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

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

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”!)

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

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 = -

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 -

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) = +

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