Download Basic Chemistry – Terminology and Reactions

L. R. & S. M. VISSANJI ACADEMY
SECONDARY SECTION - 2016-17
CHEMISTRY - GRADE: VIII
Basic Chemistry – Terminology and Reactions
SYMBOLS
 A symbol is the short form that stands for the atom of a specific element.
RADICALS
 A radical is a group of atoms of the same or different elements that behaves as a single
unit with a positive or negative charge.
VALENCY
 The valency of an atom or ion is the number of electrons it shares, loses or gains in a
chemical reaction to become stable i.e. the number of bonds it forms with other atoms.
OR
 Valency is the combining capacity of an atom or of a radical.
VARIABLE VALENCY
Certain elements exhibit more than one valency i.e. they show variable valency.
Reason for variable valency: An atom of an element can sometimes lose more electrons
than are present in its valence shell i.e. there is loss of electrons from the penultimate shell
too. Therefore, such an element is said to exhibit variable valency.
MOLECULAR FORMULA
Molecular Formula denotes the number of atoms of each element present in a compound.
CHEMICAL EQUATIONS
1. WORD EQUATIONS
A shorthand way of showing what goes on in a chemical reaction is to write a WORD
EQUATION.
Substances you start with are called REACTANTS. These appear on the left hand side of
the equation.
Substances that are formed are called PRODUCTS. These appear on the right hand side of
the equation.
In a word equation ‘+’ means ‘and’ and the
means ‘produces’.
e.g. magnesium burns in oxygen to produce magnesium oxide.
magnesium
+
oxygen
magnesium oxide
(Reactants)
(Product)
2. CHEMICAL EQUATIONS
A chemical equation is when the words in an equation are replaced by the chemical formula.
e.g.
magnesium + oxygen
magnesium oxide
So the chemical equation is
Mg
+ O2
MgO
Formula for magnesium is Mg as it is an element
Formula for oxygen is O2 as it is a diatomic element
Formula for magnesium oxide is MgO (use valency rules)
BALANCING AN EQUATION
When we write chemical equations many of them will not have the same number of each
atom at the start of the reaction as we have at the end.
We call this an unbalanced equation.
e.g. Unbalanced Equation :- CH4 + O2 → H2O + CO2
If we count how many atoms of each type are on each side of the equation you will see they
are not the same.
Reactants side
Products side
1C
1C
4H
2H
2O
3O
In order to balance an equation we have to follow these steps.
Step 1: Start by finding out how many atoms of each type are on each side of the equation.
Step 2: Next, look for an element which is in only one chemical on the left and in only one on
the right of the equation.
Step 3: Balance that element by multiplying the chemical species on the side which doesn't
have enough atoms of that type by the number required to bring it up to the same as the
other side. The number must go in FRONT of the formula.
Step 4: Now look for the next element or species that is not balanced and do the same
thing.
Example 1: How to balance the equation,
CH4 + O2 → H2O + CO2
Reactants side
Products side
1C
1C
4H
2H
2O
3O
Equation is NOT balanced
2. Balance the H atoms by putting 2 in FRONT of the H2O so we have 4 H atoms on each
side.
Note: This will also increase the number of O atoms. CH 4 + O2 → 2H2O + CO2
Reactants side
Products side
1C
1C
4H
4H
2O
4O
3. Balance the O atoms by putting 2 in FRONT of the O2 so we have 4 O atoms on each
side.
CH4 + 2O2 → 2H2O + CO2
Reactants side
Products side
1C
1C
4H
4H
4O
4O
The equation is now balanced
CHEMICAL REACTIONS
I. DIRECT COMBINATION REACTION
A reaction in which two or more substances combine to form a single substance is called
Direct Combination Reaction.
For example:
Word equation:
Potassium + Chlorine
Potassium Chloride
Chemical Equation:
K(s)
+ Cl2(g)
2 KCl(s)
Examples:
II. DECOMPOSITION REACTION
The breaking up of a compound either into elements or simpler compounds is called as
Decomposition Reaction.
For example:
1. Word equation: Calcium carbonate
Chemical equation:
2. Word equation:
Calcium Oxide + Carbon dioxide
CaCO3(s)
Mercury oxide
Chemical equation:
CaO(s) + CO2(g)
Mercury + Oxygen
2 HgO(s)
2 Hg(l) + O2(g)
 Decomposition of metal carbonates
1) CaCO3 → CaO + CO2
2) MgCO3 → MgO + CO2
3) Al2(CO3)3 → Al2O3 + 3CO2
4) ZnCO3(s) → ZnO(s) + CO2(g)
5) FeCO3 → FeO + CO2
6) Fe2(CO3)3 → Fe2O3 + 3CO2
 Decomposition of metal hydroxides
1)
2)
3)
4)
Ca(OH)2 → CaO + H2O
Mg(OH)2 → MgO + H2O
2Al(OH)3 →Al2O3 + 3H2O
Zn(OH)2 → ZnO + H2O
7) PbCO3 → PbO + CO2
8) H2CO3 → CO2 + H2O
9) CuCO3 → CuO + CO2
10) Cu2CO3 → Cu2O + CO2
11) 2HgCO3 → 2Hg + 2CO2 + O2
12) 2AgCO3 → 2Ag + 2CO2 + O2
5)
6)
7)
8)
Fe(OH)2 → FeO + H2O
2Fe(OH)3 → Fe2O3 + 3H2O
Pb(OH)2 → PbO + H2O
Cu(OH)2 → CuO + H2O
 Decomposition of Metal Nitrates
1)
2)
3)
4)
5)
KNO3 → KNO2 + O2
NaNO3 → NaNO2 + O2
Ca(NO3)2 → 2CaO + 4NO2 + O2
2Mg(NO3)2 → 2MgO + 4NO2 + O2
4Al(NO3)3 →2Al2O3 +12NO2 + 3O2
6) 2Zn(No3)2 → 2ZnO + 4NO2 + O2
7) 2Pb(NO3)2 → 2PbO + 4NO2 + O2
8) 2Cu(NO3)2 → 2CuO + 4NO2 + O2
9) 2Hg(NO3)2 → Hg + 2NO2 + O2
10) 2AgNO3 → 2Ag + 2NO2 + O2
III. SIMPLE DISPLACEMENT REACTION
 The chemical change in which a more active element displaces a less active element
from its salt solution is called Simple Displacement Reaction.
 Occur between a metal element and a compound. The metal element replaces an
element in the compound. Common elements that swap are two metals or a metal
and hydrogen
For example:
1. Word equation: Magnesium + Zinc
nitrate
Chemical equation: Mg(s) + Zn(NO3)2(aq)
Magnesium + Zinc
nitrate
Mg(NO3)2(aq) + Zn(s)
The element magnesium, Mg is replaced by the element zinc, Zn in the compound
zinc nitrate.
Zinc becomes the product.
2. Word equation : Magnesium + Hydrochloric
acid
Chemical equation: Mg (s) + 2 HCl (aq)
Magnesium + Hydrogen
chloride
MgCl2 (aq) + H2(g)
In this reaction the element magnesium is replaced by the element hydrogen, H in
hydrochloric acid. Hydrogen gas, H2 becomes the product.
ACTIVITY SERIES
An activity series is a list of substances ranked in order of relative reactivity. For
example, magnesium metal can knock hydrogen ions out of solution, so it is
considered more reactive than elemental hydrogen:
Mg (s) + 2 HCl (aq)
MgCl 2 (aq) + H 2 (g)
Zinc can also displace hydrogen ions from solution:
Zn (s) + 2 HCl (aq)
ZnCl 2 (aq) + H 2 (g)
Examples:
IV. DOUBLE DECOMPOSITION REACTION
 A type of chemical change in which two compounds react to form two new
compounds by mutual exchange of radicals ( or ions ) is called Double
Decomposition Reaction.
 Occur between two compounds. The two positive metal ions in each reactant swap pl
aces with one another.
For example:
1.Word
equation: Potassium Carbonate + Barium Chloride
Potassium + Barium
Chloride
carbonate
Chemical
equation: K2CO3(aq) +
BaCl2(aq)
2 KCl (aq) + BaCO3(s)
+
2‐
2+
‐
Ions:
K , CO3
Ba , Cl
K+ , Cl‐
Ba2+, CO32‐
Examples:
V. IRREVERSIBLE REACTION
In an irreversible reaction, the reactants react to form the products, which cannot
revert back into reactants.
H2O2
2H2O
+
O2
VI. REVERSIBLE REACTION
In reversible reactions, as the reactants react with other reactants to form products,
the products react to form reactants.
Examples:
2SO2
+
O2
2SO3
3Fe
+
4H2O
Fe3O4 + 4H2
VII. CATALYTICAL REACTION
The chemical reactions that involve the use of a catalyst are called as Catalytic
Reactions.
CATALYST: Catalyst is a compound which alters the rate of reaction but does not
take part in it.
Example:
H2O2
2H2O + O2
VIII. EXOTHERMIC REACTION
The reactions which proceed with the evolution of heat energy are called as
Exothermic Reaction.
Examples: 2SO2
+
O2
2SO3
N2
+
3H2
2NH3
IX. ENDOTHERMIC REACTION
The reactions which proceed with the absorption of heat energy are called as
Endothermic Reaction.
Examples: N2
+
O2
2NO
C
+
H2O
CO + H2
X. OXIDATION REACTION
Definition 1: Oxidation is defined as a chemical process that involves addition of
oxygen. Example: Mg
+
O2
2MgO
Definition 2: Oxidation is defined as a chemical process that involves removal of
hydrogen. Example: H2S + Cl2
2HCl + S
Definition 3: In the electronic concept, oxidation is defined as a process in which an
atom or an ion loses electron(s). Example: Zn
Zn2+ + 2eXI.
REDUCTION REACTION
Definition 1: Reduction is defined as a chemical process that involves addition of
hydrogen. Example: CuO + H2
Cu + H2O
Definition 2: Reduction is defined as a chemical process that involves removal of
oxygen. Example: Cl2 + H2S
2HCl + S
Definition 3: In the electronic concept, reduction is defined as a process in which an
atom or an ion gains electron(s). Example:Cu2+ + 2eCu
XII. REDOX REACTION
A reaction in which oxidation and reduction take place simultaneously is known as
Redox Reaction.
1. OXIDIZING AGENT
An oxidizing agent is one that oxidizes other substances either by accepting
electrons, or by providing oxygen or by removing hydrogen.
2. REDUCING AGENT
A reducing agent is one that reduces other substances either by providing
electrons, or by providing hydrogen or by removing oxygen.
Addition of Hydrogen
H2S + Cl2
[ Reduction ]
2HCl + S
Removal of Hydrogen [ Oxidation ]
Reducing agent
Oxidising agent