Chem Class- January 12, 2010

To start the class off, we went over the answers to our "Classifyung Chemical Equations" worksheet. Then Mr. Doktor combined ammonium nitrate and water and the beaker turned cold. This was an example of an endothermic reaction. Then he lighted a bunsen burner nad burned magnesium (Equation: Mg + O2 -----> MgO). The magnesium produced light and was an example of an exothermic reaction. After that, he burned steel wool (Equation: Fe + O2 ----> Fe2O3). It lighted on fire and sparks flew from it. This was another example of an exothermic reaction.

We also took notes:
Heat and Enthalpy

• 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 (Mr. Doktor dropped a textbook from different heights to demonstrate that enthalpy is sotred chemical energy)
• Enthalpy of gasoline > Enthalpy of water
• Enthalpy symbol is H and change in enthalpy is ΔH

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

Also, when given an equation, the coefficients can stand for moles or molecules.
Example: N2 + 3H2 -----> 2NH3 + 46.3 kJ
So 1 mol of N2 produces 46.3 kJ, 3 mol of H2 produces 46.3 kJ and there are 2 mol of NH3
This gives you the following conversion factors: 46.3 kJ/1 mol N2, 46.3 kJ/ 3 mol H2 and 46.3 kJ/2 mol NH3
So, if you were asked to find the amount of heat released in...say 5.0 mol of H2 that is consumed, here's how it'll work: 4.63 kJ/ 3 mol H2 x 5.0 mol of H2 = 77 kJ