# Hess law and combustion

Can someone please explain this to me? I get when we are using Hess law to work out the enthalpy of formation we write all the elements that are present in the reaction in their molar quantities. But in this combustion one, they split into CO2 and H20 because it's the products of combustion I get that part as well. But what is the 3O2 on both the arrows where did that come from And for the oxygen it's not balanced im so confused with this also y are the arrows pointing down in this one and not one going to reactants and one to product like the previous page?
(edited 11 months ago)
Original post by Rohan007best
Can someone please explain this to me? I get when we are using Hess law to work out the enthalpy of formation we write all the elements that are present in the reaction in their molar quantities. But in this combustion one, they split into CO2 and H20 because it's the products of combustion I get that part as well. But what is the 3O2 on both the arrows where did that come from And for the oxygen it's not balanced im so confused with this also y are the arrows pointing down in this one and not one going to reactants and one to product like the previous page?

I also don't understand Hess' law cycles. My teacher never really explained them properly, but I've never had a problem in the exam because I'm not sure they ever ask you to draw one yourself. Every Hess' law question I've seen you can use the rule for summing enthalpies of formation in products - reactants or summing enthalpies of combustion in reactants - products. This is stated in the specification as an expected question and it always comes up as a table. From the spec- "use Hess’s law to perform calculations, including calculation of enthalpy changes for reactions from enthalpies of combustion or from enthalpies of formation."

I've never seen a question that requires you to understand how to draw or fill in or explain an actual cycle diagram like these, unless it's a question about Born-Haber cycles or finding enthalpies of solution in thermodynamics. I hope this was somewhat comforting. If you need more explanation on the alternative formulae I've given please ask.
Original post by DanielK1456
I also don't understand Hess' law cycles. My teacher never really explained them properly, but I've never had a problem in the exam because I'm not sure they ever ask you to draw one yourself. Every Hess' law question I've seen you can use the rule for summing enthalpies of formation in products - reactants or summing enthalpies of combustion in reactants - products. This is stated in the specification as an expected question and it always comes up as a table. From the spec- "use Hess’s law to perform calculations, including calculation of enthalpy changes for reactions from enthalpies of combustion or from enthalpies of formation."

I've never seen a question that requires you to understand how to draw or fill in or explain an actual cycle diagram like these, unless it's a question about Born-Haber cycles or finding enthalpies of solution in thermodynamics. I hope this was somewhat comforting. If you need more explanation on the alternative formulae I've given please ask.

hi the aqa chem book i have uses 2 formulas first is total enthalpy change = energy absorbed to break bonds - energy released when forming products so if the reaction is more exothermic just add a - symbol to your answer and if it's endothermic add a + second is q=MCDeltaT are these the 2 we need to know?
Original post by Rohan007best
Can someone please explain this to me? I get when we are using Hess law to work out the enthalpy of formation we write all the elements that are present in the reaction in their molar quantities. But in this combustion one, they split into CO2 and H20 because it's the products of combustion I get that part as well. But what is the 3O2 on both the arrows where did that come from And for the oxygen it's not balanced im so confused with this also y are the arrows pointing down in this one and not one going to reactants and one to product like the previous page?

I just did another question on this I get the answers right because I get how to write the equation and calculate the combustion but when making the hesscycle I don't understand how the oxygens on the arrows work because they don't balance with the CO2 + H20 u get when splitting the combustion equation into its products @typicalnerd do u by any chance know how this works?
Original post by Rohan007best
I just did another question on this I get the answers right because I get how to write the equation and calculate the combustion but when making the hesscycle I don't understand how the oxygens on the arrows work because they don't balance with the CO2 + H20 u get when splitting the combustion equation into its products @typicalnerd do u by any chance know how this works?

Honestly if you're getting the answers right try not to stress about drawing the cycle. I'm not sure how trhe cycle drawing works either. If the question you did could come up on a real paper then you should be absolutely fine.
Original post by Rohan007best
hi the aqa chem book i have uses 2 formulas first is total enthalpy change = energy absorbed to break bonds - energy released when forming products so if the reaction is more exothermic just add a - symbol to your answer and if it's endothermic add a + second is q=MCDeltaT are these the 2 we need to know?

Ok so those are correct however when using Hess' law there are two equations which can 'fast track' you to the answer. If you are given an unfamiliar reaction and enthalpies of formation for each substance in the reaction, then to get the total enthalpy change for the reaction by summing the enthalpies of formation of the products and then taking away the sum of the enthalpies of formation of the reactants. Make sure when you do this to multiply by any balancing numbers on reactants and substances in the equation as the definition of enthalpies of formation (and combustion) specify it's for 1 mole. E.g. if 2CO2 is in the products, multiply the enthalpy of formation given by 2 for that substance. This works almost the exact same if given enthalpies of combustion except you reverse the equation. Instead you sum enthalpies of combustion for the reactants and take away the sum of enthalpies of combustion for the products. If you want a practice question, look up Q10 on Paper 1 2020. It's a Gibbs Free energy calculation, but the first stage is finding enthalpy change for the reaction when given enthalpies of formation. I hope this makes sense, and sorry for making you read all the way through if you already knew this!
Original post by DanielK1456
Ok so those are correct however when using Hess' law there are two equations which can 'fast track' you to the answer. If you are given an unfamiliar reaction and enthalpies of formation for each substance in the reaction, then to get the total enthalpy change for the reaction by summing the enthalpies of formation of the products and then taking away the sum of the enthalpies of formation of the reactants. Make sure when you do this to multiply by any balancing numbers on reactants and substances in the equation as the definition of enthalpies of formation (and combustion) specify it's for 1 mole. E.g. if 2CO2 is in the products, multiply the enthalpy of formation given by 2 for that substance. This works almost the exact same if given enthalpies of combustion except you reverse the equation. Instead you sum enthalpies of combustion for the reactants and take away the sum of enthalpies of combustion for the products. If you want a practice question, look up Q10 on Paper 1 2020. It's a Gibbs Free energy calculation, but the first stage is finding enthalpy change for the reaction when given enthalpies of formation. I hope this makes sense, and sorry for making you read all the way through if you already knew this!