Spontaneous Reaction: Study with Video Lessons, Practice Problems & Examples

Thermodynamics studies the transformation of heat and energy in chemical reactions, focusing on spontaneity. A spontaneous reaction occurs naturally without external energy, like a boulder rolling downhill, while a non-spontaneous reaction requires continuous energy input, such as a car needing fuel. Spontaneous reactions move towards equilibrium, and their rate can vary significantly. Key variables include free-energy change (ΔG), entropy change (ΔS), and equilibrium constant (K). Understanding these concepts is crucial for analyzing chemical processes and their feasibility in real-life scenarios.

Spontaneous Reaction Concept 1

Hey guys, in this new video, we're going to take a look at thermochemical processes. Now, we're going to say that the section of physical science concerned with the transformation of heat and other forms of energy from one type to another is called thermodynamics. And what we're going to say here is when it comes to a chemical reaction, just realize sometimes they occur, other times they don't. And it's really based on the conditions at the moment that a chemical reaction can or won't happen. Now, what we're going to say is this chapter is concerned with one keyword, spontaneous. Is a reaction spontaneous or is a reaction non spontaneous? The deeper into this chapter we get, we're going to learn that there are certain variables such as ΔG, ΔS and even k. All of these together in some way help to determine if a reaction is spontaneous or not. Now, when we say spontaneous, just remember, we're going to say a reaction that requires no outside energy source is classified as a natural process. So think about it. Let's think of a boulder, a huge rock rolling down a hill. We're going to say that huge boulder doesn't require any type of energy to roll down that hill. It's using its own momentum in order to do that. We're going to say that since it doesn't require any type of energy for it to happen, then it's a spontaneous reaction. Now on the flip side, let's say we have a reaction where we have to continuously feed it energy in order for it to occur. If you're supplying a continuous amount of energy to something, we're going to classify it as a non spontaneous reaction. And we're going to say non spontaneous reactions are unnatural. They constantly need energy for them to occur. Let's say we want to run our car. Well, our car cannot spontaneously run itself without any type of energy. We have to give it a battery. We have to give it gasoline. Without these sources of energy, the car can't ignite and start off and move on its own. So just remember, the movement of a car will be classified as a non spontaneous reaction. Now, just remember, spontaneity deals with thermodynamics, so we're shifting our reaction from one side to another in a balanced equation. But remember, spontaneous has nothing to do with kinetics. Kinetics deals with speed. A spontaneous reaction could take a minute to occur. It could take a million years to occur. The fact that it occurs without the need for outside energy makes it a spontaneous reaction. The length of time it takes for it to happen has nothing to do with whether it's spontaneous or not. Now, knowing this, let's take a look at the first example. Here it says, which of the following statements is not true? We're going to say the reverse of a spontaneous reaction is always non spontaneous. Well, just remember we're talking about thermodynamics here. Thermodynamics deals with shifting of reactions, so we go in both directions. We can go in the forward direction or we can go in the reverse direction. Just remember, if you're spontaneous in one direction, then you would be non spontaneous in the reverse. So the first one is true. Next, we're going to say spontaneous reaction always moves towards equilibrium. Again, we're talking about thermodynamics. Thermodynamics deals with equilibrium. We're going to say if a reaction is spontaneous, it wants to get to equilibrium because when you're at equilibrium you're balanced. Everything is copacetic, everything is good. So we're going to say here, a spontaneous reaction does move towards equilibrium. Next, a highly spontaneous reaction can occur at a fast or a slow rate. Now, we're going to say like we said before, spontaneity has nothing to do with kinetics. A spontaneous reaction could take a minute to happen or it could take a million years to happen. The fact that it can happen makes it a spontaneous reaction. Spontaneous reactions occur. This leaves us with our last choice here. Here it says that it is possible to create a non spontaneous reaction. This might seem weird but non spontaneous reactions are again unnatural. They do not occur. We can supply constant energy to something but once you take away that energy source, the reaction is going to cease. It's going to stop. It's not going to continue. So when we say it is not possible to create a non spontaneous reaction, what we're saying here is, I can't create something and then take away the energy and expect it to continue to happen. That's what this sentence is really saying. So d would be false. We cannot create a non spontaneous reaction. Once I have a non spontaneous for it to continue to occur I need to constantly feed it energy. Once I take away that energy it no longer happens so I really can't create a non spontaneous reaction because you cannot supply energy forever towards a reaction. You don't have enough sources to do that. Now, seeing how we answer these questions, I want you guys to attempt to do the next one on your own. And just remember, think about it in your head in order to answer the question, does this happen in real life? If it happens in real life, it's most likely a spontaneous reaction. If it doesn't quite make sense, oh, this shouldn't happen, you know, my car shouldn't just start on its own and drive itself home without the use of a battery or gasoline. Then you're going to say that's a non spontaneous reaction. So again, natural processes occur, natural processes are spontaneous. Unnatural processes don't occur in real life, they don't make any sense for them to happen. Those are non spontaneous. Use that type of logic in order to solve this practice question.

Which of the following statements is/are true?

a) The rusting of iron by oxygen is a non-spontaneous reaction.

b) The addition of a catalyst to a reaction increases spontaneity.

c) The movement of heat from a cold object to a hot object is a non-spontaneous reaction.

d) The diffusion of perfume molecules from one side of a room to the other is a non-spontaneous reaction.

e) None of the above. 

• A. C
• B. A,C
• C. D
• D. B,C

Here’s what students ask on this topic:

What is a spontaneous reaction in thermodynamics?

A spontaneous reaction in thermodynamics is a process that occurs naturally without the need for external energy input. This means the reaction can proceed on its own once it has started. For example, a boulder rolling down a hill is a spontaneous process because it doesn't require any additional energy to continue moving. In chemical terms, spontaneity is determined by the Gibbs free energy change (ΔG). If ΔG is negative, the reaction is spontaneous. Spontaneous reactions move towards equilibrium and can occur at varying rates, from very fast to extremely slow.

How do ΔG, ΔS, and K determine if a reaction is spontaneous?

The spontaneity of a reaction is primarily determined by the Gibbs free energy change (ΔG). If ΔG is negative, the reaction is spontaneous. ΔG is calculated using the equation:

$\mathrm{\Delta G}=\mathrm{\Delta H}-T\mathrm{\Delta S}$

where ΔH is the enthalpy change, T is the temperature in Kelvin, and ΔS is the entropy change. A positive ΔS (increase in disorder) and a negative ΔH (exothermic reaction) favor spontaneity. The equilibrium constant (K) also plays a role; if K is greater than 1, the reaction tends to be spontaneous under standard conditions.

What is the difference between spontaneous and non-spontaneous reactions?

Spontaneous reactions occur naturally without the need for continuous external energy input. They move towards equilibrium and are driven by a negative Gibbs free energy change (ΔG). An example is a boulder rolling downhill. Non-spontaneous reactions, on the other hand, require continuous energy input to proceed. They do not occur naturally and have a positive ΔG. An example is a car running, which needs a constant supply of fuel and battery power to operate.

Can a spontaneous reaction be slow?

Yes, a spontaneous reaction can be slow. Spontaneity is determined by thermodynamics, specifically the Gibbs free energy change (ΔG), and not by the reaction rate. A spontaneous reaction has a negative ΔG, meaning it can occur without external energy input. However, the rate at which it occurs is governed by kinetics, not thermodynamics. Therefore, a spontaneous reaction can take anywhere from a few seconds to millions of years to complete.

Why do spontaneous reactions move towards equilibrium?

Spontaneous reactions move towards equilibrium because they aim to reach a state of maximum stability and minimum free energy. At equilibrium, the forward and reverse reaction rates are equal, and the system's Gibbs free energy (ΔG) is at its lowest possible value. This state is thermodynamically favorable, as it represents a balance where no net change occurs over time. Thus, spontaneous reactions naturally progress towards this balanced state to achieve equilibrium.