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

K12 Standards

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

NC K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.4.2

Chemical Equations

As reações químicas envolvem transformações químicas de uma ou mais substâncias em materiais diferentes. Uma reação é tradicionalmente representada por uma equação química, que utiliza fórmulas químicas para representar os átomos, as moléculas ou os compostos envolvidos. As substâncias presentes no início de uma reação e consumidas durante o processo são denominadas de reagentes.Os reagentes estão indicados no lado esquerdo da equação. As substâncias formadas durante a reação são denominadas de produtos e estão indicados no lado direito da equação. A seta que separa os reagentes dos produtos mostra a direção da reação.Os diferentes estados da matéria são representados com abreviaturas entre parênteses. Uma reação química respeita a lei da conservação da massa, onde as massas totais de todos os elementos participantes são conservadas. Por isso, o número cumulativo de átomos de cada elemento vai corresponder aos reagentes e produtos.As equações químicas são equilibradas utilizando coeficientes numéricos para refletir o número atual de átomos envolvidos na reação. É importante alterar apenas os coeficientes numéricos e não os subscritos numéricos que definem a identidade de um composto ao mesmo tempo que equilibram qualquer equação química. As orientações gerais ajudam a ilustrar e a ajustar uma equação química de forma eficaz.Por exemplo, a combustão de ácido benzoico líquido, formando dióxido de carbono gasoso e água são representadas inicialmente utilizando uma equação química esquelética. Em seguida, os átomos são equilibrados ajustando primeiro os que se encontram dentro dos compostos e depois os átomos dos elementos livres. A inclusão dos coeficientes sete e três antes das fórmulas para o dióxido de carbono e a água equilibra os átomos de carbono e de hidrogênio.Um coeficiente fracionário de quinze por dois ajusta os átomos de oxigênio. Os coeficientes fracionários são convertidos em números inteiros através da multiplicação da equação completa pelo denominador. O número coletivo de átomos de cada elemento é igual, em ambos os lados, numa equação equilibrada.Nas equações químicas que representam reações que envolvem compostos iônicos, os cátions são equilibrados primeiro, seguidos dos ânions. Os íons poliatômicos são sempre equilibrados como uma unidade. Por exemplo, o sulfato de alumínio aquoso reage com o hidróxido de cálcio aquoso formando o hidróxido de alumínio sólido e o sulfato de cálcio sólido.Utilizando uma equação esquelética, os cátions de alumínio e de cálcio são equilibrados, seguidos dos ânions de hidróxido de poliatômico e dos íons de sulfato. Finalmente, os coeficientes são ajustados para obter uma equação geral equilibrada.

equações químicas

As equações químicas representam a identidade e as quantidades relativas de substâncias envolvidas em uma reação química. As substâncias que sofrem a reação são chamadas de reagentes, e as suas fórmulas são colocadas do lado esquerdo da equação. As substâncias formadas pela reação são chamadas produtos, e as suas fórmulas são colocadas do lado direito da equação. Sinais de adição (+) separam as fórmulas de produtos e reagentes individuais e uma seta (→) separa os lados dos reagentes e dos produtos (esquerdo e direito) da equação. Os números relativos de espécies de reagentes e de produtos são representados por coeficientes numéricos, colocados imediatamente à esquerda de cada fórmula. Normalmente, não é apresentado um coeficiente de 1. Os menores coeficientes possíveis de número inteiro são comumente usados em uma equação química, e são interpretados como razões. Por exemplo, o metano e o oxigénio reagem para produzir dióxido de carbono e água em uma razão de 1:2:1:2. 

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1019/1019_Equations-01.png" style="height: 40px;" />

A razão indica que os menores coeficientes possíveis de metano, oxigénio, dióxido de carbono e água são 1, 2, 1, e 2, respectivamente. Os coeficientes podem ser interpretados em relação a qualquer unidade de quantidade (número), e assim esta equação pode ser lida corretamente de muitas maneiras, incluindo:

i. Uma molécula de metano e duas moléculas de oxigénio reagem para produzir uma molécula de dióxido de carbono e duas moléculas de água.

ii. Um mole de moléculas de metano e 2 moles de moléculas de oxigénio reagem para produzir 1 mole de moléculas de dióxido de carbono e 2 moles de moléculas de água. 

Os estados físicos dos reagentes e produtos em equações químicas muitas vezes são indicados com uma abreviação entre parênteses a seguir às fórmulas. As abreviaturas padrão incluem ‘s’ para sólidos, ‘l’ para líquidos, ‘g’ para gases, and ‘aq’ para substâncias dissolvidas em água.

<h4>Equilíbrio de equações</h4>

Em uma equação equilibrada, os números de átomos para cada elemento envolvido na reação são os mesmos do lado dos reagentes e dos produtos, cumprindo assim a lei da conservação da matéria. Uma equação equilibrada pode ser confirmada adicionando os números de átomos em cada lado da seta e comparando estas somas para garantir que são iguais. Note-se que o número de átomos para um dado elemento é calculado multiplicando o coeficiente de qualquer fórmula que contenha esse elemento pelo subscrito do elemento na fórmula. Se um elemento aparecer em mais de uma fórmula em um determinado lado da equação, o número de átomos representados em cada um deve ser calculado e adicionado então em conjunto.

Para equilibrar a equação, os coeficientes da equação podem ser alterados conforme necessário. Às vezes é conveniente usar frações em vez de números inteiros como coeficientes intermediários no processo de equilibrar uma equação química. Quando o equilíbrio é alcançado, todos os coeficientes da equação podem então ser multiplicados por um número inteiro para converter os coeficientes fracionários em inteiros sem alterar o equilíbrio dos átomos. 

Por exemplo, a reação do etano (C2H6) com oxigénio produz água e dióxido de carbono, que pode ser representada pela seguinte equação desequilibrada:

<img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/1019/1019_Equations-02.png" style="height: 40px;" />

A equação desequilibrada contém: 

<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;">Átomos</td>
<td style="color: black; border: 1px solid black; text-align: center;">Reagentes</td>
<td style="color: black; border: 1px solid black; text-align: center;">Produtos</td>
<td style="color: black; border: 1px solid black; text-align: center;">Equilibrada?</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;">Não</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;">Não</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;">Não</td>
</tr>
</tbody>
</table>

Para equilibrar o número de átomos de carbono e de hidrogénio, multiplica-se CO2 pelo coeficiente 2 e multiplica-se H2O pelo coeficiente 3, respectivamente. Isto altera o número total de átomos de oxigénio no produto para 7. Para equilibrar o número de átomos de oxigénio, multiplica-se o oxigénio pelo coeficiente fracional 7/2. Este equilíbrio inicial dos átomos de carbono, hidrogénio, e oxigénio, alterando os coeficientes para reagentes e produtos, fornece a equação provisoriamente equilibrada: 

<img alt="Eq3" src="https://www.jove.com/files/core/CDNSource/lessons/1019/1019_Equations-03.png" style="height: 40px;" />

Uma equação equilibrada convencional com apenas coeficientes inteiros é derivada multiplicando cada coeficiente por 2, para obter a equação:

<img alt="Eq4" src="https://www.jove.com/files/core/CDNSource/lessons/1019/1019_Equations-04.png" style="height: 40px;" />

Este texto é adaptado de <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>.

Equações Químicas

As equações químicas representam as identidades e quantidades relativas de substâncias envolvidas em uma reação química. As substâncias que sofrem reação são ...

Education

JoVE Core

Química

Molecules, Compounds, and Chemical Equations

Moléculas, Compostos e Equações Químicas

molecules-compounds-and-chemical-equations

KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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

Matter is made of elements, and elements are built from atoms. An atom is the smallest unit of an element that still keeps that element’s properties. A silver coin, for example, is made of silver atoms. Each atom keeps the unique properties of silver. Elements are pure substances made of identical atoms. They cannot be broken down into simpler substances by chemical changes. Most elements are atomic elements, which use single atoms as their basic unit. Carbon and sodium are examples of atomic...

molecular models

Molecular Model Types in Chemistry

Molecular models help chemists and students visualize the structures and shapes of atoms and molecules. They represent the molecular architecture of chemical compounds in a way that is easier to study than formulas alone. Skeletal models are simple two-dimensional drawings of compounds. They show the molecular framework, or the bonds, without drawing every atom. In this model, many carbon and hydrogen atoms are not shown directly. Their positions are implied by the ends and junctions of the...

organic compounds

Organic Molecules in Everyday Chemistry

Organic molecules are carbon-based compounds found in living things and in many everyday products. Most organic compounds have carbon as the main element, bonded to hydrogen and other carbon atoms. Some carbon-containing substances, such as carbonates, cyanides, and simple oxides like CO and CO2, are not classified as organic. Carbon can form up to four strong bonds, which helps explain why organic compounds can make chains and rings of many sizes and shapes. These molecules appear in...

formula mass and mole concepts of compounds

formula-mass-and-mole-concepts-of-compounds

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Chemical formula mass and percent composition help describe the makeup of compounds. A chemical formula shows which atoms are present and how many of each atom appear in a substance. For covalent compounds, the formula shows the atoms in one molecule. That is why formula mass can also be called molecular mass. Chloroform, CHCl3, is one example. Its formula shows one carbon atom, one hydrogen atom, and three chlorine atoms. The molecular mass is the sum of their average atomic masses. Ionic...

experimental determination of chemical formula

experimental-determination-of-chemical-formula

Finding Empirical and Molecular Formulas

Chemical formulas show the elemental makeup of a compound. When a formula is unknown, scientists often start by measuring the masses of the elements in the sample. Those mass data are then used to find the simplest whole-number ratio of atoms, which gives the empirical formula. To get from mass to atom ratio, the masses of the elements are first converted to moles using molar masses. Chemical formulas describe numbers of atoms, not masses, so this step is necessary. In one example, 1.71 grams...

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 - Química dá vida a conceitos gerais de química usando vídeos de animação fáceis de entender. Além disso, nos vídeos Cientistas em ação, demonstramos experimentos de pesquisa clássicos e originais realizados atualmente por cientistas em laboratórios em todo o mundo.

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

CH.3.b

CH.3.c

PS.3

PS.3.d

Minnesota Academic Standards in Science

2.2.1 9C.1.2.1.2

Arizona Science Standards

HS+C.P1

HS+C.P1U1.5

HS+C.P1U1.7

new-jersey-student-learning-standards-for-science

New Jersey Student Learning Standards for Science

Texas Essential Knowledge and Skills

CHEM.7

CHEM.7.B

CHEM.9

CHEM.9.A

Alabama Course of Study Science

9-12.PS.3.5.d

Next Generation Science Standards

Florida Department of Education

SC.912.P.8

SC.912.P.8.9

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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

HS. Chemical Reactions

HS-PS1-4.

HS-PS1-7.

HS-PS1-12.

Georgia Standards of Excellence

SPS3.

Chemical Reaction

SC3.

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Elements: Chemical Symbols and Isotopes

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Nuclear Binding Energy

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The Energies of Atomic Orbitals

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PS.Chm.1.3

Periodic Classification of the Elements

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The Periodic Table

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Atomic Mass

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Molar Mass

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Atomic Radii and Effective Nuclear Charge

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Ionic Radii

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Ionization Energy

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Electron Affinity

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Construct an explanation to infer the atomic size, reactivity, electronegativity, and ionization energy of an element based on its position in the Periodic Table.

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Analyze and interpret data to explain the mechanisms and properties of the two main types of intramolecular (ionic and covalent) bonds.

PS.Chm.3.1

Analyze the structure of atoms and isotopes.

Reading Chemical Equations and Balancing Them

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