Chemical Reactions Notes

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What are Chemical Reactions?

nature and identity of initial substance changes

so, whenever a change occurs, a chemical reaction occurs

When a chemical reaction takes place, the following happens

change in state of the initial object (solid, liquid, gas)

change in color

evolution of a gas ( as a result of the reaction)

change in temperature

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What are Chemical Equations?

a shorter form of describing a chemical reaction

the simplest way is a word-equation

Magnesium + Oxygen --> Magnesium Oxide

2 parts of the equation

Left side -reactants

Right side - product

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Using Chemical Formula

a shorter way of representing the equation

use the chemical symbols of the reactants and products

Mg + O2 --> MgO

Skeletal Chemical Equations

number of atoms in LHS is not equal to no of atoms in RHS

the equation is not balanced

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Balanced Chemical Equations

Mass can neither be created nor destroyed

the total mass of reactants = total mass of products

no. of atoms remain the same - before and after the chemical reaction

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Steps to balance equation

Fe + H2O --> Fe3O4 + H2

Step 1: Identify each formula in the equation

H2O

Fe3O4

Step 2:

Identify the elements in the equation

List the number of atoms in LHS and RHS

Fe --> LHS = 1 atom ; RHS =1 atom

H --> LHS = 2 atoms ; RHS =2 atom

O --> LHS =1 atom ; RHS = 4 atoms

no. of atoms differ because of the chemical formula

3Fe needs to combine with 4O to form a stable ferrous oxide molecule

Step 3:

Identify the compound with maximum no. of atoms (can be on any side of the equation)

Fe3O4

Step 4:

Choose an element in the compound

Step 5:

no. of atoms in LHS of O --> 1

no. of atoms in RHS of O --> 4

balance in LHS (as it is less)

to balance 1x4 H2O ( as O is present in that compound)

Equation now becomes

Fe + 4H2O --> Fe3O4 + 4H2

Step 6:

Repeat steps 4 and 5 for all elements in the equation

For H

no. of atoms in LHS --> 8 (4x2)

no. of atoms in RHS --> 2

balance RHS ( as it is less)

to balance 1x4 H2 (4x2=8)

Equation becomes: Fe +4H2O --> Fe3O4 + 4H2

For Fe

no. of atoms in LHS --> 1

no. of atoms in RHS --> 3

balance LHS ( as it is less)

to balance 1x3 Fe

Equation becomes : 3Fe + 4H2O --> Fe3O4 +4H2

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Completing the equation

Follow these final steps:

Count number of atoms on both sides- they should be equal

Write the physical states of the compound -s,l,g

Add temp, pressure, catalyst if required

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Types of Chemical Equation

There are the following types:

Combination Reaction: single product formed with two or more reactions

Decomposition Reaction: single, complex reactant gives simpler products

Displacement Reaction: the reactant replaces a compound in another reactant

Exothermic chemical Reaction: reaction produces a lot of heat

Thermal decomposition: decomposition reaction in the presence of heat

Endothermic reactions: reaction in which energy (heat, light, electricity) is absorbed

Double displacement reaction: displacement of reactant and exchange of ions as well

Precipitation reaction: any reaction that forms a precipitate (insoluble substance)

Oxidation Reaction: if a reactant gains oxygen during a reaction

Reduction Reaction: if a reactant loses oxygen during a reaction

Redox Reaction: if both oxidation and reduction occur

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Examples

Combination Reaction

CaO(s) + H2O(l) → Ca(OH)2(aq)

C(s) + O2(g) --> CO2(g)

2H2(g) + O2 --> 2H2O

2Mg + O2 --> 2MgO

Exothermic Reaction

CaO(s) + H2O(l) → Ca(OH)2(aq)

CH4(g) + 2O2(g) --> CO2(g) + 2H2O(g)

C6H12O6 (aq) +6O2 (aq) --> 6CO2(aq) + 6H2O (l) +energy (respiration)

decomposition of vegetable into compost

Decomposition Reaction

2FeSO4(s) (Heat)→ Fe2O3(s) + SO2(g) + SO3(g) (on heating)

CaCO3(s) (Heat)→CaO(s) + CO2(g) (on heating)

2Pb(NO3)2(s) (Heat)→ 2PbO(s) + 4NO2(g) + O2(g) (on heating)

2AgCl(s) (Sunlight) → Ag(s) + Cl2(g)

2AgBr(s) (Sunlight) → 2Ag(s) + Br2(g)

Endothermic Reactions

2FeSO4(s) (Heat)→ Fe2O3(s) + SO2(g) + SO3(g) (on heating)

CaCO3(s) (Heat)→CaO(s) + CO2(g) (on heating)

2Pb(NO3)2(s) (Heat)→ 2PbO(s) + 4NO2(g) + O2(g) (on heating)

2AgCl(s) (Sunlight) → Ag(s) + Cl2(g)

2AgBr(s) (Sunlight) → 2Ag(s) + Br2(g)

breaking of bonds in decomposition requires energy, hence they are always endothermic

Displacement Reaction

Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s)

Zn(s) + CuSO4(aq)→ ZnSO4(aq) + Cu(s)

Pb(s) + CuCl2(aq) → PbCl2(aq) + Cu(s)

Double Displacement Reaction

Na2SO4(aq) + BaCl2(aq) → BaSO4(s) + 2NaCl(aq)

Oxidation Reaction

2Cu + O2→ (Heat) 2CuO

in real life - corrosion and rancidity (spoiling of food)

Reduction Reaction

Cuo + H2 --> (Heat) Cu +H2O

Redox Reaction

Cuo + H2 --> (Heat) Cu +H2O

ZnO + C → + Zn CO

MnO2 + 4HCl --> MnCl2 + 2H2O +Cl2

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Effects of oxidation

corrosion

rancidity

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What happens when....?

Burn magnesium ribbon in air

Observation: magnesium ribbon burns with a dazzling white flame and changes into a white powder.

Conclusion: MgO is formed due to the reaction between magnesium and oxygen present in the air.

Add potassium iodide solution to lead nitrate

Observation: lead nitrate reacts with potassium iodide to give lead iodide and potassium nitrate.

The color of the products is different from that of the reactants.

Conclusion: Pb(NO3)2 + 2KI → PbI2 + 2KNO3.

The change in colour is because lead iodide and potassium nitrate have been formed after the reaction. PbI2 is a yellow ppt. KNO3 is a colourless solution

Add hydrochloric acid or sulphuric acid to zinc granules

A gas is evolved and the conical flask is warm

Conclusion: From this, we conclude that when a chemical reaction takes place there is a change in the state, colour, and temperature.

Zn + 2HCl → ZnCl2 + H2↑ + heat

Zn + H2SO4 → ZnSO4 + H2↑ + heat

Water is added to calcium oxide

Observation: The beaker becomes hot (exothermic reaction).

It reacts vigorously with water.

Slaked lime is formed by the reaction of the combination of calcium oxide with water

CaO(s) + H2O(l) → Ca(OH)2(aq) + heat

(quicklime) (Slaked lime)

Ferrous sulphate crystals are heated

The green colour of the ferrous sulphate crystals has changed there is also the characteristic odour of burning sulphur.

2FeSO4(s) + Heat → Fe2O3 + SO2(g) + SO3(g)

Lead nitrate is heated

We observe the emission of brown fumes.

These fumes are of nitrogen dioxide (NO2). The reaction that takes place is

2Pb(NO3)2(s) ---Heat→ 2PbO(s) + 4NO2(g) + O2(g)

(Lead nitrate) (Lead oxide) (Nitrogen dioxide) (Oxygen)

Electrolysis of water ( separating hydrogen and oxygen)

The test tube containing hydrogen gas has double the volume of the test tube containing oxygen gas.

Hydrogen gas burns with a light blue flame with a pop sound.

its a decomposition reaction

2 H2O(l) → 2 H2(g) + O2(g)

Silver chloride is kept in sunlight

Observation: White silver chloride turns grey in sunlight.

Conclusion: This is due to the decomposition of silver chloride into silver and chlorine by light

2AgCl(s) ----Sunlight→ 2Ag(s) + Cl2(g)

Iron nails are immersed in a copper sulphate solution.

the iron nail dipped in the copper sulphate solution in test tube A becomes brownish in colour and the blue colour of copper sulphate solution fades in test tube A. Wh

Sodium Sulphate and Barium Chloride are mixed together:

A white substance, which is insoluble in water, is formed.

This is a double displacement reaction shown by the equation