Introduction: Change is the law of nature. Scientists classify these changes as physical changes and chemical changes. When a chemical change occurs, a chemical reaction is said to have taken place.
Physical change: A change in which the physical properties of the substance changes but the chemical composition does not change. The substance is restored to its original state as soon as the cause of change is withdrawn.
Fig. Physical change i.e melting of ice
Chemical change: In a chemical change, at least one of the reacting substance changes into new substances with a different composition. The new substances can not be changed back to the original substance even if the cause of change is withdrawn.
Fig. Chemical change i.e Burning of candle
Difference between physical change and chemical change:
Chemical reaction: The processes in which a substance or substances undergo a chemical change to produce new substance or substances, with entire new properties, is known as chemical reaction.
The nature and identity of products totally changes from the reactants. Observations which determines whether a chemical reaction has taken place or not.
(a) Chemical reaction must be associated with change in temperature i.e. Heat should be either evolved or absorbed.
(b) The reaction must occur between fixed quantities of the reactants.
(c) The chemical reaction should follow the law of conservation of mass.
(d) The products obtained must-have properties different from those of the reactants.
(i) When potassium nitrate is heated, it gives potassium nitrite and oxygen.
(ii) When sodium reacts with water sodium hydroxide is produced and hydrogen gas is liberated.
As description of a chemical reaction in a sentence form is quite long so when it is written in shorter form by using words and some signs (+ for addition & arrow (→) to show the direction, then it is called Word equation.
Word equation: A chemical equation which represents a chemical reaction briefly in words is called word equation.
Example: The word equation is
Sodium + Water → Sodium hydroxide + Hydrogen
Fig. Reactants combines to form products
Reactant: The substance or substances which takes part in a chemical reaction are called reactants.
Products: The new substance or substances formed as a result of a chemical reaction are called products.
In the above reaction sodium & water are reactants and sodium hydroxide & hydrogen are products.
Rules for writing a word equation:
(a) The substances taking part in chemical reaction reactants are always written on the left-hand side of arrow.
(b) The substances formed after the chemical reaction, products are always written on the right hand side of arrow.
(c) A plus sign (+) is put in between the reactants or the products. If their number is two or more.
(d) An arrow (→) is put between the reactants and products, the arrow shows the direction of the reaction in which the reaction proceeds. The arrow is read as "to yield" or "to form".
In the word equation when symbols and chemical formulas of the reactants and products are used then it is called a chemical equation.
Example: Na + H2O → NaOH + H2
i.e. A chemical equation is a statement that describes a chemical reaction in terms of symbols and formulas. In this equation the law of conservation of mass is not obeying. Such chemical equation is called skeletal equation or unbalanced equation.
A chemical equation expressed in symbols and formula, such that the number of atoms of different elements towards the side of the reactants is not equal to the number of atoms of the products is called skeletal equation or unbalanced equation.
To make this equation meaningful, this equation is balanced then it is called balanced chemical equation.
Balancing chemical equations:
A balanced chemical equation is the one which contains an equal number of atoms of each element on both sides of the equation.
The simple equations are balanced by "hit and trial method" which is done in following steps.
Step (i) Count the no. of atoms of various elements on both sides of the equation.
Example: Fe + H2O → Fe3O4 + H2
Step (ii) Start balancing with the compound which contains maximum number of atoms. It may be a reactant or product. In that compound select the element which has maximum number of atoms.
According to this rule Fe3O4 has maximum number of atoms & oxygen has 4 atoms so it is selected.
So the partly balanced equation is
Fe + 4H2O → Fe3O4 + H2
Step (iii) Fe and H are not balanced in the above reaction so the above reaction repeat the above process for both i.e.
Now the equation becomes as
Fe + 4H2O → Fe3O4 + 4H2
Step (iv) Balance the Iron atoms similarly.
Now the equation is
3Fe + 4H2O → Fe3O4 + 4H2
Step (v) Finally check the correctness of the balanced equation by counting the number of atoms on both sides of the equation.
Step (vi) To make chemical equation more informative physical states of the reactants and products are mentioned as for solid (s), liquid (l), gas (g) and for aqueous solution of reactant or product (aq) is written.
Now the equation becomes as
3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)
Symbol (g) with water is written to show that water is used in the form of steam in this equation.
If a gas is evolved in a reaction it can be shown by the symbol (↑) after the formula i.e. arrow pointing upwards e.g.
2Na(s) + 2H2O(l) → 2 NaOH(aq) + H2(g) or H2 (↑)
The symbol (↓) or ppt is be written for precipitate.
NaCl + AgNO3 → NaNO3 + AgCl (↓) or ppt.
Reversible reaction is represented by (⇌) symbol and irreversible reaction by the (→) symbol.
The heat evolved in chemical reaction is written on the right side by putting positive (+) sign and heat absorbed in the chemical reaction is written on the right hand side by putting negative (-) sign.
N2 + 3H2 ⇌ 2NH3 + 22400 calories (Exothermic reaction)
N2 + O2 ⇌ 2NO - 43200 calorie (Endothermic reaction)
Some times the reaction conditions, such as temperature, pressure, catalyst etc. are written above or below the arrow in the equation e.g.
N2 + 3H2 2NH3 + 22400 calorie heat
Exothermic and Endothermic Reactions:Fig. Difference between endothermic and exothermic reactions
(i) Exothermic reaction: The reaction in which heat is liberated (or given out) is called an exothermic reaction.
Example: C(s) + O2(g) → CO2(g) + Heat (393 kJ/mol)
(ii) Endothermic reaction: The reaction in which heat is absorbed (or taken in ) is called an endothermic reaction.
Example: C(s) + 2S(g) → CS2(g) - Heat (92 kJ/mol)
The reaction with + Heat term on the product side is called exothermic reaction, while those with - Heat term on the product side are called endothermic reactions.
During respiration, the digested food gets oxidised and the energy is released. That is why, it is considered as an exothermic reaction.
Balancing of a chemical equation is necessary because no matter (hence, no atom) is lost or gained during a chemical reaction.
Writing the symbols of Physical States of substances in Chemical equation:
By writing the physical states of substances a chemical equation becomes more informative.
Writing the condition in which reaction takes place: The condition is generally written above and/or below the arrow of a chemical equation.
Thus, by writing the symbols of physical state of substances and condition under which reaction takes place, a chemical equation can be made more informative.
Q.1. Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.
(a) N2(g) + 3H2(g) 2NH3(g)
(b) 2H2S + 3O2 → 2H2O + 2SO2
(d) 2K (s) + 2H2O (1) → 2KOH + H2 (g)
Q.2. Balance the following chemical equations.
(a) HNO3 + Ca(OH)2 → Ca(NO3)2 + H2O
(b) NaOH + H2SO4 → Na2SO4 + H2O
(a) 2HNO3 + Ca(OH)2 → Ca(NO3)2 + 2H2O
(b) 2NaOH + H2SO4 → Na2SO4 + 2H2O
Q.3. Write the balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride.
Q.4. Write the balanced chemical equation for the following and identify the type of reaction in each case -
(a) Potassium bromide(aq) + Barium iodide(aq) → Potassium iodide(aq) + Barium bromide(s)
(b) Zinc carbonate (s) → Zinc oxide (s) + Carbon dioxide(g)
(c) Hydrogen(g) + Chlorine(g) → Hydrogen chloride(g)
(d) Magnesium(s) + Hydrochloric acid(aq) → Magnesium chloride(aq) + Hydrogen(g)
(a) 2KBr(aq) + BaI2(aq) → 2KI(aq) + BaBr2(aq)
This reaction is a double-displacement reaction.
(b) ZnCO3(s) → ZnO(s) + CO2(g)
This reaction is a decomposition reaction
Q.5. Why should a magnesium ribbon be cleaned before burning in air?
Ans. Magnesium reacts with the constituent gases of the atmosphere to form various compounds which get deposited over its surface. The ribbon is cleaned before burning to remove the layer of these compounds so that pure magnesium can burn in air.