The nature and the identity of the initial substance have somewhat changed in the folowing situations.
- milk is left at room temperature during summers.
- an iron tawa/pan/nail is left exposed to humid atmosphere.
- grapes get fermented.
- food is cooked.
- food gets digested in our body.
- we respire.
1.1 CHEMICAL EQUATIONS
When a Magnesium ribbon is burnt in Oxygen, it gets converted to Magnesium oxide. This description of a chemical reaction having long form. So it needs to write Word-equation.
The word-equation for the above reaction can be written as below–
Here,
- The substances magnesium and oxygen, undergo chemical change are the reactants.
- The new substance is magnesium oxide, formed during the reaction, as a product.
- A word-equation shows change of reactants to products through an arrow mark placed between them.
- The reactants are written on the left-hand side (LHS) with a plus sign (+) between them.
- Similarly, products are written on the right-hand side (RHS) with a plus sign (+) between them.
- The arrowhead points towards the products, and shows the direction of the reaction.
1.1.1 Writing a Chemical Equation
Now, by recalling the formulae of magnesium, oxygen and magnesium oxide, the above word-equation can be written as –
Mg + O_{2} \rightarrow MgO. . . . . . . (1.2)- Now count and compare the number of atoms of each element on the LHS and RHS of the arrow.
- If the number of atoms of each element the same on both the sides then the equation is balanced.
- If not, then the equation is unbalanced because the mass is not the same on both sides of the equation.
- Such a chemical equation is a skeletal chemical equation for a reaction. The above equation 1.2 is a skeletal chemical equation for the burning of magnesium in air.
1.1.2 Balanced Chemical Equations [Hit and Trial (HT) Method]
Conservation of mass: Mass can neither be created nor destroyed in a chemical reaction.
That is, the total mass of the elements present in the products of a chemical reaction has to be equal to the total mass of the elements present in the reactants.
In other words, the number of atoms of each element remains the same, before and after a chemical reaction. Hence, we need to balance a skeletal chemical equation.
The word-equation for the reaction of Zinc and Sulphuric acid may be represented as –
Zinc + Sulphuric acid \rightarrow Zinc sulphate + HydrogenThe above word-equation may be represented by the following chemical equation –
Zn + H_{2}SO_{4} \rightarrow ZnSO_{4} + H_{2}Now count the number of atoms of different elements on both sides of the arrow mark.
| Elements | Number of atoms in reactants (LHS) | Number of atoms in products (RHS) |
| Zn | 1 | 1 |
| H | 2 | 2 |
| S | 1 | 1 |
| O | 4 | 4 |
Let us try to balance the following chemical equation –
Fe + H_{2}O \rightarrow Fe_{3}O_{4} + H_{2}STEP-I:
- Draw boxes around each formula.
- Do not change anything inside the boxes while balancing the equation.
STEP-II:
- List the number of atoms of different elements present in the unbalanced equation.
| Elements | Number of atoms in reactants (LHS) | Number of atoms in products (RHS) |
| Fe | 1 | 3 |
| H | 2 | 2 |
| O | 1 | 4 |
STEP-III:
- Start balancing with the compound that having maximum number of atoms. It may be a reactant or a product.
- In that compound, select the element which has the maximum number of atoms.
- Using these criteria, we select Fe_{3}O_{4} and the element oxygen in it. There are four(4) oxygen atoms on the RHS and only one(1) on the LHS.
To balance the oxygen atoms –
| Atoms of Oxygen | In reactants (LHS) | In products (RHS) |
| (i) Initial | 1 (in H_{2}O) | 4 (in Fe_{3}O_{4}) |
| (II) To balance | 1 X 4 | 4 |
To balance oxygen atoms we can put coefficient ‘4’ as 4 H_{2}O. Now the partly balanced equation becomes –
Fe + 4 H2O
STEP-IV:
To equalise the number of H atoms, make the number of molecules of hydrogen as four on the RHS.
| Atoms of Hydrogen | In reactants (LHS) | In products (RHS) |
| (i) Initial | 8 (in 4 H_{2}O) | 2 (in H_{2}) |
| (II) To balance | 8 | 2 X 4 |
The equation would be –
STEP-V:
Now we have to balance Fe.
| Atoms of iron | In reactants (LHS) | In products (RHS) |
| (i) Initial | 1 (in Fe) | 3 (in Fe_{3}O_{4}) |
| (II) To balance | 1 X 3 | 3 |
To equalise Fe, we take three atoms of Fe on the LHS.
STEP-VI:
Finally, we have to to check the correctness of the balanced equation. Let’s count atoms of each element on both sides of the equation.
The numbers of atoms of elements on both sides are
equal.
This is a balanced chemical equation.
This method of balancing chemical equations is called Hit-and-Trial (HT) method as we make trials to balance the equation by using the smallest whole number coefficient.
STEP-VII: Writing Symbols of Physical States.
- To make a chemical equation more informative, the physical states of the reactants and products are mentioned along with their chemical formulae.
- The gaseous, liquid, aqueous and solid states of reactants and products are represented by the notations (g), (l), (aq) and (s), respectively.
- The word aqueous (aq) is written if the reactant or product is present as a solution in water.
Now the above equation will be,
3Fe(s) + 4H_{2}O(g) \rightarrow Fe_{3}O_{4}(s) + 4H_{2}(g)Sometimes the reaction conditions, such as temperature, pressure,
catalyst, etc., for the reaction are indicated above and/or below the arrow
in the equation. For example –
1.2 TYPES OF CHEMICAL REACTIONS
1.2.1 Combination Reaction
The reaction in which a single product is formed from two or more reactants is known as a combination reaction.
Calcium oxide reacts vigorously with water to produce slaked lime (calcium hydroxide) releasing a large amount of heat.
CaO(s) + H_{2}O(l) \rightarrow Ca(OH)_{2}(aq) + HeatIn this reaction, calcium oxide and water combine to form a single product, calcium hydroxide.
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 whitewashing and gives a shiny finish to the walls.
The chemical formula for marble is CaCO_{3}.
Examples:- (Combination Reactions)
(i) Burning of coal
C(s) + O_{2}(g) \rightarrow CO_{2}(g)(ii) Formation of water from H_{2}(g) and O_{2}(g).
2H_{2}(g) + O_{2}(g) \rightarrow 2H_{2}O(l)Exothermic Chemical Reaction: The reactions in which heat is released along with the formation of products are called exothermic chemical reactions.
Examples:- (Exothermic Reactions)
(i) Burning of natural gas.
CH_{4}(g) +2 O_{2}(g) \rightarrow CO_{2}(g) + 2H_{2}O(g)(ii) Respiration is an exothermic process.
C_{6}H_{12}O_{6}(aq) + 6O_{2}(aq) \rightarrow 6CO_{2}(aq) + 6H_{2}O(l) + Energy(iii) The decomposition of vegetable matter into compost is also an example of an exothermic reaction.
1.2.2 Decomposition Reaction
The reaction in which a single reactant breaks down to give simpler products is called decomposition reaction.
Here, Ferrous sulphate crystals (FeSO_{4}, 7H_{2}O) lose water when heated and the colour of the crystals changes. It then decomposes to ferric oxide (Fe_{2}O_{3}), sulphur dioxide (SO_{2}) and sulphur trioxide (SO_{3}). Ferric oxide is a solid, while SO_{2} and SO_{3} are gases.
Calcium oxide is called lime or quick lime.
Thermal Decomposition:- When a decomposition reaction is carried out by heating, it is called thermal decomposition.
Examples:- (Decomposition Reaction)
White silver chloride turns grey in sunlight. This is due to the decomposition of silver chloride into silver and chlorine by light.
2AgCl(s) \xrightarrow{Sunlight} 2Ag(s) + Cl_{2}(g)Silver bromide also behaves in the same way.
2AgBr(s) \xrightarrow{Sunlight} 2Ag(s) + Br_{2}(g)This reactions are used in black and white photography.
We have seen that the decomposition reactions require energy either in the form of heat, light or electricity for breaking down the reactants.
Endothermic reactions: Reactions in which energy is absorbed are known as endothermic
reactions.
1.2.3 Displacement Reaction
The reaction in which an element displaces or remove another element from from a compound is known as displacement reaction.
The iron nail becomes brownish in colour and the blue colour of copper sulphate solution fades.
In the above reaction, iron has displaced or removed another element, copper, from copper sulphate solution.
Examples: (Displacement reactions)
Zinc and lead are more reactive elements than copper. They displace copper from its compounds.
1.2.4 Double Displacement Reaction
The reactions in which there is an exchange of ions between the reactants are called double displacement reactions.
Here, the white precipitate of BaSO_{4} is formed by the reaction of SO_{4}^{2-} and Ba^{2+}. The other product formed is sodium chloride which remains in the solution.
Precipitation reaction: Any reaction that produces a precipitate can be called a precipitation reaction.
1.2.5 Oxidation and Reduction
When one reactant gets oxidised while the other gets reduced during a reaction, such reactions are called oxidation-reduction reactions or redox reactions.
When we heat a china dish containing cpper powder, it becomes coated with black copper(II) oxide.
This is because oxygen is added to copper and copper oxide is formed.
2Cu + O_{2} \xrightarrow{Heat} 2CuOIf hydrogen gas is passed over this heated material (CuO), the black coating on the surface turns brown.
CuO + H_{2} \xrightarrow{Heat} Cu + H_{2}OIn this reaction the copper(II) oxide is losing oxygen and is being reduced. The hydrogen is gaining oxygen and is being oxidised.
Example: (Redox reactions)
(i) Zno + C \rightarrow Zn + CO
(ii) MnO_{2} + 4HCl \rightarrow MnCl_{2} + 2H_{2}O + Cl_{2}
Oxidised: if a substance gains oxygen or loses hydrogen during a reaction, it is oxidised.
Reduced: If a substance loses oxygen or gains hydrogen during a reaction, it is reduced.
1.3 HAVE YOU OBSERVED THE EFFECTS OF OXIDATION
REACTIONS IN EVERYDAY LIFE?
1.3.1 Corrosion
When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion.
- The black coating on silver and the green coating on copper are other examples of corrosion.
- Corrosion causes damage to car bodies, bridges, iron railings, ships and to all objects made of metals, specially those of iron.
- Corrosion of iron is a serious problem.
Rusting of iron:- The iron articles are shiny when new, but get coated with a reddish brown powder when left for some time. This process is commonly known as rusting of iron.
1.3.2 Rancidity
When fats and oils are oxidised, they become rancid and their smell and taste change. This process is called rancidity.
- Usually substances which prevent oxidation (antioxidants) are added to foods containing fats and oil.
- Keeping food in air tight containers helps to slow down oxidation.
- The chips manufacturers usually flush bags of chips with nitrogen gas to prevent the chips from getting oxidised.
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