CH-1 chemical reaction and equation

CHAPTER 1
Chemical Reaction And Equation      

Chemical Reaction - The transformation of chemical substance into another chemical substance is known as Chemical Reaction. For example: rusting of iron, burning of coal, burning of candle, burning of paper, setting of milk into cottage cheese or ghee, digestion of food etc.

Exothermic Reaction - A chemical reaction which is accompanied by evolution of heat energy is known as exothermic reaction.
Endothermic Reactions - A chemical reaction which is accompanied by absorption of heat energy is known as exothermic reaction.

.Rusting of iron.

magnesium is burnt in air to form magnesium oxide. This chemical reaction can be represented as

. We can observe or recognise a chemical reaction by observing    change in state, colour, by evolution of gas or by change in         temperature.

. We can observe or recognise a chemical reaction by observing change in       state, colour, by evolution of gas or by change in temperature.

Balancing a chemical equation -

To balance any chemical equation, we have to follow these steps:

(1) Write the number of atoms of elements present in reactants and in products in a table; as shown here.

(2) Attack on the problem.

In above equation, number of elements of iron and oxygen are not equal on two sides.

Let’s take oxygen first.

(3) Attack the side where there is less number of atoms of that particular element.

In above equation, number of oxygen atom is minimum on LHS.

(4) Thus to balance the equation, we need to multiply the oxygen by 4 on LHS.

(5) Now again check the number of atoms of each element present in reactants and in products.

You can see that 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.

(6) After that 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.

(7) 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.

Q.1 Balance the given equations.

Types of Reaction

I. Combination Reaction :– The reaction in which two or more substances combine to form a new single substance.
e.g.

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Decomposition Reaction - Reactions in which one compound decomposes in two or more compounds or element are known as decomposition reaction. Decomposition reaction is just opposite of combination reaction.

When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide

 Displacement Reaction - Reactions in which atoms or ions move from one compound to other to form new compound are known as displacement reaction. Displacement reaction is also known as Substitution Reaction or Single displacement /Replacement Reaction.

When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc chloride.

Double Displacement Reaction - Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions.

When the solution of barium chloride reacts with the solution of sodium sulphate, white precipitate of barium sulphate is formed along with sodium chloride.

Oxidation :Oxidation is the gain of oxygen or loss of hydrogen

Reduction: Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound is called reduction. The compound or element which goes under reduction is called to be reduced.

Effects of Oxidation -

1. Respiration is oxidation reaction in which food is oxidized to produce energy.

2. Iron gets oxidized to form rust; which leads to corrosion of iron in the long run.

3. Most of the metals react with atmospheric oxygen and it leads to formation of a layer on the metal article. The metal gets corroded in the long run.

4. Rusting of iron can be prevented by painting the iron article. This can also be prevented by applying a layer of zinc over iron article. This process is known as galvanization.

5. Fried food gets oxidized when exposed to air. This spoils the taste of the food and the food becomes unfit for consumption. The spoiling of fried food because of oxidation is called rancidity. Fried food is often packed in airtight packets to prevent rancidity.

6. We are able to utilize various types of fuel because of oxidation. Oxidation of fuel helps in producing energy.