▀▄▀▄Chemical Reactions & Equation▄▀▄▀
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So we already had posted a introductive part of this - 1st chapter of chemistry ( class 10)..
But in this blog you will find the complete chapter.
Let's Begin.....
• Chemical Reaction : The process in which a substance undergoes change to produce new substances with new properties are known as chemical reaction.
• Chemical Equation : The qualitative representation of a Chemical reaction in a short hand or concise form in term of symbols and formulae, is called a chemical equation. For example. The burning of magnesium wire in oxygen(air) to produce magnesium oxide can be written as under in the form of a word equation.
( Reactants) ( Product)
Magnesium + oxygen ➡Magnesium oxide
Word equations represent simply change of reactants into products. They do not give the true picture of the chemical reactions.
• Skeletal Chemical Equation or Symbol Equation : A chemical equation written in the form of symbols and formulae is called a skeletal chemical equation. Such a chemical equation may or may not be balanced .Example:
Magnesium + oxygen ➡Magnesium oxide
(word equation)
Mg + O2 ➡ MgO (skeletal equation)
In the skeletal equation given above there are two oxygen atoms (O2 = 2 atoms of oxygen) in the reactants and only one oxygen atom in products so it is not a balanced equation.
• Balanced Chemical Equation : A chemical equation in which number of atoms of each elements on L.H.S. (i.e. reactants) and R.H.S. (i.e.products) is equal is called a balanced chemical equation for example, magnesium reacts with sulfuric acid to form magnesium sulphate and hydrogen, can be represented as follows.
Magnesium + Sulfuric acid ➡Magnesium Sulphate + Hydrogen (word equation)
Mg + H2 SO4➡ MgSO4 + H2 ⬆ (Balanced equation)
The balancing of a chemical equation is essential or necessary to fulfill the requirement of the Law of Conservation of Mass.
• Balancing of Chemical Equations :
The process of making the number of different elements on both side of the equation equal is known as balancing of chemical equation.
Hit and Trial Method : The reaction between hydrogen and oxygen to form water can be represented as under :-
Hydrogen + oxygen ➡water (word equation)
H2 + O2➡ H2O (skeletal equation)
This equation is unbalanced and can be balanced by hit and trial method as follows :-
Oxygen will be balanced if 2 is written before H2O (product).
It will give us the following equation.
H2 + O2 ➡ 2H2O
In doing so the number of hydrogen atoms becomes unbalanced. On the L.H.S. there are only 2 atoms of hydrogen while on RHS we have 4 atoms of hydrogen. To balance hydrogen atoms write 2 before H2 .
2H2 + O2 ➡ 2H2O (balanced equation)
In a balanced chemical equation, homoatomic gases with Q atomicity two or more are always written in molecular form e.g. H2 , O2 , N2 , Cl2 , O3 etc. Atomicity may be defined as the number of atoms in one molecule of the gas.
• Essential of a Chemical Equation :
A chemical equation-
(i) should represent an actual chemical change
(ii) should be balanced, and
(iii) should be molecular, i.e., all the substances should be in the form of molecules.
• Implications of a Chemical Equation or Information by a Chemical Equation :
A chemical equation conveys a large number of informations as described below-
(i) It tells us about the names of the reactants and products.
(ii) It expresses the number of molecules of reactants and products.
(iii) It expresses the relative weight of the reactants and the products.
(iv) It expresses the volume of the gaseous reactants and Products.
Types of chemical reactions :
• Decomposition reactions : Decomposition reactions are those reactions in which a compound breaks down into simpler compounds (or substances). This type of reaction is simply the reverse of combination reactions. These reactions require energy in the form of heat, light, electricity etc.
Kinds of decomposition reactions :
Various kinds of decomposition reactions are :
(i) Thermal decomposition : This type of reaction takes place on heating a substance. For example,
Heat
ZnCO3 (s) ➡ Zno(s) + CO2(g)
(ii) Electrolytic decomposition : This type of reaction takes place on passing electric current for example -
( electric current)
2H2O(l) ➡ 2H2(g) + O2(g)
( acidified water)
(iii) Catalytic decompositions : This type of reaction takes place in presence of a catalyst. For example-
( catalyst)
CH4(g) ➡ CO+ 2H2 (g)
(iv) Photochemical decomposition : This type of reaction takes place in presence of light. For example-
(Light)
2AgBr ➡ 2Ag+ Br2
• The reaction in which a hydrated salt loses molecules of water of crystallisation is known as dehydration reaction.
(Heat)
FeSO3. 7H2O ➡ FeSO4(s) + 7H2O(g)
(Green) ( Dirty White)
• Simple Displacement Reaction and Simple Substitutions:
A displacement reaction is a reaction in which an atom, or group of atoms, present in a molecule is displaced by another atom. Some examples of displacement reactions are :
(i)CuSO4( aq) +Mg(s)➡MgSO4(aq)+Cu(s)
(ii) Fe2O3+ 2Al ➡ Al2O3+ Fe
A more reactive metal will displace a less reactive metal from its compound in solution. There are the sorts of things you observe in this type of reaction :
• the more reactive metal gradually dissolves
• the less reactive metal coats the more reactive metal
• the solution may change colour
• Double Displacement Reactions or Double Decomposition :
The reactions in which mutual exchange of radicals takes place are known as double decomposition reactions. As a result of double decomposition reactions two new substances are formed.
Examples :
(i) When sodium chloride reacts with conc. sulphuric acid,two new substances (sodium sulphate and hydrogen chloride) are formed.
2NaCl+ H2SO4 ➡ Na2SO4+ 2HCL
(II) When potassium bromide and silver nitrate reacts together, silver bromide and potassium nitrate are formed.
KBr + AgNO3 ➡ KNO3 + AgBr
The double-displacement reactions have two major features in common. First, two compounds exchange ions or elements to form new compounds. Second, one of the products is either a compound that will separate from the reaction mixture in some way (commonly as a solid or gas) or a stable covalent compound, often water.
• Precipitation Reactions : A precipitation reaction occurs when two solutions are mixed together and a solid separates from the solution. The solid part that forms and separates from the mixture is called the precipitate the reaction shown below is a precipitation reaction.
BaCl2 + 2Na2SO4 ➡ BaSO4 ⬇+ 2NaCl
• Oxidation-Reduction Reactions
Oxidation reactions : Oxidation is defined as a process which involve addition of oxygen or removal of hydrogen.
Addition of oxygen-
N2 + O2➡ 2NO
Removal of hydrogen :
Cl2 + H2S ➡ 2HCL+ S
Reduction : The term reduction is defined as a process which involve the removal of oxygen or addition of hydrogen.
Removal of oxygen :
CuO + H2 ➡ Cu+ H2O
Addition of hydrogen :
H2+S ➡ H2S
• Oxidation - Reduction in terms of Electronic Concept :
Oxidation is a loss of electrons
Reduction is a gain of electrons.
Oxidizing agent accepts electrons. Some common oxidising agents are KMnO4, K2, Cr2O7 etc.
Reducing agent lose electrons. Some common reducing agents are LiAlH4 , NaBH4 etc.
• Redox Reactions : Those reactions in which oxidation and reduction takes place simultaneously, are known as redox reactions.
Example-
Addition of oxygen ( oxidation)
_________________
| |
| ( heat) ⬇
Cuo + H2 ➡ Cu + H2O
| ⬆
|_________________|
Reduction ( removal of hydrogen)
Redox processes are very important as they play important role in governing a number of natural processes like corrosion,combustion, biological processes like respiration etc.
• Exothermic reactions : The chemical reactions which proceed with the evolution of heat energy are called exothermic reactions.
The heat energy produced during the reaction is indicated by writing +q on the product side. In general, exothermic reactions may be represented as :
A + B ➡ C + D + Q(heat energy)
(i) All combustion reactions are exothermic.
(ii) Respiration is an exothermic reaction.
(iii) The decomposition of vegetable matter into compost is also an example of exothermic reaction.
• Endothermic reactions : The chemical reactions which proceed with the absorption of heat energy are called endothermic reactions.
The heat energy absorbed during the reaction can be indicated by writing +q with the reactants or it can be indicated by writing –q (or the actual numerical value) with the products.
In general, an endothermic reaction can represented as:
A + B ➡ C + D – Q(heat),
where Q is the heat absorbed.
(i) Decomposition reactions are generally endothermic.
(ii) The number of endothermic reactions is much less than the exothermic reactions.
• Combustion Reactions : A chemical reaction in which a substance burns or gets oxidised in the presence of air or oxygen is called combustion reaction. For example, kerosene,coal, charcoal, wood etc, burn in air and thus undergo Combustion. All substances acting as fuels undergo combustion reactions. These are highly exothermic and are accompanied by release of energy which is quite useful in our daily life. All combustion reactions are not accompanied by flame. Combustion accompanied by release of energy.
• Homogeneous reactions : In case all the reactants and products of a chemical reaction are in the same physical state then such a reaction is known as homogeneous reactions.
For representation of a physical state of reactants/products We write (g) for gases, ( l ) for liquids and (s) for solids state.
Example :
H2 (g) + Cl2 (g) ➡ 2HCl (g)
• Heterogeneous reaction : If in a chemical reaction, all the products and reactants are not in the same physical state then such a reaction is known as heterogeneous reaction.
Example :
( heat)
CaCO3 (s) ➡ CaO(s) + CO2 (g)
• Corrosion (Erosion by chemical action) : Corrosion is the degradation of metals and generally called rust. Corrosion causes damage to car bodies, iron railings, ships and to all objects made of metals, specially those of iron. Corrosion of iron is a serious problem. Corrosion is the primary means by which metals deteriorate. Most metal corrode on contact with water (and moisture in the air), acids, bases, salts, oils, aggressive metals polishes, and other solid and liquid chemicals. To minimize corrosion, protective coatings are applied to prevent the direct contact of moisture and oxygen with that metal.
• Rancidity : The most important cause of deterioration in fats and fatty foods is oxidation of fats. What we perceive is an unpleasant change in the flavour and odour of a food, called rancidity. Factors which accelerate fat oxidation include trace metals (iron, zinc, etc.), salt, light, water, bacteria, and moulds. Fat oxidation can be retarded by use of antioxidants by use of spices such as sage and rosemary, and by use of light and or air tight wrapping.
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