Energy Calculations

I'm back and better than ever!

It is now time for the mole to resurface and assist us in some calculations...

Energy in Equation:
- Energy absorption or release can be placed directly in an equation
Eg. CH4 + 2O2 = CO2 + 2H2O + 812 kJ
- Exothermic reactions have energy term on the RHS and negative Enthalpy, or energy released
- Endothermic reactions have energy term on the LHS and positive Enthalpy, or energy absorbed

Energy Calculations:
- Enthalpy is energy change of a reaction expressed in kJ per mole of one of the chemicals
- Using the above reaction as an example, we use coefficients of balanced equations for this exothermic reaction
-812 kJ/1 mol CH4 or -812 kJ/2 mol O2 or -406 kJ/1 mol O2
For products:
-812 kJ/1 mol CO2 or -812 kJ/2 mol H2O or -406 kJ/1 mol H2O

Now that we have the moles, we can use it for tons of different calculations!
Don't forget your sig figs because they have returned too!!!
Using the above reaction, calculate the amount of energy released when 0.35 moles of H2O is produced?

0.35 moles X -812 kJ/2 mol H2O = -140 kJ or 140 kJ released

Now calculate the moles of CH4 needed to produce 2100 kJ of energy.

2100 kJ X 1 mol CH4/-812 kJ = 2.6 moles CH4


Calculate how many grams of O2 would be needed to produce 1500 kJ of energy.

-1500 kJ X 1 mol O2/-406 kJ X 32 grams/1 mol = 120 grams of O2


Now, as you can probably tell, the return of the mole means that we are now able to do tons of Energy Calculations. STP, Particles, Formula Units, Grams, Atoms, etc. So, this is a good thing, right...? Of course!

Need a bit more help? Here's a video: