Today we’re covering:
 What is Gibbs Free Energy?
 Predicting the effect of temperature on the spontaneity of a reaction
Let’s go!
From the previous post, we know that:
For a reaction to be spontaneous/feasible, the TOTAL entropy of the system + surroundings MUST increase.
Generally*, you can know if a reaction has a positive ΔS_{total }if:
 The entropy of the system increases
ΔS_{system} > 0  The reaction releases heat to the surroundings (exothermic)
ΔH_{r} < 0
*However, there are exceptions.
The most accurate way to tell if a reaction is spontaneous is to look at its
Gibbs Free Energy Change.
What is Gibbs Free Energy?
The amount of available energy in a reaction that can be used to do work.
It is denoted by ‘G’.
A change in G is defined as:
ΔG = TΔS_{total
}or
ΔG = ΔH – TΔS_{system}
Proof 
ΔS_{total} = ΔS_{system }+ ΔS_{surroundings}
TΔS_{total} = TΔS_{system }– TΔS_{surroundings}
Since ΔS_{surroundings }= –ΔH_{reaction}/T, TΔS_{surroundings }= –ΔH_{reaction}
TΔS_{total} = TΔS_{system }– ΔH_{reaction} ΔG = ΔH – TΔS_{system} 
How do you use ΔG to tell if a reaction is spontaneous?
Look at the value of ΔG, if:
ΔG < 0 (negative) 
Reaction IS spontaneous.
Since the reaction increases the total entropy of system + surroundings, it disperses energy. When entropy increases, useful energy is dispersed as wasteful energy. 
ΔG > 0 (positive) 
Reaction IS NOT spontaneous.
However, the reaction CAN occur if energy is constantly supplied by an external source. The reaction CAN also occur in REVERSE (i.e. the products form the reactants) since the ΔG of the reverse reaction is negative. 
ΔG = 0  Reaction is in equilibrium.
Reaction can occur both forwards & backwards, but has no net change in amounts of reactants & products. 
Calculating ΔG manually:
STEP 1: Calculate ΔH_{r} 

STEP 2: Calculate ΔS_{system} 
can be calculated from given standard enthalpies 
STEP 3: Identifying T 

STEP 4: Using the Formula 
ΔG = ΔH – TΔS_{system} 
STEP 5: Is the reaction spontaneous? 

Calculating ΔG using Hess’ Law
Just like ΔH, ΔG can be calculated by using alternate pathways.
Just as how certain reactions have Standard Enthalpies,
they also have Standard Gibbs Free Energy Changes:
The Steps in calculating ΔG using Hess’ Law are the same as calculating ΔH
(see here)
From Cambridge International A Level Chemistry
What increases the spontaneity of a reaction?
Based on the equation ΔG = ΔH – TΔS_{system},
to make ΔG more NEGATIVE, you need:
 A LOWER (more negative) ΔH
 A HIGHER Temperature
 A HIGHER ΔS_{system}
What can you do to make a nonspontaneous reaction spontaneous?
Since you can’t change ΔS_{system }or ΔH directly,
you have to INCREASE the temperature.
Q: Doesn’t increasing T affect ΔS_{system }or ΔH?
A: Yes, but very slightly, such that the difference is negligible.