Chemical reactions and equations Science Notes
Importent points :
- Elements are divided into three classes i.e. metals, nonmetals and metalloids. When two or more elements combine chemically in a fixed proportion by weight, a compound is formed.
- The properties of a compound are altogether different from those of the constitutional elements.
- The number of electrons that an atom of an element gives away or takes up while forming an ionic bond, is called the valency of that element.
- While writing the molecular formulae of different compounds, the symbol of the radicals and their valence should be known.
- The number of the ions is written as subscript on the right of the symbol of the ion. By cross multiplication of valencies chemical formula is obtained
Chemical reactions :
→ During chemical reactions composition of the matter changes and that change remains permanent and during physical change only the state of matter changes and this change is often temporary in nature. Identify physical and chemical changes from the phenomena given in the following table.
|Phenomenon||Physical change||Chemical change|
|1. Transformation of ice into water||✓|
|2. Cooking of food||✓|
|3. Ripening of fruit||✓|
|4. Milk turned into curd||✓|
|5. Evaporation of water||✓|
|6. Digestion of food in the stomach|
|7. Size reduction of naphtha balls exposed to air||✓|
|8. Staining of Shahbadi or Kadappa tile by lemon juice||✓|
|9. Breaking of a glass object on falling from a height||✓|
Chemical reaction : A process in which some substances undergo bond breaking and are transformed into new substances by formation of new bonds is called a chemical reaction.
Rules writing chemical reaction :
→ Chemical equation : The representation of a chemical reaction in a condensed form using chemical formulae is called as the chemical equation.
Rules used in writing a chemical equation :
- The reactants are written on the left hand side (LHS), while the products are written on the right hand side (RHS).
- Whenever there are two or more reactants, a plus sign (+) is written between each two of them. Similarly, if there are two or more products, a plus sign is written between each two of them.
- Reactant side and product side are connected with an arrow (→) pointing from reactants to products. The arrow represents the direction of the reaction. Heat is to be given from outside to the reaction, it is indicated by the sign ∆ written above the arrow.
- The conditions like temperature, pressure, catalyst, etc., are mentioned above the arrow (→) pointing towards the product side.
- The physical states of the reactants and products are also mentioned in a chemical equation. The notations g. 1, s, and aq are written in brackets as a subscript along with the symbols /formulae of reactants and products.
- The symbols g. 1, s, and aq stand for gaseous, liquid, solid and aqueous respectively. If the product is gaseous, instead of (g) it can be indicated by an arrow↑pointing upwards.
- If the product formed is insoluble solid, then instead of (s) it can be indicated by an arrow↓pointing downwards.
- Special information or names of reactants/ products are written below their formulae.
Balancing a chemical equation :
→ In a chemical reaction, the number of atoms of the elements in the reactants is same as the number of atoms of those elements in the product, such an equation is called a balanced equation.
→ Example: AgNO3 + Nacl → AgCl + NaNO3
In the above reaction, the number of atoms of the elements in the reactants is same as the number of atoms of elements in the products.
Types of chemical reactions :
Types of chemical reactions :
- Combination reaction
- Decomposition reaction
- Displacement reaction
- Double displacement reaction.
→ Combination reaction : When two or more reactants combine in a reaction to form a single product, it is called a combination reaction.
→ An example of combination reaction : The ammonia gas reacts with hydrogen chloride gas to form the salt in gaseous state, immediately it condenses at room temperature and gets transformed into the solid state.
→ Decomposition reaction: The chemical reaction in which two or more products are formed from a single reactant is called decomposition reaction.
→ Examples of decomposition : Thermal decomposition: The reaction in which a compound is decomposed by heating it to a high temperature is called thermal decomposition At high temperature, calcium carbonate decomposes into calcium oxide and carbon dioxide.
→ Electrolytic decomposition : The reaction in which a compound is decomposed by passing an electric current through its solution or molten mass is called an electrolytic decomposition.
When an electric current is passed through acidulated water, it is electrolysed giving hydrogen and oxygen.
→ It is possible to produce hydrogen by decomposition of water by means of heat, electricity or light.
→ Displacement reaction : The reaction in which the place of the ion of a less reactive element in a compound is taken by another more reactive element by formation of its own ions, is called displacement reaction.
→ An example of displacement reaction : When zinc granules are added to the blue coloured copper sulphate solution, the zinc ions formed from zinc atoms take the place of Cu2+ ions in CuSO4, and copper atoms, formed from Cu2+ ions comes out i.e. the more reactive zinc displaces the less reactive Cu from copper sulphate.
→ Double displacement reaction: The reaction in which the ions in the reactants are exchanged to form a precipitate are called double displacement reaction.
→ An example of double displacement reaction : Solutions of sodium chloride and silver nitrate react with each other forming a precipitate of silver chloride and a solution of sodium nitrate.
→ White precipitate of AgCl is formed by exchange of ions Ag+ and Cl– between the reactants.
→ Exothermic process. The process in which heat is given out is called an exothermic process. When NaOH dissolves in water, there is evolution of heat leading to a rise in temperature.
NaOH(s) + H2Ol → NaOH(aq) + Heat
→ Endothermic process. The reaction in which heat is absorbed is called an endothermic process. When KNO3(s) dissolves in water, there is absorption of heat during the reaction and the temperature of the solution falls.
KNO3(s) + H2O(l) + Heat → KNO3(aq).
→ Rate of chemical reaction : Some reactions are completed in short time, i.e., occur rapidly, while some other require long time for completion, i.e, occur slowly. It means that the rate of different reaction is different.
- One or more chemical reactions take place during every chemical change.
- Strong acid and strong base react instantaneously.
- In our body, enzymes increase the rate of physiological reactions.
- If the rate of the chemical reaction is fast, it is profitable for the chemical factories.
- The rate of chemical reaction is important with respect to environment.
- The ozone layer in the earth’s atmosphere protects the life on earth from the ultraviolet radiation of the sun. The process of depletion or maintenance of this layer depends upon the rate of production or destruction of ozone molecules.
Factors affecting the rate of a chemical reaction :
- Nature of reactants
- Size of the particles of reactants
- Concentration of the reactants
- Temperature of the reaction
→ Oxidation : The chemical reaction in which a reactant combines with oxygen or loses hydrogen to form the product is called oxidation reaction. The chemical substances which bring about an oxidation reaction by making oxygen available are called oxidants or oxidizing agents. In the combustion of carbon, oxygen is an oxidant.
→ A variety of oxidants : K2 Cr2O7 / H2SO4, KMnO4 / H2SO4 are the commonly used chemical oxidants. Hydrogen peroxide (H2O2) is used as a mild oxidant. Ozone (O3) is also a chemical oxidant. Nascent oxygen is generated by chemical oxidants and it is used for the oxidation reaction.
O3 → O2 + [O]
H2O2 → H2O + [O]
K2 Cr2 O7 + 4H2SO4 → K2 SO4 + Cr2(SO4)3 + 4H2O + 3[O]
2KMnO4 + 3H2SO4 → K2SO4 + 2MnSO4 + 3H2O + 5[O]
→ Nascent oxygen is a state prior to the formation of the O2 molecule. It is the reactive form of oxygen and is represented by writing the symbol as [O].
→ Reduction : The chemical reaction in which a reactant gains hydrogen or loses oxygen to form the product is called reduction. The chemical substance that brings about reduction is called a reductant, or a reducing agent.
→ When hydrogen gas is passed over black copper oxide a reddish coloured layer of copper is formed.
CuO + H2 → Cu + H2O
→ Redox reaction : The reaction which involves simultaneous oxidation and reduction is called an oxidation-reduction or redox reaction.
- In a redox reaction, one reactant gets oxidised while the other gets reduced during a reaction.
- Redox reaction = Reduction + Oxidation
- In redox reaction, the reductant is oxidized by the oxidant and the oxidant is reduced by the reductant.
MnO2 + 4HCl → MnCl2 + 2H2O + Cl2↑
→ Corrosion : Due to various components of atmosphere, oxidation of metals takes place, consequently resulting in their damage. This is called ‘corrosion’. Iron rusts and a reddish coloured layer is collected on it. This is corrosion of iron. This is also termed as rusting of iron. Its formula is Fe2 O3.H2O.
→ Prevention of corrosion : Corrosion damages buildings, bridges, automobiles, ships, iron railings and other articles made of iron. It can be prevented by using an anti-rust solution, coating surface by the paint by processes like galvanising and electroplating with other metals.
→ Rancidity : Fats and oils in food, is kept for a long time, gets oxidised, it is found to have foul odour called rancidity.