Chemical Reaction and equation class 10 notes

Chemical reaction are the processes in which new substance with new properties are formed. During a chemical reaction, atoms of one element do not change into those of another. Only a rearrangement of atoms takes place in a chemical reaction.

We will now discuss reactants and products of a chemical reaction

The substances which take part in a chemical reaction are called reactants.
The new substances produced as a result of chemical reaction are called products.
In a chemical reaction, reactants are transformed into products. The products thus formed have properties which are entirely different from those of the reactants.

When a magnesium ribbon is heated, it burns in air with a dazzling white flame to form a white powder called magnesium oxide. Actually, on heating, magnesium combines with oxygen present in air to form magnesium oxide.

$magnesium_{(As .ribbon)}+oxgen_{(from.air)}\overset{heat}{\rightarrow}magnesium.oxide_{(white.powder)}$

The burning of magnesium in air to form magnesium oxide is an example of a chemical reaction before burning in air, the magnesium ribbon is cleaned by rubbing with a sand paper. This is done to remove the protective layer of basic magnesium carbonate from the surface of magnesium ribbon.

Chemical reaction can be carried out only in a science laboratory. A large number of chemical reaction keep on occurring in our daily life.

Ex:-Souring of milk(when left at room temperature during summer), Formation of curd from milk, cooking of food, Digestion of food in our body, Process of respiration, burning of fuel, burning of candle wax, and Ripening of fruits.

Characteristic of chemical Reaction

The conversation of reactants into products in a chemical reaction is often accompanied by some features which can be observed easily.

The important characteristic of chemical reaction are

Evolution of a gas
Formation of a precipitate
change in colour
Change in temperature
Change in state

1. Evolution of gas:-

When zinc granules react with dilute sulphuric acid then bubbles of hydrogen gas are produced, so the chemical reaction between zinc and dilute sulphuric acid is characterised by evolution of hydrogen gas.

The chemical reaction between sodium carbonate and dilute HCl acid is characterised by the evolution of carbon dioxide gas.

$Zn+H_{2}SO_{4}\rightarrow ZnSO_{4}+H_{2}$

Note:-A precipitate is a solid product which separate out from the solution during a chemical reaction

2. Formation of a precipitate:-

Some chemical reaction is characterised by the formation of a precipitate.

Ex:-When potassium iodide solution is added to a solution of lead nitrate, then a yellow precipitate of lead iodide and lead nitrate is characterised by the formation of a yellow precipitate of lead iodide.

Pb(NO3)2(aq) + 2KI(aq) → PbI2(s) + 2KNO3(aq)

Yellow precipitate of lead iodide

3. Change in colour:-

Some chemical reaction is characterised by a change in colour

1:-The chemical reaction between citric acid(C6H8O7) and purple coloured potassium permanganate (KMnO4) solution is characterised by a change in colour from purple to colourless.

2:-The chemical reaction between sulphur dioxide gas(SO2) and acidified potassium dichromate solution (K₂Cr₂O₇)by change in colour from orange to green.

4. Change in Temperature:-

1:-The chemical reaction between quicklime(CaO) and water to form Salked lime(Ca(OH)2) is characterised by a change in temperature(which is rise in temperature). It is an exothermic reaction(which means heat producing reaction)

2:-The chemical reaction between zinc granules and dilute sulphuric acid is also characterised by a change in temperature(which is rise in temperature)

3:-The chemical reaction between barium hydroxide and ammonium chloride to form barium chloride, ammonia and water is characterised by a change in temperature(which is fall in temperature). It is an exothermic reaction(which means heat absorbing reaction)

5. Change in state:-

The combustion reaction of candle wax is characterised by a change in state from solid to liquid and gas(because wax is a solid, water formed by the combination of wax is a liquid at room temperature where carbon dioxide produced by the combustion of wax is a gas)

Chemical Equation:-

The method of representing a chemical reaction with the help of symbols and formulae of the substances involved in it is known as a chemical equation.

$Zinc+Sulphuric.acid\rightarrow Zinc.Sulphate+Hydrogen$

Balanced and Unbalanced chemical equation:-

1. A balanced chemical equation has an equation number of atoms of different elements in its, Reactants and Products.

2. An unbalanced chemical equation has an unequal number of atoms of one or more elements in the reactants and products.

Note:-On the basis of law of conservation of mass, chemical reaction equation balanced.

To make equation more informative:

The chemical equation can be made more informative in three ways:-

By indicating the “physical states” of the reaction and products
By indicating the “Heat changes” taking place in the reaction.
By indicating the “condition “under which the reaction take place.

1. To indicate the physical states of reactants and products in an equation:-

Solid state is indicated by the symbol(s)
Liquid state is indicated by the symbol(l)
Aqueous solution(solution made in water) is indicating the symbol(aq)
Gaseous state is indicating by the symbol (g)
In reaction if solid product(precipitate) formed is written in short form as (ppt)
Gas librate(↑)
If heat for a reaction to take place, then the heat sign delta (△) is put over the arrow to the equation.

Example:-

2:-To indicate the Heat changes in an Equation:-

There are two types of reaction on the basis of heat changes involved 1. Endothermic 2. Exothermic.

Exothermic Reaction:-

Those reactions in which heat is evolved are known as exothermic. All the combination reaction are exothermic reaction.

$C(s)+O_{2}(g)\rightarrow CO_{2}(g)+Heat$

The burning of carbon in oxygen is an exothermic reaction

. $CH_{4}(g)+2O_{2}(g)\rightarrow CO_{2}(g)+2H_{2}O(g)+Heat$

The burning of natural gas is an exothermic reaction.

$C_{6}H_{12}O_{6}(aq)+6O_{2}(g)\rightarrow 6CO_{2}(g)+6H_{2}O(l)+Heat$

Respiration is an exothermic process because energy is produced during this process. This glucose then undergoes slow combustion by combining with oxygen in the cells of our body to produce energy in a process called respiration. In addition to other function, this energy maintain our body heat.

Endothermic Reaction:-

This reaction in which heat is absorbed are known as endothermic reaction. All decomposition reaction are endothermic reactions.

$N_{2}(g)+O_{2}(g)+heat\rightarrow 2NO(g)$

When nitrogen and oxygen are heated to a very high temperature(about 3000℃) they combine to form nitrogen monoxide and lots of heat is absorbed in this reaction.

$6CO_{2}(g)+6H_{2}O(l)\xrightarrow[Chlorpphyll]{sunlight}C_{6}H_{12}O_{6}(aq)+6O_{2}(g)$

Photosynthesis is an endothermic reaction.

3. To indicate the conditions under which the reaction takes place.

If heat is required for a reaction to take place, then the heat sign delta (Δ) is put over the arrow of the equation. If the reaction taken place in the presence of a catalyst, then the symbol or formula of the catalyst equation.

$2KClO_{3}(s)\xrightarrow[MnO_{2}]{\Delta }2KCl(s)+3O_{2}(g)$

When potassium chlorate(KClO3) is heated in the presence of manganese dioxide catalyst, It decomposes to form potassium chloride and oxygen gas. MnO2, used as a catalyst.

$CO_{2}(g)+2H_{2}(g)\xrightarrow[Zno+Cro_{3}]{300atm,300\degree C}CH_{3}OH(l)$

Methanol(or methyl alcohol) is manufactured from carbon monoxide and hydrogen.

$6CO_{2}(g)+6H_{2}O(l)\xrightarrow[Chlorpphyll]{sunlight}C_{6}H_{12}O_{6}(aq)+6O_{2}(g)$

The green plants make food by photosynthesis.

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