Thermochemical Cycle things. ΔHf, ΔHc

Hess' Law states that the enthalpy change for a chemical reaction is the same, whatever ther route taken for the reactants to products. This means they can be made into cycles.

When in enthalpy of formation, (the enthalpy it takes to form one mole of compund from its constituent elements, under standard conditions, all reactants/products in standrad states), you write the balanced equation of your reactions on top, and the constiuent elements on the bottom. The arrows point up. Then, to follow the path of it, fill in the values, and flip the first arrow, and the negative/positive charges round. Add it up and you'll get the ΔH, from the ΔHf.

When it's the enthalpy of combustion, (the enthalpy it takes to burn one mole of compound in oxygen under standard conditions, with all products and reactants in their standard states), the balanced equation is on the top, and the arrows point downards. You'd the flip the second arrow and charges when you add all stuffs upppp.

So,

• Write an equation for the reaction
• Write down the constituent elements or CO2 and H2O at the bottom, balanced.
  
• Put in all the ΔHf/ΔHc values you have.
  
• Turn the correct arrows around
  
• Follow the path round, adding up.