6.3: Hess’s Law

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. • N2 + 2O2→ 2NO2 all(g); H = +68kJ

3. 6.3: Hess’s Law • The overall rxn: • N2 + 2O2→ 2NO2 all(g); H = +68kJ • The two step rxn: • (1) N2 + O2→ 2NO all(g); H = +180kJ • (2) 2NO + O2 → 2NO2 all(g); H = -112kJ • net: N2 + 2O2 → 2NO2 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 XeF4 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