Name: oxidation states in redox reactions
Uploaded: 2020-03-26T07:22:24.000Z
Duration: 4 min 56 s
Channel: JoVE

K12 Standards

Texas Essential Knowledge and Skills

High School

Chemistry

CHEM.9

CHEM.9.B

Redox Reactions

Redox, que é a abreviação de redução-oxidação, é um tipo de reação química classificada pela transferência líquida de elétrons. Nessa reação, uma molécula perde elétrons, chamada de oxidação, e a outra molécula ganha elétrons, chamada de redução.
Para ajudá-lo a diferenciar entre os dois, lembre-se da frase 'OIL-RIG', que significa 'oxidação está perdendo, redução está ganhando'. A molécula oxidada é chamada de agente redutor porque reduz o outro reagente. Da mesma forma, a molécula que é reduzida é chamada de agente oxidante porque oxida a outra molécula.
Agora que temos a terminologia resolvida, vejamos um exemplo de reação redox, a formação do mineral óxido de magnésio. Durante a reação, cada átomo de magnésio perde dois elétrons. Assim, o magnésio é oxidado. Cada átomo de oxigênio ganha dois elétrons; assim, o oxigênio é reduzido.
No entanto, nem todas as reações são reações redox. Por exemplo, a reação do óxido de cálcio com o dióxido de carbono para formar carbonato de cálcio não é uma reação redox. Então, como podemos identificar uma reação redox?
Para fazer isso, rastreamos o número de oxidação de cada elemento à medida que ele passa do reagente ao produto. O número de oxidação é a carga hipotética que um átomo teria se suas ligações com diferentes elementos fossem iônicas, o que significa que os elétrons são atribuídos ao átomo mais eletronegativo. A soma dos números de oxidação em uma molécula é igual à sua carga total.
Vamos olhar para trás, para o óxido de magnésio. É um composto neutro, então a soma dos números de oxidação do magnésio e do oxigênio é igual a zero. O magnésio pode dar dois elétrons, então seu número de oxidação é mais dois. O oxigênio pode aceitar dois elétrons, então seu número de oxidação é menos dois.
E quanto à reação? Os compostos elementares neutros puros têm um número de oxidação zero. Assim, tanto o magnésio quanto o oxigênio começam com números de oxidação de zero. Os números de oxidação do magnésio e do oxigênio mudaram durante a reação, então esta é uma reação redox.
Agora, vamos olhar para a reação do carbonato de cálcio que vimos anteriormente. Ambos os reagentes são neutros, então a soma dos números de oxidação para ambos os compostos é zero. Como vimos com o óxido de magnésio, o cálcio tem um número de oxidação de mais dois e o oxigênio menos dois. Então, o carbono na molécula de dióxido de carbono tem um número de oxidação de mais quatro e cada oxigênio menos dois.
Que tal o produto? O cálcio é mais dois e o carbono mais quatro, assim como nos reagentes. Cada oxigênio é menos dois, totalizando menos seis, com o número de oxidação líquido zero. Como nenhum dos números de oxidação mudou, esta não é uma reação redox.
Agora vamos apresentar os quatro tipos de reações redox. A primeira é uma reação de deslocamento único, onde um átomo desloca outro. Você verá isso em uma reação de termite, onde um metal é reduzido e o outro metal é oxidado.
O próximo tipo é uma reação de combustão, que ocorre entre um combustível e um oxidante para formar produtos oxidados e calor. Você vê isso no laboratório durante a combustão do metano com oxigênio ao usar um bico de Bunsen.
A terceira é uma reação de síntese, onde dois reagentes se combinam para formar um produto, como na síntese de amônia, onde o nitrogênio é combinado com hidrogênio para formar amônia.
Finalmente, o quarto tipo é uma reação de decomposição, onde um reagente absorve energia suficiente para quebrar suas ligações para formar compostos menores. É o que acontece com os fogos de artifício, onde o clorato de potássio se decompõe em cloreto de potássio e oxigênio após o aquecimento.
Neste laboratório, você executará e identificará vários tipos de reações redox que transformam cobre sólido em óxido de cobre e, em seguida, de volta em cobre sólido.

reações redox

<h4>Oxidação e Redução</h4>
Algumas reações químicas podem ser classificadas como reações de redução-oxidação ou reações redox. A oxidação é o processo da matéria, como um átomo ou molécula iônica, perdendo um ou mais elétrons, e a redução é o processo da matéria ganhando um ou mais elétrons.
<h4>Estados de oxidação</h4>
Cada átomo em uma molécula tem seu próprio estado de oxidação ou número de oxidação. O estado de oxidação descreve o quão oxidada uma molécula é em relação à sua forma elementar livre. O estado de oxidação é expresso como a carga que um átomo teria se cada uma de suas ligações com outros elementos fosse puramente iônica. Isso significa que os elétrons na ligação são atribuídos ao átomo mais eletronegativo. O estado de oxidação de um átomo em sua forma elementar livre é definido como 0.
Existem algumas regras que são seguidas para determinar o estado de oxidação. Os elementos do Grupo I e do Grupo II normalmente têm estados de oxidação de +1 e +2, respectivamente. O hidrogênio e o oxigênio normalmente têm estados de oxidação de +1 e -2, respectivamente, e os halogênios geralmente têm um estado de oxidação de -1. Além disso, os estados de oxidação dos átomos em uma molécula sempre somam a carga na molécula. Assim, o estado de oxidação de um átomo não listado acima pode ser deduzido com frequência. Por exemplo, considere o dióxido de carbono (CO2), que é uma molécula neutra. Se cada uma das duas moléculas de oxigênio contribui com -2, o estado de oxidação do carbono deve ser +4 para cancelar o -4 dos oxigênios.
Para uma abordagem mais geral, desenhe a estrutura de Lewis da molécula, identifique as ligações entre os diferentes átomos e atribua cada ligação ao átomo mais eletronegativo. Em seguida, conte o número de elétrons em cada átomo, com cada ligação contribuindo com dois elétrons. Subtraia o número de elétrons que estão atualmente no átomo do número padrão de elétrons de valência para esse átomo para obter o número de oxidação.
Considere o dióxido de carbono novamente. Cada oxigênio tem dois pares de elétrons isolados e está conectado ao carbono central por uma ligação dupla. O oxigênio é mais eletronegativo que o carbono, então cada ligação C = O, que representa quatro elétrons, é atribuída ao seu oxigênio. Assim, a cada oxigênio é atribuído um total de oito elétrons (quatro dos pares isolados e quatro da ligação dupla), e o carbono não recebe nenhum. O número padrão de elétrons de valência para oxigênio é seis, então o número de oxidação para cada oxigênio é 6 – 8 = -2. O número padrão de elétrons de valência para o carbono é quatro, então o número de oxidação para o carbono é 4 – 0 = +4.
<h4>Reações redox</h4>
Nem todas as reações químicas são classificadas como reação redox. Uma reação redox é qualquer reação na qual há uma mudança no estado de oxidação de um átomo. Assim, para verificar se uma reação é redox, determine os estados de oxidação de cada átomo nos reagentes e produtos e procure por quaisquer alterações.
Muitas reações redox envolvem uma transferência de elétrons diretamente de uma molécula ou átomo para outro. Nessas reações, se uma molécula, ou átomo, ganha um elétron, outra molécula, ou átomo, deve perder um elétron. Uma maneira simples de lembrar as definições de oxidação e redução é através da frase OIL-RIG, que significa: Oxidation Is Losing – Reduction Is Gaining.
A molécula que ganha um elétron está sendo reduzida, mas é chamada de oxidante ou agente oxidante porque está oxidando a outra molécula. Da mesma forma, a molécula que perde um elétron está sendo oxidada, mas é chamada de agente redutor ou redutor porque reduz a outra molécula.
Existem quatro tipos principais de reação que normalmente envolvem processos redox.
<ol>
	<li style="font-size:17px">Reação de deslocamento único: Um átomo desloca outro átomo que faz parte de um composto e o substitui.</li>
	<li style="font-size:17px">Reação de combustão: Um composto é reduzido por um oxidante forte, normalmente gás oxigênio. As reações de combustão que ocorrem entre hidrocarbonetos e compostos orgânicos normalmente produzem dióxido de carbono e água.</li>
	<li style="font-size:17px">Reação de síntese: Dois reagentes formam um único produto.</li>
	<li style="font-size:17px">Reação de decomposição: Um único reagente se divide em dois ou mais produtos.</li>
</ol>
<h4>Referências</h4>
1. Harris, D. C. (2015). Análise Química Quantitativa. Nova York, NY: W. H. Freeman and Company.

Reações redox

Oxidação e Redução
Algumas reações químicas podem ser classificadas como reações de redução-oxidação ou reações redox. A oxidação é o processo de ...

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oxidation states in redox reactions

Oxidation states help identify redox reactions. In a redox reaction, oxidation is the loss of one or more electrons, and reduction is the gain of one or more electrons. These reactions are often tracked by changes in oxidation state, also called oxidation number.

Each atom in a molecule has an oxidation state. This number shows how oxidized an atom is compared with its free elemental form. It is written as the charge an atom would have if every bond were purely ionic. In that model, bonding electrons are assigned to the more electronegative atom. An atom in its elemental form has an oxidation state of 0.

Several rules help determine oxidation states. Group I elements usually have +1, and Group II elements usually have +2. Hydrogen is typically +1, oxygen is typically -2, and halogens usually are -1. The oxidation states in a molecule or ion must add up to the total charge, so unknown values can often be found from the rest.

Carbon dioxide is a clear example. CO2 is neutral, so its total oxidation states must add to zero. Each oxygen is usually -2, so carbon must be +4 to balance the two oxygens. Another way to find oxidation numbers is to draw the Lewis structure, assign each bond to the more electronegative atom, and count electrons on each atom.

In CO2, oxygen is more electronegative than carbon, so each C=O bond is assigned to oxygen. Each oxygen gets eight electrons total, including lone pairs and bonding electrons. Oxygen has six valence electrons by default, so its oxidation number is 6 - 8 = -2. Carbon has four valence electrons by default and is assigned none, so its oxidation number is 4 - 0 = +4.

A reaction is a redox reaction when an atom’s oxidation state changes from reactants to products. To check for redox, compare oxidation states on both sides of the reaction. Many redox reactions also involve direct electron transfer between atoms or molecules. If one species gains an electron, another must lose one.

The phrase OIL-RIG helps remember the process: oxidation is losing, and reduction is gaining. The species that gains electrons is reduced, but it is also called the oxidizing agent or oxidant because it oxidizes the other species. The species that loses electrons is oxidized, but it is also called the reducing agent or reductant because it reduces the other species.

Four major reaction types often involve redox processes. In single displacement reactions, one atom replaces another atom in a compound. In combustion reactions, a compound is reduced by a strong oxidant, usually oxygen gas, and hydrocarbons or other organic compounds often produce carbon dioxide and water. In synthesis reactions, two reactants form one product. In decomposition reactions, one reactant breaks into two or more products.

Oxidation States in Redox Reactions

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Oxidation Numbers

In redox reactions, the transfer of electrons occurs between reacting species. Electron transfer is described by a hypothetical number called the ...

Oxidation and Reduction
Some chemical reactions can be classified as reduction-oxidation reactions, or redox reactions. Oxidation is the process of ...

Acids, Bases and Neutralization Reactions

An acid-base reaction is one in which a hydrogen ion, H+, is transferred from one chemical species to another. Such reactions are of central importance to ...

Reaction Stoichiometry

A balanced chemical equation provides a great deal of information in a very succinct format. Chemical formulas provide the identities of the reactants and ...

Limiting Reactant

The relative amounts of reactants and products represented in a balanced chemical equation are often referred to as stoichiometric amounts. However, in ...

Reaction Yield

The theoretical yield of a reaction is the amount of product estimated to form based on the stoichiometry of the balanced chemical equation. The ...

Stoichiometry, Product Yield, and Limiting Reactants

Chemical equations represent how a chemical reaction proceeds from reactants to products through physical or chemical change using chemical formulas.
...

CHEM.9.C

Science concepts. The student understands how matter is accounted for in chemical reactions. The student is expected to:

Oxidation States in Redox Reactions

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