzana w izolowanym pojemniku wypełnionym wodą o znanej masie. Elektryczne zapłon reagenta inicjuje reakcję spalania wewnątrz bomby.

Uwolnione ciepło jest pochłaniane przez wodę i różne elementy zespołu kalorymetru, które razem tworzą otoczenie. Powoduje to wzrost temperatury o 6.42 °C.

Ciepło pochłonięte przez cały zespół kalorymetru, qcal, jest równe jego pojemności cieplnej, Ccal, pomnożonej przez zmianę temperatury, ΔT.

Jeżeli doświadczalnie wyznaczona wartość dla Ccal wynosi 3,20 kJ/°C, to podstawienie jej wraz z wartością ΔT daje ciepło kalorymetru jako 20,5 kJ.

Ciepło uzyskane przez kalorymetr jest dokładnie równe ciepłu uwalnianemu w reakcji. Bomba jest zapieczętowana, reakcja zachodzi przy stałej objętości, a wykonana praca wynosi zero. Zatem ciepło reakcji jest równe jej wewnętrznej zmianie energii, -20,5 kJ.

Podzielenie tej wartości przez liczbę moli w 0,512 grama naftalenu (3,99 × 10−3 moli) daje zmianę energii wewnętrznej na mol naftalenu, która wynosi −5140 kJ/mol.

<ol>
	<li>Hornyak, Frederick M. "Kalorymetr z bombą błyskową." Journal of Chemical Education 38, nr 2 (1961): 97.</li>
	<li>Watkins, Geo B. "Całkowity węgiel w węglu określony przez analizę gazu z kalorymetru bombowego." Chemia Przemysłowa i Inżynieryjna 19, nr 9 (1927): 1052-1054.</li>
	<li>Olney, David J. "Symulacja kalorymetru bombowego." (1990): 922. 
	 </li>
</ol>

kalorymetria stałoobjętościowa

Kalorymetry są przydatne do określenia ciepła uwolnionego lub pochłoniętego w wyniku reakcji chemicznej. Kalorymetry kubkowe są przeznaczone do pracy przy stałym (atmosferycznym) ciśnieniu i są wygodne do pomiaru przepływu ciepła (lub zmiany entalpii) towarzyszących procesom zachodzącym w roztworze pod stałym ciśnieniem. Inny typ kalorymetru, który działa przy stałej objętości, potocznie nazywany kalorymetrem bombowym, służy do pomiaru energii wytwarzanej w reakcjach, w których powstają duże ilości ciepła i produktów gazowych, takich jak reakcje spalania. (Termin „bomba” pochodzi z obserwacji, że reakcje te mogą być na tyle energiczne, że przypominają eksplozje, które mogłyby uszkodzić inne kalorymetry).

Pierwsza zasada termodynamiki sugeruje, że zmiana energii wewnętrznej (ΔE) reakcji jest sumą ciepła (q) i pracy (w).

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/797/797_Equations-01.png" style="height: 40px;">

W reakcjach gazowych wykonywana praca ma charakter ciśnienia i objętości, co powoduje zmiany objętości reakcji.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/797/797_Equations-02.png" style="height: 40px;">

Kalorymetry bombowe są zaprojektowane do pracy przy stałej objętości, tak że objętość reakcji nie może się zmienić (ΔV = 0).

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/797/797_Equations-03.png" style="height: 40px;">

Zatem wykonana praca wynosi zero, a ciepło (qv) zmierzone za pomocą kalorymetru bombowego jest równoważne zmianie energii wewnętrznej reakcji.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/797/797_Equations-04.png" style="height: 40px;">

Kalorymetr bombowy składa się z wytrzymałego stalowego pojemnika zawierającego reagenty i sam jest zanurzony w wodzie. Próbkę umieszcza się w bombie, którą następnie napełnia się tlenem pod wysokim ciśnieniem. Do zapalenia próbki wykorzystuje się małą iskrę elektryczną. Energia wytworzona w wyniku reakcji jest pochłaniana przez stalową bombę i otaczającą ją wodę. Mierzy się wzrost temperatury (ΔT) i wraz ze znaną pojemnością cieplną kalorymetru (Ccal) oblicza się ciepło pochłonięte przez cały zespół kalorymetru (qcal).

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/797/797_Equations-05.png" style="height: 40px;">

Ponieważ kalorymetr jest izolowany i ciepło nie jest tracone do otoczenia, ciepło uzyskane przez kalorymetr jest równe ciepłu wydzielonemu w wyniku reakcji.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/797/797_Equations-06.png" style="height: 40px;">

Ze względu na stałe warunki objętościowe ciepło wydzielone w reakcji odpowiada wewnętrznej zmianie energii.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/797/797_Equations-07.png" style="height: 40px;">

Jest to zmiana energii wewnętrznej określonej ilości reagenta ulegającego spalaniu. ΔErxn na mol konkretnego reagenta oblicza się dzieląc tę wartość przez liczbę moli, które faktycznie przereagowały.

Kalorymetry bombowe wymagają kalibracji w celu określenia pojemności cieplnej kalorymetru i zapewnienia dokładnych wyników. Kalibrację przeprowadza się za pomocą reakcji o znanym q, takim jak odmierzona ilość kwasu benzoesowego zapalonego przez iskrę z niklowego drutu bezpiecznikowego, ważoną przed i po reakcji. Zmiana temperatury wywołana znaną reakcją służy do określenia pojemności cieplnej kalorymetru. Kalibrację przeprowadza się zazwyczaj każdorazowo przed użyciem kalorymetru do gromadzenia danych badawczych.

Kalorymetria stałoobjętościowa

Kalorymetry są przydatne do określania ciepła uwalnianego lub pochłanianego w wyniku reakcji chemicznej. Kalorymetry do filiżanek kawy są przeznaczone do pracy przy stałym ...

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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

energy basics

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Coffee Cup Calorimeter Heat Changes

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constant volume calorimetry

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...

hess's law

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Calculating reaction heat with Hess's law lets chemists find enthalpy changes without measuring every reaction directly. Some reactions are hard or impossible to test in the lab. Even when a reaction can be measured, it is often easier to calculate the heat change from other known enthalpy values. Hess's law says that if a process can be written as the sum of several stepwise processes, the enthalpy change for the total process equals the sum of the enthalpy changes for the steps. This works...

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Enthalpy changes are often listed for reactions that start and end under the same conditions. Chemists use a standard state as a reference set of conditions for comparing properties. In IUPAC terms, the standard state means 1 bar of pressure and 1 M solutions, but it does not set a temperature. Many thermochemical tables still use 1 atm as the reference pressure. Because the difference between 1 bar and 1 atm is very small, enthalpy change usually stays almost the same. A superscripted o in...

enthalpies of reaction

Calculating reaction heat with Hess's law uses standard enthalpies of formation to find the enthalpy change of a reaction. When the formation values for the reactants and products are known, the overall reaction can be broken into simpler stepwise reactions. This approach helps show how the heat change for the full reaction is built from smaller parts. First, the reactants are thought of as being decomposed into their component elements. The enthalpy change for this step is proportional to the...

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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