Physical changes :
Those changes which are accompanied with change in physical properties of the substances but no new substance is formed are called physical changes e.g., melting of ice, boiling of water, etc.
Chemical changes :
Those changes in which the original substances lose their nature and identity to form new chemical substances with different properties are called chemical changes e.g., burning of candle, cooking food, etc
Chemical reactions :
The process involving a chemical change is known as a chemical reaction. e chemical substances taken initially are called reactants and the chemical substances which are formed during a chemical reaction are called products. us, chemical reaction is a process in which breaking of chemical bonds (present in the reactant molecules) and making of new chemical bonds (in the product molecules) occur e.g., burning of magnesium ribbon in air.
Mg + O2------>MgO
Reactants Product
Characteristics of chemical reactions :
Th e following observations help us to determine whether a chemical reaction has taken place or not :
Change in state :
Certain chemical reactions are accompanied with the change of state e.g., when a mixture of hydrogen gas and oxygen gas is ignited with an electric spark at room temperature, liquid water is formed
Change in colour :
Certain chemical reactions are accompanied with the change of colour e.g., when red lead oxide is heated yellow lead monoxide is formed.
Evolution of a gas :
Some chemical reactions are accompanied with the evolution of a gas e.g., reaction between a metal (like zinc, magnesium or iron) and dilute sulphuric acid produces hydrogen gas.
Change in temperature :
Some chemical reactions occur with change in heat energy or with change in temperature. –Reactions which result in rise in temperature i.e. in which heat is evolved are called exothermic reactions.
Chemical equation :
A method of representing a chemical reaction in terms of symbols and formulae of the substances participated is known as chemical equation. ere are two ways to represent a chemical equation :
In terms of words : When a chemical equation is written in terms of words, it is called a word equation. e chemical reaction between granulated zinc and hydrochloric acid can be written in terms of words as.
Zinc + Hydrochloric acid (Reactants) → Zinc chloride + Hydrogen (
Products)
Conventions used in chemical equations :
– The reactants are written on the le hand side along with plus (+) sign between them.
– Similarly, products are written on the right hand side along with plus (+) sign between them.
– An arrow (→) separates the reactants from the products.
– Th e arrowhead points towards the products and tells about the direction of the reaction.
Balanced and unbalanced chemical equations :
Balanced chemical equation : The equation which contains an equal number of atoms of each element on both sides of the arrow is called a balanced chemical equation.
2Mg + O2 → 2MgO
A balanced chemical equation must obeys the law of conservation of mass.
Unbalanced chemical equation :
The equation in which the number of atoms of each element on both sides of the arrow is not equal.
H2 + O2→ H2O
Balancing a chemical equation : Balancing of a chemical equation means to equalise the number of atoms of each element on both sides of the equation.
Consider the following steps for balancing the chemical equation :
For example, iron reacts with water (steam) to form iron(II, III) oxide and hydrogen.
Step-I : Write the chemical equation in the form of a word equation. Keep the reactants on the left hand side and the products on the right hand side. Separate them by an arrow (→) with head pointing from the reactants to products.
Iron + Steam → Iron (II, III) oxide + Hydrogen
Step-II : Write down the symbols and formulae of the various reactants and products which gives skeletal chemical equation.
Fe + H2O→ Fe3O4 + H2
Step-III : Listing number of atoms of different
Step-IV : Select the compound with maximum number of atoms to start balancing. In that compound, balance the element with maximum number of atoms (e.g., oxygen in the given equation). It may be a reactant or a product.
Fe + 4H2O→ Fe3O4 + H2
Step-V : To balance the atoms of an element, put a whole number coefficient before the formula of the compound. If selection of the biggest formula appears inconvenient, balance the atoms of that element which occurs at minimum number of places on both sides of the equation. Atoms of the element which occur at maximum places are balanced at last.
To balance H-atoms on both sides :
Fe + 4H2O→ Fe3O4 + 4H2
To balance Fe-atoms on both sides :
So, the equation would be 3Fe + 4H2O→ Fe3O4 + 4H2
Step VI : For checking the correct balanced equation, we count atoms of each element on both sides of the equation.
As the number of atoms of each element on both sides of the equation are equal, the equation is balanced.
3Fe(s) + 4H2O(g)→ Fe3O4(s) + 4H2(g)
How can a chemical equation be made more informative?
A chemical equation can be made more informative by adding some extra information to the chemical equation which may be summarised as :
- Write the state symbols for the reactants and products taking part in a chemical reaction.Use symbols (g) for gases, (s) for solids, (l) for liquids and (aq) for aqueous solutions.
- Indicate the gas evolved in the reaction by (↑).
- Indicate the precipitate obtained in the reaction by (↓).
- Mention the heat evolved by (+) sign and heat absorbed by (–) sign on the product side.
Mention the reaction conditions, temperature as t°C or K, pressure as atm, catalyst, etc. above or below the arrow.
When reactants are converted into products and products cannot be converted back to reactants then the reaction is called an irreversible reaction and is represented by(→).
On the other hand, if in a chemical reaction, reactants are converted into products in forward direction and again products are converted back into reactants in backward direction then, it is called a reversible reaction and is represented by (⇋).
Types of chemical reactions :
As we know, in chemical reactions, bonds present in reactants break and new bonds form in the products. is exchange of species can take place in a number of ways resulting in different types of reactions, which can be explained as follows :
Combination reactions :
The reactions in which two or more substances combine to form a single substance under suitable conditions.
Examples :
Combustion of coal
C(s) + O2(g) → CO2(g)
Combination of nitric oxide with oxygen to form nitrogen dioxide.
2NO(g) + O2(g) → 2NO2(g)
Nitric oxide → Nitrogen dioxide(brown gas)
Combination of ammonia with hydrogen chloride gas to form
a white solid mass of ammonium chloride.
NH3(g) + HCl(g) →NH4Cl(s) (Ammonium chloride(white)
Decomposition reactions : Those reactions in which a single substance breaks down to give two or more smaller substances under suitable conditions.
Three types of decomposition reactions are as follows :
– Thermal decomposition reactions :
These reactions occur in presence of heat
2FeSO₄(s)sheat → Fe₂O₃(s)+SO₂(↑ )g+SO₃( ↑)g
Ferrous sulphate(green) → Ferric oxide (reddish brown) + Sulphur dioxide(smell of burning sulphur) + Sulphur trioxide
Examples :
NaOH(aq) (Base) + HCl(aq) (Acid )→NaCl(aq) (Salt)+ H2O
CuO(s) Base + 2HCl(aq) Acid →CuCl2(aq) (Salt)+ H2O
Redox reactions : Reactions in which oxidation and reduction take place simultaneously are called redox reactions.
–Oxidation : Those reactions in which the addition of oxygen to a substance or removal of hydrogen from a substance takes place are called oxidation reactions.On the other hand, the substance which either gives oxygen or removes hydrogen in an oxidation reaction is known as an oxidising agent.
–Reduction : Those reactions in which addition of hydrogen to a substance or removal of oxygen from a substance takes place are called reduction reactions.On the other hand, the substance which either gives hydrogen or removes oxygen in a reduction reaction is known as reducing agent.
Effect of oxidation reactions in everyday life : As oxygen is the most essential element for sustaining life, it is involved in variety of reactions which has wide range of effects on our daily life. e two effects are discussed below :
Corrosion : It is a process in which metals are decayed gradually by the action of air, moisture and acids on their surface.
Basically, it is caused by oxidation of metals by oxygen present in the air.
Example : Rusting of iron,
4Fe(s) + 3O2(g) + 2xH2O(l) 2Fe2O3·xH2O(s)
Iron +Air+ Moisture → Hydrated Iron(III) oxide
2Cu+ CO₂(g)+O₂(g)+ HO₂(l) (Moisture) →CuCO₃·Cu(OH)2(Basic copper carbonate(green))
Chemical Equations (1 mark).
Write a balanced chemical equation for the reaction between sodium carbonate and hydrochloric acid indicating the physical state of the reactants and the products.(Foreign 2010)
2. Balance the following chemical equation :Pb(NO3)2(s)heat--> PbO(s) + NO2(g) + O2(g)(Delhi 2009)
3. Balance the following chemical equation :Fe(s) + H2O(g)---> Fe3O4(s) + H2(g) (AI 2008)
4. Balance the following chemical equation :FeSO4heat ----> Fe2O3 + SO2 + SO3(AI 2008)
5. Balance the following chemical reaction :MnO2 + HCl-----> MnCl2 + Cl2 + H2O(AI 2008)
1.2 Types of Chemical Reactions (1 mark)
6. In electrolysis of water, why is the volume of gas collected over one electrode double than that of gas collected over the other electrode?(AI 2009)
7. What change in colour is observed when white silver chloride is led exposed to sunlight? What type of chemical reaction is this? (Foreign 2009)
8. What happens chemically when quick lime is added to water? (Delhi 2008) (2 marks)
9. What happens when an aqueous solution of sodium sulphate reacts with an aqueous solution of barium chloride? State the physical conditions of reactants in which the reaction between them will not take place. Write the balanced chemical equation for the reaction and name the type of reaction. (Delhi 2010)
10. What is a redox reaction? When a magnesium ribbon burns in air with a dazzling flame and forms a white ash, is magnesium oxidised or reduced? Why? (Delhi 2009)
11. (a) What is the colour of ferrous sulphate crystals? How does this colour change while heating?(b) Name the products formed on strongly heating ferrous sulphate crystals. What type of chemical reaction occurs in this change?(Delhi 2009)
12. (i) What is observed when a solution of potassium iodide is added to a solution of lead nitrate taken in a test tube?(ii) What type of reaction is this?(iii) Write a balanced chemical equation to represent the above reaction. (AI 2009)
13. What is an oxidation reaction? Identify in the following reaction : ZnO + C o Zn + CO(i) the substance oxidised and(ii) the substance reduced. (Delhi 2008)
14. Give an example of decomposition reaction. Describe an activity to illustrate such a reaction by heating. (AI 2008)
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