chemical reactions and equations

chemical reactions

chemical reactions

• processes in which new substances with new properties are formed

• rearrangement of atoms takes place

• old bonds are broken to form new bonds

why metals are rubbed with sand paper before burning in air

it is done in order to remove the protective layer of metal oxide formed on the surface, so that it may readily combine with the oxygen in air

examples of chemical reactions in daily life

souring of curd, fermentation of grapes, cooking of food, digestion of food, respiration, rusting of iron, burning of fuels, ripening of fruits

examples of reactions in which gas is evolved

Zn + H2SO4 → ZnSO4 + H2

Na2CO3 + HCl → NaCl + H2O + CO2

examples of reactions in which precipitate is formed

Pb(NO3)2 + KI → KNO3 + PbI2 (yellow ppt.)

BaCl2 + H2SO4 → HCl + BaSO4 (white ppt.)

Ca(OH)2 + CO2 → CaCO3 (white ppt.) + H2O

examples of reactions in which there is a change in color

sulphur dioxide + potassium dichromate (acidified) → change in color from orange to green

citric acid + potassium permanganate → change in color from purple to colorless

(these are complex reactions which we dont need to learn this year)

Pb(NO3)2 + KI → KNO3 + PbI2 (change in color from colorless to yellow)

compounds of calcium and their common names

CaO - calcium oxide [quick lime]

Ca(OH)2 - calcium hydroxide [slaked lime/lime water]

CaCO3 - calcium carbonate [limestone]

observable characteristics of chemical reactions

• evolution of gas

• formation of precipitate (occurs either when two aqueous solutions are mixed and one of the products is insoluble OR when a gas is passed through an aqueous solution)

• change in color

• change in temperature (occurs when the reaction either absorbs or releases heat energy)

• change in state

examples of reactions in which there is a change in temperature

CaO + H2O → Ca(OH)2 [exothermic]

C + O2 → CO2 [exothermic]

Zn + H2SO4 → ZNSO4 + H2 [exothermic]

Ba(OH)2 + NH4Cl → BaCl2 + NH3 + H2O [endothermic]

N2 + O2 → NO [endothermic[

C6H12O6 + O2 → CO2 + H2O [exothermic]

all combustion reactions [exothermic]

all decomposition reactions [endothermic]

examples of reactions in which there is a change in state

combustion reaction of candle wax

chemical equations

• a balanced equation has equal masses of elements in both reactants and products

• an unbalanced equation has unequal masses of elements in both reactants and products

equations are balanced due to the law of conservation of mass, which states that matter can neither be created nor destroyed

balanced and unbalanced chemical equations

• a balanced equation has equal masses of elements in both reactants and products

• an unbalanced equation has unequal masses of elements in both reactants and products

equations are balanced due to the law of conservation of mass, which states that matter can neither be created nor destroyed

diatomic elements

• elements which occur only as a diatomic molecule

• O2, H2, N2, F2, Cl2, Br2, I2

manufacture of oxygen from potassium chlorate

KClO3 + heat → (MnO2 catalyst) KCl + O2

manufacture of methanol

CO + H2 → (300 atm; 300C; ZnO + CrO3) CH3OH

types of chemical reactions

• combination reaction

• displacement reaction

• oxidation and reduction reaction

• decomposition reaction

• double displacement reaction

examples of combination reactions

A + B → AB (reactions in which two or more substances combine to form a single substance)

Mg + O2 → MgO

H2 + O2 → H2O

CaO + H2O → Ca(OH)2

SO2 + O2 → SO3

process of whitewashing with lime

the substance used to whitewash our homes is quicklime. quicklime is put in a drum of water to form slaked lime, which is applied to the walls. slowly, the slaked lime reacts with CO2 in air to form a shiny layer of limestone

Ca(OH)2 + CO2 → CaCO3 + H2O

examples of decomposition reactions

AB → A + B [reactions in which a compound splits into two or more simpler substances]

every decomposition reaction is endothermic

CaCO3 → CaO + CO2 (thermal decomposition)

KClO3 → KCl + O2 (thermal decomposition)

AgCl → Ag + Cl2 (photolytic decomposition)

AgBr → Ag + Br2 (photolytic decomposition)

NaCl → Na + Cl2 (electrolytic decomposition)

Al2O3 → Al + O2 (electrolytic decomposition)

FeSO4.7H2O (green) → FeSO4 + 7H2O ; FeSO4 (white) → Fe2O3 (brown) + SO2 + SO3 (smelly) (thermal decomposition)

Pb(NO3)2 (colourless) → PbO (yellow) + NO2 (brown fumes) + O2 (thermal decomposition)

decomposition in our body: starch decomposes to form glucose, proteins decompose to form amino acids

examples of displacement reactions

AB + C → AC + B [reactions in which one element takes the place of another element in a compound]

it depends on the reactivity of the elements

CuSO4 + Zn → ZnSO4 + Cu

CuSO4 + Fe → FeSO4 + Cu

Na + H2O → NaOH + H2

KI + Cl2 → KCl + I2

Fe2O3 + Al → Al2O3 + Fe

examples of double displacement reactions

AB + CD → AC + BD [reactions in which two compounds react by an exchange of ions]

they are also usually precipitation reactions

AgNO3 + NaCl → NaNO3 + AgCl (white ppt.)

BaCl2 + CuSO4 → CuCl2 + BaSO4 (white ppt.)

AlCl3 + NH4OH → NH4Cl + Al(OH)3 (white ppt.)

examples of redox reactions

oxidation: addition of oxygen, removal of hydrogen, addition of non-metallic element ; oxidising agent: substance which gets reduced

reduction: addition of hydrogen, removal of oxygen, addition of metallic element ; reducing agent : substance which gets oxidised

CuO + H2 → Cu + H2O (oxidised: H2; reduced: CuO; oxidising agent: CuO; reducing agent: H2)

corrosion

• it is the process by which metals are eaten up gradually by the action of air, moisture or chemical on their surface

• it weakens the iron and steel structures like railings, bridges, ships etc.

• rusting/corrosion of iron: Fe + O2 + xH2O → Fe2O3.xH2O