Enthalpy, Entropy, and Free Energy

Enthalpy H – heat lost or absorbed in a chemical reaction H – heat lost or absorbed in a chemical reaction + H – endothermic absorbs heat + H – endothermic absorbs heat - H – exothermic gives off heat - H – exothermic gives off heat

Entropy A measure of the disorder, or randomness, in a chemical reaction

Entropy and Phase Gases – most disorder Liquid – less disorder Solid – little disorder

S = S final - S initial We will not do calculations but just look at signs: + S – products more disordered than reactants + S – products more disordered than reactants - S – products less disorder than - S – products less disorder than reactants reactants

What would be the sign of S for the following: 1. Liquid water becomes solid water Answer: Since the product is LESS DISORDERED, the sign of S is negative

2. Solid bromine sublimes into gaseous bromine Answer: Since the product is MORE DISORDER, the sign of S is positive

Gibbs Free Energy A measure that reflects the enthalpy, entropy, and temperature of a reaction that is used to determine spontaneity. - G = Spontaneous reaction - G = Spontaneous reaction + G = Nonspontaneous reaction + G = Nonspontaneous reaction

G = H – T S The above equation is what is used to calculate the free energy change when given enthalpy, temperature, and entropy changes. For now, we will not do calculations, but you do need to know the equation!

IMPORTANT All reactions would prefer to have LOW ENERGY (- H) HIGH DISORDER (+ S) Spontaneity ( G) depends on the above two factors.

Some scenarios: 1. A reaction results in low energy (- H) high disorder (+ S) It will always be SPONTANEOUS because it is getting everything it wants! So G will be negative! Woo Hoo! Yeah!

2. A reaction results in high energy (+ H) low disorder (- S) It will always be NONSPONTANEOUS because it is getting nothing it wants! So G will be positive.

3. A reaction results in low energy (- H) low disorder (- S) One is good and one is bad so temperature is going to be important. In order for this reaction to be spontaneous, the temperature will have to be LOW.

4. A reaction results in high energy (+ H) high disorder (+ S) One is good and one is bad so temperature is going to be important. In order for this reaction to be spontaneous, the temperature will have to be HIGH.

Summary of Spontaneity Scenario H S High T Low T NonSp! + G Spont! - G 4++Spont! NonSp! + G Spontaneous at all Temps!!! - G Nonspontaneous at all Temps!!! + G

H = H = S = S = T = T = G = G =