Gibbs Free Energy

Memorization Hack 💡

ΔG says: "Hey, this is how much energy input needed to make this happen" 

So, if you need a negative amount of energy, we do not need to put any energy into the system for the reaction to occur. Therefore, it is SPONTANEOUS! And vice versa. 

Keq is when ΔG is zero. Keq is the equilibrium constant, which tells us that there is no push for the reaction to move one way or another. Therefore, ΔG must be zero! 

Notes:

ΔG is the change (what the delta tells us) in Gibbs free energy. 

This allows us to predict the reaction direction and the biological feasibility of this reaction occurring. (aka.. Is it going to favor products/reactions, and how likely is this to happen?)

Remember: 

ΔG > 0 (aka.. ΔG is positive), the reaction is NONspontaneous 

When we say it is nonspontaneous, we are saying that it is not going to occur without help. In fact, energy must be absorbed into the system for this reaction to occur, meaning it is endergonic 

ΔG < 0 (aka…ΔG is negative) the reaction is spontaneous 

Again, when we say it is spontaneous, it means that it is going to occur without the input of energy. In fact, energy is released from this reaction, meaning it is exergonic

To remember the term exergonic, I think of “exer” as “exit”, meaning that the energy is released and “leaves” the system. 

ΔG = 0, this means the reaction is at equilibrium. 

*Keep in mind, no matter how big or small ΔG is, it does not tell us about the SPEED of the reaction. 

→ size does not matter!

So, as the video mentioned, there is a standard Gibbs free energy equation that you are probably familiar with: 

ΔG=ΔH-TΔS

Where ΔH= change in enthalpy; ΔS= change in entropy; and T is temperature in kelvin

However, this system is not the most efficient for looking at the thermodynamics of enzymes that are under different standard conditions inside of our body. 

Therefore, scientists decided to test the standard free energy change under biological conditions denoted as: ΔG°’

The next equation we can use is: 

ΔG = ΔG°’+ RT ln(Q)

ΔG°’= free energy change under biological conditions

R= gas constant 8.315J/molK (often you need to convert this to kJ)

T= temp in Kelvin

Q= reaction quotient (concentration of products/concentration of reactants). 

Equilibrium Constant 

Keq = the value for Q when the ΔG = 0

This lets you get the equation

→ ΔG°’ = -RT ln(Keq)

This reaction tells us how far a reaction WANTS to go under standard conditions

A large Keq → large negative ΔG°’. This means that the reaction strongly favors products

-Since Keq still technically equals [Products]/[Reactants], when Keq is greater than 1, it means that there are more products than reactants. This means that the reaction is likely to happen, aka, exergonic (negative ΔG°’)

A small Keq → positive ΔG°’. This means that the reaction favors the reactants.