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Chemical Reactions and Equations, Types of Chemical Equations, To Make…
Chemical Reactions and Equations
Types of changes
Physical Change
The process in which no new substances with new properties are formed is known as physical change. Example tearing of paper
Chemical Change
The processes in which new substances with new properties are formed are known as chemical change or chemical reaction.
Chemical Reaction
When a chemical change takes place a chemical reaction takes place. Chemical reactions involve the making and breaking of bonds between atoms to produce new substances.No new atoms appear or disappear during a chemical reaction
The new substances produced as a result of a chemical reaction are called products. The substances that take part in a chemical reaction are called reactants.
The burning of magnesium in the air to form magnesium oxide is an example of a chemical reaction.
Before burning in air, the magnesium ribbon is cleaned by rubbing with sandpaper.
This is done to remove the protective layer of basic magnesium carbonate from the surface of the magnesium ribbon.
Mg + O2 → 2MgO
Substances which take part in a chemical reaction are called reactants.
Example: Mg and O2.
New substance formed after a chemical reaction is called a product.
Example: MgO
Characteristics of Chemical Reactions
Evolution of gas: The chemical reaction between zinc and dilute sulphuric acid is characterised by the evolution of hydrogen gas.
Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g) ↑
The chemical reaction between citric acid and purple coloured potassium permanganate solution is characterised 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.
The combustion reaction of candle wax is characterised by a change in state from solid to liquid and gas (becoz wax is a solid, water formed by the combustion is 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 characteristics.
The chemical reaction between quicklime water to form slaked lime is characterized by a change in rise temperature
The chemical reaction between zinc granules and dilute sulphuric acid is also characterised by a change in temperature
The chemical reaction between sulphuric acid and barium chloride solution is characterised by the formation of a white precipitate of barium sulphate.
BaCl2(aq) + H2SO4(aq) → BaSO4(s) (ppt) + 2HCl(aq)
Combination Reaction
: Reactions in which two or more reactants combine to form one product are called Combination Reactions.
A general combination reaction can be represented by the chemical equation given here:
A + B → AB
When magnesium is burnt in the air (oxygen), magnesium oxide is formed. In this reaction, magnesium is combined with oxygen.
Mg(s) + O2(g) → 2MgO(s)
Magnesium + Oxygen → Magnesium Oxide
When carbon is burnt in oxygen (air), carbon dioxide is formed. In this reaction, carbon is combined with oxygen.
C(s) + O2(g) → CO2(g)
Carbon + Oxygen → Carbon dioxide
Decomposition Reaction
Reactions in which one compound decomposes in two or more compounds or elements are known as Decomposition Reaction.
A decomposition reaction is just the opposite of a combination reaction.
AB → A + B
When calcium carbonate is heated, it decomposes into calcium oxide and carbon dioxide.
CaCO3(s) heat 🡪 CaO(s) + CO2(g)
When ferric hydroxide is heated, it decomposes into ferric oxide and water
2Fe(OH)3(s) hear→ Fe2O3(s) + 3H2O(l)
The decomposition of a substance on heating is known as Thermal Decomposition.
Example: 2Pb(NO3)2(s) heat→ 2PbO(s) + 4NO2(g) + O2(g)
Electrolytic Decomposition: Reactions in which compounds decompose into simpler compounds because of the passing of electricity are known as Electrolytic Decomposition.
This is also known as Electrolysis.
Example: When electricity is passed in water, it decomposes into hydrogen and oxygen
2H2O (l)+ Electric Current 🡪 2H2(g) + O2 (g)
Photolysis or Photo Decomposition Reaction: Reactions in which a compound decomposes because of sunlight are known as Photolysis or Photo Decomposition Reaction.
Example: When silver chloride is put in sunlight, it decomposes into silver metal and chlorine gas.
2AgCl(s) (white) Sunlight→ 2Ag(s) (grey) + Cl2(g)
Displacement reactions
The chemical reactions in which a more reactive element displaces a less reactive element from a compound is known as Displacement Reactions. Displacement reactions are also known as Substitution Reaction or Single Displacement/ replacement reactions.
A + BC → AC + B
Displacement reaction takes place only when ‘A’ is more reactive than B. If ‘B’ is more reactive than ‘A’, then ‘A’ will not displace ‘C’ from ‘BC’ and reaction will not be taking place.
When zinc reacts with hydrochloric acid, it gives hydrogen gas and zinc chloride.
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
When zinc reacts with copper sulphate, it forms zinc sulphate and copper metal.
Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
Types of Chemical Equations
Balanced Chemical Equation
Unbalanced Chemical Equation
Combination Reaction
Decomposition Reaction
Thermal Decomposition:
Electrolytic Decomposition
Photolysis or Photo Decomposition Reaction
Thermal Decomposition
Displacement Reaction
Double Displacement Reaction
Precipitation Reaction
Neutralization Reaction
Oxidation and Reduction Reactions
To Make Equations More Informative
Gaseous state is represented by symbol (g).
Liquid state is represented by symbol (l).
Solid state is written by symbol (s).
Aqueous solution is written by symbol (aq).
Writing the condition in which reaction takes place: The condition is generally written above and/or below the arrow of a chemical equation.