CHEMICAL REACTION: process in which new substance are formed with new chemical or properties is called as chemical reaction or chemical change.
Example: rusting of iron, digestion, respiration, making tea, etc.……
- The substances which take part in a chemical reaction are called reactants.
- The new substances produced as a result of a chemical reaction are called products.
2H2 + O2
→
2H2O
In the given equation, hydrogen and oxygen is reactant and water is product,
chemical properties are changed as hydrogen is highly flammable and oxygen helps in combustion whereas water is use to fire off.
Physical properties are also changed as hydrogen and oxygen are gas but water is liquid.
Characteristics of Chemical Reactions:
(i) Evolution of gas: The chemical reaction between zinc and dilute sulphuric acid is characterised by the evolution of hydrogen gas.
Zn(s) + H2 SO4(aq) → ZnSO4(aq) + H2(g) ↑
(ii) Change in Colour: The chemical reaction between citric acid and purple coloured potassium permanganate solution is characterized by a change in colour from purple to colourless.
The chemical reaction between Sulphur dioxide gas and acidified potassium dichromate solution is characterized by a change in colour from orange to green.
(iii) Change in state of substance: The combustion reaction of candle wax is characterised by a change in state from solid to liquid and gas (because the wax is a solid, water formed by the combustion of wax is a liquid at room temperature whereas, carbon dioxide produced by the combustion of wax is a gas). There are some chemical reactions which can show more than one characteristic.
(iv) Change in temperature: The chemical reaction between quick lime water to form slaked lime is characterized by a change in temperature (which is a rise in temperature).
The chemical reaction between zinc granules and dilute sulphuric acid is also characterised by a change in temperature (which is a rise in temperature).
(v) Formation of precipitate: The chemical reaction between sulphuric acid and barium chloride solution is characterised by the formation of a white precipitate of barium sulphate.
BaCl2(aq) + H2 SO4(aq) → BaSO4(s) (ppt) + 2HCl(aq)
CHEMICAL EQUATION: A
chemical reaction can be expressed symbolically by using chemical equation.
Eg. 2H2 + O2 → 2H2O
or you can also say writing any chemical reaction with the help of symbols is known as chemical equation.
Balancing of a Chemical Reaction:
Conservation of mass
According to the law of conservation of mass, no atoms can be created or destroyed in a chemical reaction, so the number of atoms for each element in the reactants side has to balance the number of atoms that are present in the products side.
In other words, the total mass of the products formed in a chemical reaction is equal to the
total mass of the reactants participated in a chemical reaction.
Balanced chemical equation
The chemical equation in which the number of atoms of each element in the reactants side
is equal to that of the products side is called a balanced chemical equation.
Fe + H2O
→
Fe2O3 + H2 UNBALANCED CHEMICAL EQUATION
3Fe + 4H2O
→
Fe2O3 + 4H2 BALANCED CHEMICAL EQUATION
Types of chemical reactions
●Combination
●Decomposition
●Displacement ● Double displacement
●Redox
● Endothermic
●Exothermic
● Precipitation
Combination reaction: In a combination reaction, two elements or one element and one compound or two compounds combine to give one single product.
Eg. A + B → AB
H2 + Cl2 → 2HCl
element + element → compound
2CO + O2 → 2CO2
compound + element → compound
NH3 + HCl → NH4Cl
compound + compound → compound
Eg. CaO + H2O → Ca (OH)2 + HEAT, 2H2 +O2 → 2H2O , SO2 + O2 → SO3
Decomposition reaction
A single reactant decomposes on the application of heat or light or electricity to give two or more products.
Eg. AB → A + B
CaCO3 → CaO + CO2
Types of decomposition reactions:
a. Decomposition reactions which require heat - thermolytic decomposition or thermolysis.
Eg. 2Pb (NO3)2(s) heat → 2PbO(s) + 4NO2(g) + O2(g)
b. Decomposition reactions which require light - photolytic decomposition or photolysis.
Eg. 2AgCl(s) (white) −→ 2Ag(s) (grey) + Cl2(g) ( in the presence of sunlight)
c. Decomposition reactions which require electricity - electrolytic decomposition or electrolysis.
Eg. 2H2O(l) → 2H2(g) + O2(g)
Exothermic and Endothermic Reactions:
Exothermic Reaction: Reaction which produces energy is called Exothermic Reaction. Most of the decomposition reactions are exothermic.
Example:
Respiration is a decomposition reaction in which energy is released.
When
quick lime (CaO) is added to water, it releases energy.
Endothermic Reaction: A
chemical reaction in which heat energy is absorbed is called Endothermic
Reaction.
Example: Decomposition of calcium carbonate.
Displacement Reaction: The chemical Reaction in which an element displaces another element from its solution.
Eg. AB + X → XB + A
Fe(s) + CuSO4(aq) → FeSO4 + Cu(s)
– The nail becomes brownish in colour and the blue colour of Copper Sulphate
solution fade.
– Other examples Zn(s) + CuSO4 → ZnSO4 + Cu(s)
Pb(s) + CuCl2 → PbCl2 + Cu(s)
– Zinc and lead are more reactive elements than copper. They displace copper from its compounds.
Double Displacement Reaction: The reaction in which two different atoms or group of atoms are mutually exchanged.
Eg. AB + XY → AY + XB
eg. Na2SO4 + BaCl2 → BaSO4(s) + 2NaCl
Al2(SO4)3(aq) + 3Ca (OH)2(aq) → 2Al (OH)3(aq) + 3CaSO4(s)
NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l)
Ki + Pb (NO3)2 → PbI + K(NO3)2
Precipitation reaction
An insoluble compound called precipitate forms when two solutions containing soluble salts are combined.
For example, Pb (NO3)2(aq) + 2KI (aq) → 2KNO3(aq) + PbI2(↓) (s)(yellow)
AgNO3 + NaCl → AgCl + NaNO3
Neutralization Reaction: The reaction in which an acid reacts with a base to form salt and water by an exchange of ions is called Neutralization Reaction.
Example NaOH + HCl → NaCl + H2O
Oxidation and Reduction Reactions:
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.
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 in called to be Reduced.
Oxidizing agent:
The substance which gives oxygen for oxidation is called an Oxidizing agent.
The substance which removes hydrogen is also called an Oxidizing agent.
Eg. ZnO + C → Zn + CO
Reducing agent:
The substance which gives hydrogen for reduction is called a Reducing agent.
The substance which removes oxygen is also called a Reducing agent.
The reaction in which oxidation and reduction both take place simultaneously is called Redox reaction.
When copper oxide is heated with hydrogen, then copper metal and hydrogen are formed.
CuO + H2 → Cu + H2O
(i) In this reaction, CuO is changing into Cu. Oxygen is being removed from copper oxide. Removal of oxygen from a substance is called Reduction, so copper oxide is being reduced to copper.
(ii) In this reaction, H2 is changing to H2O. Oxygen is being added to hydrogen. Addition of oxygen to a substance is called Oxidation, so hydrogen is being oxidised to water.
The substance which gets oxidised is the reducing agent.
The substance which gets reduced is the oxidizing agent.
Why Magnesium ribbon is cleaned before burning in the air ?
The magnesium ribbon which we use usually has a coating of ‘magnesium oxide’ on its surface which is formed by the slow action of oxygen of air on it. So, before burning in air, the magnesium ribbon is cleaned by rubbing with a sand paper. This is done to remove the protective layer of magnesium oxide from the surface of magnesium ribbon so that it may readily combine with the oxygen of air (on heating).
What is the protective layer formed over magnesium ribbon when kept openly in air? magnesium oxide or basic magnesium carbonate? explain.
Magnesium is a very reactive metal, so when usually exposed to air it reacts with oxygen and a layer of magnesium oxide is deposited on it. Magnesium oxide is quite stable, so prevents further reaction with oxygen.
2Mg + O2 → 2MgO
Why the magnesium ribbon should be burned by keeping it as far as possible from the eyes?
The dazzling (very bright) white light given out during the burning of magnesium ribbon is harmful to the eyes. So, the magnesium ribbon should be burned by keeping it as far as possible from the eyes.
Observation on burning magnesium ribbon
Dazzling white light appears and white precipitate left.