chemistry important points

7/30/2019 Chemistry Important Points

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This presentation is based on IGCSE Chemistry

(0620) Syllabus 2013

To help every one perform very well in Paper 3/2

A general awareness of the 14 topics of the syllabus is

assumed

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Atoms are the smallestparticles of an element

Elements are arranged in

the periodic table in theorder of increasing protonnumbers

Nucleon number = p + n(proton + neutron))

Atoms contain the followingparticles

particlerelative

mass

relative

charge

Proton

p1 + 1

Electrone

negligible - 1

Neutron

n 1 0


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Metals & Non-metals, a comparison

No. metals non-metals

1 sonorous not sonorous

2malleable &ductile

brittle

3

good conductor of

heat andelectricity

poor conductor ofheat and electricity

4produce hydrogenwith acids

no reaction withacids

5 form basic oxides form acidic oxides

Alloys are mixtures of metals with otherelements. Alloying helps to modify theproperties of metals. Usually alloys areharder than pure metals as the layers will

be prevented from sliding due to thedifference in the atomic size of theelements present.

Chemical test for metals:

Gas produced by the addition of a diluteacid in a metal piece will put off a burningsplint with a pop sound

Chemical test for non-metals :

Aqueous solutions of oxides will be acidic

Important details


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NO. TYPEOFSALT METHODOFPREPARATION

1 Group I metal or Ammonium salts TITRATION METHOD

2 Water soluble salts(except the first group metal salts

or ammonium salts)

NEUTRALISATION METHOD

Reaction of the metal / metal oxide/metal hydroxide/ metal carbonate

with the acid3 Water insoluble salts PRECIPITATION METHOD

Two proper solutions are mixedtogether to make the salt.

Salt Preparation

N.B: Be careful to select the correct method and reagents


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Term Expected response

Define A formal statement is expected as answer

ExplainWrite reasoning or some reference to theory, depending on the context.Not a repetition of the details given in the question

Describe Write the main points of the topic, how or why it is include reference toobservations associated with it, wherever applicable

Outline Restricting the answer to giving essentials

Predict

A short answer with no supporting statement required, by

making a logical connection between pieces of information (often)given

Deducesimilar way to predict except that some supporting statement(reasoning) is required, e.g. reference to a theory or principle

CalculateA numerical answer is required. In general, working should be shown,especially where two or more steps are involved. 6


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Explain diffusion: Spreading of substances by thefree movement of particles, in gas and liquid

e.g. A perfume spreads in the room Remember (1) free random movement is

(2)hindered by collision with air molecules,diffusion faster in gas than in liquid.

Diffusion rate isdirectly proportional to temperature,inversely proportional to molecular mass

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Describe paper chromatography of coloured andcolourless substances, locating agents

Rfvalue its calculation and use

A substance can be identified by comparing (1)spotsor (2)Rf values[always less than 1]

Criteria of purity: liquid- fixed b.p

solid - fixed m.p 8


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Macromolecular structures:graphite, diamond, silicon (IV) oxideDescribe the structure and properties

Note: Carbon, Silicon and Germanium formtetrahedral macromolecular structures withsimilarities in physical properties

Giant ionic structures: In ionic compounds and metalsMetallic bonding: explain the malleability, ductilityand electrical conductivity of metals

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Graphite - 1. Contains layers of macromolecules. 2. In a layer each carbon

atom is bonded to three other carbon atoms by single covalent bonds.

3. The remaining electrons in each atom is used to form weak bonds

between layers.

Graphite is soft because the layers can slide over each other since the

bonds are weak between them

Graphite conducts electricity as the electrons used to form weak bonds

between layers are mobile.

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Diamond - 1. Tetrahedral macromolecular structure. 2. Each carbon is

bonded to four other carbon atoms by strong covalent bonds

Diamond is hard because of the uniform strong Carbon Carbon bonds.

Diamond doesnt conduct electricity as all valence electrons are involved in

the formation of strong bonds

Diamond conducts heat as the vibrational energy can be transferred easily

between the interconnected atoms

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Silicon (IV) oxide (Silica) has a tetrahedral macromolecular structure whereeach Si atom is connected to four oxygen atoms and each oxygen atom to 2silicon atom.

Silicon

Oxygen

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Ionic Compounds Covalent Compounds

1 Crystalline solids Gas, liquid or soft solid

2

High melting & Boiling

points(non-volatile)

Low melting & Boiling points(volatile)

3Water soluble(Insoluble in organic

solvents like ethanol)

Water insoluble(soluble in organic solvents

like ethanol)

4Good electrical conductorsin liquid state, not in solid

Poor electrical conductor inany state

5 Fast reactions in solution Slow reactions


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Bond breaking is endothermic [BBN]andbond forming is exothermic[BFX]

Note: Use this idea in the appropriate place where it will be specifically required.

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Increase of temperature shows an exothermic reaction.Decrease of temperature shows an endothermic reaction.


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Electrolysis is the decomposition of an ionic substance using electricity.

Compound should be either molten or dissolved in water for mobility of ions

Generally metals and hydrogen are formed at cathode (by reduction),

Non-metals are formed at anode (by oxidation)

Molten ionic compounds give metal at the cathode and non-metal at anode,

e.g:- electrolysis of molten lead (II) bromide gives lead (silvery drops) at

cathode and bromine (brown vapour) at anode


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Electrolysis of a dilute solution

Hydrogen (at cathode)

Oxygen (at anode)

The electrolyte gets concentrated

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Useful processes involving electrolysis(1) Aluminium extraction (3) Electrolysis of sodium chloride(2) Electroplating (4) Purification of copper

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Electroplating: - (1) to make the object more attractive and(2) to prevent corrosion.

For example to electroplate an object like an iron ring with copper

Cathode: - the object (iron ring)

Anode: - Copper

Electrolyte: - concentrated aqueous copper compound

Anode reaction: Cu Cu2+ + 2e

Cathode reaction: Cu2+ + 2e Cu

Note: Electrolyte solution provides metal ions for coating,

Anode provides fresh ions to replenish


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Main points

An electrochemical cell is a convenient and portable source of energy

It is made by keeping two metal pieces in an electrolyte

The more reactive of the two metals will be the negative electrode

Electrons flow from negative to positive

At negative electrode - oxidation reaction

At positive electrode - reduction reaction

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Q:Suggest two ways of increasing thevoltage of this cell

A: replace zinc with magnesium or

replace iron with copper oruse more concentrated sulfuric acid(any two)

What are the limitations of electrochemical cells as power source?


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Oxidising agents

Acidified KMnO4 [Purple to Colourless]

Acidified K2Cr2O7 [Orange to Green]

Note: Oxidising agent will get reduced and reducing agent will getoxidisedReducing agents

Carbon monoxide

Carbon

Tests for these agents


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Increases with

(1)increase in temperature

(2) increase in concentration

(3)addition of a suitable catalyst

(4) decrease in particle size for solids

(5)increase of pressure for gases.

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TEMPERATURE: (1)Kinetic energy of particles increase with

temperature, (2) travel more distance resulting in(3) increased effective collisions

CONCENTRATION: (1)Number of particles in unit volume increases,this (2) increases the chance for collisions between particles

CATALYST: (1)Offer an alternate route with (2) less activationenergy. So (3) more collisions become effective

PARTICLE SIZE (IN SOLIDS): (1) Small particles provide increasedsurface area for contact, (2) increasing the chance for collision

PRESSURE (IN GAS): (1) As pressure increases gas particles comecloser (2) increasing the chance for collisions

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Rate increases with light intensity

Silver halides to silver (used in photographic films)

Photosynthesis

Describe photosynthesis as the reaction betweencarbon dioxide and water in the presence of chlorophyll

and sunlight (energy) to produce glucose and oxygen

6 CO2 + 6 H2O C6H12O6 + 6 O2

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(1) Gas syringe to measure the volume of gas formed( when the reaction is over volume becomes constant) (2) Weighing balance to measure the decrease in mass(at the end mass remains constant)

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Reaction rate decreasesgradually in normal chemicalreactions as the concentration

of reactants decrease withtime

There are two major exceptions

(1) Fermentation

(2) Reaction of aluminium with acids


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Equilibrium: when (1)concentrations remain constant as(2)the rate of formation of products equals the rate of decomposition

Note: Use the example, if given, to clarify the above statement

Factors affecting equilibrium

Temperature: High temperature supports endothermic reactions

Pressure: High pressure helps the side with less number of gasmolecules

Concentration: As concentration increases equilibrium shifts to theopposite side

Note: A catalyst will not affect the yield only change the rate ofreaction 28

ACIDS & BASES


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ACIDS & BASES

ACIDS ARE PROTON DONORS & BASES PROTON ACCEPTORS

Acids

Metal + Acid Salt + Hydrogen

Metal oxide/ hydroxide + Acid Salt + Water

Metal Carbonate + Acid Salt + Water + Carbon dioxidet Bubbles will be seen in reaction of acid with metal and

metal carbonate

Bases

(1) neutralise acids(2) produce ammonia when heated with any ammoniumcompound

NaOH + NH4Cl NH3 + NaCl + H2O


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No. Type of oxide Example Property

1 AcidicSO2,NO2,SiO2

Reacts with alkali, not with acid

2 BasicCaO,MgO,Na2O

Reacts with acid, not with alkali

3 AmphotericAl2O3,

ZnOReacts with both acid and alkali

4 Neutral

H2O,

CO Does not react with acid or alkali30


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P

AN

D

A


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Group number is equal to the number of electrons in thevalence shell

For groups I to IV valency = group number

For groups V to 0 valency = (8 group number)

Elements in groups V to VII, usually, do not use allelectrons in bonding.

Metallic character decreases from L to R, whereas non-

metallic character increases from L to R

Typical ionic compounds are formed between elements in

groups I and VII

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Group I metals


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Group I metals

Soft,

Highly reactive (forms alkali and H2 with cold water)

Reactivity and Density increases from top to bottom,

Melting point decreases from top to bottom,

White compounds in solid state and colourless in

solution

Water-soluble compounds- all compounds are watersoluble

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Group VII elements

Diatomic non-metals

Colour becomes darkerdown the group,

Reactivity decreases fromtop to bottom

Element D

A

R

K

E

R

C

O

LO

R

Colour

Fluorine Lightyellow

Chlorine Greenish

yellow

Bromine Red- brown

Iodine Dark brown

Astatine Black

For example chlorine can displace bromine

from a solution of its compound


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REACTIVITY

Group I Group VII

I

NC

R

E

A

S

E

S

D

EC

R

E

A

S

E

S

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High densities (more than 5g/cm3),

High melting points,

Hard metals

Forming coloured compounds,

Catalysts often(as elements and compounds)

Variable valency


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All of them are unreactive gases

Explain why are zero group elements unreactive?

Ans: A stable electron arrangement with completed outer shell

(with 2 electrons in He and 8 electrons/octet for all others)

Explain why nitrogen has valency of three

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R

E

A

CT

I

V

I

T

Y

S

ER

I

E

S

The tendency ofmetals to form

positive ions

decrease withreactivity


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Carbonates

Na and K carbonates do not decompose by heating

Less reactive carbonates form metal oxide and carbondioxide

CaCO3 CaO + CO2

Hydroxides

Na and K hydroxides do not decompose by heatingLess reactive hydroxides form metal oxide and water

Ca(OH)2 CaO + H2O


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Sodium and potassium nitrates decompose on heating to form metalnitrite and oxygen

2NaNO3 2NaNO2 + O2

Less reactive nitrates decompose to form metal oxide, nitrogendioxide and oxygen

2Zn(NO3)2 2ZnO + 4NO2 + O2

Observation: Red-brown gas with a pungent smell will be formed

Note: The apparent unreactivity of aluminium is due to the oxidecoating ( which is non-porous)


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Electrolysis for metals like K,Na,Ca,Mg, Al,[they are more reactive than carbon,

the yield will be very low]

Other less reactive metals by reduction withcarbon, most often in a blast furnace

Advantages of this method: less expense, self-sustaining process, produces in large quantity

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Ore of iron: HAEMATITE, contains Fe2O3

Raw materials used & Reactions involved Role of limestone

Making steel: passing oxygen, CO2,SO2 gases

escape, remaining acidic solid oxides are removed

by adding lime, necessary quantity of carbon is

added to make steel[alloys are harder than pure metals due to thedifference in atomic size]

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Ore: Zinc blende, contains zinc sulfide Extraction process and reactions involved Uses of zinc: battery case, alloys like brass, galvanising,

sacrificial protection etc.

Why is galvanising better than painting

(1)Zinc reacts faster than iron producing electrons which(2)

spread all over the metal surface and (3)prevent theoxidation of iron atoms(4) Equations Zn Zn2+ + 2e

Fe Fe2+ + 2eThe second reaction will be reversed by the presence of extra electrons from zinc

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Ore Bauxite, contains Al2O3

Cryolite is use to reduce the melting point of Al2O3 & increaseelectrical conductivity so that production cost will decrease

Explain the formation of CO,CO2 and F2 at anode & its replacementat intervals.

How is the temperature maintained during aluminium extraction?

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USES OF METALS

Copper

Used in electric cables

(1)Good conductor of electricity

(2) Ductile

can be drawn to thin wires

Aluminium

Used in aircraft frames

(1)Low density reduces fuel expense

(2)Does not corrode easily- easy tomaintain

Aluminium

Used in over-head power

cables

(1) Low density reduces sagging

(2) Good electrical conductivity

Note: Iron core is to strengthen the cable

Aluminium

Used in cooking vessels

(1) Does not corrode easily

(2) Good conductor of heat


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Anhydrous from to

Anhydrous from to

Uses of water at home and industry

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Describe the composition of clean air as being approximately 79 %nitrogen, 20 % oxygen and the remainder as being a mixture of noblegases, water vapour and carbon dioxide

Fractional distillation of liquefied air gives nitrogen and oxygen as theydiffer in boiling points

Sources of methane: (1) decomposition of vegetation and waste gases

(2) from digestion in animals

Green house gases: (1) carbon dioxide and(2) methane and may contribute to climate change


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Common pollutants in the air as being carbon monoxide, sulfur

dioxide, oxides of nitrogen and lead compounds

Source of : carbon monoxide from the incomplete combustion of

carbon-containing substances

Sulfur dioxide from the combustion of fossil fuels which contain

sulfur compounds leading to acidrain

Oxides of nitrogen from car exhausts (due to high temperature

inside the engine nitrogen reacts with oxygen)

State the adverse effect of common pollutants on buildings and on

health


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To remove CO, NOx and unburned hydrocarbons

Redox reaction

In presence of catalyst (Pt for example) NOx decomposes to N2 and

O2 2NO N2 + O2

Hydrocarbons are oxidised to carbon dioxide and water

CO is oxidised to CO2

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Conditions for rusting: Water and oxygenMethods of rust prevention

(1) Painting and other coatings to exclude oxygen(2) Galvanising

(3) Sacrificial protection


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Haber process for ammonia manufactureTemperature: 4500C,

Pressure:200 atmospheres,

Catalyst: Fe Contact process for H2SO4Temperature: 4500C,

Pressure :1-2 atmosphere,

Catalyst :V2O5Describe the reactions

SO3 is dissolved in H2SO4 not in water, why?

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Fertilizers usually contain N,P,and K

Example of a fertilizer made from hydrochloric acid:

ammonium chloride

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Sources of sulfur

Use of sulfur in the manufacture of sulfuric acid

Uses of sulfur dioxide (1)as a bleach in the manufacture of woodpulp for paper and as a (2) food preservative (by killing bacteria)

Describe the properties of dilute sulfuric acid as a typical acid

Sulfuric acid is a dibasic acid

Note: 10ml sulfuric aid will have the effect of 20ml hydrochloric acid of

the same concentration


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Calcium carbonate is used in the manufacture of(1) iron and of (2) cement

Lime (calcium oxide) is manufactured from limestone (calcium

carbonate) - by thermal decomposition: heating limestone toapproximately 9000C in a stream of air

Some uses of lime and slaked lime (which are basic)(1)in treating acidic soil to neutralise and(2)neutralising acidic industrial waste


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Define the terms:

Hydrocarbon, Saturated, Unsaturated

Homologous series

Isomerism


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Examples of fuels: coal, natural gas and petroleumMethane is the main constituent of natural gasPetroleum is a mixture of hydrocarbons and it is separated into usefulfractions by fractional distillation

The uses of the fractions are: refinery gas for bottled gas for heating and cooking gasoline fraction for fuel (petrol) in cars naphtha fraction for making chemicals kerosene/paraffin fraction for jet fuel

diesel oil/gas oil for fuel in diesel engines fuel oil fraction for fuel for ships and home heating systems lubricating fraction for lubricants, waxes and polishes bitumen for making roads


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Source: mainly petroleum

Generally unreactive

Physical properties C1 to C4 gasC5 to C17 liquid

above C17 solidChemical properties

(1) Highly inflammable(2) Substitution reaction with chlorine,

condition: presence of sunlight


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General molecular formula: CnH2n, where n=2,3,etcManufacture: by cracking of alkanes , temp: aprox 4500C,

catalyst: Al2O3Very reactive unlike alkanes

Describe the reactions of alkenes (in terms of addition reactions)with bromine, hydrogen and steam


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Functional group: OHGeneral molecular formula: CnH2n+1OH

Describe the formation of ethanol

(1)Fermentation [350

C, yeast, absence of air] &(2)Catalytic addition of steam to ethene [catalyst: Phosphoric acid]

Properties of ethanol: example of alcohol(1) highly flammable

(2) forms esters with carboxylic acids

Uses of ethanol as a solvent and as a fuel

Propanol


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Ethanoic acid

Physical properties:

Formation:By the oxidation of ethanol(1) by fermentation and(2) with acidified potassium manganate (VII)

Properties (1) A typical weak acid(2) Reacts with alcohol to form ester


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Preparation of estersHeat alcohol and carboxylic acid in presence of concentrated H2SO4 ascatalyst. This is a reversible reaction.

Alcohol + carboxylic acid ester + water

Name & structure of ester

Structure of methyl propanoate


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Addition polymers from unsaturated monomersCondensation polymers(1) Structure of Terylene (a polyester)

Note: Fats and oil have the same linkage as in Terylene but with different units

(1) Structure of Nylon (a polyamide)

Note: Proteins and Nylon possess amide linkage but with different unitsMonomers of Nylon


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Proteins are condensation polymers of amino acids.Proteins contain amide linkages as NylonStructure of proteins

Hydrolysis of proteins: by heating with concentrated HCl,

Separation of products by chromatography,Detection using locating agentsIdentification by comparing the spots of known samples or Rf values

Amide linkage


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Carbohydrates are compounds of Carbon, Hydrogen and Oxygen, theratio of Hydrogen to Oxygen should be as in water .

Complex carbohydrates are made of a large number of sugar units,

considered as joined together by condensation

polymerisation

Acid hydrolysis, heating with con HCl, of complex carbohydrates (e.g.starch) gives simple sugars, the procedure of separation is same as thatof proteins

The fermentation of simple sugars produces ethanol & carbon dioxide


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Fats and oils contain esters of glycerol with fatty acids .

Hydrolysis of fats and oils by heating with aqueous sodium hydroxidegives and glycerol (bye product)

Ester present in fats & oils


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One mole of atoms is the relative atomic mass expressed in grams

One mole of molecules/compounds is the relative molecular/formula mass in

grams

n = m/Ar (for atoms)

n = m/Mr (for molecules)

Molarity, M = n/V (V should be in dm3)

Note In gas reactions mole ratio = volume ratio. This is the case in solutions

with equal concentration. Percentage yield = (actual yield/ theoretical yield) X 100

Percentage Purity = (mass of the pure substance/total mass)X100

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chemistry important points

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