Spontaneous vs Nonspontaneous Reactions - Online Tutor, Practice Problems & Exam Prep

Spontaneity in chemical reactions refers to the natural tendency of a process to occur without the need for continuous external energy. A spontaneous reaction favors the formation of products at equilibrium, such as the slow conversion of diamond to graphite over millions of years. Conversely, a non-spontaneous reaction, like the formation of diamonds from graphite, requires significant energy input to proceed. Importantly, spontaneity does not equate to the speed of a reaction—some spontaneous reactions can be slow. Photosynthesis, for example, is non-spontaneous as it requires sunlight to produce glucose from ${\mathrm{CO}}_2$ and water, while cellular respiration is spontaneous, converting glucose and oxygen into ${\mathrm{CO}}_2$ and water naturally. Understanding these concepts is crucial for grasping thermodynamics in chemistry, which is distinct from kinetics, the study of reaction rates.

Understanding Spontaneity

Now spontaneity It determines whether a reaction or process under certain conditions occurs or not. Now when we say spontaneous versus non spontaneous, what exactly do we mean? Well, when we say a spontaneous reaction, we mean that it is a natural process which does not require a constant outside energy source. Now a spontaneous reaction, we're going to say that it favors a formation of products at equilibrium. So your reactants go towards making products.

When we say non spontaneous reaction, a non spontaneous reaction is an unnatural process which requires a constant outside energy source in order for it to occur. We're going to say here it does not favor formational products in equilibrium. It rather stay in its reactant form. Now here's spontaneity does not determine the speed of a reaction or process. When we take a look at a reaction, there's two things you need to keep in mind. We're talking about thermodynamics which deals with spontaneity and non spontaneous reactions. So that determines if the reaction can go or not.

On the other side, we look at kinetics which looks at the speed of a reaction. It doesn't matter if a reaction takes a million years to happen or if it takes a second to happen, if it can happen naturally, it's a spontaneous reaction. Speed does not matter. So if we take a look here, we have two types of reactions here. We're going to say we have diamond, carbon, diamond, going towards basically graphite, carbon graphite. We see that on the top arrow it's spontaneous. This is a natural process. Over time, diamonds can transform into graphite.

So that's just I guess a cheaper form of carbon if you want to say. But the good thing is this takes millions of years. So you can buy diamonds and you don't have to worry about them converting into graphite overtime. Like it will take millions of years for that to happen. So here we'd say the rate in the forward reaction is going to be slow. It happens very slowly. And we're going to say that if you're spontaneous in One Direction, then you're non spontaneous in the opposite direction. So going from graphite to diamond is a non spontaneous process. It does not occur naturally.

We have to basically input a lot of energy, a lot of heat, a lot of pressure in order to transform graphite into diamonds, because the process is not a natural process. That's what makes diamonds also so precious. Now here we have another type of reaction. Here we have CO2 plus water, giving us glucose plus oxygen. Here we'd say that this is an example of photosynthesis, right? So we have sunlight here. Sunlight is the outside energy source that's needed. Without it, this cannot occur.

So we'd say here this reaction happens pretty fast. If we have the outside energy source present, it's just going to push it forward, not going in the opposite direction where glucose plus oxygen gives us CO2 and water. So here in that direction we are a spontaneous reaction. So it's easy for glucose and oxygen to basically produce CO2 plus water. So this is cellular respiration on our part. OK, So we exude or or or breathe out CO2 into the environment. OK, so that is a natural process, right?

So again remember, a spontaneous process is natural. It can occur without the need of outside energy. A non spontaneous one is not natural and it needs a constant flow of energy for it to even have a possibility of forming. Without that energy source it does not naturally occur and there will be no products being formed. So keep this in mind. Also, spontaneous reactions can take a second to happen or less, where they can take millions of years. The fact that it can happen makes it spontaneous.

Spontaneous vs Nonspontaneous Reactions Example

Here it says to label each process as spontaneous or non-spontaneous. Remember a spontaneous process occurs naturally without the need of outside energy. A non-spontaneous process is an unnatural process that requires a constant flow of energy for it to occur. So all you have to do is imagine if we're describing this process in the real world and you imagine it happening if you can, then it's a natural process and it's spontaneous.

So here it says the sugar cube dissolved in room temperature water. We can picture ourselves taking a sugar cube, placing heated water. We know what would happen. The sugar cube would dissolve in the water. Since it can happen in the natural world, it's spontaneous. Next, mixing of three gases in a container. Gases move randomly when put in a container. So if we took three different gases and put it in a container, we can imagine them mixing together randomly. They don't just stay separate from one another, they just move around freely, so eventually they would mix. So this would be spontaneous as well because no outside energy is needed.

Heat flows from the cold object to warm object. So think about this, what's the natural flow of heat? Heat traditionally moves from a hotter object to a colder object. Here this is saying the exact opposite. So this would be a non-spontaneous reaction because it's an unnatural process. It's not something we would typically see in the natural world. Ice Cube melts at -1°C, well at 0°C. This is the temperature in which water starts to freeze. So if we go even lower than that, then we would expect the water to continue to freeze. So we shouldn't expect it to start melting. The temperature is too low, so here this will be a non-spontaneous process as well.

A ball rolls down a hill. Now here we're going to add a little bit more to this. So we have a ball and let's just say that a gust of wind or something basically pushes it over the cliff. What would be the natural progression of the ball? The ball wouldn't just hover in the air, it would naturally go down the hill because it's naturally going down the hill. It would be a spontaneous process. Here it's aided by gravity, which is not an outside source. It's a natural force.

Finally, we have plant respiration where we have oxygen, glucose, produce, CO2, water and we have some ATP involved. Now we said that when it comes to photosynthesis we need an outside energy source, so it's a non-spontaneous process. The opposite of photosynthesis will be cellular respiration or plant respiration. This is a natural process, so it would be a spontaneous process and that's how we describe each one of these processes as being either spontaneous or non-spontaneous.

Here’s what students ask on this topic:

How can one tell the difference between spontaneous and nonspontaneous reactions?

To differentiate between spontaneous and nonspontaneous reactions, you need to consider the thermodynamic properties of the system, specifically the Gibbs free energy change ($\Delta G$). A spontaneous reaction is one that occurs without the need for continuous external energy; it tends to happen naturally once initiated. For a reaction to be spontaneous, the change in Gibbs free energy must be negative ($\Delta G<0$) under constant temperature and pressure.

In contrast, a nonspontaneous reaction requires an input of energy to proceed and will not occur on its own under the given conditions. These reactions have a positive change in Gibbs free energy ($\Delta G>0$).

Temperature can also play a role in spontaneity. Some reactions may be spontaneous at certain temperatures and nonspontaneous at others due to the temperature dependence of $\Delta G$.

In summary, check the sign of the Gibbs free energy change: if $\Delta G$ is negative, the reaction is spontaneous; if $\Delta G$ is positive, the reaction is nonspontaneous. Temperature must also be considered, as it can affect the spontaneity of a reaction.

Which of the following reactions will be spontaneous at low temperatures and nonspontaneous at high temperatures?

A reaction that is spontaneous at low temperatures but nonspontaneous at high temperatures is characterized by having a negative change in enthalpy (ΔH) and a positive change in entropy (ΔS). This is because the spontaneity of a reaction is determined by the change in Gibbs free energy (ΔG), which is given by the equation:

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

where T is the temperature.

When ΔH is negative (exothermic reaction) and ΔSis positive (increase in disorder), at low temperatures the TΔSterm will be relatively small, so the negative ΔH will dominate, making ΔG negative and the reaction spontaneous. However, as the temperature increases, the TΔSterm becomes larger. If it becomes large enough to outweigh the negative ΔH, then ΔG will become positive, and the reaction will no longer be spontaneous. Thus, a reaction with a negative ΔH and a positive ΔSfits the criteria of being spontaneous at low temperatures and nonspontaneous at high temperatures.

Which of these reactions is/are predicted to be spontaneous, nonspontaneous, or at equilibrium?

To determine if a reaction is spontaneous, nonspontaneous, or at equilibrium, you need to consider the Gibbs free energy change (ΔG) for the reaction. The Gibbs free energy equation is:

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

where ΔH is the change in enthalpy, T is the temperature in Kelvin, and ΔSis the change in entropy.

• If ΔG is negative ($-\mathrm{\Delta G}$), the reaction is spontaneous, meaning it can occur without any continuous external influence.
• If ΔG is positive ($+\mathrm{\Delta G}$), the reaction is nonspontaneous, meaning it will not occur without the input of energy.
• If ΔG is zero ($\mathrm{\Delta G}=0$), the system is at equilibrium, indicating that the forward and reverse reactions occur at the same rate, and there is no net change in the concentrations of reactants and products over time.

To apply this to a specific reaction, you would need to know or be able to calculate the values of ΔH, T, and ΔS. Remember that the sign and magnitude of these values, as well as the temperature at which the reaction is occurring, will influence the spontaneity of the reaction.