Chemical Reactions and Equations Notes – Class 10 Science
Chemical Reactions and Equations
The transformation of chemical substance into another chemical substance is known as Chemical Reaction. For example: rusting of iron, setting of milk into curd, digestion of food, respiration, etc.
In chemical reaction new substance is formed which is completely different in properties from the original substance, so in a chemical reaction chemical change takes place.
Signs of Chemical Reaction
- Change of state of substance
- Change of color of substance
- Evolution of heat
- Absorption of heat
- Evolution of gas
- Evolution of light
- Evolution of sound, etc.
Reactant and Product:
Reactant: Substances which take part in a chemical reaction are called reactants.
Product: New substance formed after a chemical reaction is called product.
Chemical Equation: Representation of chemical reaction using symbols of substances is called chemical equation.
Example: A + B ⇨ C + D
In this equation, A and B are called reactants and C and D are called the products. Arrow shows the direction of chemical reaction. Condition, if any, is written generally above the arrow.
(When hydrogen reacts with oxygen, it gives water. This reaction can be represented by following chemical equation.
Hydrogen + Oxygen → Water
H2 + O2 → H2O
Chemical Equation is a way to represent the chemical reaction in concise and informative way.
In the other words, Chemical equations can be made more concise and useful if we use chemical formulae instead of words. A chemical equation represents a chemical reaction.
Chemical equation can be divided into two types – Balanced Chemical Equation and Unbalanced Chemical Equation.
Balanced Chemical Equation: A balanced chemical equation has number atoms of each element equal on both sides. For e.g. :
Unbalanced Chemical Equation: If the number of atoms of each element in reactants is not equal to the number of atoms of each element present in product, then the chemical equation is called unbalanced chemical equation.
Balancing a chemical equation:
To balance the given or any chemical equation, follow these steps:
Step I: To balance a chemical equation, first draw boxes around each formula. Do not change anything inside the boxes while balancing the equation.
Fe + H2O → Fe3O4 + H2
Step II: List the number of atoms of different elements present in the unbalanced equation.
|Elements||No. of atoms in reactants LHS||No. of atoms in Products RHS|
Step III: It is often convenient to start balancing with the compound that contains the 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 Fe3O4 and the element oxygen in it. There are four oxygen atoms on the RHS and only one on the LHS.
To balance the oxygen atoms –
|Atoms of Oxygen||In reactants||In products|
|(i) Initianal||1 in (H2O)||4 in (Fe3O4)|
|(ii) To balance||1X4||4|
To equalise the number of atoms, it must be remembered that we cannot alter the formulae of the compounds or elements involved in the reactions. For example, to balance oxygen atoms we can put coefficient ‘4’ as 4 H2O and not H2O4 or (H2O) 4. Now the partly balanced equation becomes–
Fe + 4 H2O → Fe3O4 + H2
Step IV: Fe and H atoms are still not balanced. Pick any of these elements to proceed further. Let us balance hydrogen atoms in the partly balanced equation.
To equalise the number of H atoms, make the number of molecules of hydrogen as four on the RHS.
|Atoms of Hydrogen||In reactants||In products|
|(i) Initianal||8 in (4 H2O)||2 in (H2)|
|(ii) To balance||8||2 X 4|
The equation would be –
Fe + 4 H2O → Fe3O4 + 4 H2
Step V: Examine the above equation and pick up the third element which is not balanced. You find that only one element is left to be balanced, that is, iron.
|Atoms of Iron||In reactants||In products|
|(i) Initianal||1 in (Fe)||3 in (Fe3O4)|
|(ii) To balance||1 X 3||3|
To equalise Fe, we take three atoms of Fe on the LHS.
3 Fe + 4 H2O → Fe3O4 + 4 H2
Step VI: Finally, to check the correctness of the balanced equation, we count atoms of each element on both sides of the equation.
3Fe + 4H2O → Fe3O4 + 4H2
The numbers of atoms of elements on both sides of Equation are equal. This equation is now balanced. This method of balancing chemical equations is called hit-and-trial method as we make trials to balance the equation by using the smallest whole number coefficient.
Step VII: Writing Symbols of Physical States Carefully examine the above balanced Equation. Does this equation tell us anything about the physical state of each reactant and product? No information has been given in this equation about their 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
The balance equation becomes :
3Fe(s) + 4H2O(g) → Fe3O4(s) + 4H2(g)
Note that the symbol (g) is used with H2O to indicate that in this reaction water is used in the form of steam.
Usually physical states are not included in a chemical equation unless it is necessary to specify them.
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 –
Types of Reaction
- Double Displacement Reaction
- Exothermic Reaction
- Endothermic Reaction
- Oxidation Reduction
Double Displacement Reaction: Reactions in which ions are exchanged between two reactants forming new compounds are called double displacement reactions.
AB + CD ⇨ AC + BD
Example: When solution of barium chloride reacts with the solution of sodium sulfate, white precipitate of barium sulfate is formed along with sodium chloride.
NaOH + HCl ⇨ NaCl + H2O
Double displacement reaction, in which precipitate is formed, is also known as precipitation reaction. Neutralization reactions are also examples of double displacement reaction.
Exothermic and Endothermic Reaction
Reactions which produce energy are called exothermic reaction. On the other hand, reactions which absorb energy are called endothermic reaction. Most of the combination reactions are endothermic. Most of the decomposition reactions are exothermic. Respiration is a decomposition reaction in which energy is released. When quicklime (calcium carbonate) is added to water, it decomposes and releases energy. Cooking involves chemical reactions which are endothermic as cooking is possible because of heating.
Oxidation and Reduction Reaction:
Oxidation: Addition of oxygen or non-metallic element or removal of hydrogen or metallic element from a compound is known as oxidation.
Elements or compounds in which oxygen or non-metallic element is added or hydrogen or metallic element is removed are called to be oxidized.
Oxidizing agent : Compounds which can add oxygen or a non-metallic compound or remove hydrogen or metallic element are known as oxidizing agents.
Reduction: Addition of hydrogen or metallic element or removal of oxygen or non-metallic element from a compound is called reduction. The compound or element which goes under reduction is called to be reduced.
In a chemical reaction oxidation and reduction both take place simultaneously and such reactions are also known as REDOX REACTIONS. In the word REDOX, ‘Red’ stands for reduction and ‘Ox’ stands for oxidation.
Significance of Oxidation Reduction in Everyday Life:
- Respiration is oxidation reaction in which food is oxidized to produce energy.
- Iron gets oxidized to form rust; which leads to corrosion of iron in the long run.
- Most of the metals react with atmospheric oxygen and it leads to formation of a layer on the metal article. The metal gets corroded in the long run.
- Rusting of iron can be prevented by painting the iron article. This can also be prevented by applying a layer of zinc over iron article. This process is known as galvanization.
- Fried food gets oxidized when exposed to air. This spoils the taste of the food and the food becomes unfit for consumption. The spoiling of fried food because of oxidation is called rancidity. Fried food is often packed in airtight packets to prevent rancidity.
- We are able to utilize various types of fuel because of oxidation. Oxidation of fuel helps in producing energy.