Listen and Learn Science/Chemical Reactions

Chemical Reactions.
A chemical reaction, is a process that leads, to the transformation, of one set of substances, to another. In a chemical reaction, elements and compounds react, with each other, resulting in the formation, of other elements, and compounds. This reaction involves chemical bonding. This means, the elements get attached, to each other, at the atomic level. In nature, elements tend to have, an affinity to bond, with other elements. So, we will find most of the elements, in nature as compounds. There is an exception to this. Noble gases, and elements, do not combine, with other elements. Example, Neon. Other elements have varying degrees, of affinity to combine, with other elements. This degree of affinity is related, to the atomic structure, of the element.Specifically it is related, to the number of electrons, in the outer most shell, of the atom. This gives a valence to an element.

Alloys.
An alloy is a mixture, of two or more, elements. An alloy is not a compound of the elements. There is no chemical bonding of the elements, in an alloy. Steel is an example, of an alloy of iron and Carbon. Bronze is an alloy of copper and tin. Alloys can be very useful materials with different properties. For example steel is much stronger, than iron. Bronze is much stronger than copper or tin.

Solutions.
When sugar is dissolved in milk, There is no chemical reaction. A solution of milk, and sugar, is formed. If we add water to this, again, there is no chemical reaction. We just have a solution, of diluted milk and sugar.

Valence
The tendency of an element to combine with another element, is called the valence of the element. The Valence of hydrogen is different from, the Valence of Carbon, which is different from, the Valence of Oxygen. So, is the case, with other elements. Interestingly, the Valence of an element, correlates to the way, the electrons are distributed in the shells, surrounding the nucleus. The first shell, called the K shell, can hold 2 electrons. The second shell, called the L shell, can hold 8 electrons. The third shell, called the M shell, can hold 18 electrons. If the number of electrons in the outer most shell is occupied to full capacity, the atom does not have a tendency to combine with other atoms. Also, if the number of electrons in the outer most shell is equal to 8, the atom does not have a tendency to combine with other atoms. In all other cases, the atom has a tendency to combine with other atoms.

Noble Gases.
Noble gases do not react with other elements. They are neutral, and inert.

Helium has an, atomic number of 2. It has two electrons, in the first, or K shell. The K shell is full. So, helium does not combine, with other elements. We can also say, the Valence of helium is, 0.

Neon has an, atomic number of 10. It has a total, of 10 electrons. The K shell has 2 electrons. The L shell has 8 electrons. Since the outer most shell, has 8 electrons, neon does not have a tendency, to combine with other elements. We can also say, the Valence of neon is, 0. Other examples of inert gases are, Argon, and Krypton.

Valence of elements.
Hydrogen, has an atomic number, of 1. It has one electron, in the K shell. The K shell has a capacity of 2, so there is scope, for hydrogen to accommodate, one more electron. When there is such a capacity, atoms tend to share, an electron from another atom. In this case, it has a capacity to share, one electron. The Valence, or combining power, of hydrogen is 1.

Carbon, has an atomic number, of 6. It has two electrons, in the K shell. It has 4 electrons, in the L shell. The capacity of the L shell, is 8. So, Carbon has the capacity, to share four more electrons. The Valence of Carbon, is 4.

Oxygen, has an atomic number, of 8. It has 2 electrons, in the K shell. It has 6 electrons, in the L shell. The capacity of the L shell, is 8. So, Oxygen has the capacity, to share two more electrons. The Valence of Oxygen, is 2.

Bonding.
A chemical reaction, involves chemical bonding. Bonding is a way, elements share the electrons, in the outer most shell. Hydrogen combines with Oxygen, in a chemical reaction, to form water, or H 2 O. Two atoms of hydrogen, combine with one atom of Oxygen, to form H 2 O.  Each of the hydrogen atoms, shares one electron, with the Oxygen atom. After the sharing, hydrogen feels fulfilled, in the K shell. Now the K shell, has one of its own electron, and one shared electron. The Oxygen atom gets, two shared electron, from two hydrogen atoms. The Oxygen atom, feels fulfilled, in the L shell. It has six of its own electrons, and two shared electrons, making a total of 8 electrons, in the L shell. We can say Oxygen, has one bond, with each of the two, hydrogen atoms. Each bond is represented, with a dash. So, H dash, O dash, H, Is the bonded formula, for water, or H 2 O.  This bond of sharing, one electron is called, a covalent bond.

Carbon combines with Oxygen, to form Carbon dioxide. One atom of Carbon combines, with two atoms of Oxygen, to form C O 2. One Carbon atom shares, two electrons, with each Oxygen atom. Now Carbon has four, of its own electrons, and four shared electrons, in the L shell. So the Carbon atom gets fulfilled, with the 8 virtual electrons, in the L shell. The Oxygen atom has six, of its own electrons, and two shared electrons, in the L shell. So the Oxygen atom gets fulfilled, with the 8 virtual electrons, in the L shell. This sharing, of two electrons, is called a double bond. This is represented, with two parallel dashes, just like a, equal to, sign. So, O double bond Carbon, double bond O, is C O 2.

Nitrogen combines, with hydrogen, to form, ammonia, or N H 3. Nitrogen, has an atomic number, of 7. It has 2 electrons, in the K shell. It has 5 electrons, in the L shell. It has an affinity, for 3 more electrons, in the L shell. 3 hydrogen atoms share, 1 electron each, with a nitrogen atom. This gives a virtual, 8 electrons, to nitrogen, in the L shell. 5 of its own electrons, and 3 shared electrons. This combination of Nitrogen, and hydrogen, results in a stable compound, N H 3 or ammonia. Since 3 electrons are shared, this bond is represented, as a triple bond. So N triple bond H 3, is the way for representing, ammonia.

Chemical reactions, is all about sharing of electrons, and bonding. In an chemical reaction, A bond might be formed, Existing bonds might be broken, And some other, new bonds may be formed. This concept is true, for all chemicals reactions, however big or complex, the chemical reaction is.

This also gives us the idea, that chemical reactions, are not random. They occur, due to some scientific reason, of elements and compounds, to react with one another. Chemical reactions, results in a rearrangement, of chemical bonds.

Nomenclature of a chemical reaction.
We can state that, Carbon combines with Oxygen, to form Carbon dioxide. This is a qualitative way, of stating a chemical reaction. We can also state it, like an equation. C + O 2, right arrow, C O 2. This is an example, of a chemical equation. Carbon and Oxygen, are called, the reactants. The right arrow, can be interpreted, as "results in". In this module, we will read the right arrow as "results in". Carbon dioxide is the product. We can show, an up arrow, after C O 2, to signify that, it is a resultant gas. If heat is required for the reaction, we can say so, on top of the right arrow. If energy is released, in the reaction, we can say, C O 2 + energy. If the reaction results in a precipitate, we can represent, it with a down arrow.

Catalysts are substances, which facilitate a chemical reaction. Catalysts do not participate, in the reaction. They only help the reaction, to happen. If a catalyst is required, for a reaction, we can specify that also, in the chemical equation.

A chemical equation, is therefore a concise, and convenient way, of representing, a chemical reaction, completely.

Balancing a chemical reaction.
When a chemical reaction takes place, no matter can be lost. No new matter can be created. This means, all the elements, that are present in the reactants, should be present, in the products. Magnesium reacts with Oxygen, to produce magnesium oxide. If we say M g + O 2, right arrow, M g O. This will not be fully correct. The reactant O 2, has 2 Oxygen atoms. M g O, has only one, Oxygen atom. The other, Oxygen atom cannot disappear, so we have to balance the equation. So, 2M g + O 2, results in, 2 M g O. Reactants have 2 magnesium atoms, and 2 Oxygen atoms. Product has 2 magnesium atoms, and 2 Oxygen atoms. The chemical reaction, is balanced. Some examples, of balanced chemical reactions. Hydrogen reacts with Oxygen, resulting in water, or H 2 O. 2H 2 + O 2, results in, 2 H 2 O.  Aluminium reacts with chlorine, to form aluminium chloride. 2A l + 3, C l 2, results in, 2A l, C l 3. We note that, simple arithmetic of multiplying, the number of reactants and products, to preserve the number of atoms, of the elements, balances the equation. 2 aluminium atoms, + 3 chlorine atoms, results in, 2 aluminium chloride molecules. Zinc reacts with hydrochloric acid, resulting in zinc chloride, and hydrogen. Z n + 2 H C l, results in, Z n C l 2 + H 2.

Types of chemical reactions.
There is some common type, of chemical reactions. It is worth knowing, some common types, of chemical reactions.

Chemical combination
.

Sulphur combines with Oxygen, to form sulphur dioxide. S + O 2, results in, S O 2. Nitrogen combines with hydrogen, to form ammonia. N 2 + 3H 2, results in, 2 N H 3. Ammonia combines with hydrochloric acid, to form ammonium chloride. N H 3 + H C l, results in, N H 4 C l. All these are examples, of 2 substances combining, to form, a single substance. These reactions are called, chemical combination reactions.

Chemical decomposition.
Mercuric oxide decomposes, into mercury, and Oxygen. 2H g O, results in, 2H g + O 2. In these examples, one substance decomposes, into two or more, substances.

Potassium permanganate has the formula, K M n O 4. It decomposes into potassium magnate, magnesium oxide, and Oxygen. 2K M n O 4, results in, K 2 M n O 4, + M n O 2, + O 2.

These reactions are called, chemical decomposition reactions.

Chemical displacement.
Copper sulphate, has the formula, C u S O 4. Iron sulphate or ferrous sulphate, has the formula, F e S O 4. Copper sulphate reacts with iron, to form ferrous sulphate, and copper is precipitated. C u S O 4 + F e, results in, F e S O 4 + C u. Iron has replaced copper, in this reaction.

Chlorine reacts with potassium iodide, to form potassium chloride, and iodine is precipitated. C l 2 + 2K I, results in, 2K C l, plus I 2. Chlorine has replaced Potassium, in this reaction.

The formula, for sulphuric acid, is H 2 S O 4. Zinc reacts with sulphuric acid, to form Zinc sulphate, and hydrogen is released. Z n + H 2 S O 4, results in, Z n S O 4 + H 2. Zinc has replaced hydrogen, in this reaction. In all these examples, one substance replaces, another substance, in a compound. Normally, the more reactive substance, replaces the less, reactive substance. That is, iron is more reactive, than copper. Chlorine is more reactive, than iodine. Zinc is more reactive, than hydrogen. All these reactions are examples, of chemical displacement reactions.

Chemical double displacement.
The formula, for magnesium sulphate, is M g S O 4. The group S O 4, is referred to, as sulphate. The formula for sodium Carbonate, is, N a, 2C O 3. The group C O 3, is referred to, as Carbonate. Magnesium sulphate reacts, with sodium Carbonate, to form magnesium Carbonate, and sodium sulphate. M g S O 4 + N a, 2C O 3, results in, M g C O 3 + N a, 2S O 4. In this reaction, magnesium and sodium, have replaced each other.

The formula, for calcium chloride, is C a C l 2. The formula for sodium Carbonate, is N a 2, C O 3. Calcium chloride reacts, with sodium Carbonate, resulting in calcium Carbonate, plus sodium chloride. C a C l 2 + N a 2, C O 3, results in, C a C O 3 + 2N a C l. In these examples, the two elements replace each other, in the compounds. This is called, double displacement. These reactions are examples, of chemical double displacement reactions.

Redox.
When Oxygen is added, to a substance, it is said to be, oxidised. When Oxygen is removed, from a substance, it is said to be, reduced. Oxidation and reduction are very common, chemical reactions. These reactions are called, redox reactions. R e d stands, for reduction. O x stands, for oxidation.

Magnesium reacts with Oxygen, to form magnesium oxide. 2 M g + O 2, results in, 2 M g O.

The formula, for iron oxide, is F e 2 O 3. Iron oxide reacts with Carbon, to form iron, and Carbon dioxide is released. 2F e O 3 + 3C, results in, 2F e + 3C O 2. We say that iron oxide, is reduced to iron, by removing Oxygen. This we call it, as a reduction reaction.

Traditionally addition of Oxygen, to a substance, was called oxidation. Removable of Oxygen, was called reduction. The definition has since, expanded. Oxidation is now considered, as loss of electrons. Reduction is now considered, as gain of electrons. For example, 2N a + C l 2, results in, 2 N a, C l. Sodium is considered, as being oxidised. Chlorine is considered, as being reduced. Oxidation and reduction reactions, or redox reactions, are important because, many chemical reactions are redox reactions.

Applications of chemical reactions.
Chemical reactions are involved, in many energy producing applications. Chemical reactions are used, in industry to produce, some substance that we want. These substances are produced, from other substances, which exists. In a chemical process, a series of multiple chemical reactions, may be involved. That is, we may use, multiple steps to get, the required substances that we need.

Energy from Chemicals.
One of the most common, chemical reactions is combustion. When a substance ignites, it burns, by combining with Oxygen. This produces heat energy. Heat energy is very useful, to human beings. Possibly, this was the first man made, chemical reaction. This happened, when we discovered fire. Fire was able to burn, dried wood, to produce heat. Fire was used by man, for cooking. Fire was also used to keep him warm, in cold climates.

Today we use, coal in thermal power plants. When Coal is burnt, with Oxygen, it produces heat. This is a chemical reaction. This heat is used, to produce steam. The steam drives turbines, which produce electricity. Coal is a fossil fuel. It was produced naturally, millions of years ago, from organic matter. Coal is today used as a major source, of chemical energy, in thermal power plants.

Most vehicles use, combustion engines. These combustion engines, use petrol, or diesel, as fuel. When petrol undergoes combustion, in an engine, it produces heat. This is a chemical reaction. The gases rapidly expand. This energy is converted, to mechanical energy, to propel the vehicle. Most forms of transportation we use, use combustion engines. These car engines, truck engines, jet engines, etc, use combustion engines. Petrol and diesel are the most, widely used fuel for transportation. Petrol and diesel are extracted, from naturally occurring, crude oil. Crude oil, is a fossil fuel. It was produced naturally, millions of years ago, from organic matter. The availability of Fossil fuels, are limited. Fossil fuels cannot, be regenerated. In the future, we have to look for alternatives, to fossil fuels, to meet our energy and transportation needs.

Extraction of minerals.
Many of the minerals that we need, are present in the natural state, as compounds. These naturally occurring compounds are called, ores. We mine the ores, from the earth. We use a chemical process, to extract the mineral, we want, from the ore. For example, Iron is present naturally, in iron ore, as Iron oxide or F e 2, O 3. This naturally present Iron ore, is called Hematite. We can mine this ore. This is taken to a steel plant. In the steel plant furnace, Carbon is combined with Oxygen, to produce Carbon monoxide. 2C + O 2, results in, 2 C O. In the next step, at high temperature, Iron oxide reacts, with Carbon monoxide, to produce iron, and Carbon dioxide. F e 2, O 3 + 3 C O, results in, 2 F e + 3 C O 2. In this way, molten Iron is extracted, from the furnace. This is a simple example, of a multiple step chemical process. Useful Iron is extracted, from an otherwise unusable compound, Iron oxide or F e 2, O 3.

Copper is present, as copper sulphide, C u 2 S, in copper ore. This copper ore is called, Chalcocite. With a suitable chemical process, copper can be extracted, from this ore.

Aluminium is present as aluminium oxide, as A l 2 O 3. This is called bauxite. With a suitable chemical process, aluminium can be extracted, from this ore.

Manufacture of Chemical compounds.
Chemical compounds have unique properties. Property of a compound is usually very different, from the properties of the elements, that constitute it. For example, common salt is sodium chloride. The properties of sodium chloride, is very different from sodium, or chlorine. This is one of the reasons, that chemistry becomes, very interesting. We can discover and manufacture, many compounds, which are useful to us. We already use, many such artificially manufactured compounds, in our daily life. Some examples are Soap, Toothpaste, Ink, Plastics, Glue, Cosmetics, etc. Chemicals are used in the manufacture of fertilisers and pesticides. Fertilisers and pesticides are widely used in Agriculture. There are many such chemicals compounds, that we use, and which are used, in industry. Many more chemical compounds, useful to humans, are being discovered.

Pharmaceutical drugs.
Most allopathic medicines we take, are artificially manufactured, chemical compounds. These drugs have a wide variety, of uses. They are used to cure, many illness that we could get. The pharmaceutical industry, basically, uses a number of chemical reactions, to produce these drugs. Scientists are still discovering, new and better drugs.

Biochemistry.
The process of life, involves a series, of Bio chemical reactions. These bio chemical reactions are happening silently, every minute in our lives. All living organisms are involved, in some kind of bio chemical reactions. Let us take a simple example, All plants absorb sunlight. Sunlight is a form of energy. Plants also breathe in Carbon dioxide, and take in water, through their roots. Carbon dioxide combines with water, with sunlight, resulting in glucose, and Oxygen. 6 C O 2 + 6 H 2 O + sunlight, results in, C 6, H 12, O 6 + 6 O 2. So plants breathe in Carbon dioxide, and breathe out Oxygen. They produce glucose, which is a source of food, and energy.

Animals and human beings, breathe in Oxygen. In cells of the human beings, energy is being created. A typical example of this is, Glucose + Oxygen, results in, Carbon dioxide + water + energy. C 6, H 12, O 6 + 6 O 2, results in, 6 C O 2 + 6 H 2 O + energy. The released energy is used, for our day to day living. We need about, 80 watts of energy, just to live. This is supplied, by the food we eat. Glucose is just one example, of food. Carbohydrates, proteins, and fats, are other, essential foods that we eat. Carbohydrates, proteins, and fats, are also organic chemical compounds. These compounds are a way to store energy, required for life. Our body digest these chemical compounds, through a series, of bio chemicals reactions, to produce energy.

Our body is built, from organic chemical compounds. Most of these are proteins. There are many types of proteins. Proteins are chains, of amino acids. Body synthesis these proteins. They are used for building the body, and for replacement, for maintaining the body. These bio chemicals reactions are happening, silently, all the time.

Human Genome.
Where does the human body get the knowledge, to build the organs of the body. The organs could be the heart, the brain, the kidney, the liver, muscles, bones etc. All these organs, are built up through, a series of bio chemical reactions. The genome is the encyclopaedia, of the knowledge of life. The human genome comprises, of 23 chromosomes. These are like, 23 chapters, of an encyclopaedia. Each chromosomes, is a string of 1000s, of D N A molecules. D N A, is deoxyribonucleic acid. This is called a gene. D N A molecules, are two strands of molecules, coiled around each other, in the form of a double helix. It is like a pair of snakes, coiled around each other. The human genome has, 60 to 80 thousand genes. Each gene is like a chapter, in the encyclopaedia. The genes contain the knowledge, to build the organs of life. The genes themselves comprise, of only four basic, organic compounds. These four basic compounds are called, as G A C and T. Long chains of different combinations, of G A C T molecules, make the D N A.  All these chains put together, in 23 chromosomes, form the genome. This genome is the encyclopaedia, of the knowledge of life. Each of the hundred trillion cells, in the human body, have the genome, in its nucleus. The genome is, a few nanometers in size. They teach the chemicals in the body, to build, organise, maintain, and run, the whole of life. It is amazing, that the knowledge of life, can be contained, in a chemical compound. This makes Bio-chemistry, a mysteriously interesting, branch of science. We are just beginning, to discover, the mystery that is life. Our level of knowledge is still, in the first chapter, of this discovery. Future chapters are waiting, for future scientists, to write it.