- 6/5/2025
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00:01Hello everyone. Welcome to this exciting lecture series on electrochemistry.
00:09In today's lecture, we will study about the spontaneous reactions, non-spontaneous processes.
00:16We will look into some of the examples also to further clarify our concept.
00:22Let us talk about the spontaneous reactions first.
00:25It is a fundamental concept in understanding how and why certain reactions happen naturally.
00:32First of all, a spontaneous reaction is a chemical reaction that once started, occurs without needing to be driven by an external force or energy input.
00:44This means that after the reaction gets initiated, perhaps by a small amount of energy,
00:49it can continue on its own without needing constant help from outside of the system.
00:56The system itself has enough internal drive to move forward like how a ball rolls downhill once it is pushed.
01:05In other words, we can see that the reactions naturally proceed in the direction of the product formation under some certain conditions.
01:15So, these reactions are the spontaneous reactions that drive by its own.
01:21But this is an important distinction.
01:25Spontaneous does not mean that it will be fast.
01:29Some spontaneous reactions can be very slow.
01:32This is a common misconception.
01:34Just because something will happen on its own does not mean it will happen fast.
01:39Spontaneous reactions could take seconds or years.
01:44Rifting is a good example.
01:47It is a spontaneous reaction but it can take weeks or months to fully occur.
01:54Ok.
01:56The pace of reaction is determined by the chemical kinetics of the reaction rather than the spontaneity.
02:03Ok.
02:04Here, we are distinguishing between two different branches of chemistry.
02:10Thermodynamics tells us whether the reaction can happen.
02:14That is the spontaneity.
02:16And while kinetics tells us how fast it will happen.
02:20Another key point is that in spontaneous process that each reactant molecule naturally tend to form product molecules.
02:33And this tendency is closely related to the stability of the system.
02:37Systems evolve towards states that are more stable or energetically more favorable.
02:45And that is what drives this spontaneity.
02:47So, when we say a reaction is spontaneous.
02:52We are really saying the reaction is thermodynamically favorable.
02:55Not necessarily fast.
02:57And that brings us to the next part.
03:00How we can determine the spontaneity in terms of energy and the entropy.
03:05We will also study this concept in the coming slides.
03:08Ok.
03:09Now first, we will look into the key characteristics of the spontaneous reactions.
03:21Ok.
03:25At first, let us look closer at how spontaneity in a chemical reaction is explained thermodynamically.
03:32And this is where Gibbs free energy becomes our main focus.
03:37Spontaneous reactions are closely related to thermodynamics.
03:43And more specifically, to changes in energy within a chemical system.
03:48Thermodynamic principles allow us to predict whether a process will occur naturally under a given set of conditions or not.
03:56The main quantity used to understand this is Gibbs free energy.
04:03It is denoted by ΔG.
04:05It represents the amount of usable energy in a system that can perform work during a chemical reaction.
04:13Whether ΔG is positive, negative or zero, it will give us a direct clue about the spontaneity of the reaction.
04:20First, we see that if ΔG is less than zero, the reaction will be spontaneous.
04:27This means that the system naturally proceeds toward the product formation without any need for the external energy input.
04:36It releases free energy moving to a more stable and lower energy state.
04:41Secondly, we see that if ΔG is greater than zero, the reaction will be non-spontaneous.
04:50It requires energy from an outside source to proceed.
04:54Because the system is moving toward a higher energy state, it will be less stable.
04:59When ΔG becomes equal to zero, the system will be at equilibrium.
05:05The rates of the forward and reverse reactions are equal and there is no net change in the concentration of reactants and the products.
05:14Understanding the Gibbs free energy and how it relates to the spontaneity helps us predict reaction behavior and determine whether a reaction will proceed on its own naturally or not.
05:27Now, we will look into the second characteristics of the spontaneity reaction, which will be our Gibbs free energy.
05:38We will now try to study it in the details.
05:41Gibbs free energy, which is denoted by G, is a fundamental concept in thermodynamics that tells us about the energy available in a system that will do useful work during a chemical reaction.
05:53It is a measure of system's thermodynamic potential.
05:58The equation can be written as ΔG will be equal to ΔH minus T ΔS.
06:06This equation helps us to evaluate whether the reaction is energetically favorable or not.
06:12In this equation, ΔH will be the change in the enthalpy.
06:18It is the heat content of the system.
06:20It shows whether the reaction absorbs heat or it will reduce heat.
06:27In other words, the reaction will be endothermic or it will be exothermic.
06:32ΔS is the change in entropy, which represents the degree of disorder or randomness in the system.
06:40A positive ΔS means increased randomness, which nature generally favors.
06:45Positive value of ΔS will be favorable by nature.
06:51In the equation, T is the absolute temperature.
06:56It is measured in Kelvin.
06:58It plays a key role in scaling the impact of the entropy.
07:01This equation that we see here, it tells us that a reaction's spontaneity depends on both the heat exchange and the change in the disorder of the system.
07:14They are adjusted for the temperature.
07:16When the system releases heat, that is, ΔH is negative, and increase in entropy, that is, ΔH is positive, the reaction is almost always spontaneous.
07:28Moving further, we will discuss the two key important thermodynamic factors that influence the spontaneity of the reaction, which are the enthalpy and the entropy.
07:45Enthalpy refers to the heat content in a reaction.
07:50When a reaction releases heat, means ΔH is less than zero.
07:55The reaction will tend to be spontaneous.
07:58Such reactions are called exothermic reactions because they give of energy, and these are the reactions that nature generally favors.
08:05Entropy measures the disorder or randomness of the system.
08:12When a reaction causes an increase in disorder, so ΔH is greater than zero or positive, the reaction also tends to be spontaneous.
08:22This is because the system naturally moves toward the more disorder.
08:26A reaction is most likely to be spontaneous when both conditions occur.
08:31The enthalpy change is negative, that is, the heat is relieved, and the entropy change is positive, which means that disorder increases.
08:43This combination means that the reaction gives of energy and the randomness of the system will increase, which align with the natural tendency of most of the spontaneous processes.
08:59Temperature is an important factor that influences whether the reaction is spontaneous or not.
09:11It plays a key role in the spontaneity of the reaction.
09:17When ΔH is negative, which means the reaction is exothermic, and ΔH is positive, the reaction is spontaneous at all temperatures.
09:25This is because the reaction releases heat and increases heat and increases this order, both favors the spontaneity of the reaction.
09:36Okay, when ΔH is positive, that is, the reaction absorbs heat, making it endothermic, and ΔH is positive also, which means that this order increases.
09:46The reaction becomes spontaneous only at higher temperatures.
09:47The reaction becomes spontaneous only at higher temperatures, that is, because at higher temperatures, the increase in disorder has a bigger effect on making the reactions favorable.
10:01Another case is that, when ΔH is negative, it means the reaction is exothermic, and ΔS is also negative, that is, the disorder decreases.
10:09The reaction is spontaneous, but only at very low temperatures, where the heat relieved can drive the process despite the decrease in the disorder.
10:20And finally, we see that, if ΔH is positive, that is, the reaction is endothermic, and ΔS is negative, which means the disorder is decreasing.
10:30The reaction can never be spontaneous under any temperature conditions, as neither factor favors the reaction naturally.
10:41So, the temperature combined with the enthalpy and the entropy changes determines if and when a reaction will spontaneously occur.
10:49Now, we will look into some of the examples of the spontaneous reactions to further clarify the concept.
11:02The first example that we see will be the combustion of a fuel.
11:06In this reaction, we have selected the methane as a fuel, which reacts with the oxygen.
11:11So, here is the equation.
11:14In this reaction, methane burns in the presence of oxygen to form carbon dioxide and the water vapors.
11:21The reaction is absorbed to be exothermic, which means that it releases a significant amount of energy to the environment.
11:31Okay.
11:33So, this is a factor that favors the spontaneity of the reaction.
11:36Additionally, we see that there are three number of atoms or the molecules on the left side and the three number of molecules on the right side.
11:47Although the number stays the same, the products are more disordered due to the release of energy and new molecular arrangements, which indicates an increase in the entropy.
11:58Okay.
11:59So, here we see that heat is released and the entropy is also increased, making it a highly spontaneous at standard conditions, according to the conditions that we have studied in the previous slides.
12:15Okay.
12:16So, that is the reason why fuel combustion occurs so readily once initiated.
12:22No further external input is needed to keep it going because the reaction is highly spontaneous.
12:31Now, let us explore another common example of spontaneous reaction, which is the rusting of iron.
12:38Okay.
12:39Iron gets oxidized to form the rust.
12:41The chemical reaction involved in the rusting formation is given below.
12:514.
12:52Iron atoms are being oxidized with the three oxygen molecules to form Fe2O3.
12:58The reaction takes place now when iron is exposed to oxygen and moisture in the air, and this reaction happens over a large amount of time or a large span of time.
13:11Iron gets oxidized, forming iron three oxide, okay, commonly known as the rust.
13:18Even though the process is slow, especially compared to something like fuel combustion, it is a spontaneous reaction.
13:24It means it occurs naturally at room temperature without needing external energy source.
13:33The spontaneity of the rusting is due to main two factors.
13:39The release of energy during the oxidation process, which makes delta H negative, and an overall increase in the entropy, because the system is forming a more disordered molecule,
13:51less organized product in the environment is being formed.
13:56Even though the solid rust itself appears to be more disordered, it will be thermodynamically less ordered.
14:04This reflects that the spontaneity depends on the total entropy change of the system and the surrounding also.
14:12Let us look at another everyday example of spontaneous reaction, which is the dissolution of salt in the water.
14:25When common salt is dissolved in water, the chemical process can be written as NCl gives off Na plus sign and Cl minus sign.
14:34In this reaction, solid sodium chloride dissociates into its signs, which is the cation, Na plus sign and Cl.
14:44The process happens naturally and without the need for external energy, making it a very spontaneous reaction.
14:52So the question is, why is this reaction spontaneous?
14:59Even though breaking ionic bonds in the NaCl requires energy, that energy is compensated by the hydration of ions where water molecules surround and stabilize them.
15:11And most importantly, there is a significant increase in entropy or disorder of the system, because a highly ordered solid form turns into a freely moving ions in the solution.
15:24So the spontaneity is due to the disorder of the system.
15:29This increase in randomness or the molecular motion contributes positively to the system, gives free energy change, making it a reaction, making it a very thermodynamically favorable reaction, which will correspond to the spontaneous reactions.
15:48At the end, after studying the concept of spontaneous process, let us look at the concept of non-spontaneous process, which is also referred to as the endogenic reactions.
16:05Non-spontaneous reactions is basically opposite to the spontaneous reactions.
16:10It does not happen on its own.
16:12These are the chemical reactions in which the standard change in the Gibbs free energy is positive.
16:22This means that the system absorbs energy from the surroundings instead of releasing it, which is not a very thermodynamically favorable process.
16:33Because the system and the reaction does not proceed on its own, it requires an external input of the energy.
16:40That is why we say that the reaction is non-spontaneous.
16:45It won't happen unless we push it with some external energy.
16:49It has a positive value for the delta G, which is the Gibbs free energy.
16:58A classic example is the photosynthesis.
17:00The conversion of carbon dioxide into oxygen happens in the plant's leaves, and it also forms glucose in the process.
17:11This reaction does not happen on its own, and it is powered by the energy from the sunlight.
17:17Without the solar input, the reaction would not proceed because delta G is positive, meaning it is not energetically favorable under standard condition.
17:28And it will not happen unless there is sunlight.
17:32So, here we will graphically look into the reaction's spontaneity and the non-spontaneity.
17:43Here we can see the graph between energy and the reaction coordinate in a system.
17:48At first, if the reactants have a higher energy, after going through the intermediate or after overcoming the activation energy, reactants go into the products.
18:01Okay. So, the energy of the product is less than the energy of the reactants.
18:06It means that the delta G is less than zero, and in this condition, energy will be relieved.
18:14When in a system, energy is relieved and delta G value is less than zero, it means that there is more disorder than the reactants.
18:23The reaction will be, or any process will be, spontaneous process.
18:28Also, endogenic reactions are the non-spontaneous processes.
18:33These are the processes where delta G is greater than zero.
18:39Here we can see that reactants have lower energy.
18:44And after achieving the activation energy and going beyond it, reactants are being converted into the products.
18:53Here delta G is positive and the products have more energy than the reactants.
18:58This reaction is basically a non-spontaneous reaction, which requires this amount of energy to build the gap and energy is added to the system.
19:09And when the energy is being added to the system, the system will not happen on its own and the reaction will be non-spontaneous.
19:17So, with the help of these examples, we have clearly studied the spontaneous and non-spontaneous processes.
19:29I hope you have learned something new from these lectures.
19:33This marks the end of our discussion and this lecture.
19:37Thank you very much.
19:38Thank you very much.
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