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.

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

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

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.

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.

6.3: 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