SCIENCE
(Chemistry)
Chapter 1: Chemical Reactions and Equations
To get notes visit our website
mukutclasses.in
,CHEMISTRY CHEMICAL REACTIONS AND EQUATIONS
Chemical Reactions and Equations
• Most of the substance around us undergoes various changes. Some of these changes are
temporary with no new substance being formed. They are called physical changes.
Example: Water changes to steam on boiling but no new substance is formed(Even
though steam and water look different when they are made to react with a piece of Na,
they react the same way and give the exact same products). This involves only a change
in state (liquid to vapour).
• A substance is said to undergo a chemical change when the chemical properties of a
substance alter. As a result, there is either formation or breaking of atomic bonds at the
molecular level. Some characteristics of a chemical change are:
o New substances are produced during a chemical reaction.
o Changes in energy are involved.
o During the reaction, there occurs a change in mass.
o There is a permanent alteration.
Examples:
• Cooking of food
• Rusting of iron
• Heating of Lead nitrate
• Souring of milk
• Ripening of fruit.
Chemical Equation
Writing Chemical Equations
• In a chemical reaction, the reactants are written on the left hand side and the products
on the right hand side of the equation.
• An arrow (→) pointing towards the products is inserted between the reactants and the
products. It also represents the direction of the reaction.
• A single arrow (→) indicates the direction in which the reaction proceeds.
• A double arrow ( ) indicates a reversible reaction, i.e. the products recombine to form
the reactants.
• A plus sign (+) is inserted between two or more reactants or products formed.
• If reactions are carried out under specific conditions of temperature, pressure, catalyst
etc., then these conditions are mentioned on the arrow.
• The chemical equation can be made more informative by mentioning the physical states
of the reactants and products.
• If gas is liberated as a product, then it is represented by an arrow pointing upwards (↑). If
, CHEMISTRY CHEMICAL REACTIONS AND EQUATIONS
the product formed is in the form of a precipitate, it is represented by an arrow pointing
downwards (↓).
Balancing the Chemical Equations
• In a balanced chemical equation, the total number of atoms of each element of the
reactants on the left hand side of the equation is equal to the number of atoms of the
products formed on the right hand side of the equation.
• The total mass of the reactants is equal to the total mass of the products or the number
of atoms of each element before the reaction and after the reaction is equal.
Steps Involved in Balancing a Chemical Equation
Consider the chemical reaction between magnesium and oxygen to understand the steps
involved in balancing a chemical equation.
Step 1
Let us first write the word equation for this reaction.
Magnesium + Oxygen → Magnesium oxide
Step 2
Write the chemical equation for the reaction between magnesium and oxygen.
Mg + O2 → MgO
Step 3
Count the number atoms of an element occurring on both L.H.S. and R.H.S. in this equation.
Mg + O2 → MgO
Component Reactant Product
Magnesium 1 1
Oxygen 2 1
This is an unbalanced equation.
Step 4
• To balance a chemical equation, first draw boxes around each formula. Do not change
anything inside the boxes while balancing the equation.
• Choose a reactant or a product which has the maximum number of atoms in it. In that
compound, select the element which has the maximum number of atoms. In this
equation we shall select MgO i.e. magnesium oxide and the element oxygen in it.
• To balance the oxygen atoms, let us multiply magnesium oxide molecule by 2 on the
right hand side. The equation can now be expressed as,
(Chemistry)
Chapter 1: Chemical Reactions and Equations
To get notes visit our website
mukutclasses.in
,CHEMISTRY CHEMICAL REACTIONS AND EQUATIONS
Chemical Reactions and Equations
• Most of the substance around us undergoes various changes. Some of these changes are
temporary with no new substance being formed. They are called physical changes.
Example: Water changes to steam on boiling but no new substance is formed(Even
though steam and water look different when they are made to react with a piece of Na,
they react the same way and give the exact same products). This involves only a change
in state (liquid to vapour).
• A substance is said to undergo a chemical change when the chemical properties of a
substance alter. As a result, there is either formation or breaking of atomic bonds at the
molecular level. Some characteristics of a chemical change are:
o New substances are produced during a chemical reaction.
o Changes in energy are involved.
o During the reaction, there occurs a change in mass.
o There is a permanent alteration.
Examples:
• Cooking of food
• Rusting of iron
• Heating of Lead nitrate
• Souring of milk
• Ripening of fruit.
Chemical Equation
Writing Chemical Equations
• In a chemical reaction, the reactants are written on the left hand side and the products
on the right hand side of the equation.
• An arrow (→) pointing towards the products is inserted between the reactants and the
products. It also represents the direction of the reaction.
• A single arrow (→) indicates the direction in which the reaction proceeds.
• A double arrow ( ) indicates a reversible reaction, i.e. the products recombine to form
the reactants.
• A plus sign (+) is inserted between two or more reactants or products formed.
• If reactions are carried out under specific conditions of temperature, pressure, catalyst
etc., then these conditions are mentioned on the arrow.
• The chemical equation can be made more informative by mentioning the physical states
of the reactants and products.
• If gas is liberated as a product, then it is represented by an arrow pointing upwards (↑). If
, CHEMISTRY CHEMICAL REACTIONS AND EQUATIONS
the product formed is in the form of a precipitate, it is represented by an arrow pointing
downwards (↓).
Balancing the Chemical Equations
• In a balanced chemical equation, the total number of atoms of each element of the
reactants on the left hand side of the equation is equal to the number of atoms of the
products formed on the right hand side of the equation.
• The total mass of the reactants is equal to the total mass of the products or the number
of atoms of each element before the reaction and after the reaction is equal.
Steps Involved in Balancing a Chemical Equation
Consider the chemical reaction between magnesium and oxygen to understand the steps
involved in balancing a chemical equation.
Step 1
Let us first write the word equation for this reaction.
Magnesium + Oxygen → Magnesium oxide
Step 2
Write the chemical equation for the reaction between magnesium and oxygen.
Mg + O2 → MgO
Step 3
Count the number atoms of an element occurring on both L.H.S. and R.H.S. in this equation.
Mg + O2 → MgO
Component Reactant Product
Magnesium 1 1
Oxygen 2 1
This is an unbalanced equation.
Step 4
• To balance a chemical equation, first draw boxes around each formula. Do not change
anything inside the boxes while balancing the equation.
• Choose a reactant or a product which has the maximum number of atoms in it. In that
compound, select the element which has the maximum number of atoms. In this
equation we shall select MgO i.e. magnesium oxide and the element oxygen in it.
• To balance the oxygen atoms, let us multiply magnesium oxide molecule by 2 on the
right hand side. The equation can now be expressed as,
