CHEMICAL REACTION
Physical and Chemical Changes
Physical Change:
A physical change is a temporary change in which an object changes its shape, size, volume, state, etc., and no new substances are formed. It can be reversed.
Chemical Change:
A chemical change is a permanent change in which new substances are formed. It cannot be reversed easily.
Chemical Reaction and Equation
Chemical Reaction:
A chemical change in which exchange of molecules of matter, decomposition, or combination takes place to form new substances is called a chemical reaction.
For example:
Hydrogen + Oxygen → Water
H₂ + O₂ → H₂O
Chemical Equation:
A chemical equation has two parts separated by an arrow head (→) in the middle.
- The left side of a chemical reaction is called the reactant, which includes chemicals that will undergo change.
- The right side is called the product, which includes chemicals formed during the reaction.
Example:
H₂ + O₂ → H₂O
Reactants: H₂ and O₂
Product: H₂O
Types of Chemical Reactions
1. Combination or Addition or Synthesis Reaction
A chemical reaction in which two or more substances combine to form a new single substance.
Examples:
C + O₂ → CO₂
N₂ + H₂ → NH₃
2. Decomposition or Dissociation Reaction
A chemical reaction in which a single compound breaks into two or more simpler substances.
This occurs due to heat, electricity, light, or catalysts.
Examples:
CaCO₃ → CaO + CO₂
2H₂O → 2H₂ + O₂
2H₂O₂ → 2H₂O + O₂
2AgBr → 2Ag + Br₂
3. Displacement or replacement reaction.
The chemical reaction in which an element or radical of a compound is replaced by another more reactive element or radical to form new compounds is called displacement reaction.
In this chemical reaction more reactive elements displaces the less reactive elements.
There are two types of displacement reactions. They are given below :-
a. Single displacement reaction :-
The chemical reaction in which an element or radical from a compound is displaced by another element or radical to form a new compound is called single displacement reaction.
For example,
Zn + 2HCl → ZnCl₂ + H₂
In above chemical reaction Zinc displaces chlorine from HCl to form Zinc chloride and Hydrogen gas.
b. Double displacement reaction :-
The chemical reaction in which two reacting compounds exchange their corresponding radicals to form two new compounds is called double displacement reaction.
For example,
AgNO₃ + NaCl → NaNO₃ + AgCl
In above chemical reaction Silver and Sodium exchanged their anions or NO₃ and Cl exchanged their cations.
4. Acid-base or neutralization reaction :-
A chemical reaction in which an acid reacts with a base to produce salt and water is called acid-base reaction.
Acid-base reaction is also called a neutralization reaction because acid and base neutralize each other and form salt and water.
Both salt and water are neutral compound.
For example,
HCl + NaOH → NaCl + H₂O
In above chemical reaction HCl reacts with NaOH to form NaCl and H₂O.
Rate of chemical reaction :-
The decrease in the concentration of reactants or increase in the concentration of products per unit time is called rate of chemical reaction. Some chemical reactions occur very fast whereas some chemical reactions occur very slow. For example,
NaCl + AgNO₃ → AgCl + NaNO₃
If we mix aqueous solutions of sodium chloride and silver nitrate they react instantly to produce silver chloride and sodium nitrate. Where as rusting of an iron is an example of slow chemical reaction.
Factors affecting rate of chemical reaction.
1.Temperature :- If we supply heat then temperature increases. In general supplying heat means transferring thermal energy to the reactants which increases the kinetic energy of the molecules. The increased in kinetic energy makes the molecules movement fast. As a result reacting molecules come close to each other rapidly and collide with each other quickly and the reaction occurs faster. For example.
CaCO₃ ⟶ CaO + CO₂
2. Pressure :- Pressure mainly applicable for the chemical reaction whose reactants are in gaseous state. When the gases reactants are mix for chemical reaction, initially their molecules are far apart. If we increase pressure of gas in the vessels containing reactants, often volume will decrease as result reacting molecules comes closer and collide with each other rapidly, which makes the rate of chemical reaction faster. For example.
N₂ + 3H₂ → 2NH₃
3. Surface area :- The chemical reaction increases if there is more contact between reactants. If the surface area increased there is increase in contact area as a result rate of chemical reaction increases. For example, choosing powdered form of reactants makes the collision of reactant molecules faster than a single bulk piece of reactants, that’s why increase in surface area increases the rate of chemical reaction.
4. Catalyst :- A catalyst is a chemical which increases or decreases the rate of chemical reaction without changing its concentration and composition at the end of the chemical reaction. There are two types of catalysts :-
(a) Positive catalyst
(b) Negative catalyst
a. Positive catalyst :- The catalyst which increases the rate of chemical reaction is called positive catalyst. For example,
2KClO₃ ⟶ 2KCl + 3O₂
(without catalyst high temperature)
2KClO₃ ⟶ 2KCl + 3O₂
(with catalyst low temperature).
In above chemical reaction MnO₂ acts as a positive catalyst.
A. Give reason.
1. The rate of reaction becomes faster if a bulk solid is finely powdered.
Answer :- As we know that rate of chemical reaction increases on increasing the collision between reactants. Since finely powdered reactants has a greater surface area than a bulk solid that’s why there is more collision between reactants and the rate of reaction becomes faster in finely powdered reactants than a bulk solid.
2. The rate of reaction decreases as time passes.
Answer :- The rate of chemical reaction decreases as time passes because the amount of reactant also decreases as time passes.
3. Acid-base reaction is called neutralization reaction.
Answer :- When acid reacts with base it forms salt and water as a products. Since both products salt and water are neutral in nature that’s why acid-base reaction is called neutralization reaction.
4. Hydrogen peroxide is not stored in a transparent bottle.
Answer :- Hydrogen peroxide easily decomposes in the presence of sunlight. To avoid decomposition of Hydrogen peroxide by sunlight it is not stored in a transparent bottle.
2H₂O₂ → 2H₂O + O₂
B. Write the balanced equation for the reactions in the following conditions
- When potassium chlorate is heated in the presence of catalyst MnO₂
Answer:
2KClO₃ → 2KCl + 3O₂ - When calcium carbonate is heated
Answer:
CaCO₃ → CaO + CO₂ - When carbonic acid is heated
Answer:
H₂CO₃ → H₂O + CO₂ - Aqueous solutions of silver nitrate and calcium chloride are mixed
Answer:
2AgNO₃ + CaCl₂ → 2AgCl + Ca(NO₃)₂ - Mercuric oxide is heated
Answer:
2HgO → 2Hg + O₂ - Potassium bromide and chlorine gas are mixed together
Answer:
2KBr + Cl₂ → 2KCl + Br₂ - Calcium oxide reacts with water
Answer:
CaO + H₂O → Ca(OH)₂ - A mixture of nitrogen and hydrogen is heated strongly in the presence of catalyst with a high pressure
Answer:
N₂ + 3H₂ → 2NH₃ - Hydrogen peroxide is exposed to sunlight
Answer:
2H₂O₂ → 2H₂O + O₂
- Silver bromide is exposed to sunlight.
Answer :-
2AgBr ⟶ 2Ag + Br₂ - Lead nitrate is heated.
Answer :-
Pb(NO₃)₂ ⟶ PbO + NO₂ + O₂ - Silver nitrate is heated.
Answer :-
2AgNO₃ ⟶ 2Ag + O₂ + 2NO₂ - Copper nitrate is heated.
Answer :-
2Cu(NO₃)₂ ⟶ 2CuO + 4NO₂ + O₂ - Copper carbonate is heated.
Answer :-
CuCO₃ ⟶ CuO + CO₂ - Ammonium hydroxide is heated.
Answer :-
NH₄OH ⟶ H₂O + NH₃ - Iron is dipped in a copper sulfate solution.
Answer :-
Fe + CuSO₄ ⟶ FeSO₄ + Cu - Zinc pieces are dipped in a sulphuric acid.
Answer :-
Zn + H₂SO₄ ⟶ ZnSO₄ + H₂