11Colloids and catalysis

📚
COLLOIDAL STATE
▢ Crystalloids vs Colloids:

Table 1: Crystalloids & colloids

Substance
Diffusion through animal/vegetable membrane
Examples
Crystalloids
Rapid diffusion
Egg, urea, sugar, salts
Colloids
Very slow / no diffusion
Gelatin, glue, silicic acid
▢ Definition: Colloidal state = heterogeneous system; solute particles \(10\,\text{Å}\) to \(10^3\,\text{Å}\) = \(10^{-7}\) to \(10^{-5}\,\text{cm}\); intermediate between true solution & suspension.
▢ Particle Size:

Table 1: Size comparison

True solution
Colloid
Suspension
\(<10^{-7}\,\text{cm}\)
\(10^{-7}\) to \(10^{-5}\,\text{cm}\)
\(>10^{-5}\,\text{cm}\)
Transparent
Translucent
Opaque
▢ Phases:
  • •
    Dispersed phase = internal / discontinuous phase
  • •
    Dispersion medium = external / continuous phase
  • •
    Colloidal solution = dispersed phase + dispersion medium

Table 1: Eight types of colloidal systems

S.N.
Dispersed phase
Dispersion medium
Type
Examples
1
Solid
Solid
Solid sol
Coloured glass, mixed salts, precious stones, alloys, gems
2
Solid
Liquid
Sol
Blood cells in plasma, ink, glue/gum, liquid boot polish, \(As_2S_3\) sol, gold sol, muddy water, starch, paints, protein
3
Solid
Gas
Aerosol
Smoke, dust in air, \(NH_4Cl\) fumes
4
Liquid
Solid
Gel / jelly / solid emulsion
Toothpaste, curd, honey, dry foot polish, butter, metal hydroxide gels, \(Fe(OH)_3\), cheese, vanishing cream
5
Liquid
Liquid
Emulsion
Milk, cream, ice-cream, cod liver oil
6
Liquid
Gas
Liquid aerosol
Cloud, fog, smog, sprays, mist
7
Gas
Solid
Solid foam
Bread, cake, pumice stone, rubber, occluded gases in metals, foam plastics
8
Gas
Liquid
Foam / froth
Cola-drinks \((pH\ 2.8-2.9)\) due to \(H_3PO_4 + H_2CO_3\); beer, soap lather, whipped cream, shaving cream
Medium-based names
❖ Water: Hydrosol / aquasol
❖ Alcohol: Alcosol
❖ Benzene: Benzosol
❖ Organic liquid: Organosol
Important examples
  • •
    Gases: high diffusion power → homogeneous mixture → no colloidal solution
  • •
    Milk & vanishing cream: oil dispersed in water
  • •
    Cream, ice-cream, cod liver oil: water dispersed in oil
  • •
    Butter: water dispersed in fat
  • •
    Old gel shrinkage + liquid loss = weeping of gel / syneresis
📚
LYOPHOBIC & LYOPHILIC COLLOIDS

Table 1: Lyophobic vs lyophilic sols

Feature
Lyophobic
Lyophilic
Affinity for dispersion medium
Very little
Great
Stability
Less stable
Stable
Reversibility
Irreversible
Reversible
Other name
Suspensoids
Emulsoids
Viscosity
Less viscous
More viscous
Surface tension
Low
Lower than dispersion medium
Molecular mass
High
High
Tyndall effect
High
Less than lyophobic
Brownian movement
High
Less than lyophobic
Examples
Metal sulphides, metal hydroxides
Glue, starch, gelatin, albumin
📚
PREPARATION OF COLLOIDAL SOLUTIONS
▢ Dispersion Methods:
❖ Meaning: Coarse aggregates → colloidal size
❖ Types:
    ◈ Mechanical dispersion: Large particles → smaller colloidal particles
    ◈ Bredig's arc method / electro-dispersion:
    • •
      Metal sols: Ag, Pt, Au, Cu
    • •
      Not for alkali & alkaline earth metals
    • •
      Metal electrode dipped in dispersion medium + electric arc → metal vapour → condensed into colloidal state
    ◈ Ultrasonic dispersion: Ultrasonic waves → colloidal dispersion
    ◈ Peptization:
    • •
      Precipitate → colloidal sol by small amount of electrolyte
    • •
      Fresh \(Fe(OH)_3\) gel + \(FeCl_3\) → colloidal \(Fe(OH)_3\) sol
    • •
      Cause: adsorption of electrolyte ions by precipitate particles
    • •
      Peptizing agents: sugar, gum, gelatin, electrolytes
▢ Condensation Methods:
❖ Meaning: Very small particles → colloidal particles
❖ Types:
  • •
    Exchange of solvents
  • •
    Change of physical state
  • •
    Chemical methods: double decomposition, oxidation, reduction, hydrolysis
❖ Examples:
  1. \(2H_2S + O_2 \rightarrow 2H_2O + 2S\)
  2. \(2AuCl_3 + 3SnCl_2 \rightarrow 3SnCl_4 + 2Au\) ; gold sol
  3. \(FeCl_3 + 3H_2O \rightarrow Fe(OH)_3 + 3HCl\) ; colloidal sol
📚
PURIFICATION OF COLLOIDAL SOLUTIONS
▢ Definition: Removal/reduction of impurities: electrolytes or soluble substances → minimum level.
▢ Dialysis:
  • •
    Dissolved crystalloid removed from colloidal solution through suitable membrane
  • •
    Membranes: animal membrane, parchment paper, cellophane sheet
  • •
    Electrolytes removed by electrodialysis
  • •
    Dialysis accelerated by hot water / electric field
  • •
    Separates glucose & protein
▢ Other Methods:
  • •
    Ultrafiltration
  • •
    Ultracentrifugation
📚
PROPERTIES OF COLLOIDAL SOLUTIONS
▢ General:
  • •
    Heterogeneous character → two phases
  • •
    Visibility: invisible to naked eye; visible by microscope
  • •
    Filtrability: pass through ordinary filter paper; not through parchment/fine membranes
▢ Surface Tension & Viscosity:
  • •
    Lyophobic sol: nearly same as dispersion medium
  • •
    Lyophilic sol: higher viscosity + lower surface tension
▢ Colligative Properties:
  • •
    Colloidal particles = physical aggregates, not simple molecules
  • •
    Number of particles small vs true solution
  • •
    Very low osmotic pressure
  • •
    Very small \(\Delta T_f\) depression / \(\Delta T_b\) elevation
▢ Tyndall Effect:
  • •
    Optical property; all colloids show Tyndall effect [IOM 2000]
  • •
    Beam through colloid visible as bright streak
  • •
    Illuminated path = Tyndall cone
  • •
    Simplest test: colloid or not
  • •
    Principle used in ultramicroscope
  • •
    Tail of comets = Tyndall cone due to scattering by tiny solid particles
  • •
    Longer wavelength transmitted more; shorter wavelength refracted/scattered more
  • •
    True solution: no visible light path
  • •
    Sugar solution: no Tyndall effect → true homogeneous solution
▢ Brownian Movement:
  • •
    Ceaseless zig-zag motion of colloidal particles
  • •
    Smaller particle size + lower viscosity → faster motion
  • •
    Cause: unbalanced bombardment by molecules of dispersion medium
  • •
    Helps sol stability; particles do not settle
  • •
    Ordinary suspension: no Brownian movement
▢ Charge on Colloidal Particles:
  • •
    Colloidal particles electrically charged: positive / negative
  • •
    Electric current through colloid → particles move toward electrode = electrophoresis / cataphoresis
  • •
    Electrophoresis → coagulation of colloid [MOE 2064]
  • •
    Only medium migrates, particles stationary → electro-osmosis [MOE]
  • •
    Electrophoresis determines nature of charge on colloidal particles
  • •
    Electrophoretic mobility = distance travelled by colloidal particle in 1 second under potential gradient \(1\,V\,cm^{-1}\)
▢ Coagulation:
❖ Definition: Colloidal state → suspension state; precipitation / flocculation.
❖ Examples:
  • •
    Positive \(Fe(OH)_3\) sol + \(NaOH\) → coagulation by \(OH^-\)
  • •
    Negative sol: positive ion effective
  • •
    Positive sol: negative ion effective
  • •
    \(As_2S_3\) = negative sol; coagulating power: \(Al^{3+} > Ba^{2+} > Na^+\)
❖ Hardy-Schulze Rule:
  • •
    Precipitating power depends on ion valency
  • •
    Higher valency → greater precipitating power
  • •
    For negative sol: \(M^{3+} > M^{2+} > M^+\)
  • •
    For positive sol: \([Fe(CN)_6]^{4-} > SO_4^{2-} > Cl^-\)
❖ Flocculating Value:
  • •
    Minimum concentration in millimoles/L of electrolyte required for complete coagulation/flocculation
  • •
    Smaller flocculating value → greater precipitating power
  • •
    \(\text{Coagulation value} \propto \dfrac{1}{\text{coagulating power}}\)
▢ Protective Action:
  • •
    Lyophilic sols more stable than lyophobic sols
  • •
    Lyophilic colloids protect lyophobic sols
  • •
    Lyophilic particles form layer around lyophobic particles → protection from electrolytes
  • •
    Protective power measured by gold number
  • •
    Gold number = mg of lyophilic colloid just preventing precipitation of \(10\,mL\) gold sol by \(1\,mL\) of \(10\%\,NaCl\)
  • •
    Lower gold number → higher protective power
  • •
    Gelatin gold number \(0.005-0.01\) → maximum protective power
  • •
    Starch gold number \(25-50\) → minimum protective power
📚
APPLICATIONS OF COLLOIDS
▢ Natural Applications:
  • •
    Blue colour of sky
  • •
    Fog, mist, rain
  • •
    Food articles
  • •
    Blood
  • •
    Soils
  • •
    Formation of delta
▢ Technical Applications:
  • •
    Electrical precipitation of smoke
  • •
    Purification of drinking water
  • •
    Medicines
  • •
    Tanning
  • •
    Cleaning action of soaps & detergents
  • •
    Photographic plates & films
  • •
    Rubber industry
  • •
    Paints, inks, plastics, lubricants, cement
📚
COLLOIDS: READ & DIGEST
▢ Important Points:
  • •
    Micelles = associated colloid; low concentration → solution; high concentration → colloid
  • •
    Ice cream: principle of excessive cooling
  • •
    Rubber in benzene = macromolecular colloid
  • •
    \(Fe(OH)_3\) sol in water = hydrophobic colloid
  • •
    Surfactant molecules form micelles
  • •
    Excess electrolyte → coagulation
  • •
    Soap solution = associated colloid
  • •
    Silver sol prepared by Bredig's arc method
  • •
    Bredig's arc not used for sodium sol: sodium reacts violently with water
  • •
    Alum + muddy water → coagulation of clay → clear water
  • •
    Silver iodide used for artificial rain: crystal structure similar to ice
  • •
    Foam = gas dispersed in liquid; examples: whipped cream, soap
  • •
    Smoke = solid dispersed in gas [IOM 2005]
  • •
    Particle size: suspension > colloid > solution [IOM 2047]
  • •
    Milk preserved by adding few drops of formaldehyde solution
  • •
    Purification of blood in body based on dialysis
  • •
    Sol particle filtration = ultrafiltration
  • •
    Ion coagulating capacity depends on nature of charge + amount of charge
  • •
    Electrolyte addition → destruction of colloids
  • •
    Colloid of silver used to cure eye disease under trade name protogol
  • •
    Colloidal sulphur used as germ killer
  • •
    Medicines more effective in colloidal state
  • •
    Colloidal gold injected as tonic to raise vitality
  • •
    Artificial rain: oppositely charged colloidal dust sprayed over cloud
  • •
    Cod liver oil = water-in-oil emulsion
  • •
    Cream separated from milk by demulsification
  • •
    Positive colloids: haemoglobin, metallic hydroxide, ferric hydroxide
  • •
    Negative colloids: starch, gelatin, Congo red, blood, \(As_2S_3\), metal sols Au/Ag/Pt/Cu
  • •
    Digestion of fats in intestine = emulsification
  • •
    Gel standing → small liquid amount exuded = syneresis
  • •
    Gold sol formed by Bredig's arc + reduction method
📚
ADSORPTION
▢ Definition: Accumulation of molecular species at surface of solid/liquid.
▢ Terms:
❖ Adsorbate: Species accumulated on surface
❖ Adsorbent: Material on whose surface adsorption occurs
Rate of adsorption
High initially → decreases till equilibrium
▢ Adsorption vs Absorption:

Table 1: Adsorption and absorption

Feature
Adsorption
Absorption
Distribution
Surface only
Throughout bulk
Concentration
Higher on surface
Uniform throughout
Speed
Instantaneous
Slow
▢ Sorption:
  • •
    Adsorption + absorption simultaneously = sorption
  • •
    Sorption of gases by metals = occlusion
▢ Types:

Table 1: Physical adsorption vs chemisorption

Feature
Physical adsorption / physisorption
Chemical adsorption / chemisorption / Langmuir adsorption
Forces
Van der Waals forces
Chemical bonds
Reversibility
Generally reversible
Almost irreversible
Specificity
Less specific
Highly specific
Temperature
Favoured at low temperature; decreases with rise in temperature
Slow at low temperature; increases with rise due to activation energy
Pressure
Increases with pressure
High pressure favourable
Heat of adsorption
Low: \(20-40\,kJ\,mol^{-1}\) / about \(5\,kcal\,mol^{-1}\)
High: \(40-400\,kJ\,mol^{-1}\) / \(10-100\,kcal\,mol^{-1}\)
Surface area
Increases with surface area
Increases with surface area
Layer
Multimolecular possible
Monomolecular / unimolecular layer
▢ Applications:
    ◉ _*type: bullet
  1. Production of high vacuum
  2. Gas masks: activated charcoal adsorbs poisonous gases
  3. Humidity control: silica gel, aluminium gel
  4. Separation of inert gases by coconut charcoal
  5. Softening of hard water
  6. Froth flotation process
  7. Curing diseases
  8. Chromatography, dyeing, catalysis
📚
CATALYSIS
▢ Definition: Catalysts alter reaction velocity by mere presence without change in mass/composition; phenomenon = catalysis.
▢ Catalytic Reaction Types:

Table 1: Homogeneous vs heterogeneous catalysis

Type
Condition
Examples
Homogeneous catalysis
Reactants + catalyst in same phase
  1. \(2SO_2(g) + O_2(g) \xrightarrow{NO(g)} 2SO_3(g)\)
  2. \(CH_3COOCH_3(l) + H_2O(l) \xrightarrow{HCl(l)} CH_3COOH(l) + CH_3OH(l)\)
  3. \(C*{12}H*{22}O*{11}(l) + H_2O(l) \xrightarrow{H_2SO_4(l)} C_6H*{12}O_6(l) + C_6H*{12}O_6(l)\)
Heterogeneous catalysis
Reactants & catalyst in different phases
  1. \(2SO_2(g) + O_2(g) \xrightarrow{Pt(s)} 2SO_3(g)\)
  2. \(N_2(g) + 3H_2(g) \xrightarrow{Fe(s)} 2NH_3(g)\)
  3. \(4NH_3(g) + 5O_2(g) \xrightarrow{Pt(s)} 4NO(g) + 6H_2O(g)\)
  4. \(4HCl(g) + O_2(g) \xrightarrow{Cu_2Cl_2(s)} 2H_2O(g) + 2Cl_2(g)\)
Special catalysts
  • •
    Wilkinson's catalyst: \([(PPh_3)_3RhCl]\) → hydrogenation of alkene
  • •
    Ziegler-Natta catalyst: \(TiCl_4\) in trialkyl aluminium
▢ Types of Catalysis:

Table 1: Catalysis types

Type
Meaning
Examples
Positive catalysis
Foreign substance accelerates rate
Fe, Ni, Pt, \(MnO_2\)
Negative catalysis
Substance retards reaction rate
\(H_2O_2\) decomposition retarded by ethyl alcohol, glycerol, acetanilide, \(H_3PO_4\)
Auto-catalysis
Product acts as catalyst in initial slow reaction; rate increases later
\(CH_3COOC_2H_5 + H_2O \rightarrow CH_3COOH + C_2H_5OH\); \(CH_3COOH\) = autocatalyst; nitroglycerine decomposition: \(MnSO_4\) = autocatalyst
Induced catalysis
One reaction influences rate of another reaction not occurring ordinarily
Induction by simultaneous reaction
▢ Characteristics of Catalysts:
  • •
    Unchanged in mass & chemical composition at end
  • •
    Small quantity sufficient for almost unlimited reaction
  • •
    Cannot initiate reaction
  • •
    Generally specific
  • •
    Cannot change equilibrium position in reversible reaction
  • •
    Helps achieve equilibrium in less time
  • •
    Promoters enhance catalytic efficiency
  • •
    Catalytic poisons destroy catalyst activity
  • •
    Optimum temperature required for maximum activity
  • •
    Positive catalyst lowers activation energy + provides new lower-energy pathway
▢ Promoters:
  • •
    Mo in Haber's process
  • •
    \(Cr_2O_3\) in methyl alcohol manufacture from water gas
▢ Catalytic Poisons:
  • •
    \(Al_2O_3\) in reacting gases reduces activity of platinized asbestos
  • •
    Arsenic \((As)\) = catalytic poison in contact process for \(H_2SO_4\)
▢ Theories of Catalysis:
  • •
    Intermediate compound formation theory
  • •
    Adsorption theory
▢ Enzyme Catalysis:
  • •
    Enzymes = biochemical catalysts
  • •
    Catalytic activity very high
  • •
    Each enzyme catalyses specific reaction
  • •
    Sensitive to temperature & pH
📚
ADSORPTION ISOTHERMS
▢ Definition: Relation between amount adsorbed by adsorbent and pressure/concentration of adsorbate at constant temperature.
▢ Freundlich Adsorption Isotherm:
  • •
    Amount of gas adsorbed does not increase as rapidly as pressure
▢ Langmuir Adsorption Isotherm:
  • •
    Adsorption occurs on surface only
  • •
    Surface completely covered by unimolecular layer
  • •
    Heat of adsorption same for all sites; independent of fraction of surface covered [I.E 2008]
  • •
    Molecules/atoms on different sites do not interact
  • •
    Unimolecular theory valid only at low pressure + high temperature
  • •
    At very high pressure → amount adsorbed reaches constant limiting value
▢ Adsorption Isobar: Graph between amount adsorbed \((x/m)\) and temperature at constant pressure.
📚
FACTORS AFFECTING ADSORPTION OF GASES BY SOLIDS
▢ Gas Nature:
  • •
    Easily liquefiable gases = high critical temperature → more adsorption
  • •
    Examples: \(SO_2, NH_3, HCl, CO_2\)
  • •
    Less liquefiable gases → less adsorption
  • •
    Examples: \(H_2, O_2, N_2\)
  • •
    High critical temperature → stronger Van der Waals forces → more adsorption
▢ Temperature:
  • •
    Adsorption is exothermic
  • •
    Increase in temperature → decrease in adsorption
  • •
    Physical adsorption \(\propto \dfrac{1}{Temperature}\)
  • •
    Chemical adsorption first increases, then decreases with temperature
▢ Pressure:
  • •
    Adsorption increases with pressure
  • •
    With pressure: fast increase → slow increase → independent of pressure
📚
CATALYSTS IN INDUSTRY

Table 1: Industrial processes and catalysts

Process
Catalyst / promoter
Haber's process: ammonia
Finely divided iron + Mo promoter
Ostwald's process: nitric acid
Platinised asbestos
Lead chamber process: \(H_2SO_4\)
Nitric oxide
Contact process: \(H_2SO_4\)
Platinised asbestos / vanadium pentoxide
Deacon's process: chlorine
Cuprous chloride \((Cu_2Cl_2)\)
Bosch's process: hydrogen
Ferric oxide + chromic oxide promoter
Synthesis of methanol
Zinc oxide + chromic oxide promoter
Hydrogenation of vegetable oils
Finely divided nickel
Bergius process: petrol
Iron oxalate
Ethyl alcohol from molasses
Yeast: invertase + zymase
\(O_2\) from \(KClO_3\)
\(MnO_2\)
Fischer-Tropsch process: hydrocarbons from \(H_2\) + CO
Co, \(ThO_2\)
📚
ADSORPTION & CATALYSIS: READ & DIGEST
▢ Important Points:
  • •
    Among \(N_2, CO, CO_2, HCl\): \(HCl\) adsorbed maximum by activated charcoal due to highest critical temperature
  • •
    Gas adsorption on solid surface generally exothermic because entropy decreases
  • •
    Hydrogen adsorption by palladium = occlusion; gas occupies interstitial positions in solid lattice
  • •
    Solvay's process for caustic soda: no catalyst required
  • •
    Dyeing of fibres involves sorption
  • •
    Chemisorption forms monomolecular / unimolecular layer
  • •
    Physical adsorption of gas on charcoal increases with pressure and decreases with temperature
  • •
    Ethyl alcohol retards oxidation of chloroform
  • •
    Curve of adsorption variation with pressure at constant temperature = adsorption isotherm
  • •
    Catalytic activity = ability of catalyst to accelerate chemical reaction
  • •
    Intermediate compound formation theory explains homogeneous catalysis
  • •
    Heterogeneous catalysis explained by adsorption theory
  • •
    \(SnCl_2/HCl\) catalyst in Stephen's reduction; aldehyde preparation
  • •
    Modern theory of catalysis based on number of free valencies
  • •
    Kjeldahl's method: \(CuSO_4\) catalytic agent
Q1.
The size of particles in suspension, true solution and colloidal solution varies in the order
Q2.
A colloidal system has particles of what size?
Q3.
Cloud or fog is an example of colloidal system of
Q4.
Milk is
Q5.
Smoke is an example of
Q6.
Butter is a colloid formed when
Q7.
Butter is
Q8.
The extra stability of lyophilic colloids is due to
Q9.
Flocculation value is expressed in terms of
Q10.
Light scattering takes place in
Q11.
Minimum quantity of electrolyte which can precipitate any colloid is
Q12.
The coagulation power of an electrolyte for As2S3 decreases in the order
Q13.
The charge on As2S3 sol is due to the adsorbed
Q14.
The migration of colloidal particles under the influence of an electric field is known as
Q15.
The sky looks blue due to
Q16.
Which one of the sols acts as protective colloid?
Q17.
Gold number represents
Q18.
What is gold number?
Q19.
Gold number is minimum in case of
Q20.
According to Langmuir adsorption isotherm, the amount of gas adsorbed at very high pressures
Q21.
Which colloid is used in treating eye disease?
Q22.
As2S3 sol is
Q23.
Small liquid droplets dispersed in another liquid is
Q24.
Alum helps in purifying water due to
Q25.
In physical adsorption, the force associated are
Q26.
Adsorption due to strong chemical forces is called
Q27.
A catalyst increases the rate of reaction because it
Q28.
Which is not a characteristic of a catalyst?
Q29.
In the reversible reaction a catalyst is the substance which
Q30.
In the hydrogenation of oils the catalyst used is
Q31.
In which of the following commercial processes a catalyst is not used?
Q32.
In the case of auto-catalysis
Q33.
The enzyme which can catalyse the conversion of glucose to ethanol is
Q34.
Rusting of iron is catalyzed by which of the following?
Q35.
In the Ostwald's process for the manufacture of HNO3, the catalyst used is
Q36.
A catalyst
Q37.
Which of the following will be most effective in the coagulation of Fe(OH)3 sol?
Q38.
Tyndall effect in colloidal solutions is due to
Q39.
Blood cells do not shrink in blood because blood is
Q40.
An example of dispersion of a liquid in a gas is
Q41.
Which of the following is not represented by sols
Q42.
Tyndall effect is exhibited by
Q43.
Surface tension of lyophilic solution is
Q44.
Adsorption is multilayer in case of
Q45.
Which of the following term is negative in adsorption?
Q46.
Which of the following is adsorbate?
Q47.
The decomposition of H2O2 can be slowed by the addition of a small amount of acetamide. The later acts as