Chemical Reactions And Equations notes class 10th | Chemistry | Science


Chemical Reactions And Equations
There are so many chemical reactions happening in our bodies and in our surroundings every day and every second. Even the form of life that we know today can never exist without the happening of these chemical reactions. So, finally, the question which arises is what are these chemical reactions? Let us find out.

Chemical Reaction: A process in which new substances are formed having new different properties is called a chemical reaction. For eg. Reaction of Hydrogen with oxygen gives us water, which is having entirely different properties than Hydrogen and oxygen.
Chemical change:
·         When the chemical properties of a substance are triggered or changed then a chemical change occurs in that substance. For eg. Digestion of food inside our bodies.
·         A new substance is formed during a chemical change.
Physical change:
·         A physical change involves a change in the physical attributes of a body. When a physical change occurs, there is no change in the chemical properties of the substance which is facing that particular physical change.
·         No new substance forms during a physical change.
Two important points to be noted
·         Substances which take part in a chemical reaction are called reactants.
·         Substances which are formed during a chemical reaction are called products.
  Examples of chemical reactions that take place in our daily lives
1.       Souring of fruits.
2.       Making of curd by milk.
3.       Rusting of different metals.
4.       Digestion of food in our body.
5.       Respiration by living organisms.
6.       Burning of fuels.
Characteristics of a chemical reaction
1.       Evolution of a gas
2.       Formation of precipitate
3.       Change in color
4.       Change in state
5.       Change in temperature.
6.       Evolution of a gas
Evolution of gas
Some chemical reactions are characterized by the evolution of a gas.
·         For example when Dilute hydrochloric acid is poured over zinc granules then hydrogen gas is produced and a chemical reaction takes place
Formation of precipitate
Some chemical reactions are characterized by the formation of precipitate. Let us know what is a precipitate.
Precipitate: Precipitate is an insoluble solid substance which is separated out during a some chemical reaction.
·         For example when aqueous solution lead nitrate is mixed with the aqueous solution of potassium iodide then a yellow the precipitate of lead iodide is formed.
Change in color
A chemical reaction is also characterized by a change in color. For example when Carbon Dioxide is passed through lime water then a chemical reaction takes place and a change in color from colorless to white is observed.
Change in State
When a chemical reaction takes place then there is a feature of its change in state which can be observed. For example when a candle burns, its state changes from solid to liquid.
Change in Temperature
Many chemical reactions are characterized by a change in the temperature which takes place during their happening. For example when Hydrochloric acid reacts with an aqueous solution of sodium hydroxide then a change in temperature occurs which results in a lot of heat production.

CHEMICAL EQUATIONS

The symbolic representation of a chemical reaction through symbols of different elements and compounds is called a chemical equation.
For example, the chemical reaction of Hydrogen with Oxygen can be represented through a chemical equation as,
H2 + O2                                                   H2O                                                                                               
The equation given above represents the chemical reaction between Oxygen and Hydrogen which results in the formation of Water.
Now let us discuss the components of a chemical equation.
Components of a Chemical Equation:
1.       The substances which are written on the left side of a chemical equation are called reactants.
2.       The substances which are written on the right side of a chemical reaction are called products.
3.       The arrow symbol which separates both the reactants and products is used to show that the chemical reaction between certain substances gave some products which are there on the left side?
Ways to make a Chemical Equation more informative
1.       A chemical equation can be made more informative by showing the states of substances involved in a chemical reaction.
2.       This task can also be achieved by representing the temperature changes during a chemical reaction in a chemical reaction.
3.       And the last way to make a chemical equation much more information is done by showing the conditions under which a particular chemical reaction took place.
1. To show the states of substances involved in a chemical reaction in a chemical equation, the symbol of states are used in front of the symbols of elements and compounds involved. This point can be illustrated through the example given below.
Zn(g) + H2SO4(aq)                               ZnSO4(aq) + H2(g)
Here in the equation given above the states of substances involved in the same chemical reaction are shown through their symbols.
Symbols to show the states of reactants and products in a chemical reaction
·          (g) to represent the state of gas
·         (l) to represent the state of liquid
·         (s) to represent the state of solid.
·         (aq) to represent the state of an aqueous solution i.e. a solution made in water.
2. To represent temperature changes in a chemical equation we need to know, whether in a chemical reaction heat is evolved or absorbed? There is also a classification of chemical reactions on the basis of heat. This classification will be shown later first let us focus on making our chemical equation more informative. The illustration through examples given below can make the point more clear.
C(g)  +  O2(g)                                              CO2(g) + Heat                                                            ............(1)
N2(g)  +  O2(g)                             2NO(g) - Heat                                                           .............(2)
Now, in Equation (1) given above, there is a chemical reaction between carbon and oxygen which results in the production of carbon dioxide along with a lot of thermal/heat energy thus, you can see for the same we have written +Heat ahead of the product formed in the respective equation. Now if you look down at Equation (2), there is a chemical between Nitrogen and Oxygen which is resulting in the formation of Nitric oxide. But here is a twist that in this chemical reaction instead of the evolution of heat that we saw in Equation (1), there is the absorption of heat thus for the same we have written -Heat ahead of product which is formed. You may have got the idea that to represent an evolution of heat energy in a chemical reaction what we do is simply write  +Heat ahead of product/products that are formed. In case if we observe an absorption of heat energy in a chemical reaction we just write -Heat ahead of products that are formed.
3. To represent the condition under which a particular chemical reaction took place in a chemical equation, we have to show  that whether the presence of any catalyst was there during the time the chemical reaction was taking place. A catalyst is a substance which alters the rate of a chemical reaction without taking part in it. To represent the presence of a catalyst we just write up their symbols upon the arrow in a chemical equation.
Balancing a Chemical Equation
Balancing a chemical equation means that to equalize the atoms of elements on both sides of the equation.
For better illustration of the point stated above, let us just head back to one of the equations that we have stated above in the article i.e. Equation (2), in it, we can clearly see that I have written 2 in front of the symbol of the chemical formula of Nitric Oxide. Why is that? So, to know the answer to this question we need to check out the reactants of the same equation. We can see that the reactants contain two atoms of Nitrogen and two atoms of Oxygen and now just remove that 2 from the formula of Nitric Oxide, now we are just left with one atom of Nitrogen and one atom of oxygen on the products side thus, in this case now we have an unequal no. of oxygen and nitrogen atoms on both the sides of equation which leads to a contradiction to our first law of chemical combination i.e. law of conservation of mass. So to remove this contradiction, we have to equalize the no. of atoms of both the respective elements on both the sides of the equation. To accomplish this task we just have to put a whole number(that can equalize the number of atoms on both the sides) in front of an element or compound in the equation. Now, The whole number that you'll put will be multiplied by the number of molecules in the element and will affect all the elements present in the compound. For example in Equation (2), when we put 2 in front of NO, it makes the number of atoms of nitrogen as well as oxygen 2 thus it affects both the elements in this compound.
Now let us talk about the classification of chemical reactions on the basis of heat changes during their happening. So, chemical reactions are classified into two types on the basis of heat changes that occur while their happening.
1.       Exothermic reactions: When certain chemical reactions take place, they produce heat energy such reactions are termed as exothermic chemical reactions.
2.       Endothermic reactions: When heat is absorbed in a chemical reaction then these reactions are called endothermic reactions.

TYPES OF CHEMICAL REACTIONS

There are 5 types of chemical reactions:
1.       Combination reactions
2.       Decomposition reactions
3.       Displacement reactions
4.       Double Displacement reactions
5.       Redox reactions
1. Combination reactions: The chemical reactions in which two or more elements or compounds combine or react with each other to form a single product are called combination reactions. For example, 2H2 + O2                                      2H2O
2. Decomposition reactions: The chemical reactions in which a single compound breaks or split into its constituent elements are called decomposition reaction. For example, 2FeSO4                       Fe2O3 + SO2 + SO3
3. Displacement reactions: The chemical reactions in which an element having higher chemical reactivity displaces another element having comparatively lower chemical reactivity are called displacement reactions. For example, Zn + H2SO4                                    ZnSO4  + H2
4. Double Displacement reactions: The chemical reactions which involve an exchange of ions between two salt solutions are called double displacement reactions. For example, BaCl2 + H2SO4                                 BaSO4 +2HCl
5. Redox Reactions: Redox reactions contain:
1.       Oxidation reactions
2.       Reduction reactions
Oxidation reactions:  
         i.            When oxygen is added to any substance involved in a chemical reaction then it is called an oxidation reaction.
       ii.            When hydrogen is removed from any substance involved in a chemical reaction then it is called reduction.
Reduction reactions:
         i.            When hydrogen is added to any substance involved in a chemical reaction then it is called an oxidation reaction.
       ii.            When oxygen is removed from any substance involved in a chemical reaction then it is called reduction.
When both oxidation and reduction reactions take place in a single chemical reaction then it is called Redox reaction.
 Oxidizing agents: Substances which get reduced but oxidizes other substances are called oxidizing agents.
Reducing agents: Substances which get oxidized but reduces other substances are called reducing agents.

Effects of Oxidation in Everyday's life

These are two main effects that oxidation causes in our day-to-day life,
Corrosion of metals and,
Rancidity
Corrosion: Corrosion is a process in which a metal starts eating itself up due to some environmental causes such as moisture, acids, air, etc. For example, Rusting of Iron.
The formula for rusting of iron,
4Fe  + 3O2 + 2xH2O                             2Fe2O3.xH2O
Rancidity: Rancidity refers to complete or incomplete oxidation or hydrolysis of fats and oils present in food when exposed to open air, light, and moisture for a longer period of time, it causes foul smell and taste in food which eventually leads to its spoilage.
Ways to prevent rancidity:
        i.            Rancidity can be prevented by keeping our food containing fats or oils in an airtight environment such as airtight utensils.
      ii.            Food containing fats or oils can be packed in packets having inert gases such as Nitrogen.
    iii.            Such food material can be stored in refrigerators.
     iv.            Anti-oxidants such as BHA( Butylated hydroxy-anisole) and BHT( Butylated Hydroxy-Toluene) can be used to prevent oxidation in food containing fats or oils
       v.            Rancidity can be prevented by storing food away from light.




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