1. What are the driving forces behind chemical reactions?
Entropy vs. Enthalpy

3. Remember … ENTHALPY (∆H) – change in heat
ENTROPY (∆S) – change in disorder

4. What is Entropy? (∆S) ENTROPY  measure of disorder of a system
degree of freedom that particles of matter have
Systems tend to favor more disordered systems (as evident by your notebooks, cars, bedrooms!)
Nature tends toward chaos! (Your notebooks are just “natural”!)

5. RANK four states of matter on basis of ENTROPY
GREAT ENTROPY  LOW ENTROPY
(DISORDER)  (ORDER)
Gas > Solution > Liquid > Solid

6. Measuring Change in Entropy
∆S = S Final – S Initial ∆S Rxn = S Products – S Reactants
∆S is positive if entropy increases
EX. SOLID  LIQUID, GAS
Products would have higher entropy
∆S = +
∆S is negative if entropy decreases
EX. LIQUID  SOLID
Products would have lower
entropy
∆S = -

7. Predicting Change in Entropy
We can predict whether entropy increases or decreases by noting the physical state of the reactants  products.
CaCO3 (s) → CaO (s) + CO2 (g)
Predict the sign for entropy (∆S): either + or -

8. CaCO3 (s) → CaO (s) + CO2 (g)
∆S is POSITIVE (+) IF:
Reaction breaks up larger molecule into smaller molecular fragments
Reaction increases the number of moles of gas in products
Solid changes to Liquid OR
Liquid changes to Gas OR
Solid changes to Gas
CaCO3 (s) → CaO (s) + CO2 (g)
Predict the sign for entropy (∆S) = +

9. What is Enthalpy? ∆H, heat energy
ENDOthermic: heat is taken in by the reactants
EXOthermic: heat is released as a product