1. Chemical Equations

Chemical Reactions and Equations: Balanced and unbalanced chemical equations and balancing of chemical equations.Consider the following situations of daily life and think what happens when , In all the above situations, the nature and the identity of the initial substance have somewhat changed. We have already learnt about physical and chemical changes of matter in our previous classes. Whenever a chemical change occurs, we can say that a chemical reaction has taken place

• Milk is left at room temperature during summers.
• An iron tawa/pan/nail is left exposed to humid atmosphere.
• Grapes get fermented.
• Food is cooked.
• Food gets digested in our body.
• We respire.

Chemical Reaction: The transformation of chemical substance into another chemical substance is known as Chemical Reaction. For example: Rusting of iron, the setting of milk into curd, digestion of food, respiration, etc.

Example: The burning of magnesium in the air to form magnesium oxide is an example of a chemical reaction.

2Mg(s) + O2(g) △→ 2MgO(s)

Before burning in air, the magnesium ribbon is cleaned by rubbing with sandpaper . This is done to remove the protective layer of basic magnesium carbonate from the surface of the magnesium ribbon. Reactant: Substances which take part in a chemical reaction are called reactants. Example: Mg and O2.

Product: New substance formed after a chemical reaction is called a product.

Example: MgO.

Chemical Reaction's Characteristics

1. Change in temperature: The chemical reaction between quick lime water to form slaked lime is characterized by a change in temperature (which is a rise in temperature).

2. Change in state of substance: The combustion reaction of candle wax is characterised by a change in state from solid to liquid and gas (because the wax is a solid, water formed by the combustion of wax is a liquid at room temperature.

3. Formation of precipitate: The chemical reaction between sulphuric acid and barium chloride solution is characterised by the formation of a white precipitate of barium sulphate.

BaCl2(aq) + H2SO4(aq) ----→ BaSO4(s) (ppt) + 2HCl(aq)

4. Evolution of gas: The chemical reaction between zinc and dilute sulphuric acid is characterised by the evolution of hydrogen gas.

 Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)

Chemical Equations - The chemical equation of the reaction is the representation of a chemical change in terms of symbols and formulae of the reactants and products.

Example: Potassium Hydrochloric Potassium Manganese Water Chlorine permanganate acid → chloride chloride

(a) Balanced Chemical Equation: A balanced chemical equation has the number of atoms of each element equal on both sides.

Example: Zn + H2SO4 → ZnSO4 + H2

In this equation, numbers of zinc, hydrogen and sulphate are equal on both sides, so it is a Balanced Chemical Equation.

(b) Unbalanced Chemical Equation: If the number of atoms of each element in reactants is not equal to the number of atoms of each element present in the product, then the chemical equation is called Unbalanced Chemical Equation.

Example: Fe + H2O → Fe3O4 + H2

Balancing a Chemical Equation: To balance the given or any chemical equation, follow these steps:

Fe + H2O → Fe3O4 + H2

Table as shown here.

Fe + 4 × H2O → Fe3O4 + H2To balance the oxygen, one needs to multiply the oxygen on the LHS by 4, so that, the number of oxygen atoms becomes equal on both sides.

Now, the number of hydrogen atoms becomes 8 on the LHS, which is more than that on the RHS. To balance it, one needs to multiply the hydrogen on the RHS by 4.

Fe + 4 × H2O → Fe3O4 + 4 × H2

After that, the number of oxygen and hydrogen atoms becomes equal on both sides. The number of iron is one on the LHS, while it is three on the RHS. To balance it, multiply the iron on the LHS by 3.

3 × Fe + 4 × H2O → Fe3O4 + 4 × H2

Now the number of atoms of each element becomes equal on both sides. Thus, this equation becomes a balanced equation.

After balancing, the above equation can be written as follows:

3Fe + 4H2O → Fe3O4 + 4H2.

CHAPTER → 1

CHEMICAL REACTIONS AND EQUATIONS

*  CHEMICAL EQUATIONS

CHEMICAL REACTIONS:- The process in which two or more substance combine with each other to form new substances with new properties is called chemical reaction.

There are two parts of a chemical reactions :-

(i) Reactants:-   The substances which take part in a chemical reaction are known as reactants.

(ii) Products:-  The new substances formed during a chemical reaction are known as products.

There are 5 ways to tell if a chemical reaction has occurred.

• Change in state.
• Change  in colour.
• Change in temperature.
• Evolution of a gas.
• Formation of precipitate.

  Chemical reaction in everyday life:-

• Digestion of food.
• Respiration.
• Rusting of iron.
• Formation of curd.
• Burning of magnesium ribbon.

Chemical Equations:-  A chemical equation is a written representation of a chemical reaction.

The representation of chemical reaction using symbols and formulae of the substances is called chemical equation.

A   +   B       →      C   +     D

Reactants                   Products   

n this equation, A and B are called reactants and C and D are called the products. The arrow shows the direction of the chemical reaction. The necessary condition such as temperature, pressure or any catalyst should be written on arrow between reactants and products.

E.g. Magnesium is burnt in air to form magnesium oxide.

(i) Word equation for above reaction would be -

 Magnesium + oxygen     →           Magnesium oxide

   ( Reactants )                                      ( Product )

Skeletal equation for above reaction would be - 

Mg +    O₂        →           MgO

BALANCING CHEMICAL EQUATIONS:-

• LAW OF CONSERVATION OF MASS :-  Mass can neither be created nor be destroyed in a chemical reaction.
• So number of elements involved in chemical reaction should remain same at reactant and products side.

For Example ,

Zn   +   H₂SO₄             →     ZnSO₄  +         H₂

(Zinc)    ( Sulphuric Acid)        (Zinc Sulphate)   ( hydrogen) 

Let us check the number of atoms of different elements on both sides of the arrow .

 

As the number of atoms of each element is same on both sides of arrow. This is   a balanced chemical equation.

 Let us take another example :-   

Fe     +    H₂O   →  Fe₃O₄   +   H₂

STEP 1 :-   Write a chemical equation.

Fe   +    H₂O  →   Fe₃O₄  +  H₂

 STEP 2:-  List the number of atoms of different elements present in the unbalanced equation.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

STEP 3 :_ Select the element which has the maximum number of atoms . now equalize the number of atoms by putting coefficient in front of it.

Fe   +      4  H₂O      →     Fe₃O₄   +     H₂

STEP 4 :-  Fe and H atoms are still not balanced choose any elements now to balance. To equalize the number of H atoms,

Fe   +     4 H₂O   →      Fe₃O₄   +     4 H₂

STEP 5 :- Now, take Fe and equalize the number of Fe atoms.

3 Fe   + 4 H₂O     →Fe₃O ₄     +    4 H₂

Now all the atoms of elements are equal on both sides.

STEP 6 :-  To make the chemical equation more information ,write the physical states of reactants and products.

Solid state = (s)

Liquid state = (l)

Gaseous State = (g)

Aqueous state = (aq)

3 Fe (S)    +   4H₂O(g)       →   Fe₃O₄S+  4H₂(g) 

STEP 7:-  Write necessary conditions of temperature pressure or catalyst on above or below arrow.

For Example:-