Chemical Reactions & Equations - keshav Chapter Notes - Class 10 PDF Download

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.

Chemical equations are a simple but powerful way to represent chemical reactions, and are essential for understanding and predicting the behavior of chemicals in a variety of settings.

Chemical equations are important because they allow us to understand the process of a chemical reaction and to predict the products that will be formed. They also help us to balance the equation, which means making sure that the same number of atoms of each element is present on both sides of the equation.

What is Chemical Reaction ?

Chemical reaction , a process in which one or more substances, the Reactants, are converted to one or more different substances, the Products.  

For example ,       Magnesium + Oxygen → Magnesium Oxide (MgO)

                                          ( Reactants )                                   ( Products )

The substances that undergo chemical change in the reaction , Magnesium and 0xygen, are the Reactants. The new substance, Magnesium Oxide, formed during the reaction, is the Product.

• These are the following changes to determine that the chemical reaction has taken place:
  •  Change in state
  • Change in colour
  • Evolution of gas
  • Change in temperature

 What is Chemical Equations and Its Types ?

Chemical Equation is representation of chemical reaction using symbols and formulae of the substances.  A chemical equation represents a chemical reaction. 

 Chemical equations can be made more concise and useful if we use chemical formulae instead of words.  For example  , the chemical formula for water is H2O, which indicates that it is made up of two hydrogen atoms and one oxygen atom.

Is a Chemical Equation is balanced or Unbalanced ?

 The Chemical  Equation is said to be balanced when  the number of atoms  on both sides of the equation is same . If not, then the equation is unbalanced because the mass is not the same on both side of the equation.

Here are the steps to write a balanced chemical equation:

(i) Write the reactants on the left side of the equation and the products on the right side, separated by an arrow.

(ii)Write the chemical formulas for each reactant and product. If you don't know the formula for a compound, you'll need to look it up.

(iii)Balance the equation by adjusting the coefficients (the numbers in front of each compound or element) until the number of atoms of each element is the same on both sides of the equation. Start with the most complex molecule or the molecule that appears in multiple places in the equation.

(iv)Check that the equation is balanced by counting the atoms of each element on both sides of the equation.

Finally, make sure that the chemical equation represents a feasible chemical reaction, i.e., it obeys the laws of chemistry and makes sense in terms of the reactants and products.

For example:            Zn + H₂SO₄  →  ZnSO₄  + H₂
                                        3Fe (s) + 4H₂O (g) →  Fe₃O₄  (s) + 4H₂ (g)

Types of chemical reaction

(1.)Combination reaction:  It occurs when two or more reactants combine to form a single product.  or we say , such a reaction in which a single product is formed from two or more reactants is known as a combination reaction.
For examples:  
 (i) Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.

(ii)  A solution of slaked lime produced by the reaction is used for white washing walls. Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of white washing and gives a shiny finish to the walls. It is interesting to note that the chemical formula for marble is also CaCO₃.

Some More Reactions:

(i)Exothermic reaction:  An endothermic reaction is a type of chemical reaction in which energy is absorbed from the surroundings in the form of heat, light, or electricity.  
 For Example : Endothermic reaction is the process of photosynthesis, in which plants use energy from sunlight to convert carbon dioxide and water into glucose and oxygen gas.  
  Reaction as:   C₆H₁₂O₆ (aq) + 6O₂(aq) →  6CO₂ + 6H₂O (l) + energy  (glucose)

(ii)Endothermic reaction: An exothermic reaction is a type of chemical reaction in which energy is released from the reaction system into the surroundings, usually in the form of heat, light, or sound.  

(2.)Decomposition reaction: 

For example:

Another example of it as , White silver chloride turns grey in sunlight. This is due to the decomposition of silver chloride into silver and chlorine by light.

NOTE: The above reactions are used in black and white photography.

(3) Displacement reaction:

Some Important Reaction as:
 (i) Reaction of iron nails with copper sulphate solution.

NOTE:  Zinc and lead are more reactive elements than copper. They displace copper from its compounds. 

(4) Double Displacement reaction:  

For example:       Na₂(SO)₄ (aq) + BaCl₂ (aq) →  BaSO₄ (s) + NaCl (aq)

Oxidation and Reduction

(i)Oxidation is the process in which an atom, ion, or molecule loses one or more electrons. During oxidation, the oxidation state of the atom or molecule increases, since it becomes more positive or less negative.   

(i)Corrosion:

Rusting – Iron when reacts with oxygen and moisture forms red substance called rust.

(ii)Rancidity:

• The taste and odour of food materials containing fat and oil changes when they are left exposed to air for long time. This is called rancidity. It is caused due to oxidation of fat and oil present in food material.
• It can be prevented by using various methods such as by adding antioxidants to the food materials, Storing food in air tight container and by flushing out air with nitrogen.