PAC 2 Flashcards
Definition of surface tension
A measurement of the cohesive energy present at an interface.
Examples of surface tension
Soaps/Detergents: lowering surface tension of water to help cleaning.
Clinical test: Bile (Jaundice) will lower the surface tension so powdered sulfur will sink in urine containing bile.
Washing with hot water: Better wetting agent that lowers surface tension.
Definition of cohesive and adhesive forces
Cohesive: forces of attraction between molecules of same substance.
Adhesive: forces of attraction between molecules of different substance.
List down surface phenomena
Adsorption, capillarity, surface tension and electrical double layer.
Contact angle of convex and concave meniscus
Convex: contact angle larger than 90°, surface tension will be positive.
Concave: contact angle smaller than 90°, surface tension will be negative.
Differences of hydrophobic and hydrophilic surface
Hydrophobic: High contact angle, poor adhesiveness, poor wettability, low solid surface free energy (High surface tension)
Effect of solute concentration on surface tension
a. Inorganic salt: increase surface tension.
b. Alcohols: decrease surface tension.
c. Surfactants: decrease surface tension but no more effect when minimum is reached.
Gibbs isotherm
For ideal or very dilute solutions with 2 components, if surface activity is negative, solution body contains more solute than its surface so expulsion of solute from surface. If surface activity is positive, solution body contains less solute than its surface so adsorption of solute from solution by the surface.
Coefficient of spreading
Coefficient of spreading B on surface of A, S_BA, is the difference between work of adhesion and work of cohesion. If S_BA is positive, liquid B will spread on surface of A like oil on water. If S_BA is negative, liquid B will remain as a drop on surface A.
Definition of adsorption
A process where molecules diffuse from the bulk of a fluid to the surface of the solid, forming a distinct adsorbed phase. Adsorbent is the solid that provides surface while adsorbate is the adsorbed gas to be adsorbed on solid.
Application of adsorption
Purification, de-pollution, solvent recovery and natural gas storage.
Common adsorbents
Molecular sieve, activated carbon, silica gel and activated alumina.
Explain physisorption
It is to determine the surface area, pore shape and size, size distribution of porous solid materials. Adsorbent and adsorbate interact weakly by van der waals forces. Physisorption increases when T decreases and P increases. Desorption occurs when conditions are reversed.
Explain chemisorption
It is the formation of chemical bonds between adsorbate and adsorbent. Simple reversal is not possible. Chemisorption increases when T increases. It is localized and occurs only on certain surface sites.
Differences between physisorption and chemisorption
Van der waals for physisorption and chemical bonds for chemisorption.
Multi adsorbed layer for physisorption and only 1 adsorbed layer for chemisorption.
Low adsorption heat for physisorption and high adsorption heat for chemisorption.
Low selectivity for physisorption and high selectivity for chemisorption.
Low temperature for physisorption to occur and high temperature for chemisorption to occur.
Factors affecting adsorption
Nature of adsorbents. Large surface area and very porous structure of adsorbents. Activating adsorbents by heating to high temperature in vacuum and impregnation.
Adsorption increases with increasing critical temperature and boiling point.
Adsorption increases with increasing surface area.
Adsorption increases with increasing molecular weight of adsorbate.
Classification of adsorption isotherms
Stage I to V
Isotherm equations
Langmuir adsorption isotherm
Assumptions: Ideal gas, monolayer adsorption, uniform solid surface, localized adsorption, adsorbed molecules do not interact with one another.
Plot graph P/y=1/a+(b/a)P
Strong adsorption, θ=1. Weak adsorption, θ=bP.
Error: Physisorption can be more than 1 layer.
Freundlich isotherm
Plot graph y=kP^(1/n)
BET isotherm
Assumptions: Same as Langmuir but allows multi layer.
Useful in surface area determination, A_s=A_m (V_m/V_(T.P) ×6.022×10^23 )
N_S=N_m×6.022×10^23
Plot graph (P/P_0 )/V(1-P/P_0 ) =1/(cV_m )+(c-1)/(cV_m ) (P/P_0 )
Definition of catalysis
It is an action by catalyst which takes part in a chemical reaction process and can alter the rate of reactions and yet catalyst will return to its original form without being consumed at the end of reaction.
Definition of catalyst
It is a substance that participates in chemical reactions by increasing the rate of reaction and remains intact after the reaction is complete.
Definition of activation energy
The energy required to overcome the reaction barrier.
Mechanism of catalyzed reaction
Reaction thermodynamics determined whether a reaction can procced, not catalyst. If ∆G°>0 then reaction will not proceed as there is no thermodynamic driving force.
Advantages of catalytic process
Increase reaction rates = fast, low investment cost, low energy consumption, reduces waste and better control of process (safety, flexible).
Properties of good catalyst
Able to promote rate of desired reactions. High selectivity which only promotes the rate of desired reaction and retard undesired reactions. High stability to resist deactivation caused by erosion, pressure shock, thermal deterioration and presence of impurities. Large surface area by making solid into porous structure.
Compositions of catalyst
Active phase where the reaction occurs. Promoter which increases catalyst’s activity and extend catalyst’s lifetime. Support or carrier that increases mechanical strength and surface area. Common supports are alumina, silica and zeolite (mixture of alumina and silica) which are acidic and may be inert.
Differences between Langmuir-Hinshelwood mechanism and Eley-Rideal mechanism
Langmuir deals with surface-catalysed reaction in which 2 or more reactants adsorb on surface while Eley deals with 1 reactant (A) adsorb on surface without dissociation and other reactant (B) approaching from gas to react with first reactant (A).
Langmuir: A(g)+B(g)↔A(ads)+B(ads)→P
Eley: A(g)↔A(ads) □(→┴(+B(g) ) ) P
Langmuir rate of reaction: r=k[A][B]=kθ_A θ_B
Eley rate of reaction: r=k[A][B]=kθ_A P_B
Langmuir: Both A and B weakly adsorbed, r_i=k^’ P_A P_B
A strongly adsorbed, r_i=(kb_B P_B)/(b_A P_A )
Eley: A weakly adsorbed/ low partial pressure of A, r_i=k^’ P_A P_B
A strongly adsorbed/ high partial pressure of A, r_i=kP_B
Definition of half-life
Time required for concentration of a reactant to decrease by half of its initial value.
Car catalytic converter
Catalytic converter is a device that uses a catalyst to convert harmful compounds into harmless compounds. Harmful compounds are hydrocarbon, carbon monoxide and nitrogen oxides. Catalyst used is platinum and palladium coated onto a ceramic honeycomb attached to exhaust pipes.
Definition of colloids
Dispersion of small particles of one material in another.
Difference between true solution, colloidal solution and suspension
True solution has smallest size of particles and suspension has largest size of particles. True solution is homogeneous while colloidal solution and suspension are heterogeneous. Particles of true solution diffuse rapidly through both filter and parchment paper; colloidal solution only diffuse through filter paper while suspension does not diffuse through both filter and parchment paper. Particles of true solution are not visible to naked eyes; colloidal solution are studied through ultra-microscope while suspension are big enough to be seen by naked eyes. True solution is transparent, colloidal solution is translucent and suspension is opaque.
Stability of colloidal dispersions
Stable colloidal dispersions are which dispersed particles remain single entities and do not coagulate. There are 2 types of stability, thermodynamic and kinetic stability.
Interactions between particles in colloidal solution
Van der waals forces, electrostatic double layer force and acid-base interaction/polymer interactions.
Types of colloidal dispersions (disperse phase-dispersion medium)
Gas-gas: NONE.
Gas-liquid: Foam (Whipped cream).
Gas-solid: Solid foam (aerogel, Styrofoam).
Liquid-gas: Liquid aerosol (fog, mist).
Liquid-liquid: Emulsion (milk, mayonnaise).
Liquid-solid: Gel (gelatin, jelly, cheese).
Solid-gas: Solid aerosol (smoke, air particles).
Solid-liquid: Sols (paint, blood, pigmented ink).
Solid-solid: Solid sols (cranberry, ruby glass).
Sols
Dispersion of solid in a liquid, 2 types: lyophilic (gum/starches/gelatin/proteins) which is more stable. Coagulation occurs only when large amount of electrolyte added. Lyophobic (sols of metal) is unstable. Coagulation occurs when small amount of electrolyte is added.
Particle shape in colloidal
Particle symmetry is factor determining overall properties of colloidal systems. Theoretical shapes are prolate, oblate, rod, disc and random coil. Easiest model to treat theoretically is sphere.
Optical properties of sols
Light travelling through colloidal dispersion will scatter in all directions in space because particles interfere with a motion, Tyndall effect. Amount of light scattered depends on size of colloidal particles and refractive index ratio.
Stern model
Stern model contains Helmholtz layer (electrical double layer) and Gouy-Chapman model (diffuse layer). Helmholtz layer is the charged solid surface in liquids. Charging mechanisms are ionization, ion adsorption and ion dissolution. A solid with contact to a liquid tends to develop potential difference across interface which causes some counter-ions to adsorb onto solid surface. Gouy-Chapman model is the thermal motion of ions as ions are point charges. Thermal fluctuations tend to drive counter-ions away from a surface.
Electrokinetics effect
Electrokinetics effects are used to evaluate the zeta potential which is the electric potential that exists at the shear plane. Shear plane is the interface between hydrated particle and bulk solution. Magnitude of zeta potential is the degree of electrostatic repulsion between adjacent similarly charged particles in dispersion. Colloids with high zeta potential are electrically stabilized and those with low zeta potential tend to coagulate/flocculate.
Technique of measuring electrokinetics
Electrophoresis, measures velocity, particles are moving because of applied electric field. pH affects movement of particles; isoelectric point is where particles uncharged.
Electroosmosis, measures velocity, liquid in capillary is moving because of applied electric field. Positive colloids to positive and negative colloids to negative.
Sedimentation potential, measures potential, particles are moving because of gravity, accelerated with ultracentrifuges.
Streaming potential, measures potential, liquid is moving because of pressure gradient. It is reverse effect of electroosmosis.
Stability of colloids
Protective colloid is adding hydrophilic colloid to hydrophobic to surround the hydrophobic particles to shield the hydrophobic system from destabilizing effects of electrolytes. Protective solvent sheath and electric charge must be removed if hydrophobic colloid was to be precipitated. Examples are gelatin and methylcellulose.
Buffer salts to maintain pH for product stability. As phosphate buffers contribute to potential instability by forming insoluble salts with metallic ions and precipitating insoluble phosphate salt from colloidal dispersion may co-precipitate colloidal particles along, substitute phosphate buffers to non-phosphate buffers.
Destabilizing colloidal suspension
Unstable colloidal suspensions form flocs as particles aggregate due to interparticle attractions. Remove electrostatic barrier that prevents aggregation of particles by adding salt to suspension or change pH of suspension to neutralize the surface charge of particles. Repulsive force that keep colloidal particles separate would be removed and allowed coagulation due to van der waals forces. Add charged polymer to flocculate negatively charged colloidal particles. Add depletants that cause aggregation due to entropic effects.
Definition of emulsions
Colloidal mixtures of 2 immiscible liquids such as oil and water, stabilized by an emulsifier.
Types of emulsions
Macroemulsions is thermodynamically unstable (metastable) and will break when left alone for a long period of time. Microemulsions is thermodynamically stable.
Stabilizing factors
Add surfactants that adsorbs strongly at interface between 2 immiscible phases. Add simple inorganic electrolytes, potassium thiocyanate KCNS. Add macromolecular stabilizers such as proteins, polysaccharides and synthetic polymers. Use finely divided solids that adhere to interface due to surface energy effects.
Destabilizing effects on emulsions
Creaming, difference in density between drop and medium. Breaking, 2 phases separate completely forming continuous layer. Coalescence, 2 drops merge into 1. Flocculation, small spheres of oil stick together forming clumps/flocs. Ostwald ripening, small droplets decrease in size while large drops grow.
Effects of surfactants
To reduce surface free energy and protect drops against coalescence. Surfactants anchor its hydrophilic part into water and its lipophilic part into oil. Hydrophilic part and lipophilic part adsorbed and arranged around interface. Interfacial tension is reduced by emulsifier, force to separate oil and water weakened, mixing of oil and water easier.
Classification of surfactants
Anionic, surface active portion bears a negative charge. Soaps/detergents/phosphates.
Cationic, surface active portion bears a positive charge. Amine salts/ammonium salt.
Zwitterionic, surface active portion bears both charges. Long chain amino acid salt/betaines.
Nonionic, surface active portion bears no charges. Long chain esters/ethers.
HLB system
To calculate the amount of emulsifier needed to stabilize an emulsion. High HLB number means emulsifier has more hydrophilic group.
HLB=7+ΣH+ΣL
HLB=((mass×HLB)+(mass×HLB))/(mass+mass)
Ways to break emulsions
Disrupt water or break emulsifier film to let surface dry/freeze. Increases water surface tension by salt. Mechanical agitation. Denature the emulsifier by heat. Precipitate the emulsifier by acid/alkali. Attack the emulsifier by enzymes.
Effect of emulsifier concentration
Emulsifiers gather on interface and surface tension decreases as concentration increases. A uniform mono molecular layer on the surface causes surface tension drops to the minimum as concentration increases. Further increase in concentration causes micelle formation. It is the excess molecules in which the lipophilic groups are positioned face to face gather and no change in surface tension. Concentration to start micelle formation is the critical micelle concentration (CMC). As concentration increases, shape of micelle changes from spherical to rod-shape to lamellar (liquid crystal).
Definition of chromatography
Physical method of separation, components separated to 2 phases, stationary phase and mobile phase that moves in a definite direction.
Use of gas-liquid chromatography (GC)
To determine organic compounds that are volatile, between 50℃-300℃, non-ionized organic molecules and organometallic compounds.
Type of column in GC
Packed column, suitable for compounds that do not require high resolution applications.
Capillary column, stationary phase lining tube walls as a thin adherent film.
Consequences of injecting mixture to column in GC
Condense on stationary phase. Dissolve in the liquid on the surface of the stationary phase. Remain in gas phase. Those more soluble in liquid will spend more time adsorbed into stationary phase, less soluble ones will spend more of their time in gas.
Definition of partition
Process where a substance divides itself between 2 immiscible solvents because it is more soluble in one than the other.
GC detectors
Thermal Conductivity Detector (TCD)
Mobile phase passes over hot tungsten/rhenium filament. Filament’s electrical resistance depends on temperature which depends on thermal conductivity of mobile phase. When analyte mixes with carrier gas, thermal conductivity decreases. Filament gets hotter, electrical resistance increases so voltage across filament changes. The change is proportional to the concentration of sample in carrier gas. Carrier gas are helium and hydrogen. It is suitable for analysis of gaseous mixture and permanent gases. It is universal and response is reproducible. However, it is not sensitive enough.
Flame Ionization Detector (FID)
Eluate burned in mixture of hydrogen and air. Carbon atoms produce CH radicals. Electrons flow from anode to cathode and current detected. It is universal but not for carbonyl and carboxylic group. It is sensitive and measures in ppb concentration range. However, samples will be destroyed. It is also insensitive to non-hydrocarbons.
Electron Capture Detector (ECD)
Uses radioactive beta emitter (electrons) to ionize some of the carrier gas and produce current between a pair of electrodes. When organic molecules with electronegative functional groups (halogens/phosphorus) pass by detector, they capture some of the electrons and reduce current measured between electrodes. It is sensitive to halogen, conjugated carbonyls, nitriles and nitro compounds. However, it is insensitive to hydrocarbons, alcohols, ketones.
Nitrogen-phosphorus
Sensitive to organics with N and P. It is modified FID.
Flame photometric
Eluate passes through hydrogen or air flame and excited atoms emit light.
Photoionization
Vacuum UV source to ionize aromatic and unsaturated compounds, electrons detected. It is insensitive to saturated hydrocarbons.
Definition of degree of resolution
How well 2 elution peaks can be differentiated in chromatographic separation. Rs greater than one means peaks can usually be differentiated successfully.
High Performance Liquid Chromatography (HPLC)
It is highly improved form of column chromatography, much better separation because of greater surface area by using smaller particle size for column packing material, for interactions between surface phase and molecules flowing past it. Solvent is forced through under high pressures instead of dripping through column under gravity. It is highly automated and extremely sensitive.
Equipment for HPLC
Mobile phase supply system. It has pump to provide high pressures. Solvent reservoirs filled with a range of miscible solvents of different polarities.
Sample injection system. Injection of sample is automated using a loop injector. In load position, sampling loop is isolated from mobile phase and open to atmosphere. Injector turned to inject position, mobile phase directed through the sampling loop and sample swept onto column.
Column. Analytical column is responsible for separation while guard column is placed before analytical column to protect it from contamination. Stationary phase properties in column is determined by nature of functional group attached to it.
There is 2 type of detectors. UV detector where most organic compounds absorb UV light of various wavelength. There is direct reading of how much light is absorbed. Differential refractometer or universal detector which measures changes in refractive index of eluent results from presence of solutes. Very sensitive to temperature changes. Not suitable to use when gradient elution is due to change in baseline and solvent’s index is close to solute’s index.
Normal phase HPLC
Column is filled with tiny silica particles while solvent is non-polar. Polar compounds in mixture passing through column will stick longer to polar silica than non-polar compounds. Non-polar ones will pass more quickly through the column.
Reversed phase HPLC
Column size same but silica modified to non-polar by attaching long hydrocarbon chains to its surface while polar solvent used. Polar molecules in mixture will spend most of their time moving with solvent, travel through column more quickly.
Definition of retention time
Time taken for compound to travel through column to detector.
Factors affecting retention time
Pressure used that affects flow rate of solvent. Nature of stationary phase. Exact composition of solvent. Temperature of column.
Application of HPLC
Suitable for non-volatile samples, able to analyze more wider range of components than GC. Useful in quantitative and qualitative analyses of industries.
