1. SUPA CHEMISTRY MRS. PAPARELLA SPRING 2016
Gibb's Free Energy
SUPA CHEMISTRY
MRS. PAPARELLA
SPRING 2016

2. DG = DH - TDS IS IT SPONTANEOUS? G = FREE ENERGY H = HEAT
T= KELVIN TEMPERATURE
S = ENTROPY (RANDOMNESS)
SPONTANEOUS REACTIONS HAVE A NEGATIVE DG
SPONTANEOUS REACTIONS ARE FAVORED WHEN THE REACTION IS
EXOTHERMIC ( -DH )
THE ENTROPY IS HIGH ( +DS)
TEMPERATURE IS HIGH

5. SOLUTION
D G = D H – (T X D S )
D G = 176 KJ/mol - (298 K X KJ/MOL K)
D G = 176 KJ/mol KJ/mol
D G = KJ/mol

6. Is the reaction spontaneous?????
D G = KJ
No ! D G must be negative for it to be spontaneous.
Remember that spontaneous reactions do not require any additional energy input. They proceed without outside intervention.

7. Predict if a reaction has a + or – D S
Just remember that your phases of matter show greater entropy if they are in the gaseous phase.
Count up the number of moles of gas on each side of the equation to see if the products have more entropy than the reactants. If they do, then the reaction has a + D S

8. Important Things to Remember
You will need to remember Hess's Law to find the Delta H and Delta S
Remember to find Delta H or Delta S you need to do
Products Summation – Reactants Summation
Make sure to pay attention to the signs!
Standard Conditions for these reactions is 298 K ( not 273K- like in gas law problems)
You must memorize the free energy equation.
Table C in the text book gives the values of Hf and S for various elements and compounds.
Hf for an element is zero
S values in Table C and probably on your exam will have the unit given as J/mol K. You need to move the decimal point back three places so that the S value has the same units as the H values!

9. In Brown, LeMay & Burstein, turn to page 717 and do 19.33 (d)
First write out the chemical equation
Use Table C to complete this chart
Species
Hf (KJ/mol)
S (j/mol K)
S (KJ/molK)
NOCl NO
Cl2

10. Your chart should look like this!
Species​
Hf(KJ/mol)​
S  (j/molK)​
S  (KJ/molK)​
NOCl​
​52.6
​264
​0.264
NO​
​90.37
​210.62 ​
Cl2​ ​0
​222.96 ​

11. Find the D H and D S using Products- Reactants
Find the D H and D S using Products- Reactants. Remember to use the correct unit for S!
D H
D S
Products – Reactants
2 [NO] + Cl [NOCl]
2mol [90.37 KJ/mol] + 0 KJ/mol - 2 mol[52.6 KJ/mol]
D H = KJ
Notice that the mol cancelled out!
Products – Reactants
2 [NO] + Cl [NOCl]
2mol[ KJ/mol K] KJ/mol K [0.264 KJ/mol K]
D S = kJ/K
the mol cancelled out

12. Solution: Remember to put the temperature in!
D G = D H –  (T X D S )​
D G = KJ - (298 K X KJ/ K)​
D G = KJ KJ​
D G = 40.9 KJ​

13. Practice & Self Check : p 716 19.21 ,19.23 19.33 a-c
Show your work set up ! Be very careful with units and copying data from Appendix C.