Heat and Enthalpy( January 12 class)

• Reactions that release heat are exothermic
• Reactions that absorb heat are endothermic
• Heat is a form of energy
• All chemicals have energy stored in them. Stored chemical energy is enthalpy.
  
• Enthalpy of gasoline > Enthalpy of water
• Enthalpy symbol is H and change in enthalpy is ΔH

2 ways to write enthalpy in reactions
exothermic: 2C8H18 + 25O2 -----> 16 CO2 + 18H2O + 5076 kJ
2C8H18 + 25O2 -----> 16 CO2 + 18H2O ΔH= - 5076 kJ
endothermic: 2 C + 2H2 + 52.3 kJ -----> C2H4 3.2
2C + 2H2 + -----> C2H4 ΔH= + 52.3 kJ

ΔH and Moles
Coefficients can stand for moles or molecules
N2 + 3H2 -----> 2NH3 + 46.3 kJ
1 mole of N2 produces 46.3 kJ
2 ' H2 ' '
3 ' NH3 ' '

Conversion factors:
46.3 kJ/1 mol N2
46.3 kJ/ 3 mol H2
46.3 kJ/2 mol NH3

Example:
Find the amount of heat released if 5.0 mol of H2 are consumed
4.63 kJ/ 3 mol H2 x 5.0 mol of H2 = 77 kJ