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.
