CBSE 10th Chemistry | Chemical Reactions and Equation Notes

September 22nd, 2015 by Amit Thakur Leave a reply »

Chemical Reactions and Equation Notes

1. Chemical Equation

It is a simple way to represent any chemical reaction in the form of word equation. A chemical reaction involve reactants and the products separated by an arrow e.g. in the given reaction

Zn + H2 SO4 clip_image002ZnSO4 + H2

Zn and H2SO4 are reactants and ZnSO4 and H2 are products.

2. Balanced Chemical Equation

A chemical reaction is said to be balanced if number of atoms of each element on the RHS and LHS of the arrow are same.

3. Balancing of a Chemical Equation

To balance a chemical equation we should, follow the following steps.

(i) First of all list the number of atoms of different elements present in the reactant and the product,

(ii) Now start from the element which contains the maximum number of atom whether it is a reactant or a product,

(iii) Now to equalise the number of atoms on both side multiply the compound with a suitable integer. Similarly balance the other elements which are not balanced,

(iv) To check wheather the reaction is balance or not count atoms of each element on both sides of the equation. If the number of atoms of elements on both sides are equal, the equation is balanced.

Example

FeSO4 clip_image004Fe2O3 + SO2 + SO3

Total no. of Fe clip_image004[1] 1 (L.H.S) – 2 (R.H.S)

Total no. of S clip_image004[2]1 ( L . H . S ) – 2 (R.H.S)

Total no. of O clip_image004[3] 4 (L.H.S) – 8 (R.H.S)

As FeSO4 contains maximum no. of atoms, thus start from this, in order to equalise no. of O atom on both side multiply FeS04 by 2. We get

2FeSO4 clip_image002[1] Fe2O3 + SO2 + SO3

Now the total no. of atoms of all the elements on both sides are equal. Thus the reaction is balanced.

4. Chemical Reactions

A chemical reaction involves the making and breaking of bonds between
atoms to produce new substances. Chemical reactions are of many types.

I. Combination Reaction

In these reactions two or more reactants combine to form a single product, e.g.

Mg + O  clip_image002[2]MgO

CaO + H20clip_image002[3]Ca(OH)2

It has been observed that during the formation of Ca(OH)2, a large amount of heat is evolved, such reactions in which heat is evolved along with the formation of products are known as exothermic reactions. Combination reactions are generally exothermic reactions.

II. Decomposition Reaction

These reactions are just opposite to the combination reactions, i.e. in such reactions a single reactant decomposes to give two or more products, e.g.

image

Decomposition reactions generally occur due to absorption of heat. i.e. endothermic. “Those reactions which proceed with absorption of heat are known as endothermic reactions”.

III. Displacement Reaction

Those reactions in which one element displaces the other element from its aqueous solution are known as displacement Reactions, e.g.

Zn (s) + CuSO4 (aq) clip_image004[4] ZnSO4(aq) + Cu (s)

IV. Double Displacement Reaction

Those reactions in which exchange of ions between the reactants occur are called double displacement reactions, e.g

NaCI+AgNO3clip_image004[5]AgCl + NaNO3

5. Oxidation and Reduction

Oxidation is a process which involve addition of oxygen or removal of hydrogen.

e.g.

C + O2clip_image004[6]CO2 (Addition of oxygen)

H2S + CI2 clip_image004[7] 2HCI + S (Removal of hydrogen)

Whereas reduction is a process which involve addition of hydrogen or removal of oxygen,

e.g.

H2 + Br2 clip_image004[8] 2HBr (Addition of hydrogen)

CuO + H2 clip_image004[9] Cu + H2O (Removal of oxygen)

5.1 Corrosion

It is the deterioration of a metal as a result of its reaction with air or water surrounding it. Corrosion of iron is called rusting. Rust is Fe2O3. x H2O.

Pure iron does not rust, also iron kept in vacuum does not rust because for rusting air is necessary. Rusting is faster in sea water than in marine water due to the presence of dissolved salt.

The rusting can be promoted by the presence of (i) impurities of metal (ii) moisture (iii) electrolyte.

5.2 Rancidity

When fats and oils are exposed to air they become rancid and their smell and taste
changes. To avoid this antioxidants are added to fats and oil. Keeping food in air tight jars also helps to prevent oxidation.

 

ADDITIONAL USEFUL FACTS

 

  • Shortcuts to identify different types of reactions

 

Reaction Type of Reaction
A + B clip_image011 AB

Reactants Single Porduct

AB clip_image011[1] A + B

Single reactant Products

A+BY clip_image004[10] B+AY

AX+BY clip_image004[10] BX+AY

A+B clip_image004[11] AB+Q

AB clip_image004[12] A+B-Q

Combination reaction

 

 

Decomposition reaction

 

Displacement reaction

Double Displacement reaction

Exothermic reaction

Endothermic reaction

 

  • Combination reactions generally occurs with evolution of heat.
  • Decomposition reactions generally occurs with absorption of heat.
  • Those reactions in which oxidation and reduction occurs simultaneously are known as redox reaction. e.g.

image

Here ZnO is reduced to Zn and C is oxidized to CO

Oxidizing and Reducing agent: The reactant which get oxidized itself but reduce others is called reducing agent, and the reactant which reduces itself and oxidizes others is called oxidizing agent.

e.g.

image

Oxidation is also known as a process of deelectronation – i.e. the process in which electrons are lost. e.g.

Zn clip_image004[13]Zn++ +2e

(loss of 2e)

Naclip_image004[14] Na+ + 1e

(loss of le)

Reduction is the process of electronation i.e. the process in which electrons are gained. e.g.

Cu++ + 2e clip_image004[15] Cu

(gain of 2e-)

Cl2 + 2e clip_image004[16] 2Cl

(gain of 2e)

In general almost all the reactions are redox reactions but there are some examples which neither show oxidation nor reduction e.g.

(i) BaCl2 + H2SO4 clip_image004[17] BaSO4 + 2HCl

(ii) CuSO4 + 2H2O clip_image004[18] Cu (OH)2 + H2SO4

(iii) NaOH + HCl clip_image004[19] NaCl + H2O

(iv) NaBr + HCl clip_image004[20] NaCl + HBr

(v) AgNO3 + HBr clip_image004[21] AgBr + HNO3

(vi) Na2O + H2SO4 clip_image004[22] Na2SO4 + H2O

All these reaction shows neither any loss of electrons (oxidation) nor gain of electrons (Reduction)

The reaction which produce a precipitate is called precipitation reaction. Precipitation results insoluble substance known as precipitate, e.g

Na2SO4(aq) + BaCl2 (aq) clip_image004[23] BaSO4(s) clip_image021 + 2NaCl(aq)

(ppt)

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