CHEMICAL REACTIONS AND EQUATIONS
CHEMICAL REACTIONS:-
• Chemical reactions are the processes in which new substances with new properties are formed.• Chemical reactions involve chemical changes.
• During chemical reactions, a rearrangement of atoms takes place between the reacting substances to form new substances having entirely different properties.
• Chemical reactions involve breaking of old chemical bonds which exist between the atoms of reacting substances, and then making of new chemical bonds between the rearranged atoms of new substances.
e.g. - The burning of magnesium in air to form magnesium oxide is an example of a chemical reaction.
Characteristics of Chemical Reactions:-
1. Evolution of a Gas:- Some chemical reactions are characterised by the evolution of a gas.e.g. - when zinc granules react with dilute sulphuric acid, then bubbles of hydrogen gas are produced.
2. Formation of a Precipitate:- Some chemical reactions are characterised by the formation of a precipitate.
e.g. - when potassium iodide solution is added to a solution of lead nitrate, then a yellow precipitate of lead iodide is formed.
3. Change in Colour:- Some chemical reactions are characterised by a change in colour.
e.g. - when citric acid reacts with potassium permanganate solution, then the purple colour of potassium permanganate solution disappears.
4. Change in Temperature:- Some chemical reactions are characterised by a change in temperature.
e.g. - when quicklime reacts with water, then slaked lime is formed and a lot of heat energy is produced.
5. Change in State:- Some chemical reactions are characterised by a change in state.
e.g. - when wax is burned in the form of a wax candle, then water and carbon dioxide are formed. Now, wax is a solid, water is a liquid whereas carbon dioxide is a gas.
CHEMICAL EQUATIONS:-
• The method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it is known as a chemical equation.OR
• A chemical equation is a short-hand method of representing a chemical reaction.
e.g- Zinc metal reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas. This reaction can be written in words as:
Zinc + Sulphuric acid -> Zinc sulphate + Hydrogen
Zn + H2SO4 → ZnSO4 + H2
Reactant: - The substances which combine or react are known as reactants.
Product: - The new substances produced in a reaction are known as products.
Balanced and Unbalanced Chemical Equations:-
1. A balanced chemical equation has an equal number of atoms of different elements in the reactants and products.
A balanced chemical equation has equal masses of various elements in reactants and products.
e.g. - Zn + H2SO4 → ZnSO4 + H2
In reactant In product
No. Of Zn atoms 1 1
No. Of H atoms 2 2
No. Of S atoms 1 1
No. Of O atoms 4 4
2. An unbalanced chemical equation has an unequal number of atoms of one or more elements in the reactants and products.
e.g. - H2 + O2 → H2O
In reactant in product
No. Of H atoms. 2 2
No. Of O atoms 2 1
Balancing of Chemical Equations:-
The process of making the number of different types of atoms equal on both the sides of an equation is called balancing of equation.We should remember the following four steps for writing equations for the chemical reactions:
Step 1 : Write down the chemical reaction in the form of a word equation, keeping the reactants on the left side and products on the right side.
Step 2 : Put the symbols and formulae of all the reactants and products in the word equation.
Step 3 : Balance the equation by multiplying the symbols and formulae by the smallest possible girls (Do not change the formulae to balance the equation).
e.g. - Methane burns in oxygen to form carbon dioxide and water.
This reaction can be written in the form of a word equation as:
Step1: Methane + Oxygen Carbon dioxide + Water
Step 2: CH4 + O2 → CO2 + H2O
In reactant In product
No. Of C atoms. 1 1
No. Of O atoms 2 3
No. Of H atoms. 4 2
Step 3: CH4 + 2O2 → CO2 + 2H2O
In reactant In product
No. Of C atoms. 1 1
No. Of O atoms 4 4
No. Of H atoms. 4 4
Additional information about chemical equations:-
The chemical equations can be made more informative in following ways.(i) By indicating the "physical state" of the reactant and products.
(ii) By indicating the "heat change" taking place in a reaction.
(ii) By indicating the "conditions" under which the reaction takes place.
(i) PHYSICAL STATE:-
The physical states of the reactants and products are shown by putting the bellow "state symbols" just after their symbols and formulae.• Solid state is indicated by symbol (S)
• Liquid state is indicated by symbol (L)
• Aqueous solutions is indicated by symbol (aq)
• Gaseous state id indicated by symbol (g)
e.g:-Zn(s) + H2SO4(aq)→ ZnSO4(aq) + H2(g)
Here Zinc is in solid state, H2SO4 and ZnSO4 are aqueous solution and Hydrogen is in gaseous state.
(ii) HEAT CHANGE:-
There are two types of reaction on the basis of heat change.A. Exothermic reaction:- Those reaction in which heat is evolved is known as exothermic reaction.
C + O2 → CO2
Note:- The heat evolved in an exothermic reaction is indicated by writing "+ Heat" or "+ Energy" on product side.
B. Endothermic reaction:- Those reaction in which heat is absorbed is known as exothermic reaction.
N2 + O2 → 2NO
Note:- The heat absorbed in an endothermic reaction is indicated by writing "- Heat" or "- Energy" on product side.
(iii) CONDITIONS:-
If heat is required for a reaction to take place, then the heat sign delta (∆) is put over the arrow of the equation.If the reaction takes place in presence of a catalyst , then the symbol or formula of the catalyst is also written above or below the arrow sign in the equation.
Here ∆ stands for heat and MnO2 stands for catalyst.
TYPES OF CHEMICAL REACTION:-
1. Combination reaction:-Those reactions in which two or more substance combine to form a single substance are called combination reaction.
e.g- C + O2 → CO2
2H2 + O2 → 2H2O
2. Decomposition reactions:-The reaction in which substance splits up into two more simpler substances are known as decomposition reaction.
Two decomposition reactions are carried out by applying heat, light or electricity which provide energy to break a compound into two or more simpler compounds.
e.g.
3. Displacement reactions:-The reactions in which one element takes place of another element in a compound, are known as displacement reaction.
e.g. CuSO4 + Zn → ZnSO4 + Cu
4. Double displacement reactions:-Those reactions in which two compounds react by an exchange of ions to form two new compounds are called double displacement reaction.
e.g. AgNO3 + NaCl -> AgCl + NaNO3
5. Oxidation band reduction reactions:-
a. Oxidation: - The addition of oxygen to a substance is known as oxidation.
The removal of hydrogen from a substance is also known as oxidation.
b. Reduction: - The addition of hydrogen to a substance called reduction.
The removal of oxygen from a substance is also called reduction.
Note:-
Oxidising agent: - the substance which oxidises others but get reduced itself is known as oxidising agent.
Reducing agent: - The substance which reduces others but get oxidised itself known as reducing agent.
e.g- CuO + H2 → Cu + H2O
Substance oxidised: H2
Substance reduced: CuO
Oxidising agent: CuO
Reducing agent: H2
Note:- The oxidation and reduction reaction are also known as REDOX reaction.
Effect of oxidation in everyday life:-
1. Corrosion of metal:- Corrosion is the process in which metals are eaten up by action of air, moisture, or a chemical on their surface.2. Rancidity:- The condition produced by aerial oxidation of fats and oils in foods marked by unpleasant smell and taste is called rancidity.
Mind map of CHEMICAL REACTIONS AND EQUATIONS
No comments:
Post a Comment
If you have any doubt, let me know ☺️