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

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

SOLUTION D G = D H – (T X D S ) D G = 176 KJ/mol - (298 K X 0.285 KJ/MOL K) D G = 176 KJ/mol - 84.93 KJ/mol D G = 91.07 KJ/mol

Is the reaction spontaneous????? D G = 91.07 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.

Predict if a reaction has a + or – D S https://www.youtube.com/watch?v=ro3AnXdajAM 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

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!

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

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 ​0.21062 Cl2​ ​0 ​222.96 ​0.22296

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] + Cl2 - 2 [NOCl] 2mol [90.37 KJ/mol] + 0 KJ/mol - 2 mol[52.6 KJ/mol] 180.74 - 105.2 D H = 75.54 KJ Notice that the mol cancelled out! Products – Reactants 2 [NO] + Cl2 - 2 [NOCl] 2mol[0.21062 KJ/mol K] + 0.22296 KJ/mol K - 2 [0.264 KJ/mol K] 0.42124+0.22296 -0.528 0.6442 -0.528 D S = 0.1162 kJ/K the mol cancelled out

Solution: Remember to put the temperature in! D G = D H –  (T X D S )​ D G = 75.54 KJ - (298 K X 0.1162 KJ/ K)​ D G = 75.54 KJ - 34.6276 KJ​ D G = 40.9 KJ​

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.