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

La variazione nell'energia interna di una reazione, delta E, è la somma sia del calore, q, sia del lavoro, w. Sebbene sia semplice misurare il calore attraverso i cambiamenti di temperatura, misurare il tipo di lavoro pressione-volume attraverso i cambiamenti di volume è scomodo. Per le reazioni chimiche gassose, se il volume non può cambiare, allora delta V è uguale a zero e, di conseguenza, w sarà zero.

Pertanto, la variazione di energia interna della reazione sarà esattamente uguale al calore trasferito. In condizioni di volume costante, il calore trasferito in una reazione viene misurato utilizzando un calorimetro a bombola. Questo è diverso dal calorimetro a tazza di caffè, che misura il calore di una reazione in condizioni di pressione costante.

Nel calorimetro a bombola, 0, 512 grammi di naftalene vengono posti in una bombola di acciaio inossidabile. Attraverso un ingresso, la bobina di accensione è posta a contatto con il campione. La bombola viene quindi riempita di ossigeno e immersa in un contenitore isolato, pieno di una massa d'acqua nota.

L'accensione elettrica del reagente avvia la reazione di combustione all'interno della bombola. Il calore sprigionato viene assorbito dall'acqua e dai vari componenti del gruppo calorimetrico, che insieme costituiscono l'ambiente circostante. Questo fa aumentare la temperatura di 6, 42 gradi Celsius.

Il calore assorbito dall'intero gruppo calorimetro, q cal, è uguale alla sua capacità termica, C cal, moltiplicato per la variazione di temperatura, delta T.Se il valore determinato sperimentalmente per C cal è 3, 20 kilojoule per grado centigrado, sostituendolo, insieme al valore per delta T, si ottiene che il calore del calorimetro è di 20, 5 kilojoule. Il calore guadagnato dal calorimetro è esattamente uguale al calore rilasciato dalla reazione. La bombola viene sigillata, la reazione avviene a volume costante e il lavoro è zero.

Pertanto, il calore della reazione è uguale alla sua variazione di energia interna, 20, 5 kilojoule negativi. Dividendo questo valore per il numero di moli in 0, 512 grammi di naftalene. 3, 99 per 10 rispetto a 3 moli negative, dà la variazione dell'energia interna per mole di naftalene, che è negativa di 5140 kilojoule per mole.

<ol>
	<li>Hornyak, Frederick M. "Un calorimetro a bomba flashbulb". Giornale di educazione chimica 38, n. 2 (1961): 97.</li>
	<li>Watkins, Geo B. "Carbonio totale nel carbone determinato dall'analisi del gas dal calorimetro a bomba". Chimica industriale e ingegneristica 19, n. 9 (1927): 1052-1054.</li>
	<li>Olney, David J. "Simulazione del calorimetro della bomba." (1990): 922. 
	 </li>
</ol>

calorimetria a volume costante

I calorimetri sono utili per determinare il calore rilasciato o assorbito da una reazione chimica. I calorimetri per tazze di caff sono progettati per funzionare a pressione costante (atmosferica) e sono convenienti per misurare il flusso di calore (o variazione di entalpia) che accompagna i processi che si verificano in soluzione a pressione costante. Un diverso tipo di calorimetro che funziona a volume costante, colloquialmente noto come calorimetro a bomba, viene utilizzato per misurare l'energia prodotta da reazioni che producono grandi quantit di calore e prodotti gassosi, come le reazioni di combustione. (Il termine “bomba? deriva dall’osservazione che queste reazioni possono essere abbastanza vigorose da assomigliare a esplosioni che danneggerebbero altri calorimetri.)

La prima legge della termodinamica suggerisce che la variazione di energia interna (ΔE) di una reazione la somma del calore (q) e del lavoro (w).

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/797/797_Equations-01.png" style="height: 40px;" />

Nelle reazioni gassose, il lavoro svolto di tipo pressione-volume, con conseguenti variazioni nel volume della reazione.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/797/797_Equations-02.png" style="height: 40px;" />

I calorimetri a bomba sono progettati per funzionare a volume costante, in modo tale che il volume della reazione non possa cambiare (ΔV = 0).

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/797/797_Equations-03.png" style="height: 40px;" />

Pertanto, il lavoro svolto zero e il calore (qv) misurato utilizzando un calorimetro a bomba equivalente alla variazione di energia interna della reazione.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/797/797_Equations-04.png" style="height: 40px;" />

Un calorimetro a bomba costituito da un robusto contenitore di acciaio che contiene i reagenti ed esso stesso immerso nell'acqua. Il campione viene posto nella bomba, che viene poi riempita di ossigeno ad alta pressione. Una piccola scintilla elettrica viene utilizzata per accendere il campione. L'energia prodotta dalla reazione viene assorbita dalla bomba d'acciaio e dall'acqua circostante. Viene misurato l'aumento di temperatura (ΔT) e, insieme alla capacit termica nota del calorimetro (Ccal), viene utilizzato per calcolare il calore assorbito dall'intero gruppo calorimetro (qcal).

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/797/797_Equations-05.png" style="height: 40px;" />

Poich il calorimetro isolato e non viene disperso calore nell'ambiente, il calore acquisito dal calorimetro uguale al calore rilasciato dalla reazione.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/797/797_Equations-06.png" style="height: 40px;" />

A causa delle condizioni di volume costante, il calore sviluppato nella reazione corrisponde alla variazione di energia interna.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/797/797_Equations-07.png" style="height: 40px;" />

Questa la variazione di energia interna per la quantit specifica di reagente sottoposto a combustione. ΔErxn per mole di un particolare reagente si ottiene dividendo il valore per il numero di moli che hanno effettivamente reagito.

I calorimetri a bomba richiedono la calibrazione per determinare la capacit termica del calorimetro e garantire risultati accurati. La calibrazione viene eseguita utilizzando una reazione con un q noto, come una quantit misurata di acido benzoico innescato da una scintilla proveniente da un filo di fusibile in nichel che viene pesato prima e dopo la reazione. La variazione di temperatura prodotta dalla reazione nota viene utilizzata per determinare la capacit termica del calorimetro. La calibrazione viene generalmente eseguita ogni volta prima che il calorimetro venga utilizzato per raccogliere dati di ricerca.

Calorimetria a volume costante

I calorimetri sono utili per determinare il calore rilasciato o assorbito da una reazione chimica. I calorimetri per tazze da caffè sono progettati per funzionare a velocità costante ...

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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

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constant pressure calorimetry

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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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standard enthalpy of formation

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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...

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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

chemistry

JoVE Core: Chimica illustra concetti chiave di chimica attraverso video animazioni concise e di immediata comprensione. Inoltre, i video della sezione "scienziati in azione" mostrano procedure metodologiche realizzate da ricercatori provenienti dai migliori laboratori internazionali.

Matter and its Interactions

PS.Chm.4.1

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Ohio’s Learning Standards and Model Curriculum

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Texas Essential Knowledge and Skills

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Bomb Calorimeter Heat Measurement

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