Class 10, Science, Chapter-1, Lecture-1, Chemical Reaction (Notes)
CHEMICAL REACTION:
The process, in which one or more substances undergo change to produce one or more new substances with new properties, is known as chemical reaction.
REACTANTS:
The substances which take part in a chemical reaction are called reactants.
PRODUCTS:
The substances which are formed in a chemical reaction are called products.
REACTANTS $\buildrel {} \over
\longrightarrow $ PRODUCTS
CHARACTERISTICS OF A CHEMICAL REACTION:
- Change in physical state.
- Change in colour.
- Evolution of gases.
- Change in temperature.
DURING A CHEMICAL REACTION:
- Energy is evolved or absorbed.
- Law of conservation of mass is obeyed.
- New products with new properties are formed.
- Fixed quantities of reactants participate in the process.
- Old bonds between the atoms of reactants are broken.
- New bonds are formed between the atoms to give products.
CHEMICAL EQUATION:
A shorthand representation of a chemical reaction in terms of the symbols and formulae of substances involved in it is called a chemical equation.
SKELETAL CHEMICAL EQUATION:
A chemical equation which has an unequal number of atoms of one or more elements in the reactants and the products is called a skeletal chemical equation
Example: $Mg + {O_2} \to MgO$
BALANCED CHEMICAL EQUATION:
A chemical equation which has an equal number of atoms of each element in the reactants and the products is called a balanced chemical equation.
Example: $2Mg + {O_2} \to 2MgO$
Chemical Equations Should Be Balanced.
Reason:
Mass can neither be created nor be destroyed in a chemical reaction. So, the mass of chemical present in the products of a chemical reaction must be equal to the total mass of the elements present in the reactants.
And the atoms are indestructible during a chemical reaction.
So, the number of atoms of each element must remain the same in the reactants and the products.
Ways of making chemical equations more informative:
- Balancing the number of atoms of each element on both sides
- Writing symbols of physical states
- Indicating reaction conditions such as temperature, pressure, and catalysts.
Example:
The equation ${6{\rm{C}}{{\rm{O}}_2}(aq) + 6{{\rm{H}}_{\rm{2}}}{\rm{O}}(l)}$ ${\mathrel{\mathop{\kern0pt\longrightarrow}
\limits_{{\rm{Chlrophyll}}}^{{\rm{Sunlight}}}} {{\rm{C}}_{\rm{6}}}{{\rm{H}}_{{\rm{12}}}}{{\rm{O}}_{\rm{6}}}(aq) + 6{{\rm{O}}_2}(aq)}$ informs that 6 molecules of carbon dioxide dissolved in water combines with 6 molecules of water in the presence of sunlight and chlorophyll to produce one molecule of glucose and 6 molecules of oxygen both dissolved in water.
Magnesium Ribbon Should Be Cleaned Before Burning in Air.
Reason:
Magnesium reacts slowly with air to form a layer of less reactive magnesium oxide (MgO).
This protective inert layer of magnesium oxide must be removed to make the burning possible.