CHEMICAL REACTIONS AND EQUATIONS

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CHEMICAL REACTIONS

• A chemical reaction is a process that leads �to the transformation of one set of  chemical substances to another.
• Chemical Reactions are associated with chemical change.
• Whenever a chemical change occurs chemical reaction is said to take place.
• In Chemical reactions a new product is formed.

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BURNING OF A MAGNESIUM RIBBON IN AIR AND COLLECTION OF MAGNESIUM OXIDE IN A WATCH-GLASS

FORMATION OF HYDROGEN GAS BY THE ACTION OF�DILUTE SULPHURIC ACID ON ZINC

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REACTIONS IN DAILY LIFE

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REACTION OF THE YEAR

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EVERY REACTIONS IS NOT TRY AT HOME

SIGNS OF CHEMICAL REACTIONS

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FEW CHEMICAL REACTION OF LEAD IN CHEMISTRY LAB

�WRITING A CHEMICAL EQUATION�A WORD EQUATION

• A WORD EQUATION describes chemical change using the names of the reactants and products.

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CHEMICAL EQUATION

• The simplest form of description of a chemical reaction in a shorter form is by
• writing it in the form of an equation. E.g.-

2Mg + O₂ 2MgO

• The reactants are written on the left side and product is written on the right side.
• An arrow is placed between them to show the direction of reaction.

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CHEMICAL EQUATION

•  CHEMICAL EQUATION :

 is the symbolic  representation of a chemical reaction in the form of � symbols and formulae.

• The substances that undergo chemical  change in the reaction  (magnesium and  
• oxygen) are the reactants.
• The new substances (magnesium oxide)  formed during the reactions is the product.The equation must represent known facts.
• The equation must contain the correct formulas for the reactants and products.
• The law of conservation of mass must be satisfied

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�SHOWING PHASES IN�CHEMICAL EQUATIONS

• The physical states of the reactants and products are mentioned along with their chemical formulae.
• The gaseous, liquid, aqueous, and solid states of reactants and products are represented by the notations (g), (l), (aq), and (s), respectively.
• Sometimes the reaction conditions , such as temperature, pressure, catalyst etc are indicated above or below the arrow in the equation

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SKELETAL EQUATION 

• Skeletal equation are those equation which  shows the reactant and product so formed without balancing them.
• It is also known as unbalanced equation.

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BALANCING OF CHEMICAL EQUATIONS

• According to the law of conservation of mass, total mass must be equal on the both sides of the equation.

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A chemical equation conveys a large number of informations as described below–

Qualitative information

It tells us about the names of the reactants and products.

Quantitative information

It expresses the number of molecules of reactants and products.

It expresses the relative number of moles of the reactants and the products.

It expresses the relative weight of the reactants and the products.

It expresses the volume of the gaseous reactants and products.

IMPLICATIONS OF A CHEMICAL EQUATION

Limitations of a Chemical Equation

A chemical equation does not gives the following informations :

. Conditions necessary for a chemical reaction : A chemical equation does not indicate about the conditions

necessary for the reaction i.e., about temperature, pressure or presence of a catalyst.

. Rate of reaction : It does not give any idea about the rate of the reaction whether slow or fast.

. Change of heat : An equation fails to give any indication whether the heat is absorbed or evolved.

. Concentration : It does not given any indication about the concentration f the reactants.

.. Nature of reaction : An equation fails to give any indication about the nature of reaction whether it is

reversible or irreversible reaction.

HOW TO BALANCE A EQUATION

• Step 1 :- Take an unbalanced equation with correct formula and symbols.
• Step 2 :- Draw boxes around all the chemical formulas.
• Step 3 :- Make an element inventory
• Step 4:- t is often convenient to start balancing with the compound
• that contains the maximum number of�atoms. It may be a reactant or a
• product.
• Step 5 :- Write numbers in front of each of the boxes until the inventory for each element is the same both before and after the reaction.
• Step 6 :- Now the equation is balanced

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TRY YOUR SELF

• Balance following chemical equation

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VALANCEY OF IONS

PRACTICE QS

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WRITE BALANCED CHEMICAL EQUATION

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TYPES OF CHEMICAL REACTIONS�

• COMBINATION REACTION
• •DECOMPOSTION REACTION
• •DISPLACEMENT REACTION
• •DOUBLE DISPLACEMENT REACTION
• •OXIDATION AND REDUCTION

/REOX REACTION

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COMBINATION REACTION

• In combination reaction , two or more substances combine to form a new
• substance
• For example –

• C(s) + O₂(g) 🡪 CO₂(g)

• 2H₂(g) + O₂(g) -🡪 2H₂O(g)

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They are generally exothermic reactions which involve evolution of heat during reaction.

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WHITE WASH

• A solution of slaked lime produced by the reaction previous is used for white washing walls.
• Calcium hydroxide reacts slowly with the carbon dioxide in air to form a thin

layer of calcium carbonate on the walls. Calcium carbonate is formed after two to three days of white washing and gives a shiny finish to the walls.

• It is interesting to note that the chemical formula for marble is also CaCO3

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DECOMPOSITION REACTION

• In a decomposition reaction , a single compound breaks down to produce two or more new substences.
• The decomposition reactions take place when energy is supplied in the form of like heat , electricity or light

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• When a substance decomposes on heating it is called thermal decomposition.

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ELECTROLYSIS

• When a substance is decomposed by passing electric current, the process is
• called electrolysis.
• • 2H₂O(I) 🡪 2H₂(g) + O₂(g)

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PHOTOCHEMICAL DECOMPOSITION

• When a substance is decomposed in presence of sunlight, it is called a

photochemical decomposition.

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EXOTHERMIC AND ENDOTHERMIC REACTIONS?

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DISPLACEMENT REACTION�•,

• In a displacement reaction a more reactive metal displaces or removes another metal from a compound. For example�

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REACTIVITY SERIES OF METALS

• Reactivity series of metals is �a series in which the metals �arranged in the decreasing �order of their reactivity.

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DOUBLE DISPLACEMENT REACTION

• The reaction in which two different atoms or group of atoms are displaced by other atoms or groups of atoms are double displacement reaction.
• For example

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• BaCl₂(aq) + H₂SO₄(aq) -🡪 BaSO₄ (s) + 2HCl (aq)
• Double displacement reaction can b

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PRECIPITATION REACTION

• Precipitation reactions occur when cations and anions in aqueous solution combine to form an insoluble ionic solid called a precipitate.

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OXIDATION-REDUCTION REACTIONS

• Oxidation is :- if a substance gains oxygen or loses hydrogen
• Reduction is :- If a substance loses oxygen or gains hydrogen
• Examples : C + O₂ ––––🡪CO₂
• _{(i) Addition of oxygen : When magnesium reacts with oxygen. It oxidises into mangesium oxide.} � Mg ₊ O_{2 ––––🡪} MgO
• _{(ii) Removal of hydrogen : Chlorine oxidises hydrogen sulphide to sulphur by the removal of hydrogen.}

Cl₂+ H₂S _{––––🡪} 2 HCl + S

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OXIDATION AND REDUCTION

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��CORROSION�

• 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.

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Reddish brown coating on iron

the green coating on copper

And The black coating on silver

RANCIDITY

• When fats and oils are oxidised, they become rancid and their smell and taste change
• Prevention of rancidity
• Antioxidants
• air tight containers
• nitrogen

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