Class 10 Chemistry
Notes---- by Sahil Swë
CHEMICAL REACTION AND EQUATION
Chemistry is defined as that branch of science which deals with the composition and properties of matter and the changes that matter undergoes by various interactions.
A chemical compound is formed as a result of a chemical change and in this process different types of energies such as heat, electrical energy, radiation etc. are either absorbed or evolved.
The process involving a chemical change is called a chemical reaction.
Chemical change – one or more new substances with new physical and chemical properties are formed.
Example: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)
(Blue) (Green)
Here, when copper sulphate reacts with iron, two new substances, i.e., ferrous sulphate and Copper are formed.
• Therefore, A chemical reaction is a process which transforms one or more substances into new substances.
• The process in which a substance or substances undergo change, to produce new substances with new properties, is known as chemical reaction.
Reactants: The substances which take part in a chemical reaction are called reactants.
Products: The new substances formed as a result of the chemical reaction are called products.
For example
2 H2 (g) + O2 (g) => 2H20 (l)
(Reactant) (Reactant) (Product)
Observations that help determine a chemical reaction
A chemical reaction can be determined with the help of any of the following observations:
a) Evolution of a gas
b) Change in temperature
c) Formation of a precipitate
d) Change in color
e) Change of state
Chemical reaction
Evolution of gas: Some chemical reactions are characterized by evolution of a gas.
• When zinc metal is treated with dilute sulphuric acid, hydrogen gas is evolved. The hydrogen gas burns with a pop sound.
Zn (s) + H2SO4 (dilute) → ZnSO4 (aq) + H2(g)
Change in temperature: When quicklime reacts with water, slaked lime is formed with a release of large amount of heat, causing change in temperature in the reaction mixture.
CaO + H2O Ca(OH)2 + Heat
Formation of precipitate : Some chemical reactions are characterized by the formation of precipitate (an insoluble substance), when the solutions of the soluble chemical compounds are mixed together. When silver nitrate solution is mixed with a solution of sodium chloride.
AgNO3 (aq) + NaCl (aq) → NaNO3 (aq) +AgCl (s)
Change of color: Certain chemical reactions are characterized by the change in colour of reacting substance.
• When red lead oxide is heated strongly it forms yellow coloured lead monoxide and gives off oxygen gas.
2Pb3O4 (s) → 6PbO(s) + O2(g)
Lead oxide Lead monoxide
(Red) (Yellow)
Change of state: Some chemical reactions are characterised by a change in state i.e. solid, liquid or gas
Two volumes of hydrogen gas reacts with one volume of oxygen gas to from water.
2H2 (g) + O2 (g) → 2H2O
Or when electric current is passed through water it splits into its elements.
2H2O → 2H2 (g) + O2 (g)
• Chemical reactions are chemical changes in which reactants transform into products by
making or breaking of bonds(or both) between different atoms.
Chemical Reactions and Equations I
Chemical Equation
By definition, a chemical equation is a written representation of chemical reaction in the form of symbols and formulas, showing both the reactants and products, their physical states, and the direction in which the reaction proceeds. In addition, many chemical equations also indicate the conditions necessary for the reaction to occur.
A chemical equation is a symbolic representation of an actual chemical change or the short-hand method of representing a chemical reaction in terms of symbols and formulae of the different reactants and products is called a chemical equation.
A chemical reaction can be represented in two different ways :
Word equation
A word equation is a chemical reaction expressed in words rather than chemical
formulas. It helps identify the reactants and products in a chemical reaction.
Symbols of elements and their valencies
• A symbol is the chemical code for an element. Each element has one or two letter atomic
symbol, which is the abbreviated form of its name.
• Valency is the combining capacity of an element. It can be considered as the number of electrons lost, gain or shared by an atom when it combines with another atom to form a
molecule.
Writing chemical equations
Representation of a chemical reaction in terms of symbols and chemical formulae of the
reactants and products is known as a chemical equation.
Zn(s) + dil. H2SO4(aq) → ZnSO4(aq) + H2(↑)
(Reactants) (Products)
• For solids, the symbol is "(s)".
• For liquids, it is "(l)".
• For gases, it is "(g)".
• For aqueous solutions, it is "(aq)".
• For gas produced in the reaction, it is represented by "(↑)".
• For precipitate formed in the reaction, it is represented by "(↓)".
An arrow sign (→) is put between the reactants and the products, pointing from reactants towards products
Balancing of a Chemical Reaction
Conservation of mass
According to the law of conservation of mass, no atoms can be created or destroyed in a
chemical reaction, so the number of atoms for each element in the reactants side has to
balance the number of atoms that are present in the products side.
In other words, 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.
Balanced chemical equation
The chemical equation in which the number of atoms of each element in the reactants side
is equal to that of the products side is called a balanced chemical equation.
Zn + H2SO4 → ZnSO4 + H2
Unbalanced chemical equation (skeletal equation):
The equation in which the number of atoms of different elements on the reactants and the product sides are not equal is called an unbalanced chemical equation. The unbalanced chemical equation is also known as skeletal equation.
For example
The burning of aluminum in oxygen to form aluminum oxide can be written as :
Al + O2 → Al2 O3
Steps for balancing chemical equations
Hit and trial method: While balancing the equation, change the coefficients (the numbers in
front of the compound or molecule) so that the number of atoms of each element is same
on each side of the chemical equation.
Short-cut technique for balancing a chemical equation
Example:
aCaCO3 + bH3PO4 → cCa3(PO4)2 + dH2CO3
Set up a series of simultaneous equations, one for each element.
Ca: a=3c
C: a=d
O: 3a+4b=8c+3d
H: 3b=2d
P: b=2c
Let's set c=1
Then a=3 and
d=a=3
b=2c=2
So a=3; b=2; c=1; d=3
The balanced equation is
3CaCO3 + 2H3PO4 → Ca3(PO4)2 + 3H2CO3
Different types of Chemical Reactions are:-
● Combination ● Decomposition ● Single Displacement ● Double displacement
● Redox ● Endothermic ● Exothermic ● Precipitation ● Neutralisation
1. In a combination reaction, two elements or one element and one compound or two
compounds combine to give one single product.
H2 + Cl2 → 2HCl
element + element → compound
2CO + O2 → 2CO2
compound + element → compound
NH3 + HCl → NH4Cl
compound + compound → compound
2H2(g) + O2(g) → 2H2O(l)
(Element) (e). ( C)
DECOMPOSITION REACTIONS:
Those reactions in which a single substance (reactant) splits up into two or more simpler substances (products) are known as decomposition reactions.
These reactions are carried out by supplying energy in form of heat, electricity or light which breaks that substance into simpler substances. Thus decomposition reactions are classified as:
• Thermolysis or thermal decomposition reactions (decomposition by heat).
Examples: CaCO3→CaO+CO2
2FeSO4 → Fe2O3 + SO2 + SO3
• Electrolysis or electrolytic decomposition reactions (decomposition by electricity)
2H2O→2H2+O2 [2:1 ] ratio
• Photolysis or photodecomposition reactions (decomposition by light)
Sunlight (hv)
AgCl (s) -----------------------> Ag (s) + Cl (g)
Sunlight (hv)
H2O2 ----------------> H2O (l) + O2 (g)
The elements involved may be metals or non-metals. In displacement reactions more reactive metal may displace a less reactive metal or a more reactive non-metal may displace a less reactive non-metal from its compound
Relative activities or reactivity’s of metals:
Metals have been arranged in decreasing order of their activities (or reactivity) in the activity series as follows:
Relative activities (or reactivity) of Non-metals:
Relative activities of non-metals like halogens is in the order:
F > Cl > Br > I
Thus, fluorine is most reactive and iodine is least reactive. So fluorine (F2) can displace chlorine (Cl2) from NaCl, Bromine (Br2) from NaBr and so on. Similarly, chlorine can displace bromine (Br2) from KBr and iodine (I2) from KI and so on.
Displacement reaction
More reactive element displaces a less reactive element from its compound or solution.
i) Zn(s) + CuSO4(aq) → ZnSO4(aq) + Cu(s)
ii) Cu(s) + 2AgNO3(aq) → Cu(NO3)2(aq) + 2Ag(s)
DOUBLE DISPLACEMENT REACTIONS
Those reactions in which two different atoms or groups of atoms are exchanged are called the double displacement reactions. These reactions generally occur between two ionic compounds in the solution. So they may be defined as :
“Those reactions in which two ionic compounds in the solution react by exchange of their ions to form new compounds are called double displacement reactions”
For example, Al2(SO4)3(aq) + 3Ca(OH)2(aq) → 2Al(OH)3(aq) + 3CaSO4(s)
Oxidation is the addition of oxygen, whereas, reduction is removal of oxygen.
Oxidation is removal of hydrogen, whereas, reduction is addition of hydrogen.
Oxidation is loss of electron, whereas, reduction is gain of electron.
Oxidation is addition of comparable electronegative element or removal of comparable electropositive elements and for reduction vice versa.
Oxidation is increase in oxidation state, whereas, reduction is decrease in oxidation state.
REDOX REACTION
Oxidation and reduction are complementary to each other and one cannot take place in the absence of the other. So the oxidation and the reduction will take place simultaneously. It is obvious that, if a substance takes electrons, there must be another substance to give these electrons. The reactions, which involve oxidation and reduction are called redox reactions. The redox reactions can be split into two half reactions namely oxidation half reaction (where oxidation takes place) and reduction half reaction (where reduction
takes place).
Redox reaction: Zn + Cu2+→ Zn2+ + Cu
Oxidation half reaction: Zn → Zn2+ + 2e−
Reduction half reaction: Cu2+ + 2e−→ Cu
Oxidizing agents are those substances, which can oxidize some other substances. To do so, it will have to gain electron (decrease in oxidation state) and hence it will be reduced. Similarly the reducing agent will give electrons and will be oxidised. So the substances undergoing oxidation are reducing agent and the substances undergoing reduction are oxidizing agents.
ZnO + C → Zn + CO
ZnO reduces to Zn → reduction
C oxidises to CO → oxidation
ZnO - Oxidising agent
C - Reducing agent
Oxidation Number
Oxidation number is also referred to as oxidation state. Oxidation number for an element is the arbitrary charge present on one atom when all other atoms bonded to it are removed. The removal of other atoms (dissociation of bonds between them) is done by assuming that the bonds are either ionic or pure covalent.
Oxidation number of an atom is defined as the charge that an atom appears to have on forming ionic bonds with other heteroatoms.
• Oxidation number of all alkali metal ions is always = +1
• Oxidation number of all alkaline earth metal ions is always = +2
• Oxidation number of all boron family metal ions is always = +3
• Oxidation number of hydrogen in proton (H+) is +1, and in hydride is -1.
• Oxidation number of oxygen in oxide ion(O2-) is -2, and in peroxide ion(O-O2-) is -1.
Precipitation reaction
An insoluble compound called precipitate forms when two solutions containing soluble salts
are combined.
For example, P b(NO3)2(aq) + 2KI(aq) → 2KNO3(aq) + P bI2(↓)(s)(yellow)
Energy changes: All chemical reactions proceed either with the absorption or release of energy.
On the basis of energy changes, there are two types of reactions:
(A) Endothermic reaction: A chemical reaction which is accompanied by the absorption of heat energy is called an endothermic reaction.
C (s) + 2S (s) → CS2
• Light energy is essential for biochemical reaction, photosynthesis, by which green plants prepare their food from carbon dioxide & water.
(B) Exothermic reaction: A chemical reaction which is accompanied by the release of heat energy is called exothermic reaction.
When magnesium wire is heated from its tip in a bunsen flame, it catches fire and burns with a dazzling white flame with release of heat and light energy.
2Mg (s) + O2 (g) → 2MgO (s) + Energy
Corrosion
Gradual deterioration of a material, usually a metal, by the action of moisture, air or chemicals in the surrounding environment.
Rusting:
4Fe(s) + 3O2(from air) + xH2O(moisture) → 2Fe2O3. xH2O(rust)
Corrosion of copper:
Cu(s) + H2O(moisture) + CO2(from air) → CuCO3. Cu(OH)2(green)
Corrosion of silver:
Ag(s) + H2S(from air) → Ag2S(black) + H2(g)
Rancidity
It refers to oxidation of fats and oils in food that is kept for a long time. It gives foul smell
and bad taste to food. Rancid food causes stomach infection on consumption.
Prevention:
• Use of air-tight containers
• Packaging with nitrogen
• Refrigeration
• Addition of antioxidants or preservatives.
