Name: reading chemical equations and balancing them
Uploaded: 2020-09-03T13:38:26.000Z
Duration: 3 min 10 s
Channel: JoVE

K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.4.2

Chemical Equations

Le reazioni chimiche comportano trasformazioni chimiche di una o più sostanze in materiali diversi. Una reazione è convenzionalmente rappresentata usando un'equazione chimica, che utilizza formule chimiche per rappresentare atomi, molecole o composti coinvolti. Le sostanze presenti all'inizio di una reazione e consumate durante il processo sono chiamate reagenti.

I reagenti sono specificati sul lato sinistro dell'equazione. Le sostanze che si formano durante la reazione sono chiamate prodotti, e sono indicate sul lato destro dell'equazione. Una freccia che separa i reagenti e i prodotti mostra la direzione della reazione.

I diversi stati della materia sono rappresentati utilizzando abbreviazioni fra parentesi. Una reazione chimica segue la legge di conservazione della massa, in cui vengono conservate le masse totali di tutti gli elementi partecipanti. Pertanto, il numero cumulativo di atomi di ciascun elemento corrisponderà sia nei reagenti che nei prodotti.

Le equazioni chimiche vengono bilanciate utilizzando coefficienti numerici per riflettere il numero effettivo di atomi coinvolti nella reazione. È importante modificare solo i coefficienti numerici, e non i pedici numerici, che definiscono l'identità di un composto bilanciando qualsiasi equazione chimica. Le linee guida generali aiutano ad illustrare e regolare un'equazione chimica in modo efficiente.

Per esempio, la combustione dell'acido benzoico liquido, che forma anidride carbonica gassosa e acqua, è rappresentata inizialmente utilizzando un'equazione chimica scheletrica. Gli atomi vengono successivamente bilanciati regolando prima quelli all'interno dei composti, seguiti dagli atomi degli elementi liberi. L'inclusione dei coefficienti sette e tre prima delle formule per l'anidride carbonica e l'acqua bilancia gli atomi di carbonio e idrogeno.

Un coefficiente frazionario di quindici per due regola gli atomi di ossigeno. I coefficienti frazionari vengono convertiti in numeri interi moltiplicando l'equazione completa per il denominatore. Il numero collettivo di atomi di ciascun elemento è uguale su entrambi i lati in un'equazione bilanciata.

Nelle equazioni chimiche che rappresentano reazioni che coinvolgono composti ionici, i cationi vengono bilanciati per primi, seguiti dagli anioni. Gli ioni poliatomici sono sempre bilanciati come unità. Per esempio, il solfato di alluminio acquoso reagisce con l'idrossido di calcio acquoso, formando idrossido di alluminio solido e solfato di calcio solido.

Utilizzando un'equazione scheletrica, i cationi di alluminio e calcio vengono bilanciati, seguiti dagli anioni dell'idrossido poliatomico e degli ioni solfato. Infine, i coefficienti vengono adeguati per ottenere un'equazione complessiva equilibrata.

equazioni chimiche

Le equazioni chimiche rappresentano le identit e le quantit relative delle sostanze coinvolte in una reazione chimica. Le sostanze che subiscono una reazione sono chiamate reagenti e le loro formule sono posizionate sul lato sinistro dell'equazione. Le sostanze generate dalla reazione sono chiamate prodotti e le loro formule sono posizionate sul lato destro dell'equazione. I segni pi (+) separano le singole formule dei reagenti e dei prodotti, mentre una freccia (→) separa i lati del reagente e del prodotto (sinistro e destro) dell'equazione. I numeri relativi delle specie di reagenti e di prodotti sono rappresentati da coefficienti numerici, posti immediatamente a sinistra di ciascuna formula. Di solito non viene mostrato un coefficiente pari a 1. In un'equazione chimica vengono comunemente utilizzati i coefficienti di numeri interi pi piccoli possibili e vengono interpretati come rapporti. Ad esempio, metano e ossigeno reagiscono per produrre anidride carbonica e acqua in un rapporto 1:2:1:2.

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1019/1019_Equations-01.png" style="height: 40px;" />

Il rapporto indica che i coefficienti pi piccoli possibili di metano, ossigeno, anidride carbonica e acqua sono rispettivamente 1, 2, 1 e 2. I coefficienti possono essere interpretati rispetto a qualsiasi unit di importo (numero), quindi questa equazione pu essere letta correttamente in molti modi, tra cui:

Una molecola di metano e due molecole di ossigeno reagiscono per produrre una molecola di anidride carbonica e due molecole di acqua.

Una mole di molecole di metano e 2 moli di molecole di ossigeno reagiscono per produrre 1 mole di molecole di anidride carbonica e 2 moli di molecole di acqua.

Gli stati fisici dei reagenti e dei prodotti nelle equazioni chimiche molto spesso sono indicati con un'abbreviazione tra parentesi dopo le formule. Le abbreviazioni standard includono "s" per solidi, "l" per liquidi, "g" per gas e "aq" per sostanze disciolte in acqua.

Equazioni di bilanciamento

In un'equazione bilanciata, il numero di atomi per ciascun elemento coinvolto nella reazione lo stesso sia dal lato del reagente che da quello del prodotto, soddisfacendo cos la legge di conservazione della materia. Un'equazione bilanciata pu essere confermata sommando i numeri di atomi su entrambi i lati della freccia e confrontando queste somme per assicurarsi che siano uguali. Tieni presente che il numero di atomi per un dato elemento viene calcolato moltiplicando il coefficiente di qualsiasi formula contenente quell'elemento per il pedice dell'elemento nella formula. Se un elemento appare in pi di una formula su un dato lato dell'equazione, il numero di atomi rappresentati in ognuna deve essere calcolato e poi sommato.

Per bilanciare l'equazione, i coefficienti di questultima possono essere modificati secondo necessit. A volte conveniente utilizzare le frazioni anzich i numeri interi come coefficienti intermedi nel processo di bilanciamento di un'equazione chimica. Una volta raggiunto l’equilibrio, tutti i coefficienti dell’equazione possono quindi essere moltiplicati per un numero intero per convertire i coefficienti frazionari in numeri interi senza sconvolgere l’equilibrio atomico.

Ad esempio, la reazione dell'etano (C_2H_6) con l'ossigeno produce acqua e anidride carbonica, che possono essere rappresentate dalla seguente equazione sbilanciata:

<img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/1019/1019_Equations-02.png" style="height: 40px;" />

L'equazione sbilanciata contiene:

<table style="margin-left: auto; margin-right: auto;">
	<tbody style="border: 1px solid black;">
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">Atomi</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Reagente</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Prodotto</td>
			<td style="color: black; border: 1px solid black; text-align: center;">Equilibrato?</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">C</td>
			<td style="color: black; border: 1px solid black; text-align: center;">2</td>
			<td style="color: black; border: 1px solid black; text-align: center;">1</td>
			<td style="color: black; border: 1px solid black; text-align: center;">No</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">H</td>
			<td style="color: black; border: 1px solid black; text-align: center;">6</td>
			<td style="color: black; border: 1px solid black; text-align: center;">2</td>
			<td style="color: black; border: 1px solid black; text-align: center;">No</td>
		</tr>
		<tr>
			<td style="color: black; border: 1px solid black; text-align: center;">O</td>
			<td style="color: black; border: 1px solid black; text-align: center;">2</td>
			<td style="color: black; border: 1px solid black; text-align: center;">3</td>
			<td style="color: black; border: 1px solid black; text-align: center;">No</td>
		</tr>
	</tbody>
</table>

Per bilanciare il numero di atomi di carbonio e di atomi di idrogeno, moltiplicare rispettivamente CO_2 per il coefficiente 2, e H_2O per il coefficiente 3. Ci modifica il numero totale di atomi di ossigeno sul prodotto a 7. Per bilanciare il numero di atomi di ossigeno, moltiplicare l'ossigeno per il coefficiente frazionario 7/2. Questo bilanciamento iniziale degli atomi di carbonio, idrogeno e ossigeno modificando i coefficienti dei reagenti e dei prodotti, fornisce l'equazione provvisoriamente bilanciata:

<img alt="Eq3" src="https://www.jove.com/files/core/CDNSource/lessons/1019/1019_Equations-03.png" style="height: 40px;" />

Un'equazione bilanciata convenzionale con coefficienti solo interi viene derivata moltiplicando ciascun coefficiente per 2, per generare l'equazione:

<img alt="Eq4" src="https://www.jove.com/files/core/CDNSource/lessons/1019/1019_Equations-04.png" style="height: 40px;" />

Questo testo adattato da <a href="https://openstax.org/books/chemistry-2e/pages/4-1-writing-and-balancing-chemical-equations">Openstax, Chemistry 2e, Section 4.1: Writing and Balancing Chemical Equations. </a>

Equazioni chimiche

Le equazioni chimiche rappresentano le identità e le quantità relative di sostanze coinvolte in una reazione chimica. Le sostanze che subiscono la reazione sono ...

Education

Didattica

JoVE Core

Chimica

Molecules, Compounds, and Chemical Equations

Molecole, composti ed equazioni chimiche

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KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

reading chemical equations and balancing them

Chemical equations show the identities and relative amounts of substances in a chemical reaction. The substances that react are called reactants, and their formulas are written on the left side of the equation. The substances formed are called products, and their formulas are written on the right side. A plus sign (+) separates formulas on the same side, and an arrow (→) shows the change from reactants to products.

The numbers in a chemical equation are shown with coefficients. A coefficient is the number written in front of a formula. A coefficient of 1 is usually not written. The smallest whole-number coefficients are commonly used, and they work as ratios. For example, methane and oxygen react to form carbon dioxide and water in a 1:2:1:2 ratio.

That ratio can be read in more than one way. It can mean 1 methane molecule and 2 oxygen molecules make 1 carbon dioxide molecule and 2 water molecules. It can also mean 1 mole of methane molecules and 2 moles of oxygen molecules make 1 mole of carbon dioxide molecules and 2 moles of water molecules. Chemical equations may also include physical states after the formulas. Common state labels are s for solid, l for liquid, g for gas, and aq for substances dissolved in water.

A balanced chemical equation has the same number of atoms for each element on both sides. This follows the law of conservation of matter. To check balance, count the atoms on the reactant side and the product side and compare the totals. To find the number of atoms in a formula, multiply the coefficient by the subscript. If an element appears in more than one formula on one side, add all of those atoms together.

Balancing a chemical equation means changing coefficients, not the formulas themselves. Fractions can be used as temporary coefficients during balancing. For the reaction of ethane with oxygen, carbon and hydrogen are balanced first by placing 2 in front of carbon dioxide and 3 in front of water. That leaves 7 oxygen atoms on the product side, so oxygen gets a fractional coefficient of 7/2. The final equation is then multiplied by 2 to turn all coefficients into whole numbers without changing the balance.

Reading Chemical Equations and Balancing Them

reading-chemical-equations-and-balancing-them

molecules and compounds

Atomic Elements, Molecules, and Bonds

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chemical equations

Chemical equations show the identities and relative amounts of substances in a chemical reaction. The substances that react are called reactants, and their formulas are written on the left side of the equation. The substances formed are called products, and their formulas are written on the right side. A plus sign (+) separates formulas on the same side, and an arrow (→) shows the change from reactants to products. The numbers in a chemical equation are shown with coefficients. A coefficient...

key terms and concepts

Learn how to read chemical equations, identify reactants and products, use coefficients and state symbols, and balance equations with whole-number ratios.

Reading Chemical Equations and Balancing Them | 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.

Physical Science

PS.PSc.2.3

Pennsylvania State Academic Standards

Chemical Reactions

3.2.9-12.C

3.2.9-12.G

Louisiana Student Standards for Science

High School

Matter And Its Interactions

HS-PS1-2

HS-PS1-7

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Ohio’s Learning Standards and Model Curriculum

Study Of Matter

Reactions of matter

Interactions Of Matter

Chemical reactions

Stoichiometry

Tennessee Academic Standards

Matter and Its Interactions

CHEM1.PS1.2

CHEM1.PS1.3

Virginia  Standards of Learning

HIgh School

CH.3

CH.3.a

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CH.3.c

PS.3

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2.2.1 9C.1.2.1.2

Arizona Science Standards

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HS+C.P1U1.5

HS+C.P1U1.7

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New Jersey Student Learning Standards for Science

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CHEM.7

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South Carolina College and Career Ready Science Standards

C-PS1-2

C-PS1-7

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New York State Science Learning Standards

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HS-PS1-4.

HS-PS1-7.

HS-PS1-12.

Georgia Standards of Excellence

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Chemical Reaction

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PS.Chm.1.3

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Atomic Mass

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Molar Mass

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Analyze the structure of atoms and isotopes.

Reading Chemical Equations and Balancing Them

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