Gibbs Free Energy
Entropy Changes in the Surroundings - Endothermic Reactions
Heat is being absorbed by the system and released by the surroundings. As a result, the decrease in temperature of the surroundings reduces the freedom of motion of the particles and makes energy less dispersed, so the entropy decreases. qsystem > 0 then qsurrounding < 0 and ΔSsurrounding < 0
Entropy Changes in the Surroundings - Exothermic Reactions
Heat is released from the system and absorbed by the surroundings. As a result, an increase in temperature of the surroundings results in more freedoms of motion of the particles, so entropy increases. qsystem < 0 then qsurrounding > 0 and ΔSsurrounding > 0
The Effect of Temperature on Reaction Spontaneity
In most cases, the enthalpy contribution to free energy is much larger than the entropy contribution. The temperature of a reaction influences the magnitude of the TΔS term, so for many reactions, the overall spontaneity depends on the temperature.
Standard Free Energy Change (ΔG°)
Occurs when all components of the system are in their standard states. ΔG°system = ΔH°system - TΔSsystem Another way to calculate ΔG° is with values for the standard free energy of formation (ΔG°f) of the components. This equation is applying Hess's Law and is just like solving for ΔH or ΔS.
Significance of the Sign of ΔG
Since the absolute temperature is always positive, for a spontaneous process, TΔSuniv > 0 so -TΔSuniv < 0 Using this relationship we can see: ΔG < 0 for a spontaneous process ΔG > 0 for a nonspontaneous process ΔG = 0 for a process at equilibrium
Entropy Changes in the Surroundings
The 2nd Law of Thermodynamics dictates that, for a spontaneous process, a decrease in entropy of the system is outweighed by an increased in entropy of the surroundings
What does a change in ΔG reveal?
The free energy change ΔG is a measure of the spontaneity of a process and of the useful energy available from it.
What does it mean when ΔSuniverse = 0 or ΔG = 0?
The process is at equilibrium. This occurs in phase changes
What does it mean when ΔSuniverse < 0 or ΔG > 0?
The process is nonspontaneous.
What does it mean when ΔSuniverse > 0 or ΔG < 0?
The process is spontaneous. A -ΔG is preferred because then the reaction will happen. If it is positive, it will not happen.
Significance of the Sign of ΔS
The sign of ΔG can tell us if a reaction is spontaneous or not. It relates to entropy: ΔSuniv > 0 for a spontaneous process ΔSuniv < 0 for a nonspontaneous process ΔSuniv = 0 for a process at equilibrium
Sign vs. Magnitude of ΔG
The sign of ΔG reveals whether a reaction is spontaneous or not, but the magnitude of ΔG tells us how spontaneous the reaction is.
Gibbs Free Energy
the energy of a system that is available to do work at a constant temperature and pressure. Also called simple free energy (G). Equation: ΔGsystem = ΔHsystem - TΔSsystem
Similar properties of ΔG°f values and ΔH°f values
ΔG°f of an element in its standard state is zero. An equation coefficient multiplies ΔG°f by that number. Reversing a reaction changes the sign, not value of ΔG°f.
When will a reaction become spontaneous as temperature decreases?
When ΔH < 0 and ΔS < 0 Ex: H2O(l) --> H2O(s) This reaction is exothermic and entropy decreases (L --> S) ΔG = ΔH - TΔS --> Since the negative sign on ΔS makes the term TΔS positive, T must be relatively low so that TΔS is less positive than ΔH is negative so that ΔG will be negative. "hot and tidy" --> spontaneous at low temp
When will a reaction be spontaneous at all temperatures?
When ΔH < 0 and ΔS > 0 Ex: 2 H2O2(l) --> 2 H2O(l) + O2(g) this reactions forms a gas and has more moles of products than reactants, thus entropy increases. ΔG = ΔH - TΔS --> The reaction is exothermic and entropy increases. Thus the negative ΔH is being subtracted and becoming more negative. Spontaneous reactions are "hot and messy"
When will a reaction be nonspontaneous at all temperatures?
When ΔH > 0 and ΔS <0 Ex: 3 O2(g) --> 2 O3(g) This reaction is endothermic and decreases in entropy due to less moles of product. ΔG = ΔH - TΔS --> ΔH is positive and ΔS is negative, thus the term adds to the positive ΔH and is more positive. Nonspontaneous reactions are "cold and tidy"
When will a reaction become spontaneous as temperatures increase?
When ΔH > 0 and ΔS > 0 Ex: H2O(s) --> H2O(l) This reaction is endothermic and entropy increases (S -> L) ΔG = ΔH - TΔS --> in order for the positive ΔH to become negative ΔG, the term -TΔS must be more negative than ΔH is positive. Thus the temperature (K) must be relatively high. "cold and messy" --> spontaneous at high temp
What reactions are temperature independent?
When ΔH and ΔS have opposite signs. When ΔH is negative and ΔS is positive, the reaction will be spontaneous at all temperatures. (exothermic + increased entropy) When ΔH is positive and ΔS is negative, the reaction will be non-spontaneous at all temperatures. (endothermic + entropy decreases)
What reactions are temperature-dependent?
When ΔH and ΔS have the same sign, the relative magnitudes of -TΔS and ΔH determine the sign of ΔG. In these cases, the direction of the change in T is crucial
