Chemical Kinetics
CONTAINTS :-
- Molecularity of Reaction
- Order of Reaction
- Determination of Order
- Comparison of Different Orders
- Generalised Rate Expression
- Rate Determining Step
Chemical kinetics involve the study of the rate of a chemical process. The rate of a reaction can be understood by studying the time course of changes in the concentration.
The rate, velocity or speed of a reaction is given by #(dc/dt). Here de is the small change in the concentration within a given time interval dt. The negative sign indicates the decrease in concentration over a period of time, which normally is the case, when the concentration refers to reactants. In case of reactions in the gaseous state, rate is evaluated by studying the time course of changes in the pressure of the system.
The rate of reaction is appreciable only when the reactants are present either in a solution phase or in a gaseous state. In these states, the molecules assume three dimensional spatial arrangement, which facilitates a selective modification of the functional groups in the chemi cal structure.
Thus, chemical reactions may be faster in solution dosage forms than in suspensions. Drugs in the solid state decomposes very slowly.
Kinetic principles have found applications in pharmacy.
 |
| Apparent zero order suspensions |
1. Drug stability:
Chemical kinetics provide the basis to predict drug stability. The extent of inactivation of drug due to various environ mental adverse conditions can be understood from the drug stability studies. It is expressed as a rate process. These studies help to predict the expiry period (shelf life) of a product.
2. Dissolution :
Normally, the drug is expected to release from the solid dosage forms (tablets etc..) and immediately go into molecular solution. This process is called dissolution. It is expressed in terms of a rate process. The greater the rate of dissolution, the faster the absorp tion
3. Drug release :
A number of prodrugs are currently entering into the market. Prodrugs are defined as those agents which do not have any therapeutic activity as such, but are converted back in vivo to their parent compounds. Prodrugs are specially designed when the parent drug is poorly soluble (for example, making a soluble derivative), or poorly bioavailable (example is, erythromycin propionate) etc. These prodrugs undergo chemical or enzymatic conversion to pharmacologically active drugs. Chemical kinetic principles are employed for predicting the release pattern of drugs from prodrugs. For example, disodium phosphate esters of 3-[hydroxymethyl]phenytoin has appeared to be a good candidate for the parenteral administration of the drug. It has to hydrolyse to release the drug.
4. Pharmacokinetics :
It involves the study of the transport of the drugs from the site of application to blood (absorption), from blood to tissue spaces and other body parts (distribution), and finally its removal from the body (elimination). All such processes are characterized by corresponding rate constants.
5. Drug action :
The interactions of drugs with biomembranes or receptors are being interpreted using kinetic models. Such models provide information regarding the quantitative differences in the drug action of different drugs of the same therapeutic category.
 |
| Zero order, First order and Second order Graphs |
Rate Process-Law of Mass Action
The hydrolysis of ethyl equation acetate may be considered in the following equation.
CH3COOC2H3 + H20 ⇄ CH3COOH + C2H5OH
Rate = -de = -d[CH3COOC2H5] = -d[H20] (1)
dt dt dt
Accordingly to the Law of Mass Action, the rate of a chemical reaction is proportional to the product of the molar concentration of the reactants each raised to a power equal to the number of molecules of the substance undergoing reaction.
In general terms, a chemical reaction for the kinetic study can be
written as :-
MA + nB ⇾ Products (2)
Rate = -1 . d[A] = -1 . d[B]
m dt n dt
= km+n [A]m[B]n (3)
in which kmin is the specific rate constant of overall order (m+n): [4] and [B] are the molar concentrations of A and B, respectively. The rate of a chemical reaction is expressed in terms of reaction rate constant. The higher the rate constant, the faster the rate of a reaction. The specific rate constant remains unchanged for a given set of experimental conditions. The rate of a reaction depends on the following factors.
Internal (Medium) Factors
- Solvent
- pH
- Ionic strength
- Catalyst
- Other ingredients
- Dissolved gases
External (Environmental) Factors
- Temperature (heat)
- Light (photochemical)
- Radiation
- Moisture
- Ambient air
- Pressure
Changes in these factors necessarily alter the interaction of the reactants at molecular level. The k value reflects the gross picture about the reaction. In the notation of kinetics, chemical reactions are ex pressed in terms of molecularity and order,
👉first Order Reaction, Order Of Reaction, Chemical Kinetics :- click here
👉Solid state decomposition, Order of Reaction, Chemical Kinetics :- click here
👉What is Half Life and Shelf Life, Order of Reaction, Chemical Kinetics :- click here
👉Apparent Zero Order Suspension, Chemical Kinetics, Physical Pharmaceutics :- click here
👉Zero Order Reaction, Chemical kinetics :- click here
👉ORDER OF REACTION, Chemical Kinetics Free PDF Note, Pharmacy Free PDF Book :- click here
👉ORDER OF REACTION, Chemical Kinetics Free PDF Note, Pharmacy Free PDF Book :- click here
👉MOLECULARITY OF REACTION :- click here
👉PRODRUG DESIGN :- click here
👉Application Of Prodrug :- click here
👉Classification Of Prodrug.:- click here
👉PRODRUGS, :- P.Valentina. Pharmacy PDF books for students free download :- click here
👉Chemical Kinetics, CVS Subrahmanyam full PDF file. bookhata free PDF books for students. :- click here
👉Metabolism And Extraction :- click here
👉What is Protein Binding :- click here
👉General Principle of Drug Action. Medicinal Chemistry Chapter-2 :- Click here
👉Semister–V, Medicinal Chemistry E-BOOK :- Click here
👉Pharmacognocy Semister-V, PDF Book :- Click here
👉Industrial Pharmacy-I PDF book, B-Pharm 5th Semister :- Click here
👉Interfacial Phenomena By CVS Subrahmanyam √ Physical Pharmaceutics √ 3rd Semister √ Pharmacy Free PDF Books Download :- Click here
👉Pharmaceutical Jurisprudence √ 5th Semister √ Pharmacy Free PDF Book Download :- click here
👉Pharmacology of drugs acting on cardio vascular system, 5th semister Pharmacology, Pharmacy Free PDF Book Download :- Click here
👉Autocoids and Related Drugs, Pharmacology, 5th Semister, Pharmacy Free PDF Book Download :- click here
👉Drug Stability, By CVS Subrahmanyam Physical Pharmaceutics, 4th Semister, Pharmacy Free PDF Book Download :- click here
👉Rheology, By CVS Subrahmanyam Physical Pharmaceutics, 4th Semister, Pharmacy Free PDF Book Download :- click here
👉Micromeritics, By CVS Subrahmanyam Physical Pharmaceutics, 4th Semister, Pharmacy Free PDF Books For Students Download :- click here
👉What is Absorption of Drug :- click here
👉What is Distribution Of Drug ? :- click here
👉Complexation and Protein Binding By CVS Subrahmanyam, 3rd Semister, Physical Pharmaceutics, Pharmacy Free PDF Book Download :- click here
👉Drug Metabolism, Med-Chem, 4th Semister, Bookhata Free PDF Books For Students download :- click here
