Learn everything about spontaneous reactions in electrochemistry with this detailed Electrochemistry Lecture 8. Understand Gibbs free energy, criteria for spontaneity, cell potential, and how spontaneous reactions drive electrochemical processes. Perfect for CSS aspirants, chemistry students, and anyone studying electrochemistry or physical chemistry. Watch the full lecture for step-by-step explanations, examples, and exam preparation tips.
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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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