AS Enthalpy Question...

First, write an equation for what it's asking you to find. Standard enthalpy of formation of propane means the enthalpy change when 1 mole of propane is formed from its elements under standard conditions. Propane is C3H8, and its elements are C and H2.

So the equation is: 3C + 4H2 --> C3H8

You then form a Hess's cycle linking the equations you already have and the equation above. To do this, find relationships between the equations e.g. in the first one, propane is reacting and in the equation above propane is being formed. When you have made a cycle (which must balance) linking them all together, you can find the standard enthalpy of formation of propane from it using Hess's Law (the enthalpy change accompanying a chemical change is independent of the route by which the chemical change occurs) i.e. you use the cycle to find two ways of getting from one set of reagents to a set of products, put the delta Hs of one route equal to that of the other and use them to find standard enthalpy change of formation of C3H8. It's quite hard to explain in words, but have a go and see what you come up with :smile: