5) The first thing you need to do is look at the reactions you're given - reactions 1 and 3 are both going in the same direction as they consume similar reactants and produce similar products, but reaction 2 is the wrong way around, so you will need to use the reverse, which will have the opposite enthalpy change:
TiO
2(s) -> Ti(s) + O
2(g) ΔH = 912kJmol
-1After that, I would look at how reactions 2 and 3 can be used to explain the steps of reaction 1.
Starting with C(s) + TiO
2(s) + 2Cl
2(g), the first thing that could happen is the TiO
2 decomposes into Ti(s) and O
2(g), so we end up with C(s) + Ti(s) + O
2(g) + 2Cl
2(g).
The question is asking about a standard enthalpy of formation, so we need to look for elements in their standard states. If you look at the list of products from that last reaction, you can see that we now have all of the elements in this form! To get to the products of reaction 1, we would first need to convert C(s) + O
2(g) -> CO
2(g) (we are given this ΔH), and then Ti(s) + 2Cl
2(g) -> TiCl
4(l) (this is the ΔH
f of TiCl
4).You could write this in a cycle like this:
.
6) Look at your balanced equation, you have 5H on the left but only 2H on the right. You have exactly the right idea and your intermediate step with 5/2H
2 is correct, I think you just forgot to carry this on to the final reaction.