Chemical Reactions and Equations
Chemical Reactions
The process in which the rearrangement of atoms takes place between the reacting substances to form new substances. These new substances have entirely different chemical properties. During a chemical reaction, the old chemical bonds which exist between the atoms of reactants break down and new chemical bonds form between the rearranged atoms to form new substances.
In other words, during a chemical reaction only a rearrangement of atoms takes place and atoms of one element do not change into another element.
Example: Zinc (Zn) reacts with sulphuric acid (H2SO4) to form zinc sulphate (ZnSO4) and hydrogen gas (H2).
Zn + H2SO4 ========> ZnSO4 + H2
Reactants Products
Difference between Reactants and Products
We are now going to discuss about reactants and products of a chemical reaction.
Reactants: The chemical substances which take part in a chemical reaction are called chemical reactants.
Products: The new chemical substances that are produced at the end of a chemical reaction are called products.
So, we can say that during a chemical reaction reactants change into new products.
Reactants ========> Products
Example: Zinc (Zn) reacts with sulphuric acid (H2SO4) to form zinc sulphate and hydrogen gas.
Zn + H2SO4 ========> ZnSO4 + H2
Reactants Products
In this reaction, zinc and sulphuric acid are the reactants and zinc sulphate and hydrogen are the products.
Characteristics of Chemical Reactions
There are some observable changes which take place as a result of chemical reactions known as characteristics of chemical reactions. The important characteristics of chemical reactions are:
- Evolution of a gas
- Change in colour
- Formation of a precipitate
- Change in temperature, and
- Change in the physical state
Examples:
Evolution of a Gas:
Gas formation is also a characteristic of a chemical change. Gases are formed as products in many chemical reactions. For example:
Zinc (Zn) reacts with sulphuric acid (H2SO4) to form zinc sulphate and hydrogen gas.
Zn + H2SO4 ========> ZnSO4 + H2 (g)
Reactants Products
In this reaction, hydrogen gas is evolved. It is the characteristic of a chemical reaction.
Similarly, the reaction between sodium carbonate (Na2CO3) and dilute hydrochloric acid (dil 2HCl) is characterized by the evolution of carbon dioxide gas.
Na2CO3 + dil 2HCl ========> 2NaCl + CO2 + H2O
Formation of a Precipitate:
The precipitate formation is the second characteristic of a chemical reaction. Precipitate is a solid substance, that settles down after formation.
For example, when potassium iodide (KI) reacts with lead nitrate [Pb(NO3)2], lead iodide is formed in the form of a yellow precipitate. So, it is a chemical reaction which is characterized by precipitate formation.
2KI + Pb(NO3)2 ========> PbI2 + KNO3
Yellow Precipitate of Lead Iodide
Similarly, the reaction between barium chloride (BaCl2) and sulphuric acid (H2SO4) is characterized by the formation of a white precipitate of barium sulphate(BaSO4).
BaCl2 + H2SO4 ========> BaSO4 + 2HCl
White Precipitate
Change in Colour:
Some chemical reactions are characterized by a change in colour in the reaction mixture. The above two precipitation reactions are also characterized by the formation of yellow and white colours respectively.
2KI + Pb(NO3)2 ========> PbI2 + 2KNO3
Yellow Precipitate
BaCl2 + H2SO4 ========> BaSO4 + 2HCl
White Precipitate
Similarly, the reaction between purple-coloured potassium permanganate (KMnO4) solution and citric acid (C6H8O7) is characterized by a change in colour from purple to colourless.
The chemical reaction between acidified potassium dichromate (K2Cr2O7) and Sulphur dioxide (SO2) is also characterized by a change in colour from orange to green.
Changes in Temperature:
During the chemical transformation, heat may be absorbed or released. So, the temperature of the reaction system me decrease or increase. This is also an indication of chemical change.
For example, when quicklime (CaO) reacts with water to form slaked lime with the evolution of heat the temperature of the reaction vessel increases. So, this reaction is characterized by the rise in the temperature.
CaO + H2O ========> Ca(OH)2 + Heat
Quicklime Slaked Lime
But, when barium hydroxide Ba(OH)2 is mixed with ammonium chloride (NH4Cl) in a test tube, then barium chloride (BaCl2), ammonia (NH3) and water (H2O) are formed. During this, a lot of heat is absorbed due to which the temperature of the reaction mixture falls and the bottom of the test tube becomes very cold. So, this chemical reaction is characterized by a fall in temperature.
Ba(OH)2 + NH4Cl ========> BaCl2 +NH3 +H2O -Heat
Change in state
In some chemical transformation, the physical state of products is different from reactants. So some chemical reactions are characterized by a change in physical states.
When wax (Solid) is burned, water (vapour) and carbon dioxide (gas) are formed. So, this reaction is characterized by a change in physical state.
Note: There are some chemical reactions which can show more than one characteristic.
Example:
When potassium iodide (KI) reacts with lead nitrate [Pb(NO3)2], lead iodide is formed in the form of a yellow precipitate.
2KI + Pb(NO3)2 ========> PbI2 + KNO3
Yellow Precipitate
So, it is a chemical reaction which is characterized by precipitate formation and colour change (Yellow).
Chemical Equations
What is a chemical equation?
A chemical equation is a method of representing a chemical reaction with the help of formulae and symbols of the reactants, products and conditions of the reaction. This can be explained with the help of the following example:
The granules of zinc metal react with dilute sulphuric acid to form zinc sulphate and hydrogen gas.
Zinc + Sulphuric acid ========> Zinc Sulphate + Hydrogen
This is called the word chemical equation. So, you can change this word equation into a chemical equation by replacing words with formulae and symbols.
We know that The symbol of zinc granules is Zn
The formula of sulphuric acid is H2SO4
The formula of zinc sulphate is ZnSO4
The formula of hydrogen is H2
We now get the following chemical equation:
Zn + H2SO4 ========> ZnSO4 + H2
Reactants Products
Rules for writing chemical equations:
- Chemical substances are written as symbols and formulae.
- We put an arrow between reactants and products.
- The arrow signs points towards the right-hand side indicates that reactants are written on the left-hand side and products are on the right-hand side.
- There is a plus (+) sign between two reactants or products.
- The catalyst is neither a reactant nor a product. However its presence is necessary for the occurrence of the chemical reaction. So, its formula is written above the arrow in the chemical equation.
- The other symbols like heat, temperature and pressure are written above the arrow.
- The side product molecules like water may also be written below the arrow with a minus (-) sign.
- The physical state of the reactants and products can be written in the form of state symbols in the bracts with substance symbols or formulae.
The different symbols used for writing a chemical equation are:
| S.No | Name | Symbol | S.No | Name | Symbol |
| 1 | Heat | 5 | Gas | ||
| 2 | Solid | (s) | 6 | Water soluble | (aq) |
| 3 | Liquid | (l) | 7 | Precipitate | Down Arrow |
| 4 | Gas | (g) | 8 | Combine | + |
So, it should be clear that a chemical equation is a short-hand method of writing a chemical equation.
Types of Chemical Equations
Chemical equations are of two types:
- Balanced Chemical Equations
- Unbalanced Chemical Equations
Balanced Chemical Equations
The chemical equation which has an equal number of atoms of different elements in the reactants and products.
Example: Zinc (Zn) reacts with sulphuric acid (H2SO4) to form zinc sulphate and hydrogen gas.
Zn + H2SO4 ========> ZnSO4 + H2 (g)
Reactants Products
Let’s check the number of atoms of all the elements in the reactants and products separately.
In Reactants In Products
Number of Zn Atoms: 1 1
Number of H atoms: 2 2
Number of S atoms: 1 1
Number of O atoms: 4 4
In the above chemical equation, there are equal numbers of atoms of Zn, H, S and O in the reactants and products. Therefore, the above chemical equation is a balanced equation.
Unbalanced Chemical Equation
The chemical equation which has an unequal number of atoms of different elements in the reactants and products is called an unbalanced chemical equation. It means that the number of atoms of reactants does not equal the number of atoms of products.
Example: When potassium iodide (KI) reacts with lead nitrate, then lead iodide is formed.
KI + Pb(NO3)2 ========> PbI2 + KNO3
Let’s check the number of atoms of all the elements in the reactants and products separately.
In Reactants In Products
Number of K Atoms: 1 1
Number of I atoms: 1 2
Number of Pb atoms: 1 1
Number of N atoms: 2 1
Number of O atoms: 6 3
In the above chemical equation, there are an equal number of atoms of K and Pb in the reactants and products but has unequal number of I, N and O atoms in the reactants and products. Therefore, the above chemical equation is an un-balanced equation.
Balancing of Chemical Equations
It is the process of making the number of different types of atoms equal on both sides open the equation is called balancing of the equation. The simple chemical equations can be balanced by the hit and trial method.
The main steps or hit and trail method are:
- Step 1: Write the formulae and symbols of the reactant and products of the given equation.
- Step 2: Now, count and write the number of atoms of each element in reactants and products
- Step 3: Write all the reactants and products in square brackets.
- Step 4: The substances have definite formulae which cannot be altered. So, we should never change the formula of an element or a compound to balance an equation we can only multiply a symbol or formula by figures like 2, 3, 4 etc.
- Step-5: Count and write the number of each atom in reactants and products
Now, the number of atoms of each element becomes equal on both sides in reactants and products.
Let’s take some examples:
Problem 1: Balance the following equation:
H2 + O2 ========> 2H2O
Step 1: Write the given unbalanced chemical equation.
H2 + O2 ========> 2H2O
Step 2: Let’s check the number of atoms of all the elements in the reactants and products separately.
In Reactants In Products
Number of H Atoms: 2 4
Number of O atoms: 2 2
In the above chemical equation, the number of oxygen atoms is equal on both sides but the number of hydrogen atoms is not equal.
Step 3: Write all the reactants and products in square brackets.
[H2] + [O2] ========> 2[H2O]
Step-4: Balancing hydrogen atoms by multiplying H2 by 2
2[H2] + [O2] ========> 2[H2O]
Step-5: Count and write the number of each atom in reactants and products
In Reactants In Products
Number of H Atoms: 4 4
Number of O atoms: 2 2
This chemical equation contains an equal number of hydrogen and oxygen atoms in the reactants and the products, so this is a balanced equation.
Problem 2: Translate the following statement into a chemical equation and then balance the equation: Hydrogen gas combines with nitrogen to form ammonia.
Solution: In this reaction, hydrogen combines with nitrogen to home ammonia this can be written as:
N2 + H2 ========> NH3
Step 2: Let’s check the number of atoms of all the elements in the reactants and products separately.
In Reactants In Products
Number of N Atoms: 2 1
Number of H atoms: 2 3
In the above chemical equation, the number of nitrogen and hydrogen atoms is unequal on both sides.
Step 3: Write all the reactants and products in square brackets.
[N2] + [H2] ========> [NH3]
Step 4: Balancing nitrogen atoms by multiplying NH3 by 2 on right-hand side
[N2] + [H2] ========> 2[NH3]
Step-5: Balancing hydrogen atoms by multiplying H2 by 3 on left-hand side
[N2] + 3[H2] ========> 2[NH3]
Step-6: Count and write the number of each atom in reactants and products
In Reactants In Products
Number of N Atoms: 2 2
Number of H atoms: 6 6
This chemical equation contains an equal number of nitrogen and hydrogen atoms in the reactants and the products, so this is a balanced equation.
Problem 3: Translate the following statement into a chemical equation and then balance the equation: Methane burns in oxygen to form carbon dioxide and water.
Solution: In this reaction, hydrogen combines with nitrogen to form ammonia this can be written as:
CH4 + O2 ========> CO2 + H2O
Step 2: Let’s check the number of atoms of all the elements in the reactants and products separately.
In Reactants In Products
Number of C Atoms: 1 1
Number of H atoms: 4 2
Number of O atoms: 2 3
In the above chemical equation, the number of carbon atoms is equal but the number of oxygen and hydrogen atoms is unequal on both sides.
Step 3: Write all the reactants and products in square brackets.
[CH4] + [O2] ========> [CO2] + [H2O]
Step-4: Balancing hydrogen atoms by multiplying H2O by 2 on the right-hand side
[CH4] + [O2] ========> [CO2] + 2[H2O]
Step-5: Balancing oxygen atoms by multiplying O2 by 2 on the left-hand side
[CH4] + [O2] ========> [CO2] + 2[H2O]
Step-6: Count and write the number of each atom in reactants and products
In Reactants In Products
Number of C Atoms: 1 1
Number of H atoms: 4 4
Number of O atoms: 4 4
This chemical equation contains an equal number of carbon, hydrogen and oxygen atoms in the reactants and the products, so this is a balanced equation.
Problem 4: Translate the following statement into a chemical equation and then balance the equation: Magnesium burns in oxygen to form magnesium oxide.
Solution: In this reaction, magnesium combines with oxygen to form magnesium oxide this can be written as:
Mg + O2 ========> MgO
Step 2: Let’s check the number of atoms of all the elements in the reactants and products separately. In Reactants In Products
Number of Mg Atoms: 1 1
Number of O atoms: 2 1
In the above chemical equation, the number of magnesium atoms is equal but the number of oxygen atoms is unequal on both sides.
Step-3: Write all the reactants and products in square brackets.
[Mg] + [O2] ========> [MgO]
Step-4: Balancing oxygen atoms by multiplying MgO by 2 on the right-hand side
[Mg] + [O2] ========> 2[MgO]
Step-5: Count and write the number of each atom in reactants and products
In Reactants In Products
Number of Mg Atoms: 1 1
Number of O atoms: 2 2
This chemical equation contains an equal number of magnesium and oxygen atoms in the reactants and the products, so this is a balanced equation.
Types of Chemical Reactions
The chemical reactions are of various kinds. Some of the important types of chemical reactions are given below:
- Combination Reactions
- Decomposition Reactions
- Displacement Reactions
- Double Displacement Reactions
- Redox or Oxidation-Reduction Reactions
- Neutralization Reaction
- Electrolysis
Combination Reactions
The chemical reactions in which two or more substances (atoms, molecules or compounds) combine to form a single substance, are called combination reactions.
These can be explained with the help of the following examples:
Example 1: When magnesium is heated with oxygen, then magnesium and oxygen combine to form magnesium oxide (Single Compound).
Mg + O2 (Air ) ========> MgO
Magnesium Magnesium Oxide
Example 2: When hydrogen burns in oxygen, hydrogen combines with oxygen to form water.
2H2 (g) + O2 (g) ========> 2H2O
Hydrogen Oxygen Water
So, we can say that the formation of water molecules by the combination of hydrogen and oxygen is an example of a combination reaction.
Example 3: When carbon burns in oxygen, then carbon combines with oxygen to form carbon dioxide.
C(s) + O2 (g) ========> 2CO2
Carbon Oxygen Carbon dioxide
So, we can say that the formation of carbon dioxide molecules by the combination of carbon and oxygen is another example of a combination reaction.
Decomposition Reactions
The chemical reactions in which a single substance breaks down into two or more simpler substances, are called decomposition reactions.
These are the opposite of combination reactions. Decomposition reactions are carried out by applying heat, light or electricity. So, these reactions are of three types:
- Thermal Decomposition Reaction
- Photo or Light Decomposition Reaction
- Electrical Decomposition Reaction
Thermal Decomposition Reaction
When the decomposition reaction is carried out by heating the substance, it is called a thermal decomposition reaction.
Example 1: When calcium carbonate is heated, it breaks down into calcium oxide and carbon dioxide. This is an example of a Thermal decomposition reaction.
CaCO3 (s) ========> CaO (s) + CO2
Calcium Carbonate Calcium Oxide
Example 2: Thermal decomposition of potassium chlorate into potassium chloride and oxygen in the presence of manganese dioxide catalyst.
KClO3(s) ========> 2KCl (s) + 3O2
Photochemical Decomposition
When the decomposition reactions take place in the presence of sunlight or light is called photochemical decomposition.
Example-1: In the presence of sunlight, the silver chloride decomposes to form silver metal and chlorine gas.
Sunlight
2AgCl (s) ========> Ag (s) + Cl2
Decomposition
Example 2: In the presence of sunlight, the silver bromide decomposes to form silver metal and bromine gas.
Electric Decomposition
When the decomposition reaction is carried out by using electricity, it is called an electric decomposition reaction.
Example-1:
When we pass an electric current through the acidified water, then it decomposes into hydrogen and oxygen.
The water molecule is decomposed in the presence of electric current, so it is called electric decomposition.
Example-2:
When we pass an electric current through the molten sodium chloride, then it decomposes into sodium metal and chlorine gas.
Example-3:
Similarly, if we pass an electric current through the molten alumina, then it decomposes into aluminium metal and oxygen gas.
Displacement Reaction
The chemical reactions in which one element takes the position of another element in a compound, are known as displacement reactions. Generally, the more reactive element displaces the less reactive element from its compound.
Example 1: Zinc displaces the Copper from copper sulphate solution because zinc metal is more reactive than copper.
CuSO4 + Zn ========> ZnSO4 + Cu
Example 2: Magnesium displaces the Copper from copper sulphate solution because magnesium metal is more reactive than copper.
CuSO4 + Mg ========> MgSO4 + Cu
Example 3: Iron displaces the Copper from copper sulphate solution because iron metal is more reactive than copper.
CuSO4 + Fe ========> FeSO4 + Cu
Example-4: Iron displaces the Hydrogen from the hydrochloric acid solution because iron metal is more reactive than hydrogen.
2HCl + Fe ========> FeCl2 + H2
Example-5: Copper displaces the silver from silver nitrate solution because Copper metal is more reactive than silver.
Cu + 2AgNO3 ========> Cu(NO3)2 + Ag
Example-6: Lead displaces the copper from the copper chloride solution because lead is more reactive than copper
CuCl2 + Pb ========> PbCl2 + Cu
Double Displacement Reaction
The chemical reactions in which two compounds react together by an exchange of their ions to form two new compounds are called double displacement reactions.
Example-1: When the solution of silver nitrate is added to the solution of sodium chloride, then two new compounds silver chloride and sodium nitrate are formed.
AgNO3 (aq) + NaCl (aq) ========> AgCl (s) + NaNO3(aq)
Example 2: When the solution of sodium hydroxide is added to the solution of hydrochloric acid, then two new compounds sodium chloride and water are formed.
NaOH (aq) + HCl (aq) ========> NaCl (s) + H2O(aq)
Example 3: When the solution of barium chloride is added to the solution of sodium sulphate, then two new compounds barium sulphate and sodium chloride are formed.
BaCl2 (aq) + Na2SO4 (aq) ========> BaSO4 (s) + NaCl(aq)
Example-4: When the solution of barium chloride is added to the solution of copper sulphate, two new compounds barium sulphate and copper chloride are formed.
BaCl2 (aq) + CuSO4 (aq) ========> BaSO4 (s) + CuCl2(aq)
Redox Reaction
(Oxidation and Reduction Reactions)
The chemical reactions in which oxidation and reduction take place simultaneously are called redox reactions. In the name redox, red stands for reduction and ox stands for oxidation.
Let’s discuss reduction and oxidation.
Oxidation:
- The addition of oxygen to a substance is called oxidation.
- The removal of hydrogen from a substance is called oxidation
- The addition of a non-metallic element is called oxidation
- Removal of metallic elements is called oxidation
Reduction:
- The addition of hydrogen to a substance is called oxidation.
- The removal of oxygen from a substance is called oxidation
- The addition of metallic elements is called reduction
- Removal of non-metallic elements is called oxidation
Generally, the oxidation and reduction reactions occur simultaneously. Let’s discuss it with some examples.
Example-1: When copper oxide is heated with hydrogen, then water and copper metal are formed. The chemical equation of this reaction is given below:
In the above reaction, copper oxide is changing into copper. This means that oxygen is being removed from copper oxide. So copper oxide is being reduced to copper metal (Reduction reaction).
While hydrogen is changing into a water molecule. This means that oxygen is being added to hydrogen. So, hydrogen is being oxidized to water (Oxidation reaction). The overall reaction may also be represented as follows:
Oxidising agent: Copper oxide because it is supplying oxygen to the hydrogen.
Reducing Agent: Hydrogen because it removes oxygen from the copper oxide.
Example 2: When hydrogen sulphide reacts with chlorine, then two substances sulphur and hydrogen chloride are formed. The chemical equation of this reaction is given below:
H2S + Cl2 ========> S + 2HCl
In the above reaction, hydrogen sulphide is changing into sulphur. This means that hydrogen is being removed from hydrogen sulphide. So, the reaction is an oxidation reaction.
While chlorine is changing into hydrogen chloride. This means that hydrogen is being added to chlorine. So, chlorine is being reduced to hydrogen chloride (reduction reaction). The overall reaction may also be represented as follows:
Oxidising agent: Chlorine because it removes hydrogen from hydrogen sulphide.
Reducing Agent: Hydrogen sulphide because it is supplying hydrogen to the chlorine.
Effects of Oxidation Reactions in Everyday Life
Oxidation reactions have damaging effects on food (Rancidity) as well as on metals (Corrosion). We will now discuss these two effects caused by the oxidation reactions.
Corrosion: The process in which metals are gradually eaten up by the action of air, moisture or a chemical on the surface.
Example: Rusting of Iron
In rusting of iron means that when an iron object comes in contact with moisture and air, a red-brown substance appears on the surface called rust. The process of rust formation on the surface of the iron object is called rusting of iron.
During rust formation, iron metal is oxidized by oxygen from air in the presence of moisture (water) to form hydrated iron (III) oxide called rust.
4Fe + 3O2 + xH2O ========> 2Fe3O4.xH2O
(Rust Formula)
The rusting of iron is a continuous process which, if not controlled, eats up the whole iron object.
Note: Corrosion also observed in other metals like aluminium, silver and copper too. But metals such as Gold and Platinum never corrode.
Harmful effects of Corrosion:
It weakens metal (iron) objects such as railings, windows, doors, vehicles, bridges, ships, and furniture. It also shorts the life of metal objects.
Prevention of Rusting
Most of the methods of prevention involve coating the metal surface with something to keep out water and air. The different methods of prevention are given below:
- Painting the metal surface
- Applying grease or oil
- Galvanization (The process of applying a thin layer of zinc on iron objects)
- By Tin or Chromium plating (By electroplating)
Rancidity:
When the food materials are prepared with oils and fats and kept for a long time, they start giving an unpleasant taste and smell. This happens when the oils and fats present in the food materials get oxidized by the oxygen present in the air and produce oxidative products of unpleasant taste and smell.
This condition is known as rancidity.
Prevention of Rancidity
- By adding anti-oxidants to foods containing fats and oils.
Examples: BHT (Butylated Hydroxy Toluene) and BHA (Butylated Hydroxy Anisole).
- By packaging fat and oil-containing foods in nitrogen gas.
- By keeping food in the refrigerator.
- By storing food in air-tight containers.
- By storing foods away from light.
Neutralization Reaction
When an acid reacts with a base, then salt and water are formed. This reaction is known as the neutralization reaction.
Example 1: When hydrochloric acid reacts with sodium hydroxide, then sodium chloride and water are formed.
HCl + NaOH ========> NaCl + H2O
Acid Base Salt Water
Example 2: When hydrochloric acid reacts with sodium hydroxide, sodium chloride and water are formed.
H2SO4 + KOH ========> KCl + H2O
Acid Base Salt Water
Electrolysis
Electrolysis is defined as a process of decomposing ionic compounds into their elements by passing a direct current through a substance in a solution form.
In this cations are reduced at the cathode and anions are oxidized at the anode. It is usually done in a vessel called an electrolytic cell containing two electrodes which is connected to a battery.
Example-1: When electric current is passed through molten sodium chloride, it ionized into sodium ions and chloride ions. The sodium ions are attracted by the cathode, from there, it takes an electron and becomes a sodium atom. While chloride ion reaches the anode, gives its electron to form chlorine molecule.
At cathode: Na+ + e– ========> Na (metal)
At anode: Cl– + Cl– ========> Cl2
Support Us: Please Donate to Support Us
Scan QR to Donate
Suggesting Post