Chemical Reactions

Redox reactions

Oxidation and reduction reactions (redox) are asked frequently in class 10 science. We shall discuss the definitions of redox reactions in this article.

Oxidation and reduction (redox) reactions form the basis of many life processes and life on earth would not be possible without these reactions. Oxidation and reduction reactions happen together and hence there are referred as redox reactions.

Oxidation can be defined in a number of ways. This can be defined on the basis of the transfer of oxygen, hydrogen or electrons between the reactants.

Redox reactions in terms of oxygen transfer.

In a reaction, when one reactant gains oxygen, it is said to be oxidised. Therefore, oxidation can be defined as a gain of oxygen. For this reactant to gain an oxygen atom, another reactant has to donate an oxygen atom. The donor is called an oxidising agent. And the donor reactant is in turn reduced. Therefore, loss of oxygen defines a reduction reaction.

Consider this example:

Fe2O3 + 3CO = 2Fe +3CO2

In this equation, iron loses its oxygen to carbon monoxide to produce carbon dioxide. Here, iron oxide is the oxidising agent. And since carbon monoxide removes an oxygen from iron oxide, it is called a reducing agent.

Redox reactions in terms of transfer of hydrogen atoms.

This definition is useful in organic chemistry. Let us consider this equation:

In organic chemistry, one will come across two terms- alcohol and aldehyde. Alcohols and aldehydes are inter-convertible. When alcohol loses hydrogen, it converts to an aldehyde and this process is called oxidation. Similarly when an aldehyde gains hydrogen, it forms alcohol. This process is called reduction. Equations in organic chemistry are complex, therefore we have simplified this reaction using ethanol and its corresponding aldehyde ethanaldehyde.

CH2CH3OH — ———–> CH3CHO

Loss of hydrogen

 

Redox reactions in terms of transfer of electrons.

This is the most useful and comprehensive definition of oxidation and reduction reactions. Based on the transfer of electrons, oxidation is defined as loss of electrons while reduction is defined as a gain of electrons. Consider this basic equation:

CuO + Mg + Cu + MgO

In the above equation, copper oxide loses oxygen to magnesium, therefore magnesium is getting oxidised and copper oxide is the oxidising agent. Now let us rewrite this equation as an ionic equation (copper oxide and magnesium oxide are both ionic, the metals aren?t).

Cu2+ + Mg = Cu + Mg2+

In this equation, the oxygen atom is simply a spectator. Thus if we look at this equation in terms of electron transfer, the copper gains 2 electrons from magnesium, hence it is being reduced and magnesium loses 2 electrons to copper, hence it is getting oxidised.

This can be more easily remembered as:

OIL(oss) RIG(ain)

Oxidation= loss of electrons

Reduction= gain or electrons

Therefore, to summarise:

  1. Oxidation is loss of electrons.
  2. An oxidising agent oxidises another agent.
  3. Thus, an oxidising agent must gain electrons.

Similarly, a reducing agent reduces another substance, thus it must lose electrons (as the reduction is a gain of electrons).

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Balanced Chemical Equations

Chemical equations form a very important component of CBSE class 10 science. Students are often asked about balancing a chemical equation. This article will discuss some of the methods used in balancing equations.

A chemical equation is a symbolic, written representation of a chemical reaction. In an equation, the reactants are on the left and the products are on the right. There are a few rules to be followed when writing a chemical reaction.

  1. The chemical formulas of the reactants and products must be mentioned correctly.
  2. As mentioned above, the reactants are placed on the left side and the products are on the right.
  3. Law of conservation of matter states that matter can neither be created nor destroyed. Therefore, the total number of atoms should be the same on both sides of the equation.

Now consider this equation:

H2 +?O2?= H2O

There are 4 atoms on the left while there are only 3 atoms on the right. This is against the law of conservation, therefore this equation will need to be modified. This process of modifying the equation to make it conformant with the above law is called balancing an equation and the equation itself is called a balanced chemical equation. So let us proceed to balance the above equation.

H2?+ O2?= H2O

H=2????????? H=2

O=2????????? O=1

As there is only 1 oxygen atom on the right, we will add a coefficient to the oxygen atom on the right. The coefficient, in this case, is 2 (the number of oxygen atoms on left).

H2?+ O2?= 2H2O

H=2????????? H=4

O=2 ?????????O=2

Now you note that the number of oxygen atoms is same, but the number of hydrogen atoms is more on the right. So we now need to add a coefficient to the hydrogen on the left.

2H2?+ O2?= 2H2O

H=4 ???????????H=4

O=2???????? ???O=2

Now there are 6 atoms on either side and the equation is balanced.

This was a fairly simple equation to balance. Let us now consider a more complex equation. This equation represents the events that happen when propane burns in the presence of oxygen to produce water and carbon dioxide.

C3H8?+ O2?= H2O + CO2

There are three elements in this equation. Carbon, oxygen and hydrogen. We will balance the carbon first, the balance the oxygen and hydrogen molecules. We will follow the same process we did in the previous equation.

C3H8?+ O2?= H2O + CO2

C=3??????????????? C=1

H=8??????????????? H=2

O=2??????????????? O=3

There are 3 carbon atoms on the left but only one on the right, so we add a coefficient, which, in this case, is 3 (the number of carbon atoms on left).

C3H8?+ O2?= H2O + 3CO2

C=3??????????????? C=3

H=8??????????????? H=2

O=2??????????????? O=7

We now note that the carbon atoms are balanced, but hydrogen and oxygen are not. We will now balance the hydrogen atoms in the equation (we do hydrogen before oxygen because hydrogen is present in only a single molecule on both the left and right side). There are 8 hydrogen atoms on left and 2 on right, so we add a coefficient to hydrogen on the right, i.e. 4 (8/2=4).

C3H8?+ O2?= 4H2O + 3CO2

C=3??????????????? C=3

H=8??????????? ????H=8

O=2??????????????? O=10

Now both carbon and hydrogen are balanced. We only need to balance the oxygen atoms and we are done. This we do by adding a coefficient to the oxygen on the left (there are 10 atoms on the right but only 2 on left). The coefficient would be 5 (10/2=5).

C3H8?+ 5O2?= 4H2O + 3CO2

C=3??????????????? C=3

H=8??????????????? H=8

O=10????????????? O=10

Using this method one can balance any type of chemical equation, ranging from simple ones like above to more complex equations. However, as equations get complex, this method will become tedious, hence there are other methods which we shall discuss in another article.

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Types of chemical reactions

When wood burns, it is a chemical reaction. When food gets bad, it is a chemical reaction. Similarly, rusting of iron is also a chemical reaction. However, in each of the above examples, the nature of the chemical change undergone by the reactants is different. There are many different chemical reactions, therefore it is often useful to classify them into different types. The common types of chemical reactions include:-

Combination or synthesis reaction: A combination reaction is a reaction where two or more reactants combine under suitable conditions to form a new substance. As a new substance is being formed, this type of reaction is also called as a synthesis reaction. Take for example the rusting of iron. During the process of rusting, iron combines with oxygen present in the air to form iron oxide. It is this iron oxide which is visible as rust. As iron is reacting with oxygen, this type of reaction is also called as an oxygenation reaction.

  1. Decomposition reaction: Have you ever wondered what happens to the food we eat? The food we eat is broken down in our intestines by proteins called as enzymes. These enzymes break food into its elemental forms (through a series of complex reactions) and it is the elemental form which is absorbed from the intestines. For example, bread is broken down into glucose and absorbed by the body. Similarly, meat is broken down in amino acid and absorbed by the body. These are all examples of the decomposition reaction. Thus, a displacement reaction is the opposite of a combination reaction.
  2. Displacement reactions: This is a type of reaction where a more reactive element replaces a less reactive element from a compound. Displacement reactions are in turn classified as cation replacement reactions or anion replacement reactions. These reactions are also called as substitution reactions.
  3. Double displacement reactions: In this reaction, elements from each of the reactant molecules are exchanged thus forming two totally new compounds.
  4. Neutralisation reactions: When equal quantities of acids and bases react with each other, salt (not necessarily common salt) and water is produced. This type of reaction is called as neutralisation reaction.
  5. Isomerisation reaction: Butane is a four carbon compound which is used in the manufacture of gasoline. When it is heated, it undergoes a change in its structure and forms isobutane. However, the chemical composition of isobutane is the same as that of butane. This type of reaction is called as isomerisation reactions.
  6. Oxidation reactions: What happens during rusting of iron? Iron reacts with oxygen to form iron oxide. This is a form of the oxidation reaction. Oxidation reactions are one of the commonest chemical reactions in the world. Burning of coal or gasoline is an oxidation reaction. Our bodies produce energy for living by oxidation reactions.
  7. Reduction reaction: Reduction reactions are the opposite of oxidation reaction. When oxygen is removed from a compound, it is called a reduction reaction. This explanation is, however, a very simplistic view of chemistry. In reality, any reaction where an element gains electrons is called a reduction reaction.

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Chemical reactions: The Basics

Have you ever wondered how striking a matchstick produces fire, or how a burning candle produces light? The tip of the matchstick is coated with a chemical substance (commonly a combination of nitrates and phosphorus) which when struck, reacts with the oxygen present in air. This reaction leads to the production of heat and light, and the product of this event is quite different from the original chemical substance. Therefore in the above instance, a chemical reaction is said to have occurred.

Now take another example- when salt is mixed in water, it dissolves. However salt can be brought back to its original state by boiling off the water. In this example, even when dissolved in water,?the chemical composition of salt remains?unchanged. There is only a change in the physical?state?of the substance?(salt).,?On the other hand, it is not possible to return the matchstick to its original state. The later is an example of a physical change while the former is a case of a chemical change. Whenever a chemical change occurs, a chemical reaction is said to have occurred.

Thus a chemical reaction can be inferred to have taken place when the following events occur:-

  1. Physical effects, such as the emission of heat and light
  2. The formation of a precipitate
  3. The evolution of gas, or
  4. A color change.

The substance or substances initially involved in a chemical reaction are called reactants. Reactants in turn are composed of tiny particles called atoms. This concept was first alluded to by John Dalton in the early 19th century. In his atomic theory, he postulated that matter is composed of small, indivisible particles called atoms. The atoms of each element are unique, and when these atoms re-arrange in a chemical reaction, new substances are formed. All chemical reactions are based on this fact.

There are two significant rules one must be aware of while studying chemical reactions:-

  • ?Law of conservation of matter
  • Law of constant composition

What is the law?of conservation of matter?

The matter is neither created nor destroyed. The ash left behind after burning of wood weighs less than the original piece of timber. Has that piece of wood has lost some mass? No, if we put this piece of wood inside a closed container and weigh the container before and after burning, we will note that the weight of the container remains the same. Thus, wood has merely transformed.

What is?the?law?of constant composition?

Now consider this example- when coal burns in air, the carbon in coal reacts with oxygen in the air to form carbon dioxide. Gasoline (petrol) is another carbon-containing compound. When gasoline burns, the carbon and oxygen react to form carbon dioxide. The carbon dioxide produced by both these processes contains one atom of carbon and two atoms of oxygen, and this ratio remains constant. Thus, we could say that one atom of carbon will always combine with two atoms of oxygen.

To summarise, A chemical reaction is a process that results in a chemical change in the reactants. In this process the product or products, are, in general, different from the reactants.

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