5.3 Hess’s Law

1. 5.3 Hess’s Law 5.3.1 Determine the enthalpy change of a reaction that is the sum of two or three reactions with known enthalpy changes Be able to use simple enthalpy cycles and enthalpy level diagrams to manipulate equations.

2. Hess’s Law • the heat evolved or absorbed in a chemical process is the same whether the process takes place in one or in several steps.

3. Combustion of methane • ∆H1 shows the magnitude of the single step reaction. • ∆H2 and ∆H3 when combined are equivalent to ∆H1, demonstrating Hess’s Law.

4. Most reactions occur in more than one step. • We focus on the OVERALL reaction based on the balanced chemical equation, but there could be many steps involved to reach the final outcome.

5. Rule #1 • If you reverse an equation the ∆Hmust switch signs. • Na (s) + ½ Cl2(g) NaCl (s) ∆H= -411 kJ • NaCl (s)  Na (s) + ½ Cl2(g) ∆H= 411 kJ

6. Rule #2 • If you need to multiply or divide any part of the equation, the whole equation must follow the same operation, including ∆H. • Na (s) + ½ Cl2(g) NaCl (s) ∆H= -411 kJ • 2 Na (s) + Cl2(g) 2 NaCl (s) ∆H= -822 kJ

7. Practice • A + B  AB ∆H1 = 20 kJ • AB + B  AB2∆H2 = 50 kJ • What is the ∆Hfor the overall reaction A + 2 B  AB2∆H3 = ? ∆H3 = 70 kJ

8. Practice:

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10. Review • Work on handout • Look over readings posted online