1. 6.3: Hess’s Law Enthalpy is a state function. So, in going from a set of reactants to a set of products, the change in enthalpy is the SAME whether the rxn takes place in ONE STEP or a SERIES OF STEPS. See the overall rxn broken out into a series of steps on p. 256.

2. 6.3: Hess’s Law BTW, notice the species NO formed in step 1 and consumed in step 2. This is called a “rxn intermediate” and we will study these in detail in Ch 12, kinetics. N 2 + 2O 2 → 2NO 2 all(g);  H = +68kJ

3. 6.3: Hess’s Law The overall rxn: N 2 + 2O 2 → 2NO 2 all(g);  H = +68kJ The two step rxn: (1)N 2 + O 2 → 2NO all(g);  H = +180kJ (2)2NO + O 2 → 2NO 2 all(g);  H = -112kJ net: N 2 + 2O 2 → 2NO 2 all(g);  H = +68kJ

4. 6.3: Using Hess’s Law: two important rules. (1) If a rxn is reversed, the sign of  H is also reversed. (2) Heat flow is an extensive property; if the coefficients in a balanced rxn are multiplied by an integer, the value of  H is also multiplied by that same integer.

5. 6.3: Hess’s Law See the XeF 4 example in the text, p. 257. Also see S/E 6.7, p. 258. A little tougher example: S/E 6.8, pp. 258 – 259. Nothing to fear, Hess’s Law problems are actually fun once you get used to solving them. See Journal Notes 6.3a They’re kind of like solving puzzles.

6. 6.3: Hess’s Law Speaking of having fun, Let’s have a pop quiz! Books and notes away. After the quiz, h/w 6.3: 52 - 58