Physical Pharmacy I Lecture 9 PDF

Ministry of Higher Education and Scientific Research Al- Amal College for Specialized Medical Sciences Pharmacy department, undergraduate, 2nd class Year 2024-2025 Physical pharmacy I Lecture 9...

Ministry of Higher Education and Scientific Research Al- Amal College for Specialized Medical Sciences Pharmacy department, undergraduate, 2nd class Year 2024-2025 Physical pharmacy I Lecture 9 Dr. Ahmed AL-mouswy Reference text: Physical Pharmacy by Alfred Martin et al. 1  Thermodynamic Reversibility A thermodynamic equilibrium is a state in which the system properties are not undergoing any changes with time. A reversible process is one carried out in such as way that, when undone, the original equilibrium of the system will be restored  Thermodynamic Reversibility An example of reversible process is increasing the pressure on a gas infinitesimally slowly so that all the work applied will be converted to an internal energy (no energy is lost permanently through other forms (e.g. by friction)). When the exact pressure increment is removed infinitesimally slowly, the process will reverse itself exactly, and the original equilibrium will be restored.  Thermodynamic Reversibility A reversible process is practically impossible (it is an ideal process). One feature of a reversible process is that it can yield the maximum amount of work; any other (irreversible) process would generate less work, because some energy would be permanently lost (e.g., by friction).  Thermodynamic Reversibility When an energy is permanently lost during a process (not converted to an internal energy), the process is irreversible. An example of irreversible process is the conversion of mechanical work into frictional heat; there is no way, by reversing the motion of a weight along a surface, that the heat released due to friction can be restored to the system. All real processes are irreversible process  Thermodynamic Spontaneity Some processes happen spontaneously, other processes do not. E.g. objects fall down spontaneously, but throwing them up requires an external work. A spontaneous process is one that occurs “naturally” (without intervention). E.g. diamond converts to graphite. A non-spontaneous process is one that does not occur “naturally” (needs intervention to occur). E.g. throwing objects up. All spontaneous processes are irreversible.  Thermodynamic Spontaneity Heat flows naturally only from hotter to colder bodies. Gases expand naturally form higher to lower pressure. Solute molecules diffuse from a region of higher to one of lower concentration. These spontaneous processes will not proceed in reverse without the intervention of some external force to facilitate their occurrence.  Thermodynamic Spontaneity The first law of thermodynamics simply state energy must be conserved when it is converted from one form to another. It was once thought that a negative ∆H (evolution of heat) was itself proof of a spontaneous reaction. Many natural reaction do occur with an evolution of heat: however, the spontaneous melting of ice at 25⁰C is accompanied by absorption of heat (∆H is positive) Therefor : the function state (enthalpy change, ∆H) in the first law, dose not determine whether the process occurs spontaneously or not.  2nd Law Of Thermodynamics This inability to predict the natural direction of a process based on energy considerations alone (e.g. ∆H) requires another state function. The second law uses a new state function called entropy change (∆S) represents the probability of the occurrence of a process and the tendency of a system to approach a certain state of energy equilibrium.  2nd Law of Thermodynamics In spontaneous process, thermal energy tend to be spread. (e.g. gas molecules spread out all over the can) (high energy spreading). (e.g. gas molecule do not group at the bottom of the can) (low energy spreading). Entropy is a measure of the degree of the spreading and sharing of the thermal energy within a system. As thermal energy spreading increases, entropy will also increase.  2nd Law of Thermodynamics The entropy change ∆S is equal to the heat change in an isothermal reversible process divided by the absolute temperature at which the heat change occurs. ∆𝑆 = 𝑸𝑸𝑟𝑒𝑣/𝑇 Q rev is the heat change in an isothermal reversible process (Joule) T is the absolute temperature (kelvin) The units of entropy are energy per degree kelvin (J K -1 or cal K -1 ). The entropy change for the system depends only on its current state and is independent of the path. Entropy change ∆S is a state function Entropy change ∆S is an extensive property  2nd Law of Thermodynamics The second law of thermodynamics state all natural processes are accompanied by a net gain entropy of the system and its surroundings (i.e. ∆S>0). ∆S net = ∆S system + ∆S surroundings Then the second law says ∆S net > 0 (spontaneous processes „irreversible process‟) ∆S net = 0 (system at equilibrium „reversible processes‟)  3rd Law of Thermodynamics The entropy of a perfect crystal at 0 K is zero because the crystal arrangement must show the greatest orderliness at this temperature. The third law of thermodynamics refers to an ideal state (0 K) which is practically impossible.  3rd Law of Thermodynamics The third law is used to set a scale for measuring the absolute entropy (S) and make it possible to calculate the absolute entropies of pure substances. The absolute entropy of a pure substance at a given temperature is the sum of all the entropy it would acquire on warming from absolute zero (where S=0) to the particular temperature.  3rd Law of Thermodynamics Standard molar entropy S⁰ is the entropy of 1 mol of a substance at standard condition (JK-1 mol-1) Standard entropies for different substances can be calculated or measured ( always > 0) For similar substances: S⁰ gas > S⁰ liquid > S ⁰ solid An increase in molecular weight lead to an increase in S⁰ CH4 S⁰ = 186.3 JK-1 mol-1 C2H6 S⁰ = 229.6 JK-1 mol-1 C3H8 S⁰ = 269.9 JK-1 mol-1  Gibb’s Free Energy  Free energy function ∆G  Free energy application  Free Energy Function ∆G Definition There are two factors involved in determining the direction of chemical change: The system seeks to minimize its energy (∆H) The system seeks to maximize its entropy (∆S) Gibbs free energy ∆G, is a state function that links the first and second law of thermodynamics and determine the direction of a chemical change. ∆G = ∆H - T ∆S  Free Energy Function ∆G Interpretation If ∆G is negative it means that the process is spontaneous If ∆G is zero, it means that the system at equilibrium. If ∆G is positive it means that the process is not spontaneous. A more negative ∆H and a more positive ∆S favors spontaneous reaction, by making ∆G more negative. If T ∆S < ∆H, and ∆H is negative, then ∆G will be negative (i.e. the process is spontaneous). If T ∆S < ∆H, and ∆H is positive, then ∆G will be positive (i.e. the process is not spontaneous). If T ∆S > ∆H, then ∆G will be negative (i.e. the process is spontaneous regardless whether ∆H is negative or positive).  Free Energy Application Example 1 ΔH and ΔS for the transition from ice to liquid water at 25 °C and 1 atm are +1650 cal/mole and +6 cal/mole deg), respectively Compute ΔG for the phase change and indicate whether the process is spontaneous. The process leads to: An increased freedom of molecular movement (ΔS is positive) A increased molecular energy (ΔH is positive) ΔG = ΔH − T ΔS ΔG = 1650 − (298 × 6) = -138 cal/mole The process is spontaneous because T ΔS is sufficiently larger than the positive value of ΔH to make ΔG negative ΔH and ΔS for the transition from liquid water to ice at −10°C and at 1 atm pressure are −1343 cal/mole and −4.91 cal/mole deg, respectively. Compute ΔG for the phase change and indicate whether the process is spontaneous. The process leads to: A decreased freedom of molecular movement (ΔS is negative) A decreased molecular energy (ΔH is negative) ΔG = ΔH − T ΔS ΔG = (−1343) − [263 × (−4.91)] = −51.67 cal/mole The process is spontaneous, as reflected by the negative value of ΔG.  Some Pharmaceutical Application Of Thermodynamics 1-Thermodynamic concepts can be used to explain the various phenomena. For example, the diffusion of the drug through a biological membrane is a spontaneous process. 2-The enthalpy, as a thermodynamic measure, is useful to understand how drug molecules dissolve in a solvent system (During dissolution). 3-Entropy is the other thermodynamic measure for the determination of the amount of energy that is not useful to do work. The higher the entropy of a system, the closer that system to its point of equilibrium and the lower its capacity to do work 22