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Deciding if a Reaction is Spontaneous - Concept

Teacher/Instructor Kendal Orenstein
Kendal Orenstein

Rutger's University
M.Ed., Columbia Teachers College

Kendal founded an academic coaching company in Washington D.C. and teaches in local area schools. In her spare time she loves to explore new places.

Tips on deciding if a reaction is spontaneous. First let’s discuss what spontaneity is exactly. So, if a reaction is spontaneous, it means it's occurring without any external stimulus. But what does that actually mean?

Well combustion reactions are considered to be spontaneous, however they do need a spark or something that initiates the reaction or starts the reaction. That’s okay. We are allowed to have this initial spark or initial push to let the reaction go, but once the reaction starts, if it continues to go without any outside intervention, we consider that to be spontaneous.

So external stimulus or you might hear in class, outside and around outside intervention. So how can you decide if something is spontaneous or not? Because we could wait for it to happen, but sometimes the reactions take years upon years, and sometimes even millions of years to actually process. So how can we decide if something is going to be spontaneous or not? This guy named Gibbs came along and he noted the two things that we needed to have, in order to decide if something is spontaneous, is enthalpy and entropy.

And this guys, this is enthalpy and remember its enthalpy because enthalpy has an H in it, and delta H is enthalpy or heat, amount of heat. And the measurement of comparing are also using entropy. And that is a measurement of chaos or disorder. Enthalpy is a measurement of energy. We like to have low energy or a least energy, so we like this to be negative, a negative number, because we like to go we like things to be exothermic. We like to lower things in energy, we don’t like things to be high in energy.

T is temperature in Kelvin this is always positive. So it’s always a positive number multiplied by delta S, which is entropy, which is amount of measurement of chaos or disorder. So the universe likes to be disorderly. So we like to increase in entropy, so you like entropy also to positive. So these are perfect conditions when delta H is negative and delta S is positive. Negative subtracted from a positive number will give us a negative number. So when delta G ends up being negative it is spontaneous. However not all reactions are going to have this beautiful, easy, explanation or combination to show that this is spontaneous.

So let’s go through the different scenarios you might see in class or you might see in lab, and decide whether it’s going to be spontaneous or not. Let’s say you have a endothermic reaction meaning the delta H is positive. This is going to be always a subtraction because that’s part of the equation. Temperature is always positive, because it's Kelvin. And let's say entropy is also still positive. If have the positive minus another positive, in order for this delta G to negative, this number must be very high, in order for the overall problem to be negative. Notice delta H, I’ll put this down here and delta S are constants for a reaction. The only thing that’s actually something that’s changeable, is temperature. So it needs to be a big or high temperature in order for this combination to be spontaneous. That’s all good enough.

Next, another combination. Let’s look at if we have an exothermic reaction subtracted by, again temperature is always positive, and let’s say we have a negative entropy. So we have one good thing, enthalpy is negative, but entropy is also negative not so good. So this will only occur negative number, subtracted by another negative number we want the first negative number to be bigger. So this needs to be small temp or a very low temperature. We want this overall number on the right-hand side, T delta S to be very small, so the negative subsides and the delta G is negative.

The last one, the last type of combination is if we an endothermic reaction which means its delta H is positive, minus, positive T because temperature is always positive, times another negative entropy. So the enthalpy is positive meaning it’s endothermic. We do not like that, entropy is negative meaning we're going to get something more orderly. So these are both bad things. This will never combine to be a negative delta G. A positive minus a negative will always a positive, this will never happen. This will always happen.

So this combination will always help you with test questions or hopefully even understanding what sort of combinations will occur in order to make a reaction spontaneous or not. So we don’t have to always have to have a combination of exothermic and positive delta S or high disorder. We can actually have a different combination of both, but temperature varies. So hopefully this helps you understand if a reaction is spontaneous or not using Gibbs free energy.

Also, just a side note, when delta G equal 0, which sometimes will, we know that the reaction is equilibrium. And what that means is, the forward reaction happens as well the backward reaction, both occur. If the delta G is positive, the forward reaction will not occur. If the delta is negative the forward reaction will occur.

So hopefully this helps you understand if a reaction is spontaneous or it’s even at equilibrium. Gibbs free energy is very helpful in discovering that. Hope that helped.