Chem entropy / Gibbs Free

What is a spontaneous process?

Proceeds without any assistance from outside the system

Water evaporates at 25 C

Irons rusts in the presence of O2 and H2O

NaCl dissolves in water

What is a non-spontaneous process?

Assistance fron outside the system is necessary in order to induce the desired change

Water does not boil at 75 C and 1 atm

Water does not freeze at 15 C

Fe2O3(s)+3C(s)-->2Fe(s)+3CO(g) at 25 C

What are exothermic reactions often?

Spontaneous

What does nature tend to favor?

Processes that cause a reduction in energy

What happens in an exothermic reaction?

The bonds in the products contain less energy than the bonds in the reactants. The excess energy is released as heat

Are endothermic reactions spontaneous?

They can be

Is evaporation spontaneous?

Yes

H2O(l)-->H2O(g) ∆Hº=+40.7 kJ

C3H8(l)-->C3H8(g) ∆Hº=+16.7 kJ

Is dissolving soluble compounds spontaneous?

Yes

NaCl(s)-->Na+(aq)+Cl-(aq) ∆Hº=+3.9 kJ

What does each spontaneous reaction involve?

An increase in entropy

What is the symbol for Gibb's Free Energy?

G

What is Gibb's Free Energy?

The energy associated with a chemical reaction that can be used to do work

What is work?

Force X Distance

What is energy?

The ability to do work

What is the formula for ∆G?

∆H-T∆S

What does ∆G stand for?

Free energy change (kJ)

What does ∆H mean?

Enthalpy change (kJ)

What does T mean?

Temperature of system (K)

What does ∆S mean?

Entropy change (kJ/K)

When is the reaction spontaneous?

If ∆G<0

How do you determine Gibb's Free Energy?

By combination of enthalpy and entropy

Which reactions are favorable?

Exothermic

Which reactions are not favorable?

Endothermic

An increase in what is favorable?

Entropy

What is not favorable?

Decrease in entropy

Determine the temperature of the system. H20(l)-->H2O(g)

∆H= 44.004 kJ/mol

∆S= 118.8 J/K mol

370.4 K

Find ∆G for a reaction if: ∆H=-218 kJ and ∆S=-765 J/K at 32 C. Is the reaction spontaneous?

15.3 kJ, non-spontaneous

What determines whether it is spontaneous most of the time?

Temperature

If the enthalpy is - and the entropy is + is it spontaneous?

always

If the enthalpy is - and the entropy is - is it spontaneous?

sometimes. spontaneous at low temperature, not at high

If the enthalpy is + and the entropy is + is it spontaneous?

sometimes. spontaneous at high temperature, not at low ones

If the enthalpy is + and the entropy is - is it spontaneous?

never