Chemical Reaction and Equation
The process in which new substance with new properties are formed.
Couse :-
Chemical reaction involved the breaking of bonds between the atom of the reacting substances and making of new bonds between the atom of products.
• During chemical reactions a large variety of rearrangement of atoms can take place to produce new substance is having entirely different properties.
• the following observations can help us to determine whether a chemical reaction has been taken place or not
1. Change in state
2. Change in colour
3. Evolution of a gas
4. Change in temperature
Chemical equation :-
the method of representing a chemical reaction with the help of symbols and formula of the substance involved in it.
Example :-
when a magnesium ribbon is burnt in oxygen it gets converted to magnesium oxide
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Reactants :- the substance that undergo chemical change are called the reactants.
Product :- the new substances formed during the reaction are called the products.
• the reactants are written on the left hand side with a plus sign between them. similarly, products are written on the right side and with a plus sign between them.
• the arrow is put between reactants and the product, and the arrowheads is pointed towards the product which shows the direction of the reaction.
Writing a chemical equation :-
Chemical equation can be made more informative and useful if we use chemical formula instead of words in word equation.
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Balanced chemical equation :-
• a balanced chemical equation has an equal number of atoms of different elements in the reactants and products.
• in other words, the number of atom of each element remains the same before and after the chemical reaction.
Example :-
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Note :- To make chemical equation more informative the physical state of the reactant and the product are mentioned along with their chemical formula. the gaseous liquid and solid states of reactant and product are presented by the notations (g), (l), (s) respectively.
• The word aqueous (aq) is return if the reactant and product is present as a solution in water.
• downward Arrow is for the formation of precipitate and upward Arrow is for the evolution of gas in the chemical equation.
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Note :-
sometime the reaction conditions such as temperature, pressure, catalyst etc for the reaction are indicated above and or below the arrow in the chemical equation.
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Types of chemical reactions :-
1. Combination reaction
in a combination reaction two or more substances combine to form a new substance.
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• Combination reactions are also termed as synthesis reaction.
2. Decomposition reaction
• in a decomposition reaction a single compound breaks down to produce two or more simpler substances.
• the decomposition reactions take place when energy is supplied in the form of heat, electricity or light.
• when a substance is decomposed by passing electric current the process is called electrolysis.
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• When a substance decompose on heating it is called thermal decomposition
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• when a substance is decomposed in presence of sunlight, it is called photochemical decomposition.
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Reactivity series of metals
reactivity series of metal is a series in which the metals are arranged in the decreasing order of their reactivity.
• a more reactive metal can displace the less reactive metal from their solution.
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3. Displacement reactions
In a displacement reaction a more reactive element displaces or remove another element from a compound.
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4. Double displacement reaction
The reactions in which two different atoms or groups of atoms are displaced by other atoms or group of atoms are called double displacement reaction.
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Oxidation and reduction
• classical ideas of oxidation and reduction are, oxidation is a process which involves
(a) addition of oxygen
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(b) removal of hydrogen
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(c) addition of an electronegative element
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(d) removal of an electropositive element
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Oxidising agent
A substance which brings oxidation.
Reduction
Reduction is a process which involves,
(a) removal of oxygen
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(b) addition of hydrogen
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(c) removal of an electronegative element
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(d) addition of electropositive element
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Reducing agent :- the substance which brings reduction.
Note :-
A substance which undergoes oxidation, act as reducing agent while a substance which undergoes reduction, act as oxidising agent.
Modern concepts of oxidation and reduction
• Two useful concepts for interpreting redox reaction have been developed.
1. Electron concept
2. Oxidation number concept
Electron concept :-
oxidation is a process in which an atom, ion or molecule loses one or more electrons or oxidation is the process of de-electronation.
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• as a result of electron or electrons lose, there is an increase in positive charge or decrease in negative charge on the atom or ions undergoing oxidation.
Reduction :-
A process in which an atom, ion or molecule ginse one or more electrons.
• Reduction is the process of electronation
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Redox reaction may be represented as
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Reducing agent
• the material which loses one or more electrons is called reducing agent.
Oxidising agent
• the material which gains one or more electrons is called oxidising agent
• the valency of an element which undergoes oxidation increases and the valency of an element
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• in the case of electrovalent compound the oxidation number of an element or radical is the same as the charge on the ion.
• this is a real charge and is developed by the loose or gain of the electron.
Oxidation number
• The charge (real or imaginary) which an atom appears to have when it is in combination.
• oxidation state is given positive sign if electrons are lost.
• oxidation state is given negative sign if electron are gained.
• oxidation State represents real charge in case of ionic compound, in covalent compound it represents imaginary charge.
• the oxidation number of an atom in free element is zero.
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• the fluorine which is the most electronegativity element has oxidation number -1 in all its component.
• oxidation number of oxygen is -2 in all compounds. except in peroxides, superoxides and oxygen fluorides.
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