Hess

1
Hesss Law

• If you add two or more thermochemical equations
  to give a final equation, then you can also add
  the heats of reaction to give the final heat of
  reaction.
• Allows you to determine the heat of reaction.

2
Manipulations

• If you reverse a reaction, change the sign of ?H.
• C O2 ? CO2(g) -393.5kJ
• CO2(g) ? C O2 393.5kJ

3
Manipulations

• If you double/triple/etc. a reaction, you also
  must double/triple/etc. ?H.
• C O2 ? CO2(g) -393.5kJ
• 2C 2O2 ? 2CO2(g) 2-393.5kJ

4
Example

• Work backwards
• Given the following, calculate ?H for
• 4NH3(g) 3O2(g) --gt 2N2(g) 6H2O(l)
• 2N2O(g) --gt O2(g) 2N2(g) ?H -164kJ
• 2NH3(g) 3N2O(g) --gt 4N2(g) 3H2O(l) ?H
  -1012kJ
• Answer -1532 kJ

5
Standard Enthalpy of Formation

• Represented by ?Hf
• Sometimes calorimeters can not be used to find ?H
  (like if the process is very slow)
• Shows how much energy is required to make 1 mole
  of a compound from its elements with all
  substances in their standard state
• Table 6.2 in your book

6
Standard State Definition

• COMPOUND
• Pressure 1 atm
• Pure liquid or solid
• If in solution, concentration 1M
• ELEMENT
• Pressure 1 atm
• Temperature 25C
• State whatever state it exists in these
  conditions

7
Calculating

• DH0reaction ?npDHf0(products)?nrDHf0(reactants)
• ? means sum of
• np is moles of productnr is moles of reactant
• If dealing with an element, the DHf0 is zero

8
Example

• Calculate the change in enthalpy
• CH4(g) 2O2(g) ? CO2(g) 2H2O(l)
• Answer
• Products -393.5kJ 2(-285.8kJ)
• -965.1kJ
• Reactants -74.86kJ 2(0.0kJ)
• -74.86kJ
• P-R -965.1kJ-(-74.86kJ) -890.24kJ

9
Fossil Fuels

• When decayed plants are burned, the energy stored
  in them can be used
• Petroleum and natural gas (hydrocarbons)
• Coal creates about 23 of U.S. energy

10
Hydrocarbons

• Prefixes are the same in organic chemistry
• Meth-
• Eth-
• Prop-
• But-
• Pent-
• Hex-
• Hept-
• Oct-
• Suffixes represent different bonding/compounds

11
Environmental Impacts

• Greenhouse effect CO2 created from combustion of
  fossil fuels absorbs infrared radiation given off
  by the earth and does not allow it to leave the
  atmosphere
• H2O can also absorb radiation (humidity), but
  earths H2O has not changed much

12
New Energy Sources

• Coal gasification creates a gaseous form of coal
  (syngas) that is easily transported and reacted
  with oxygen in a combustion reaction, releasing
  energy
• This material can be used to produce other fuels
  like methanol

13
New Energy Sources

• Coal Slurries coal ground up and mixed with
  water and used instead of solid coal in power
  plants