Chemical Reactions and Equations
Chemical Reaction :
The transformation of chemical substance into another chemical substance is known as Chemical Reaction. For example : the setting of milk into curd, digestion of food, rusting of iron, respiration, etc.
- Whenever a chemical change occurs, we can say that a chemical reaction has taken place.
- In a chemical reaction, a new substance is formed which has completely different properties from original substance, so in a chemical reaction, a chemical change takes place.
Magnesium + Oxygen —> Magnesium oxide
Substance which take part in a chemical reaction are called reactants. In above example Mg and O2 is reactants.
The new substance which is formed after a chemical reaction is called a product. In above example MgO is product.
Characteristics of Chemical Reactions :-
(a) change in state :
In combustion reaction of candle wax is is characterised by change in state from solid to liquid and gas. There are some chemical reactions which shows more than one characteristics.
(b) change in colour :
In a chemical reaction between citric acid and purple coloured potassium solution the purple colour of solution changes to colourless.
chemical reaction between sulphur dioxide gas and acidified potassium dichromate solution the colour of solution change from orange to green.
(c) evolution of gas :
In the chemical reaction between zinc and dilute sulphuric acid is characterised by the evolution of hydrogen gas.
(d) change in temperature :
In the chemical reaction between quick lime water to form slaked lime is characterized by change in temperature (which is a rise in temperature).
(e) Formation of precipitate :
In the chemical reaction between Sulphuric acid and barium chloride solution is characterised by the formation of white precipitate of barium sulphate.
Chemical Equation :
Writing the chemical reaction using symbols and formulae of The substances is called Chemical Equation.
A + B → C + D
In this chemical equation, A and B are reactants
C and D are products
When hydrogen reacts with oxygen, it gives water. This reaction can be represented in following equation :
Hydrogen + Oxygen → Water
H2 + O2 → H2O
(a) Balanced Chemical Equations :
Recall the law of conservation of mass; mass can neither be created nor destroyed in a chemical reaction. That is, the total mass of elements present in products of a chemical reaction has to be equal to the total mass of the elements present in reactants. In other words, the number of atoms of each element remains the same.
The balanced equation contain number of atoms of each element is equal in both sides.
Zn + H2SO4 → ZnSO4 + H2
In this equation, the number of zinc, hydrogen and sulphate atoms are equal in both sides, So it is called balance chemical equations.
(b) Unbalanced Chemical Equation :
The unbalanced chemical Equation doesn’t contain number of atoms of each element is equal in both sides.
Fe + H2O → Fe3O4 + H2
In this example, the number of atoms of iron is not equal in both sides and number of oxygen is not equal in both sides. Therefore, it is called unbalanced chemical equation.
Balancing a Chemical Equation :
To balance the above or any chemical equation, we have to follow these steps :
Fe + H2O → Fe3O4 + H2
step 1: To balance a chemical equation, first draw a table in which write the number of atoms presents in reactants and products.
|Name of atom||No. of atoms in reactant LHS||No. of atoms in the product RHS|
Step 2: this details are convenient to start balancing with compound that contain the maximum number of atoms. It may be reactant or a product. In that compound, select the element which has the maximum number of atoms. Using this criteria, we select Fe3O4. There are four oxygen atoms on the RHS and only one in LHS side.
To balance :
|Atoms of oxygen||In reactants||In products|
|(i) Initial||1 (in H2O)|
|(ii) To balance||1×4||4|
To equalise the number of atoms of oxygen :
Fe + 4H2O → Fe3O4 + H2
step 2: Fe and H are still not balanced. Pick any of these elements to proceed further. Let us balance hydrogen atom.
|Atoms of hydrogen||In reactants||In products|
|(i) Initial||8 (in 4H2O)||2 ( in H2)|
|(ii) To balance||8||2 × 4|
The equation would be :
Fe + 4H2O → Fe3O4 + 4H2
step 4: Examine the above equation pick up the third element which is not balanced. You find that only one element is left to be balanced, that is iron.
|Atoms of iron||In reactants||In products|
|(i) Initial||1 (in Fe)||3 (in Fe3O4)|
|(ii) To balance||1×3||3|
step 5: Finally, to check the correctness of the balanced equation, we we count atoms of each element on both sides of the equation.
3Fe + 4H2O → Fe3O4 + 4H2
The number of atoms of element is equal in both sides.
step 6: Writing Symbols of Physical states : Carefully examine the equation.
3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)
To Make Equations More Informative :
Writing the symbols of physical states of substances in a chemical equation. By writing the physicals states of substances, a chemical equation becomes more informative. :
The gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s), respectively. The word aqueous (aq)
Thus, by writing the symbols of physical state of substances and condition under which reaction takes place, a chemical equation can be made for informative.
Types of Chemical Reactions
during a chemical reaction atoms of one element do not change into those of another element. Nor do atoms disappear from the mixture or appear from elsewhere. Actually, chemical reactions involve the breaking and making of bonds between atoms to produce new substances.
1. Combination Reaction :
The reactions in which two or more reactants are combined to form one product are Combination Reactions.
General Representation of combination Reaction can be :
A + B → AB
Calcium oxide reacts vigorously with water to produce slaked lime(calcium hydroxide) releasing a large amount of heat.
In this reaction, calcium oxide and water combine to form a single product, calcium hydroxide. Such a reaction in which single product is formed from two or more reactants is known as a combination reaction.
More Examples of Combination Reactions :
(i) Burning of coal
C (s) + O2(g) → CO2(g)
(ii) Formation of water from H2(g) and O2(g)
Reactions in which heat is released along with the formation of products are called exothermic chemical reactions.
2. Respiration is an exothermic process
We all know that we need energy to stay alive. We get this energy from the food we eat. During digestion, food is broken down into simpler substances. The special name of this reaction is respiration.
3. The decomposition of vegetables matter into compost is also an example of an exothermic reaction.
2. Decomposition Reaction :
The reactions in which one compound decomposes in two or more compounds or elements are known as Decomposition Reaction.
General representation of decomposition reaction can be :
AB → A + B
(i) When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide.
(ii) When Ferrous sulphate is Heated, it decomposes Ferric oxide is formed.
Thermal Decomposition :
Thermal Decomposition is a chemical decomposition caused by heat. The decomposition temperature of a substance is the temperature at which the substance chemically decomposes. The reaction is usually endothermic as heat is required to break chemical bonds in the compound undergoing decomposition.
Electrolytic Decomposition :
Electrolytic decomposition is a chemical decomposition in which a single reactant is decomposed into more than one products by the passage of electricity.
Photolysis or Photo Decomposition Reaction :
Reactions in which a compound decomposes because of sunlight are known as Photolysis or Photo Decomposition Reation.
When silver chloride is put in sunlight, it decomposes into silver metal and chlorine gas.
White silver chloride turns grey in sunlight. This is due to the decomposition of silver chloride into silver and chlorine by light.
Silver bromide also behaves in the same way.
The above reactions are used in black and white photography.
The decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants. Reactions in which energy is absorbed are known as endothermic reactions.
3. Displacement Reaction
The chemical reactions in which a more reactive element displaces a less reactive element from a compound is known as Substitution Reaction or Single Displacement reactions
General representation of displacement reaction is :
A + BC → AC + B
Displacement reaction takes place only when ‘A’ is more reactive than ‘B’.
If ‘B’ is more reactive than ‘A’ will not displace ‘C’ from ‘BC’ and reaction will not take place.
(i) When iron nail react with copper sulphate, it gives iron sulphate and copper.
Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
(ii) Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
4. Double Displacement Reaction :
Reaction in which there is an exchange of ions between the reactants to form new compound are called double displacement reactions.
AB + CD → AC + BD
The solution of barium chloride reacts with the solution of sodium sulphate, white precipitate of barium sulphate is formed along with sodium chloride.
BaCl2(aq) + Na2SO4(aq) → BaSO4(s) (Precipitate) + 2NaCl(aq)
Neutralization Reaction :
The reaction in which an acid reacts with a base to form salt and water by exchange of ions is called Neutralization Reaction
When Sodium hydroxide (a base) reacts with hydrochloric acid, sodium chloride and water are formed.
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
In double displacement reaction, in which precipitate is formed, is also known as precipitation reaction. Neutralisation reactions are also examples of double displacement reaction.
Precipitation Reaction :
Precipitation Reaction refers to the formation of an insoluble salt when two solutions containing soluble salts are combined. The insoluble salt that falls out of solution is known as the precipitate, hence these reaction are called Precipitation Reaction.
Precipitation Reaction can help determine the presence of various ions in solution.
5. Oxidation and Reduction :
The chemical reactions in which one reactant gets oxidised while the other gets reduced during a during a reaction. Such reaction are called oxidation-reduction reactions.
|Addition of oxygen or non-metallic element or removal of hydrogen or metallic element from a compound is known as Oxidation||Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound is called Reduction.|
|Oxidizing agent||Reducing agent|
|The substance which gives oxygen for oxidation is called Oxidizing agent.|
The substance which removes hydrogen is also called Oxidizing agent.
|The substance which gives hydrogen for reduction is called a Reducing agent.|
The substance which removes oxygen is also called a Reducing agent.
Redox Reaction :
The reaction in which oxidation and reduction both take place simultaneously is called redox reaction.
If hydrogen gas is passed over this heated material (CuO), the black coating on the surface turns brown as the reverse reaction take place and copper is obtain.
CuO + H2 → Cu + H2O
- If a substance gains oxygen during a reaction, it said to be oxidised.
- If a substance losses oxygen during a reaction, it said to be reduced.
- Oxidation and Reduction take place together.
Some other examples of redox reaction are :
ZnO + C → Zn + CO
Exothermic and Endothermic Reaction :
Exothermic Reaction :
Exothermic reactions are reactions or processes that release energy, usually in the form of heat or light. In an exothermic reaction, energy is released because the total energy of the products is less than the total energy of the reactants.
Respiration is a decomposition reaction in which energy is released.
Endothermic Reaction :
Endothermic reactions are chemical reactions in which reactants absorb heat energy from the surroundings to form products. These reactions lower the temperature of their surrounding area, is called Endothermic Reaction.
Effects of Oxidation Reaction in EveryDay Life
In this process there is a slow conversion of metals into their undesirable compounds due to their reaction with oxygen, water, acids, gases etc. present in atmosphere is called Corrosion.
(i) Rusting of iron :
You must have observed that iron articles are shiny when new, but get coated with a reddish brown powder when left for some time. This process is commonly known as rusting of iron.
(ii) The black coating on silver.
The surface of silver metal gets tarnished (become dull) on exposure to air, due to the formation of a coating of black silver sulphide () on its surface by the action of gas present in the air.
(iii) the green coating on copper.
copper objects lose their lustre and shine after some time because the surface of these objects acquires a green coating of basic copper carbonate, when exposed to air.
Causes of Corrosion :
Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, specially those of iron. Corrosion of iron is a serious problem. Every year an enormous amount of money is spent to replace damaged iron.
Methods to prevent rusting :
- By painting.
- By greasing and oiling.
- By galvanisation.
The taste and odour of food materials containing fat and oil changes when they are left exposed to air for a long time. This is called Rancidity. It is caused due to the oxidation of fat and oil present in food materials.
Causes of Rancidity :
When fats and oils are oxidised, they become rancid and their smell and taste change.
Methods to Prevent rancidity :
Replacing air by nitrogen.
Refrigeration of foodstuff.
By adding anti-oxidant.
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