Table of contents | |
What is a Chemical Equation? | |
Rules for Writing a Word Equation | |
Steps to Balance Chemical Equations | |
Writing the Symbols of Physical States of Substances | |
Solved Examples |
Balancing chemical equations means adding numbers to the chemicals to make sure the same number of each type of atom is on both sides. This is important because a chemical equation must obey the law of conservation of mass and the law of constant proportions, i.e. the same number of atoms of each element must exist on the reactant side and the product side of the equation.
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.
A balanced chemical equation is one that contains an equal number of atoms of each element on both sides of the equation. The simple equations are balanced by the "hit and trial method" which is done in the following steps.
Step 1. Count the no. of atoms of various elements on both sides of the equation
Example: Fe + H2O → Fe3O4 + H2
Step 2. 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 3. 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 4. Balance the Iron atoms similarly.
Now the equation is
3Fe + 4H2O → Fe3O4 + 4H2
Step 5. Finally check the correctness of the balanced equation by counting the number of atoms on both sides of the equation.
Step 6. 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 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.
A reversible reaction is represented by the (⇌) symbol and irreversible reaction by the (→) symbol.
The heat evolved in a 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 calories (Endothermic reaction)
Sometimes 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
450°C (Fe-Mo)
Exothermic and Endothermic 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.
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.
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1. What is a chemical equation? |
2. What are the rules for writing a word equation? |
3. What are the steps to balance chemical equations? |
4. Why is it important to include the symbols of physical states of substances in a chemical equation? |
5. Can you provide a solved example of writing and balancing a chemical equation? |
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