5.3 Hess

1
5.3 Hesss 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
Hesss 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 Hesss 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 ?H must 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 ?H for the overall reaction
• A 2 B ? AB2 ?H3 ?

?H3 70 kJ
8
Practice
9
Practice
10
Review

• Work on handout
• Look over readings posted online