Name: heat changes in chemical equations
Uploaded: 2020-09-03T13:38:27.000Z
Duration: 2 min 55 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Energy

PS.Chm.7.2

Thermochemical Equations

La maggior parte delle reazioni chimiche si verificano a pressione atmosferica. In condizioni di pressione costante, la variazione di calore associata alla reazione:delta Q, è uguale alla variazione di entalpia;delta H, detta anche entalpia o calore di reazione. L'entalpia della reazione è la differenza fra le entalpie dei prodotti e dei reagenti.

Quando l'entalpia dei prodotti è maggiore dell'entalpia dei reagenti, il delta H è positivo. Tali reazioni assorbono il calore e sono endotermiche. Al contrario, se l'entalpia dei reagenti è maggiore dell'entalpia dei prodotti, delta H è negativo.

Tali reazioni rilasciano calore e sono esotermiche. Per qualsiasi reazione chimica, l'entità della variazione di entalpia associata dipende dalle quantità stechiometriche di reagenti e prodotti, come indicato dai coefficienti dell'equazione bilanciata. Un'equazione chimica bilanciata che include etichette di fase ed entalpia di reazione, delta H, è detta equazione termochimica.

Considerate la combustione del metano:una fonte primaria di carburante. La combustione del metano rilascia calore nell'ambiente circostante. La natura esotermica della reazione è indicata dalla variazione di entalpia negativa nell'equazione termochimica.

L'equazione per la combustione mostra che quando una mole di gas metano reagisce con 2 moli di ossigeno gassoso per produrre 1 mole di anidride carbonica e 2 moli di acqua liquida, 890, 8 kilojoule di calore vengono rilasciati nell'ambiente circostante. Il rapporto molare fra i reagenti o i prodotti e il calore di reazione può essere utilizzato come fattore di conversione per calcolare il calore scambiato durante la reazione. Se una bombola di gas contiene 25, 5 chilogrammi di metano, e tutto il metano contenuto nella bombola viene sottoposto a combustione, che quantità di calore verrà prodotta?

In generale, il piano concettuale è convertire la massa in mole e quindi i mole in calore di reazione. Per iniziare, 25, 5 chilogrammi vengono moltiplicati per 1000 per trovare la massa in grammi. Quindi, la massa del metano viene divisa per la sua massa molare:16, 0 grammi per mole, per produrre 1594 moli di metano.

Infine, utilizzando il fattore di conversione tra moli di metano e calore di reazione, 1594 moli di metano rilascia 14, 2 volte dieci ai 6 kilojoule di calore, che è il calore della reazione. Il risultato è negativo poiché la reazione è esotermica e il calore si disperde nella reazione.

<ol>
	<li>Canagaratna, Sebastian G. "Un aiuto visivo nei calcoli dell'entalpia". Giornale di educazione chimica 77, n. 9 (2000): 1178.</li>
	<li>Keifer, David. "Entalpia e seconda legge della termodinamica". Journal of Chemical Education 96, n. 7 (2019): 1407-1411.</li>
	<li>Khalil, Mutasim I. "Calcolo dell'entalpia di reazione con un metodo a matrice". Journal of Chemical Education 77, n. 2 (2000): 185. 
	 </li>
</ol>

equazioni termochimiche

Per una reazione chimica (il sistema) condotta a pressione costante – con il solo lavoro svolto causato dall’espansione o dalla contrazione – l'entalpia di reazione (ΔHrxn) pari al calore scambiato con l'ambiente circostante (qp).

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-01.png" style="height: 40px;" />

La variazione di entalpia una propriet estensiva e dipende dalla quantit di reagenti che partecipano alla reazione (o dal numero di moli di reagenti). La variazione di entalpia specifica della reazione e gli stati fisici delle specie reagente e prodotto sono importanti. Una reazione esotermica caratterizzata da un valore −ΔHrxn, mentre una reazione endotermica ha un valore +ΔHrxn.

Poich la quantit di calore rilasciata o assorbita da una reazione corrisponde alla quantit di ciascuna sostanza consumata o prodotta dalla reazione, conveniente utilizzare un'equazione termochimica per rappresentare i cambiamenti sia nella materia che nell'energia. In un'equazione termochimica, la variazione di entalpia di una reazione indicata come ΔHrxn e generalmente viene fornita seguendo l'equazione della reazione. L'entit di ΔHrxn indica la quantit di calore associata alla reazione mostrata nell'equazione chimica. Il segno di ΔHrxn indica se la reazione esotermica o endotermica, come scritto. Nella seguente equazione, 1 mole di idrogeno gassoso e 1/2 mole di ossigeno gassoso (a una certa temperatura e pressione) reagiscono per formare 1 mole di acqua liquida (alla stessa temperatura e pressione).

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-02.png" style="height: 40px;" />

Questequazione indica che 286 kJ di calore vengono rilasciati nell'ambiente circostante. In altre parole, vengono rilasciati 286 kJ di calore (la reazione esotermica) per ogni mole di idrogeno consumata o per ogni mole di acqua prodotta. Pertanto, l'entalpia di reazione un fattore di conversione che pu essere utilizzato per calcolare la quantit di calore rilasciata o assorbita durante reazioni che coinvolgono quantit specifiche di reagenti e prodotti.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-03.png" style="height: 80px;" />

Se i coefficienti dell'equazione chimica vengono moltiplicati per un fattore (cio se la quantit di una sostanza viene modificata), la variazione di entalpia devessere moltiplicata per lo stesso fattore.

(aumento del doppio degli importi)

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-04.png" style="height: 40px;" />

(doppia diminuzione degli importi)

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-05.png" style="height: 40px;" />

Per illustrare che la variazione di entalpia di una reazione dipende dagli stati fisici dei reagenti e dei prodotti, si consideri la formazione di acqua gassosa (o vapore acqueo). Quando 1 mole di idrogeno gassoso e mole di ossigeno gassoso reagiscono per formare 1 mole di acqua gassosa, vengono rilasciati solo 242 kJ di calore, contro i 286 kJ di calore rilasciati quando si forma acqua liquida.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/799/799_Equations-06.png" style="height: 40px;" />

Equazioni termochimiche

Per una reazione chimica (il sistema) effettuata a pressione costante - con il solo lavoro svolto causato dall'espansione o dalla contrazione - ...

Education

Didattica

JoVE Core

Chimica

Thermochemistry

Termochimica

KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

heat changes in chemical equations

Heat changes in chemical equations show how much energy a reaction releases or absorbs. At constant pressure, the enthalpy of reaction, or ΔHrxn, is equal to the heat exchanged with the surroundings, qp. Because this value depends on how much reactant takes part, it is an extensive property.

The size and sign of ΔHrxn also depend on the reaction itself and on the physical states of the substances involved. An exothermic reaction has a negative ΔHrxn value, while an endothermic reaction has a positive one. For that reason, a thermochemical equation is useful because it shows both the chemical change and the energy change together.

In a thermochemical equation, ΔHrxn is written after the chemical equation. Its magnitude tells how much heat is linked to the reaction as written. Its sign shows whether the reaction is exothermic or endothermic in that form.

For example, 1 mole of hydrogen gas and 1/2 mole of oxygen gas can react to form 1 mole of liquid water. This reaction releases 286 kJ of heat to the surroundings. That means 286 kJ is released for every mole of hydrogen used or every mole of water made, so ΔHrxn can act like a conversion factor.

If the coefficients in the chemical equation are multiplied, the enthalpy change must be multiplied by the same factor. A two-fold increase in the amounts gives a two-fold increase in ΔHrxn, and a two-fold decrease gives a two-fold decrease. The physical state matters too. When the same reaction makes gaseous water instead of liquid water, only 242 kJ of heat are released.

Heat Changes in Chemical Equations

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thermochemical equations

Heat changes in chemical equations show how much energy a reaction releases or absorbs. At constant pressure, the enthalpy of reaction, or ΔHrxn, is equal to the heat exchanged with the surroundings, qp. Because this value depends on how much reactant takes part, it is an extensive property. The size and sign of ΔHrxn also depend on the reaction itself and on the physical states of the substances involved. An exothermic reaction has a negative ΔHrxn value, while an endothermic reaction has a...

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hess's law

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key terms and concepts

Learn how ΔHrxn shows heat released or absorbed in reactions, why equation coefficients matter, and how physical state changes the enthalpy value.

Heat Changes in Chemical Equations | 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

Texas Essential Knowledge and Skills

High School

Chemistry and Physics

IPC.8

IPC.8.B

Pennsylvania State Academic Standards

Physical Science

Chemical Reactions

3.2.9-12.D

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Ohio’s Learning Standards and Model Curriculum

Interactions Of Matter

Chemical reactions

Tennessee Academic Standards

CHEM1.PS3.3

Virginia  Standards of Learning

HIgh School

CH.7

CH.7.a

CH.7.c

CHEM.13

CHEM.13.A

CHEM.13.C

Florida Department of Education

SC.912.P.10

SC.912.P.10.7

Georgia Standards of Excellence

SC3.

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Heat Changes in Chemical Equations

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