Properties of Gases and pH-partition theory PDF

Preview

Summary

first year pharmacy ...

Description

Collision Theory In order for molecules to react they must collide. As temperature increases, the molecules move faster and the collision frequency increases. Therefore, the greater the number of collisions the faster the rate. Thus reaction rate should increase with an increase in temperature. Also, the more molecules present, the greater the probability of collision and the faster the rate. Thus reaction rate should increase with an increase in the concentration of reactant molecules.

Kinetics A plot of [A] vs time is linear = zero order reaction A plot of ln [A] vs time is linear = first order reaction A plot of 1/ [A] vs time is linear = second order reaction Units of K Zero order First order Second order

Equation of State for an Ideal Gas PV = nRT P: Pressure (Pascals) P = F / A V: Volume n: number of moles R: Universal Gas Constant T: Temperature in Kelvin. Diathermic boundary = change of state when 2 objects of different temperatures are brought into contact. Adiabatic = no change of state. Thermal equilibrium: no change of state across a diathermic boundary. Charles’ Law, Boyle’s Law etc. Partial Pressure Pi = P (xi) The partial pressure is the pressure of the total gas multiplied by the molar fraction of the gas.

Lattice Structure Body centred cubic and face centred cubic. Phase: A state of matter, uniform throughout, which is similar in chemical composition and physical state. A constituent of a system is a chemical species (an ion or molecule) that is present A mixture of ethanol and water has two constituents A component is a chemically independent constituent of a system The number of components (C) in a system is the minimum number of independent species necessary to define the composition of all phases present in the system The Phase Rule: F = C – P + 2 F = Variance (amount of factors that can be changed without changing the phases in a system) C = Components (#) in a system P = Number of phases in a system Transition Temperature: The temperature at which two phases are in equilibrium Vapour Pressure: The vapour pressure of a liquid or solid is the pressure exerted by the vapour in equilibrium with the condensed phase. The temperature at which the vapour pressure of a liquid is equal to the external pressure is called the boiling point. The temperature at which the surface disappears is called the critical temperature (T). The vapour pressure at this point is called the critical pressure (PC). Melting temperature/freezing temperature: when the solid and liquid phases coexist. Triple point: when all 3 phases coexist in a system (solid, liquid and gas).7 Raoult’s Law: partial vapour pressure of each components in a liquid mixture is approximately equal to the mole fraction of the component. A molecular interpretation of the law considers the rate at which molecules leave and enter the liquid phase. The rate of A leaving is obstructed by the presence of component B. However, the presence of B does not affect the rate of A returning to the liquid phase. The number of A molecules leaving the liquid surface is proportional to the amount of them at the surface which is proportional to the mole fraction.

Rate of Passive Diffusion:

PC = Permeability constant A = Surface area of the membrane C1 and C2 are the concentrations on either side of the membrane. C1 is outside the membrane while C2 is inside. If the Concentration is higher outside it leads to a higher concentration gradient which leads to a higher rate of passive diffusion. This only works for smaller molecules.

pH-Partition Theory The dissociation constant, lipid solubility and pH of the fluid at the absorption site determines the extent of absorption from a solution. The interrelationship among these parameters is known as the pH-partition theory. The GIT is a lipid bilayer barrier. The drug is more readily absorbed in the unionised form. Most drugs are absorbed by passive diffusion. To cross a lipid membrane the drug must be soluble in the lipid material of the membrane to travel through it as well as soluble in the aqueous phase to get in/out of the membrane. The pKa of the drug and pH of GIT/bloodstream affect how readily absorbed the drug is as these will affect the degree of ionisation of the drug. The partition co-efficient is a measure of the drug’s lipophilicity. It shows how a drug is distributed across an aqueous and organic phase (solvent). The partition co-efficient is then the ratio of concentration of drug across the two phases. logP! (partition coefficient) must be lower than 5 according to the Lipinski rule of 5 to become a successful API molecule. We can measure the partition coefficient in the lab by distributing a known quantity of drug between octanol and water. The two phases are then assayed (by titration) to measure the amount of drug in each phase. Octanol is representative of the lipid bilayer in cells.

Action of local anaesthetics is important

A phase diagram in physical chemistry is a type of chart used to show conditions at which thermodynamically distinct phases can occur in equilibrium. Transition state theory (TST) explains the reaction rates of elementary chemical reactions. The theory assumes a special type of chemical equilibrium (quasi-equilibrium) between reactants and activated transition state complexes. The transition state complexes can dissociate to form the reactants or go on to form products. Kinetic theory is used to explain the rate of this conversion....