Revision Notes

A chemical reaction involves a chemical change in which the molecules of one substance break apart and join together with those of another substance to create a different compound having entirely new properties.

• The atoms and molecules which take part in a chemical reaction are called reactants.
• The new substances (the atoms and molecules) produced as a result of chemical reaction are called products.
• In a chemical reaction, reactants are converted into products. No new atoms are created, and no atoms are destroyed. In other words, only rearrangement of atoms takes place in a chemical reaction.

For example:
Hydrogen gas (H₂) can react (burn) with oxygen gas (O₂) to form water (H₂O). The chemical equation for this reaction is written as:

Here, 2 molecules of hydrogen gas combine with one molecule of oxygen gas to form two molecules of water.

Physical change: Physical change is a process in which the substance experiences change in its physical properties like shape, colour, size, appearance, state (i.e. solid, liquid, gas), etc., without making any change in their chemical composition. Eg., melting of ice into water, dissolving sugar in water.

Chemical change: Chemical change is defined as the process in which the atoms of one or more substances are broken or combined to form a new substance. Eg., Rusting of iron, fermenting of milk into curd.

DETERMINATION OF A CHEMICAL REACTION:

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

• Evolution of a gas
• Formation of a precipitate
• Change in colour
• Change in temperature
• Change in state
• New substances are formed

Precipitate: A precipitate is a solid insoluble product which separates out from the solution during a chemical reaction. E.g., When an aqueous solution of lead nitrate Pb(NO₃)₂ is added to an aqueous solution of potassium iodide (KI), a yellow precipitate of lead iodide (PbI₂) is formed along with some colorless potassium nitrate KNO₃ (which stays in solution):

Based on energy differences between reactants and products the chemical reactions are classified into two types. They are exothermic reaction and endothermic reaction:

• An exothermic process releases heat and causes the temperature of the immediate surroundings to rise. E.g., rusting iron, chemical reaction between quicklime and water to form slaked lime is characterized by release of heat energy.
• An endothermic process absorbs heat and cools the surroundings. E.g., producing sugar by photosynthesis, formation of nitric oxide from nitrogen and oxygen.

CHEMICAL EQUATIONS

• A chemical equation is a symbolic representation of the reactants and the products using their chemical formulae. Chemical equations are a way of describing a chemical reaction.

Features of a chemical equation:
For example,

Here, A and B are the reactants and C and D are the products.

• The reactants are written on the left side of the arrow and the products are written on the right side.
• The arrow shows the direction of the reaction (the direction of change from reactants to products).
• The optimum conditions that are required for the reaction to take place are generally written above or below the arrow mark.
• The number of atoms of each element involved in the reaction should remain constant.
• The number of molecules of each chemical is represented by a numerical coefficient, which is always a whole number.
• An arrow pointing downward indicates that the product is a precipitate (↓) and an arrow pointing upward shows that one of the products is a gaseous one (↑).
• The physical state of the chemicals may be written near the formula by using short forms such as (s) —— solid ; (l) —— liquid ; (g) —— gas or vapor and (aq. ) —— aqueous solution.

There are two types of chemical equations namely word equation and symbol equation

• Word equations are a way of explaining chemical reactions using the names of substances involved. No symbols or numbers are used in word equations.
• Symbol equation uses the chemical formulae of the reactants and products with their elemental symbols to represent them in a chemical reaction.

A simple example of a chemical reaction is when a magnesium ribbon is burnt in oxygen; it gets converted to magnesium oxide

Here is the equation in both words and symbols:

A chemical equation represents a chemical reaction. So reactants are on the left hand side (LHS) of arrow and there is written (+) Sign between them. Similarly, Products are on the Right hand side (RHS) of arrow with a plus (+) sign between them.

Chemical equation can be divided into two types,
(i) Balanced chemical equation
(ii) Unbalanced chemical equation

Balanced chemical equation-

• A balanced chemical equation has an equal number of atoms of different elements in the reactants and products.

For example,
Zinc metal reacts with dilute sulphuric acid to form zinc sulphate and hydrogen gas.

As there are an equal number of atoms of different elements in the reactants and products, so the above chemical equation is a balanced equation. Complete balanced chemical equation gives us information about the reactants, products and the conditions under which a reaction has taken place.

• Unbalanced chemical equation – An equation which has an unequal number of atoms of one or more elements in reactants and products is called unbalanced chemical equation.
• An unbalanced chemical equation is called a skeletal equation.

For example,
Hydrogen reacts with oxygen to form water

In the above chemical equation, there are an unequal number of oxygen atoms in reactants and products, so it is an unbalanced equation.

A chemical equation is said to be balanced when the number of atoms of each elements on the reactant side is equal to the number of atoms of corresponding elements on the product side. Unbalanced chemical equation has to be made balanced to justify the law of conservation of mass.

The Law of Conservation of Mass states that when a chemical reaction occurs, the mass is conserved. In other words, the mass of reactants and products are equal and mass is neither generated nor destroyed during a chemical reaction.

Balancing of chemical Equations

The process of making the number of different types of atoms equal on both the sides of an equation is called balancing of equation.

TYPES OF CHEMICAL REACTIONS

Chemical reactions involve the breaking and making of bonds between atoms to produce new substances

• Combination reactions:
  Those reactions, in which two or more substances combine to form a single substance, are called combination reactions.
• In a combination reaction, two or more elements can combine to form a compound; two or more compounds can combine to form a new compound; or an element and a compound can combine to form a new compound.
• General formula is A + B    →   AB
  Examples:
  Calcium oxide (CaO) reacts vigorously with water and form a single product, calcium hydroxide (slaked lime).
  CaO(s) + H₂O (l)   →   Ca(OH)₂ (s)  + Heat
• Decomposition reaction: In a decomposition reaction, a single substance decomposes to two or more simpler substances.
  – General formula is AB   →  A + B
  – Decomposition reactions are the opposite of combination reactions.
  – Decomposition reaction requires energy in the form of heat, light or electricity

Ferrous sulphate crystals (FeSO₄.7H₂O) lose water when heated and the colour of the crystals changes. It then decomposes to ferric oxide (Fe₂O₃), sulphur dioxide (SO₂) and sulphur trioxide (SO₃). Ferric oxide is a solid, while SO₂ and SO₃ are gases.

TYPES OF DECOMPOSITION REACTION

– Decomposition reaction always occurs with the inputs of energy and this energy can be in the form of heat, electric current or light.

• Thermal decomposition- When a decomposition reaction is carried out by heating, it is called ‘thermal decomposition’.
  
  Example: Decomposition of calcium carbonate to calcium oxide and carbon dioxide on heating is an important decomposition reaction
  
  
• Electrolytic decomposition- When a decomposition reaction is carried out by using electric current, it is called electrolytic decomposition.
  
  Example: When electric current is passed through acidified water, it decomposes to give hydrogen gas and oxygen gas. electricity
  
  This decomposition reaction takes place by the action of electricity. It is called electrolysis of water.
• Photolytic decomposition- When a decomposition reaction is carried out by using light energy, it is called photolytic decomposition
  
  Example: When silver chloride is exposed to light, it decomposes to form silver metal and chlorine gas.
  
  – white silver chloride turns grey in sunlight.

Displacement Reaction

– Displacement reaction occurs when a more reactive element displaces a less reactive element from its compound.

– General formula for displacement reaction

• Displacement reaction is also called substitution reaction.
  Example:
  When as strip of zinc metal is placed in copper sulphate solution, then zinc sulphate solution and copper are obtained.
  
  – The reactivity series is a list of metals arranged in the decreasing order of reactivity.
  – The most reactive metal is placed at the top and the least reactive metal is placed at the bottom.

Double Displacement Reaction

– A chemical reaction in which two atoms or a group of atoms (ions) are exchanged between the reactants to form new compounds are called double displacement reactions.

– General formula for double displacement reaction: XY + ZA → XZ + YA

– Example:
When barium chloride solution is added to sodium sulphate solution, then a white precipitate of barium sulphate is formed along with sodium chloride solution.

-An exchange of ions takes place in this reaction.

-In this reaction, barium sulphate is formed as a white, insoluble solid called precipitate which separates out suddenly from the solution.

• Any reaction in which an insoluble solid called precipitate is formed that separates from the solution is called a precipitation reaction.
• General formula for precipitation reaction:
  A + soluble salt B → precipitate + Soluble salt C

Oxidation and Reduction reactions:

• Oxidation is a chemical process in which a substance gains oxygen or loses hydrogen.

– If a substance gains oxygen or loses hydrogen during a reaction, it is said to be oxidized

– The substance which gives gains oxygen or loses hydrogen during a reaction, the substance is called the oxidizing agent.

– An oxidizing agent gets reduced

– Example:

Sulphur loses hydrogen and gets oxidized.

• Reduction is a chemical process in which a substance gains hydrogen or loses oxygen.

-If a substance gains hydrogen or loses oxygen during a reaction, it is said to be reduced.

– The substance (reactant) which is responsible for removing oxygen or gaining hydrogen for reduction during a reaction is called the reducing agent.

– A reducing agent gets oxidised.

– Examples:

(i) CuO + H₂ → Cu + H₂O

Copper loses oxygen and gets reduced

(ii)H₂S + Cl₂ → 2 HCl+ S

Sulphur loses hydrogen and gets reduced.

• Redox reaction: When oxidation and reduction takes place simultaneously in a reaction, it is known as redox reaction.

– Example:

ZnO + C → Zn + CO

Here, ZnO loses oxygen and gets reduced to Zn. Similarly, carbon gains oxygen and gets oxidized to CO

• Effect of oxidation reactions in everyday life:

Oxidation has damaging effect on metals as well as on food. There are two common effects of oxidation reactions which we observe in daily. These are:

• Corrosion of metals:

When a metal is attacked by substances around it such as moisture, acids, etc., it is said to corrode and this process is called corrosion.

– The black coating on silver and the green coating on copper are other examples of corrosion.

Prevention:

– By galvanization

– By painting metal surfaces

– By regularly oiling or greasing the metal surfaces.

Galvanisation: Coating a thin layer of Zinc on metal surfaces by electroplating.

• Rancidity: When fats and oils are oxidised, they become rancid and their smell and taste change.

Prevention:

– Antioxidants are added to foods containing fats and oils.

– Keeping food in air tight containers help to slow down oxidation.

– By removing oxygen gas and filling nitrogen gas at the time of packing the food.

– Rancidity can be retarded to a certain extent by storing food away from heat and light.

– By keeping food in the refrigerator which will eventually slow down oxidation.