CHEMICAL REACTIONS
AND EQUATIONS
CLASS X
JODHAMAL PUBLIC SCHOOL
CHEMICAL CHANGE ACCOMPANIED BY CHEMICAL
REACTIONS
Chemical Change : The change in which one or more new substances with new
physical and chemical properties are formed are called chemical change.
Chemical Reactions: They are chemical changes in which reactants transform into
products by making or breaking bonds (or both) between different atoms
Reactants (LHS) ----------------------- Products (RHS)
CHARACTERISTIC FEATURES OF CHEMICAL
CHANGE
• Change in color
• Change in physical state
• Change in temperature
• Evolution of gas
• Formation of precipitate
1. CHANGE IN COLOR
Word Equation : Lead nitrate + Potassium iodide ------ Lead iodide + Potassium nitrate
Symbol Equation : Pb(NO3)2 + 2KI ---- PbI2 + 2KNO3
Colorless
Colorless
Yellow ppt
CHANGE IN COLOR :
• Silver changes its color to slightly black in due course of time: due to the formation of
Ag2S as a result of the reaction between silver and hydrogen sulphide in the air.
2Ag
+
H2S ---------------- Ag2S + H2
2. CHANGE OF PHYSICAL STATE
• 2Mg (s) + O2 (g) --- 2MgO (s)
• Petrol (l)----------- H2O (g) + CO2 (g)
• Wax (s) ------------ H2O (g) + CO2 (g)
ACTIVITY:
• 2Mg (s) + O2 (g) --- 2MgO (s)
• Mg ribbon has generally a coating of Basic Magnesium carbonate on its surface . It is a mixture of
Magnesium hydroxide and Magnesium carbonate and this coating gets easily deposited on the
surface of the metal by the action of moist air. The layer is stable and prevents further reaction of
Mg with oxygen. In order to clean the surface and remove the layer it is rubbed with sandpaper to get
its original lustre and reactivity.
3. EVOLUTION OF GAS:
• Zn (s) +
H2SO4
---------------------------------
Granulated Zinc dilute Sulphuric acid
• CaCO3 (s)
-------------
Calcium carbonate
CaO (s)
ZnSO4
Zinc Sulphate
+
Calcium oxide
CO2
Carbon dioxide
+
H2
Hydrogen
ACTIVITY
Experiment : Zn (s) + dilute H2SO4 --------------- ZnSO4 + H2
• Zn dust provides more surface area for reaction as compared to granulated zinc. The zinc dust makes the reaction faster
with a faster evolution of H2 gas,
Observations:
• Bubbles of gas evolve accompanied by brisk effervescence
• Flask on touching appears hot as the reaction is exothermic
• The solution in the flask is colorless while a black deposit formed around the granules.
Inference :
• The gas can be detected by bringing a burning splinter near the soap bubble. It burns with a pop sound
• The gas is lighter than air.
• It is insoluble in water and can displace water and thus collected by downward displacement of water
4. CHANGE IN TEMPERATURE
• Exothermic Reactions: accompanied by evolution of heat
CaO + H2O ---------- Ca(OH)2 + heat energy
Zn + dil. H2SO4 ------------ ZnSO4 + H2 + heat energy
• Endothermic Reactions: accompanied by absorption of heat
CaCO3 (s) + heat energy ------------- CaO(s) + CO2
(turns limewater milky)
2NH4Cl + 2Ba(OH)2 + heat energy ------------- BaCl2 + 2NH4OH
5. FORMATION OF PRECIPITATE:
• Pb(NO3)2 + 2KI ---- PbI2 + 2KNO3
Yellow ppt
• NaCl (aq) + AgNO3 ----- AgCl + NaNO3
White ppt
• BaCl2 (aq) + Na2SO4 (aq) ------- BaSO4
White ppt
+
2NaCl (aq)
CHEMICAL EQUATIONS
The representation of a chemical reaction in terms of symbols and chemical formulae of
the reactants and products is known as chemical equation.
Chemical Equations have been framed to describe chemical reactions.
Chemical Equations
Word Equations
Sodium + Chlorine ----Sodium Chloride
Symbol Equations
Balanced
Unbalanced
2Na + Cl2---- 2NaCl
BALANCING OF CHEMICAL EQUATIONS
 According to Law of Conservation of Mass the total mass of the products formed in a
chemical reaction is equal to the total mass of the reactants participated in a
chemical reaction.
 To satisfy the Law of Conservation of Mass a chemical reaction always need to be
balanced.
MAKING CHEMICAL EQUATIONS FOR
INFORMATIVE
• By mentioning physical state of the reactants and products through state symbols
CaCO3 (s) + HCl (aq) ------------------ CaCl2 (aq) + H2O (l)+ CO2 (g)
AgNO3 (aq) + NaCl (aq) --------------AgCl (s) + NaNO3 (aq)
• By mentioning the concentration of the acids used in the reactions
Zn (s) + dilute H2SO4 --------------- ZnSO4 + H2
Cu (s) + conc. 4HNO3 -------------- Cu(NO3)2 (aq)+ 2H2O (l) + 2NO2
MAKING CHEMICAL EQUATIONS FOR
INFORMATIVE
• By writing the reaction conditions – Temperature, Pressure, Catalyst
N2 (g)+ 3H2 (g) ----------------------------------------- 2NH3 (g)
Temperature – 450 -500oC, Pressure- 200-400 atm Catalyst: Fe/Mo
CO(g) + 2H2 (g) -------- CH3OH (l)
Pressure : 340 atm
• By determining the heat exchange accompanying the chemical reaction
CH4(g) + 2O2 (g) ------------------ CO2 (g) + 2H2O (l) +heat energy
C6H12O6 + 6O2
----------------------- 6CO2 + 6H2O + heat energy
MAKING CHEMICAL EQUATIONS FOR
INFORMATIVE
• By mentioning the evolution of heat through arrow pointed upwards
• By mentioning the formation of precipitate through pointed downwards
INFORMATION NOT CONVEYED BY CHEMICAL
EQUATIONS
• It does not tell us about the feasibility of the reaction
Justification: Cu + H2SO4 ------ CuSO4 + H2
This reaction though seem to be feasible but actually doesnot occur because based on
reactivity series copper is placed below hydrogen and is not able to liberate hydrogen from
sulphuric acid.
• The rate of the reaction cannot be predicted from a chemical equation
TYPES OF CHEMICAL REACTIONS
• Combination Reactions
• Decomposition Reactions
• Displacement Reactions
• Double Displacement Reactions
• Oxidation and Reduction Reactions
1. COMBINATION REACTIONS
• Two or more elements or compounds combine to form a single substance as a
product
• These reactions can be
a. Between two or more elements
b. Between two or more compounds
c.
Between an element and a compound
COMBINATION REACTIONS:
BETWEEN TWO OR MORE ELEMENTS
• Formation of Sodium chloride :
2Na(s) + Cl2 (g) ---------- 2NaCl (s)
• Formation of Hydrogen chloride :
H2 (g) + Cl2 (g) ------------ 2HCl (g)
[ Reaction occurs in the presence of sunlight]
COMBINATION REACTIONS
BETWEEN TWO OR MORE ELEMENTS
• Formation of Ferrous sulphide :
Fe(s) +
S (s)
(yellow powder)
--------------
FeS (s)
( Black mass)
COMBINATION REACTIONS
BETWEEN TWO OR MORE ELEMENTS
• S (s) + O2 (g) ---------- SO2 (g)
COMBINATION REACTIONS:
BETWEEN TWO OR MORE ELEMENTS
• Formation of Ferric Chloride: When vapors of dry chlorine gas passed through iron
in the form of fine wool brown mass of ferric chloride is formed.
2Fe(s) + 3Cl2 (g) ---------------------- 2FeCl3 (s)
[brown mass]
COMBINATION REACTIONS
BETWEEN TWO OR MORE ELEMENTS
• Combustion Reactions :
a.
Combustion of coal -- C(s) + O2(g) ----------------- CO2 (g)
b.
Combustion of hydrogen – 2H2(g) + O2(g) ------------- 2H2O (l)
The water formed is initially in the gaseous state. It is later cooled to become liquid state
COMBINATION REACTION:
BETWEEN TWO OR MORE COMPOUNDS
• CaO(s) + H2O (l) ---------------- Ca(OH)2 (aq) + heat energy
(Quicklime)
(Slaked lime)
If water is dripped slowly onto a lump of calcium oxide, a hissing sound is heard as excess water
is turned to steam by the heat produced in the reaction. Slaked lime is used for white
washing the walls which makes them shine white within 2 to 3 days. It is due to the formation of a
thin layer of calcium carbonate/marble (CaCO3) as a result of slow reaction of Ca(OH)2 and CO2
present in the air.
Ca(OH)2 (aq) + CO2(g) ---------------------- CaCO3(s) + H2O (l) + heat energy
COMBINATION REACTION:
BETWEEN TWO OR MORE COMPOUNDS
• Formation of Ammonium chloride:
NH3(g) + HCl (g) --------------- NH4Cl (s)
[ white solid]
COMBINATION REACTION:
BETWEEN TWO OR MORE COMPOUNDS
• Formation of Sulphuric acid
SO3(g) + H2O(l) ----------------- H2SO4(l) + heat energy
• Formation of sodium hydroxide :
Na2O (g) + H2O (l) ------------------ 2NaOH (aq)
COMBINATION REACTION:
BETWEEN AN ELEMENT AND A COMPOUND
• 2NO(g) +
Colorless
O2(g) -------------------------
Colorless
2NO2(g)
[Reddish brown]
2. DECOMPOSITION REACTIONS
The reactions in which a single chemical compound decomposes or splits up into two
or more simpler substances (elements or compounds) is known as decomposition
reactions
Three main types of decomposition reactions include
• Thermal decomposition
• Electrolytic decomposition
• Photochemical decomposition/ Photolysis
THERMAL DECOMPOSITION
• Decomposition of Ferrous sulphate crystals (GREEN VITRIOL)
Experiment I : Crystals of Ferrous sulphate (GREEN) taken in a dry boiling tube and gently
heated over flame of a gas burner.
Observation and Results I: The light green colored crystals on gentle heating loses water of
crystallization and changes to anhydrous dirty white salt of ferrous sulphate
FeSO4, 7H2O(s) ------------------FeSO4 (s) + 7H2O(g)
Green
Dirty white
THERMAL DECOMPOSITION
• Decomposition of Ferrous sulphate crystals (GREEN VITRIOL)
Experiment II: The boiling tube was further strongly heated for another 10- 15 mins
Observation and Results II: The dirty white salt further decomposes to give a brown residence. This is
accompanied by the liberation of gases Sulphur dioxide and Sulphur trioxide with pungent and
suffocating smell.
2FeSO4 (s) ----------------- Fe2O3 (s) + SO2 (g)
Dirty white
Brown
+ SO3(g)
THERMAL DECOMPOSITION
• Decomposition of Calcium carbonate when heated strongly :
CaCO3 (s) -------------------
CaO (s)
+
CO2
(used in manufacture of cement and glass industries )
• Decomposition of lead nitrate when heated:
2Pb(NO3)2 -------------- 2PbO(s) +
White
Yellow
4NO2(g) +
O2(g)
reddish brown supporter of combustion
THERMAL DECOMPOSITION
Decomposition of Mercuric oxide :
• The amorphous orange powder changes to dark red and appears almost black.
• It gives off a colorless, odorless gas which relights a glowing wooden splint.
• A silvery mirror like surface is formed near the mouth of the test tube. On scratching the surface it
forms tiny droplets of mercury.
2HgO (s) ----------- 2Hg (s) + O2 (g)
THERMAL DECOMPOSITION
• Decomposition of Aluminium hydroxide :
2Al(OH)3 -----∆------ Al2O3 (s) + 3H2O (l)
THERMAL DECOMPOSITION
• Decomposition of Ammonium dichromate: This yellow powder when strongly heated decomposes to a
mixture of chromium oxide (grey in color) with the liberation of nitrogen gas and water (in the form of steam)
• At very high temperature generated during the decomposition reaction it immediately changes to green vapors
which are released along with steam it seems as if a volcano has erupted and is quite often termed as chemical
volcano
(NH4)2Cr2O7 (s) ------------------- Cr2O3 (s) + N2 (g) + 4H2O (g)
THERMAL DECOMPOSITION
• Decomposition of Potassium chlorate: When 2KClO3 (white) is strongly heated it
decomposes to give Potassium chloride (white ) and oxygen. If MnO2 used as a catalyst
then the reaction takes place at a lower temperature
2KClO3 (s) --------------- 2KCl(s) + 3O2 (g)
ELECTROLYTIC DECOMPOSITION :
(USED FOR METAL EXTRACTION)
• On passing electricity through acidulated water it decomposes into hydrogen and oxygen
2H2O(l) ------------------- 2H2 (g) + O2(g)
• Bubbles start forming at both the
electrodes and gases start collecting
in the test tube
• Once the test tubes are filled with
respective gases remove them one
by one carefully
PHOTOCHEMICAL DECOMPOSITION
• Decomposition of AgCl and AgBr in the presence of sunlight:
2AgCl (s)
(White)
2AgBr (s)
( Light yellow)
--------------------- 2Ag (s) + Cl2 (g)
(Grey)
–------------------- 2Ag (s) + Br2 (g)
(Grey)
Both AgCl and AgBr are used in black and white photography
• Decomposition of hydrogen peroxide (Bleaching agent):
2H2O2 (l) --------------------- 2H2O (l) + O2 (g)
This is why they are stored in dark bottles.
• Decomposition of hydrogen iodide
2HI (g) --------------------- H2 (g) + I2(g)
DECOMPOSITION REACTIONS ARE ENDOTHERMIC
DISPLACEMENT REACTIONS
• Those reactions in which a more reactive element replaces a less reactive element from
the salt solution of the less reactive element are called displacement reactions
• Both metals and non metals take part in displacement reactions
DISPLACEMENT REACTIONS
• More reactive metal/non metal may displace the less reactive metal/non metal
from its compound in the reaction.
• In this reaction C displaces A from AB to form BC. C is more reactive than A
ACTIVITY 1.7
Test tube A: Blue color of CuSO4 remains the same
Test tube B:
• Blue color of CuSO4 changes to light green.
• Iron nail dipped in CuSO4 becomes reddish brown in color
Fe (s) + CuSO4 (aq) --------------------- Cu (s) + FeSO4 (aq)
More reactive Fe displaces less reactive Cu from its salt solution.
REACTIVITY SERIES/ACTIVITY SERIES:
• The arrangement of metals in a vertical column in the decreasing order of their reactivity
is called reactivity series of metals
TYPES OF DISPLACEMENT REACTIONS
1.
Single Displacement Reactions: (Non-ionic) : Uncombined single element displaces other
element present in the compound. It can be of two different types:
a.
Cation Displacement - Metal displacement
b.
Anion Displacement – Non-metal displacement
2. Double Displacement Reactions: (Mostly ionic): It can be of three types
a.
Precipitation Reactions
b.
Gas formation
c.
Neutralisation
CATION DISPLACEMENT:
• Thermite Reaction: It is the reaction of metal oxide (Ferric oxide) with Aluminium oxide (more reactive)
which acts as a reducing agent. Since the reaction is highly exothermic, it is called thermite reaction.This reaction
isused to join railway tracks or cracked machine parts.
Fe2O3 (s) + 2Al (s) ------------------ 2Fe (l) + Al2O3 (s)
• If a strip of Mg metal is placed in Copper sulphate solution the blue color of the CuSO4 disappears and the Mg
metal turns brown as displaced copper is deposited on it. Mg is above Cu and hence more reactive than Cu.
Mg (s) + CuSO4 (aq) -------------------- MgSO4 (aq) + Cu (s)
CATION DISPLACEMENT REACTION
• Zn(s) + CuSO4 (aq) ---------------- ZnSO4( aq) + Cu (s)
Zn being more reactive displaces Cu from its salt solution. A reddish brown deposit of Cu is formed on the
surface of Zn due to the displacement of the less reactive Cu by the more reactive Zn metal in solution
•
Pb (s)
+
(Greyish black)
CuCl2 (aq) --------------- PbCl2 (aq)
(Green)
(Colorless)
+
Cu (s)
(reddish brown)
The green color of copper chloride fades and the solution becomes colorless. A red brown coating of
copper metal is deposited on the lead strip. Lead is more reactive than copper hence its able to displace it
from its solution.
• Fe (s) + Pb(NO3)2 (aq) ------------------ Fe(NO3)2 (aq) +
Pb(s)
Since Iron is more reactive than lead a single displacement reaction occurs forming Ferrous
nitrate and Lead
• Zn(s) + H2SO4 (aq) ---------------------- ZnSO4 (aq)
+
H2 (g)
CIRCUMSTANCES WHERE CATION DISPLACEMENT
DOES NOT OCCUR?
• Metals like copper and silver do not react with dilute HCl or dilute H2SO4 to evolve hydrogen gas since
they are less reactive than hydrogen being placed below hydrogen in the reactivity series
• On adding pieces of less reactive metal to a solution of a compound of a more reactive metal no
reaction takes place
Example:
i.
On adding Cu to ZnSO4 solution no reaction takes place as Cu is less reactive than Zn being placed
below Zn in the reactivity series
ii.
On dipping Ag plate in Copper nitrate solution no reaction takes place because silver is less reactive
than Cu, silver being placed below copper in the reactivity series
ANION DISPLACEMENT:
FLUORINE > CHLORINE> BROMINE> IODINE
• Cl2 (g) +
2NaBr (aq) ----------------------- 2NaCl (aq) + Br2 (g)
Chlorine being a more reactive halogen displaces Bromine a less reactive halogen from
its aqueous solutions. The red color of bromine gets displaced and the solution acquires
yellowish orange look
ANION DISPLACEMENT:
• Cl2 (g) + 2KI (aq) --------------- 2KCl (aq) +
• Br2 (g) + 2KI (aq) --------------- 2KBr (aq) +
I2 (g)
I2 (g)
DOUBLE DISPLACEMENT REACTIONS
• The chemical reactions in which one component each of both the reacting molecules
gets exchange to form the products are known as double displacement reactions
• It generally occurs between two ionic compounds in the solution in which ions
precipitate as a result of exchange of ions
DOUBLE DISPLACEMENT REACTIONS CAN LEAD
TO
• Precipitation Formation
• Gas Formation Formation
• Neutralization Reactions
PRECIPITATION REACTIONS
• It is the formation of a solid in a solution or inside another solid during a chemical
reaction.
• The process usually takes place when the concentration of dissolved ions in the solution
exceeds the solubility product
PRECIPITATION REACTIONS:
• AgNO3 (aq) + NaCl (aq) ----------------------- AgCl (s)
Silver nitrate
Sodium chloride
Silver chloride
+ NaNO3 (aq)
Silver nitrate
Curdy White
Ag+ and Na+ exchanged their anions NO3- and Cl- respectively to form precipitate of AgCl
PRECIPITATION REACTIONS
• Pb(NO3)2 (aq) + Na2SO4 (aq) ---------- PbSO4
+
2NaNO3 (aq)
White ppt
• On passing H2S (rotten egg smell) through aqueous Copper sulphate black precipitate of
Copper sulphide is formed.
CuSO4 (aq) + H2S (g) --------------- CuS
+ H2SO4 (aq)
PRECIPITATION REACTIONS
• Ba(OH)2 (s)
+ 2NH4Cl (aq) ------ BaCl2 (aq) + 2NH4OH (aq)
Cooling sensation after the reaction on touching the tubes. Reactions are endothermic.
• BaCl2 (aq) + CuSO4 (aq)
-------BaSO4
White ppt
• CoCl2 (aq) +
Cobalt (II) Chloride
+ CuCl2 (aq)
Copper Chloride
Na2CO3 (aq) -------- CoCO3
Pink/ Red ppt
+ 2NaCl (aq)
PRECIPITATION REACTIONS
• Al2(SO4)3 (aq) + 3Ca(OH)2 (aq) ----------- 2Al(OH)3
+ 3CaSO4
White ppt
• Pb(CH3COO)2 (aq) + 2HCl (aq) ---------- PbCl2
Lead acetate
White ppt
+
CH3COOH (aq)
Acetic acid
PRECIPITATION REACTIONS
• 2NH4OH (aq)
+
CuSO4 (aq) --------- Cu(OH)2
Bluish white ppt
+ (NH4)2SO4 (aq)
GAS FORMATION REACTIONS
• Double displacement reactions should also occur if an insoluble gas is formed
• HCl and ammonia are soluble in water but Hydrogen sulphide not soluble in water
• ZnS (s)
+ 2 HCl (aq) ---------- ZnCl2 (aq)
+ H2S (g)
OXIDATION- REDUCTION REACTIONS
Oxidation:
A chemical reaction which involves
 Addition of oxygen ---
2Mg + O2 ---- 2MgO
 Removal of hydrogen ----- H2S
+ Br2 ----- 2HBr + S
 Loss of electrons ----- Fe 2+ ------ Fe 3+
• REDUCTION:
A chemical reaction which involves
 Addition of Hydrogen -- 2Na + H2 ---- 2NaH
 Removal of Oxygen ----- CuO + H2 ----- Cu + H2O
 Gain of Electrons ------- Fe 3+ + e ------ Fe 2+
REDOX REACTIONS
• A chemical reaction in which oxidation and reduction takes place simultaneously is
called redox reaction
Removal of oxygen – Reduction
CO2 (g) +
H2 (g) ---------------- CO (g) +
H2O (g)
Addition of Oxygen – Oxidation
Substances that are reduced (provide O2 or remove hydrogen) in course of the reaction are called
oxidising agents.
Substances that are oxidised (remove O2 or provide H2)
NEUTRALIZATION REACTIONS
• Acid + Base ----------------- Salt + Water
• HCl (aq) + NaOH (aq) --------------- NaCl (aq) + H2O