Chapter-1
Chemical Reactions And Equations
The process in which new substances with new properties are formed from one or more substances is called Chemical Reaction.
* The substances which take part in chemical reaction are called Reactants.
* The substances which are formed in a chemical reaction are called Products.
(i) Digestion of Food
(ii) Respiration
(iii) Rusiting of iron
(iv)Burning of Magnesium ribbon
(v) Formation of curd
Chemical reaction involves :
• Change in colour
• Change in temperature
• Evolution of gas
Chemical Equation :
* A chemical reaction can be represented by chemical equation. It involves uses of symbol of elements or chemical formula of reactant and product with mention of physical state.
* The necessary conditions such as temperature, pressure or any catalyst should be written on arrow between reactant and products.
e.g., Magnesium is brunt in air to form Magnesium oxide.
2Mg + O2 → 2MgO
Balancing Chemical Equation :
* Law of conservation of Mass : Matter 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 product side.
STEPWISE BALANCING (Hit and Trial) :
Step 1. Write a chemical equation and draw boxes around each formula.
| Fe | + | H2O | → | Fe3O4 | + | H2 |
* Do not change anything inside the box.
Step 2. Count the number of atoms of each element on both the sides of chemical equation.
| Element | No. of atoms of reactant side | No. of atoms at product side |
|---|---|---|
| 1. Fe | 1 | 3 |
| 2. H | 2 | 2 |
| 3. O | 1 | 4 |
Step 3. Equalise the number of atoms of elements which has maximum number by putting in front of it.
| Fe | + | 4H2O | → | Fe3O4 | + | H2 |
Step 4. Try to equalize all the atoms of elements on reactant and product side by adding coefficient in front it.
| 3Fe | + | 4H2O | → | Fe3O4 | + | 4H2 |
* Now all the atoms of elements are equals on both sides.
Step 5. Write the physical states of reactants and products.
3Fe (s) + 4H2O (g) → Fe3O4 + 4H2 (g)
* Now all the atoms of elements are equals on both sides.
• Solid state = (s)
• Liquid sate = (l)
• Gaseous state = (g)
• Aqueous state = (aq)
Step 6. Write necessary conditions of temperature, pressure or catalyst on arrow above or below.
TYPES OF CHEMICAL REACTIONS
I. COMBINATION REACTION : The reaction in which two or more reactant combine to form a single product.
(i) Burning of coal
C (s) + O2 (g) → CO2 (g)
(ii) Formation of water
2H2 (g) + O2 → 2H2O (l)
(iii) CaO (s) + H2O (l) → Ca(OH)2 (aq)
Quick lime Slaked lime
* Exothermic Reactions : Reaction in which heat is released along with formation of products.
e.g.,
(i) Burning of natural gas
CH4 (g) + O2 (g) → 2H2O (g) + Heat
(ii) Respiration is also an exothermic reaction.
C6H12O6 (aq) + 6O (g) → 6CO2 (aq) + 6H2O (l) + energy
II. DECOMPOSITION REACTION : The reaction in which a compound splits into two or more simple substances is called decomposition reaction.
A → B + C
* Thermal decomposition : When decomposition is carried out by heating.
e.g.,
(i) 2FeSO4 -------(Heat)→ Fe2O2O3 (s) + SO2 (g) + SO3 (g)
(Ferrous sulphate) (Ferric oxide)
Green colour Red-brown colour
(ii) CaCO3 (s) -------(Heat)→ CaO (s) + CO2 (g)
(Lime stone) (Quick lime)
(iii) 2Pb (NO3)2 -------(Heat)→ 2PbO + 4NO2 (g) + O2
(Lead nitrate) (lead oxide) (Nitroge dioxide)
* Electrolytic Decomposition : When decomposition is carried out by passing electricity.
e.g.,
2H2O -------Electric current→ 2H2 + O2
* Photolytic Decomposition : When decomposition is carried out in presence of sunlight.
e.g.,
2AgCl (s) -------Sunlight→ 2Ag (s) + Cl2 (g)
2AgBr (s) -------Sunlight→ 2Ag (s) + Br2 (g)
• Silver chloride truns grey on exposure to sunlight.
• Above reaction is used in black & white photography.
* Endothermic Reactions : The reactions which require energy in the form of heat, light or electricity to break reactants are called endothermic reactions.
(III) DISPLACEMENT REACTION : The chemical reaction in which more reactive element displaces less reactive element from its salt solution.
Fe (s) + CuSO4 → FeSO4 (aq) + Cu (s)
The iron nail becomes brownish in colour by deposition of Cu and blue colour of CuSO4 changes to dirty green colour due to formation of FeSo4.
Zn + CuSO4 → ZnSO4 + Cu
Zn is more reactive than copper.
(IV) DOUBLE DISPLACEMENT REACTION : A reaction in which new compound are formed by mutual exchange of ions between two compounds.
| (i) Na2SO4 (aq) + BaCl2 (aq) → BaSO4 (s) + 2NaCl (aq) |
| (Sodium sulphate) (Barium chloride) (Barium sulphate) (Sodium chloride) |
| White precipitate of BaSO2 is formedm, so it is also called precipitation reaction. |
|
(ii) 2Kl + Pb(NO2)2 → +Pbl2 + 2KNO3 |
| (potassium lodine)(Potassium nitrate) (Lead nitrate)(Yellow ppt.) (Lead iodide)(yellow ppt.) |
|
(iii) 2KBr + Bal2 → 2Kl + BaBr2 |
| (Potassium Bromide) + (Barium iodide) → (Potassium iodide) + (Barium bromide) |
(V) OXIDATION AND REDUCTION:
Oxidation :
(i) The addition of oxygen to reactant.
(ii) The removal of hydrogen from a reactant.
C + O2 → CO2
2Cu + O2 ----Heat→ 2CuO
CuO + H2 ----Heat→ Cu + H2O
Reduction :
(i) The addition of hydrogen to reactant.
(ii) The removal of oxygen from a reactant.
In this reaction CuO is reduced to Cu and H2 is oxidized to H2O. So, oxidation and reduction taking place together is redox reaction.
Effects of Oxidation in Daily Life
1) Corrosion :
• When a metal is exposed to moisture, air, acid etc. for some time, a layer of hydrated oxide is formed which weakens the metal and hence metal is said to be corroded.
• Rusiting of iron, black coating on silver and green coating on copper are examples of corrosion.
• Corrosion can be prevented by galvanization, electroplating or by putting paints.
2) Rancidity : The oxidation of fats and oils when exposed to air is known as rancidity. It leads to bad smell and bad taste of food.
Methods to Prevent Rancidity
(i) By adding antioxidants
(ii) Keeping food in air tight containers
(iii) Replacing air by nitrogen
(iv) Refrigeration
