L-35 | Physical Pharmacy: Kinetics & Drug Stability | NIPER JEE Previous Year Question #gdc

Added: 2026-05-28

[00:00:00]Hi everyone, welcome back to GDC classes. I welcome you all to another session for our PYQ series in which we have the topic of kinetics and drug stability. So moving on to the first question of the day and these are all from the previous year. This question is from right 2023. The unit of rate constant for a second order reaction.

[00:00:20]Now from the chemical kinetics point of view all the rate reaction constants the values the equations unit for the unit of the rate constant right these all stand important right so for the second order reaction what will be the unit it will be the liter mole inverse second inverse for the first order it would be second inverse right so make sure you remember all the units all the rate equation as well as the half-life equation for example for zero order we have a upon 2k not for the first order we have 0.693 upon K. For the second order we have 1 upon AK. Now this is for our order of reaction. Then we half-life equation. Then we have the shelf life one also. For the zero order it will be.1 into a K upon K. For the first order it will be 0.105 upon K. Then for the second order it will be.1 upon A into K. All right moving on. See I told you the questions are mostly asked on these topics. Okay. Half life for zeroorder reaction is calculated as now half life for zeroorder reaction will be definitely a upon 2k not we have just covered it so we will just go through it very very quickly these are all the direct questions that they ask which order of reaction is followed by photo decomposition it was asked in 2021 and we have discussed a similar question in the GPAD preparation while we were doing the maha marathon series right so photo decomposition reaction these are generally following the First order the degradation reactions usually follow the first order reaction except the degradation of suspensions. Right? So moving on halfife for the first order reaction is represented by the formula.

[00:02:01]Right? So for the first order it was 0.693 upon K which was independent of the concentration parameter in the half-life. It was 0.693 upon K. Right? So you can spot these look similar but they have the A which is the concentration. Right? And here it is 2K. So it will be 0.693 upon K. That will be the correct answer to the question. These are all the direct ones.

[00:02:24]Okay. Arinous equation determines the stability which is mainly the plot of.

[00:02:29]Now you remember a heinous equation which mainly includes the rate constant and the temperature. Now what is the actual graph or the equation that it follows for? It will be your log K versus 1 upon temperature. Right? If you can see the equation, right? And you can see this graph also. It will be for the log K upon 1 upon temperature basics question but you cannot miss that. Okay, that becomes very clear with the equation also. Okay, rate kinetic rate of kinetics deal with again the question falls under uh 2004 and six it deals with concentration order molecularity or all of these right so rate kinetics actually deal with the order of the reaction. If you have seen we have the zero order first order second order pseudo zero order pseudo first order reactions as well. So rate kinetics mainly deal with the order of the reaction. Okay, that is how you decide which which which uh from the order which rate kinetics it falls under based on the reaction how many reactants are there right and how they are forming a product. Next in a chemical reaction the rate constant is independent of the initial concentration. A direct answer rate react rate constant independent of the initial concentration that usually happens with a zero order. That usually happens with our zero order. But even for the halflife, for first order the half life is the answer. But for the rate equation, they're asking the rate equation in which it will be independent will be the zero order. Option number D.

[00:03:55]All right. Next, for the first order, it becomes to the power of one. For the second, it becomes to the power of two.

[00:04:01]For a first order reaction, the rate constant K has the unit as we have already covered for the zero order, right? For the second order, sorry. Now that time I dictated for the first order it will be second inverse. How you can estimate that even if you don't remember you should remember the half-life equation that was 0.693 upon k right for the first order. So the k unit will become that will be 0.693 upon t half that means the inverse of time. So it will be our second inverse it will be second inverse option number d. Okay, note down for zero order, first order, second order, the equation which predicts the stability in corresponds to the room temperature from the accelerated temperature. We had just covered the log K and the one upon temperature thing which clearly was a indicative of the temperature dependence over the stability and how the degradation happens that is explained by our arhenous equation. That is explained by our arheneous equation that will be option number B. Okay, this is the equation. Okay. Which of the following is considered as an environmental factor that affects the stability? Buffer, exipient, dialectric constant or radiation. Environmental factor means that is not a part of the reaction but it has to be included from the environment. Buffer that is a chemical composition and internal part. Excellent are part of a composition again an internal part. Dialectric constant is of the solvent or the component that you are using again an internal part. But the radiation radiation that's a external or the environmental factor that we can consider. All right. So these are internal factors like solvent, pH, buffers, ionic strength, dialectric constant, exipent. These are all the internal factors. For the external factors we have temperature, light, oxygen, moisture, radiation, pressure.

[00:05:49]The most common mode of the pharmaceutical degradation. Most common hydrarolysis, polymerization, photosis or isomerization. Now polymerization and isomerization are very common but they have to be structure specific drug specific under a specific condition. So they do not stand as most common right but the function group addition becomes the most common. So hydrarolysis again because of the presence of the most obvious functional groups or even due to the atmospheric reaction. So it will be the uh hydraysis that stands as the most common of out of all shelf life is computed by fixed law a heinous equation higuchi or noise sweetening even if you don't know the answer but remaining ones you should have a great eye on. Fix law was for the diffusion. Higuchi again the release kinetics based on diffusion. It explains noise written equation is the dissolution equation right. So we have the arhenous equation that was an indicator of stability temperature and definitely for the half-life and shelf life also we can compute the destruction of microorganism by steam may be first zero second or pseudo first order. So again it is an example of the first order. I have indicated the degradations are falling under the first order category except for the suspensions right. So we have the first order reaction. Which of the following order shows the fastest rate of reaction? Now this question has been asked prior also again for the GPAT but here for the practice it has been put. So the fastest one is going to be the second order reaction based on what order it is and how it depends right. So it can depend upon the square of concentration of one reactant or the product of the concentration of two reactants. Right?

[00:07:33]So that would be much faster than the first order of the rate constant. Okay.

[00:07:38]Drugs in suspension in semiolid. Now I told you that this would be asked this has been asked and so they have put under the practice question also. So drugs under the suspension because drug now it is present in the suspension form. Now that will be continuously supplied for the degradation. So that first order now then turns to the zero order. Right? So for the degradation here we usually call it either pseudo zero order right or we generally put if you don't have that option to be zero order kinetics and it has been asked before right apparent zeroorder kinetics determination of order is done by which method graphical method correct half-life method correct substitution method correct so it falls under the all of these now at least if you don't have the explanation right it's fine their names are important in the niper we usually target the basics they they have seen the pattern they usually target the basics of a student. So make sure that you are well um aware about the topics uh what kind of topics are being constantly asked right what is a repeat pattern so I've seen they always touch the basics of the subject right so we have graphical method we have substitution method and we have the half-life method okay molecularity of a unimolecular reaction is as the name suggest unolecular means single molecule biomolecular means two molecule tertiary see three So that's how you just increase the number. So un molecular most definitely will indicate the one.

[00:09:07]Okay. It will indicate the single type of molecule. In apparent zero order like suspension the rate depends upon particle size, pH, solubility, temperature. I have told you that drug is being continuously supplied from one phase to another. So that partitioning that because of the Stokes law equation we see. So that will be the solubility difference. Okay. So that solubility difference due to that the drug can be supplied for the degradation at a constant rate into the solution from drug as a solid that acts as a reservoir that continuously supplies the drug and then it degrades. So it majorly depends upon the solubility profile. Okay.

[00:09:45]Pseudo first order reactions uh are originally what order? Right? Pseudo first means they are not first order but they had to become first order. Right?

[00:09:54]So originally uh they are second order reaction but they now fall under the first order reaction because the rate has become independent of at least one of the component. So from the second order it turned first order. Similarly we have seen a apparent zero order in the case of the degradation of suspension. Right? So that was first turn to zero here uh pseudo zero pseudo first order that was initially second order. Now shelf life is the concentration of reactant that is reduced to shelf life is denoted as T90.

[00:10:23]Half life is denoted as T50. Okay. So that means the concentration of reactant is reduced to 90%. Only 10% is remaining. That is why we refer to as T90. Right? Concentration is reduced to 90%. Unit for rate constant for second order reaction is second inverse mole per liter into second. Mole inverse liter second inverse mole liter inverse second inverse. A great great idea of attempting these questions is to remember their half like equations from that it becomes very easier to compute the rate equation and rate order and then how you can for the rate of unit of the rate reaction. So it will be mole inverse liter second inverse. Right? You can see right here for the zero order it is mole per liter per second. For the first order it is second inverse. For the second order it becomes liter mole inverse second inverse. Okay. Half life for the first order it is a repeat question. So half life for the first order was 0.693 upon k. Now this is the advantage of this chapter that most of the questions are very very direct. Okay. So we are not going to waste much time. Again the formula used for the shelf life I told you either either it's going to be the unit either it's going to be the rate constant or the shelf life. No further question apart uh extra we can go to the arhinous equation right or basic degradations. So for the shelf life it would be 0.105 upon k. All right. Now let's move on to our next question. Kinetically this is the expression of x upon a into a minus x first order second order pseudo first order or fractional order. Now this is for the second order kinetics. This is for our second order kinetic expression.

[00:12:16]You can see right here for the first zero order it is a minus 80 upon t. For the first order it was 2.303t log into a upon a minus 6. And here we have for the second order. All right.

[00:12:29]Which of the following does not play a part in determining rate of reaction?

[00:12:34]Temperatures, solvent, presence of catalyst or equilibrium constant?

[00:12:39]So for determining the rate of reaction which does not play a part, that will be the equilibrium constant. Right?

[00:12:46]Presence of catalyst why? Because it can accelerate or slow down the rate of the reaction. Solvent because of the solubility thing. Temperature you have always seen the arinius equation. All right? So D will be the correct answer.

[00:13:00]The modern definition of oxidation, loss of oxygen, gain of hydrogen, loss of one or more electron or gain of one or more electron. So the oxidation is going to be loss of one or more electron. Right?

[00:13:13]Now we generally used to earlier call it as loss of oxygen or addition of hydrogen. Right? Sorry, removal of hydrogen that was oxidation earlier.

[00:13:22]Right? So for the results of accelerated stability the energy activation should be in the range of you remember the arhaneous equation that was EA is equals to A E RA to the power minus EA upon RT that EA would be the viscosity EA would be the activation energy R was the gas constant while T was the temperature right so the activation energy would fall under the range of 11 to 30 kiloalorie per mole All right. So next, which of the following can be designated as a molecularity? 0, half, 1 and 1 and a half. We already know molecularity starts from a uni molecular reaction.

[00:14:04]How much molecules are involved? So it is one. Biomolecular it is two. Nothing comes under as half or one and a half not even zero. A second order reaction behaves as a pseudo first when the concentration becomes one reactant is significantly higher than the other then the rate becomes independent of the other one. This is what I have indicated already. The concentration of both is high both is low or equal. So I've told you of out of one of them that becomes independent of their concentration because of the other one present in the excess amount. So that second order reaction turns to pseudo first order reaction which was originally the second order. In terms of kinetic degradation suspensions are same repeat question suspensions do fall under the apparent zero order kinetics. If the option given is zero or apparent or pseudo then you choose the apparent or pseudo. If the option is only sud zero order then you choose the zeroorder reaction. So drug in the solid that act reservoir goes to the solution and then degrades. Okay. So that's it for today. It was a very small video because of the uh smaller sections that being asked from these kinetics.

[00:15:15]But I hope it has been uh fruitful for you all. Thank you so much for watching.

[00:15:18]We'll meet you in the next session.