I'm really confused, ebery question I do on hess's law/enthalpy change I get it wrong. the markscheme for this question says: -642-286-(-602/2 x -92) which gives -142jkmol-1 BUT i got -1345kjmol-1, im so confused i used the enthalpy of formation equation= products-reactants because i really dont want to use those hess law diagrams, i did -286+(-642)-(-92 x 2)+(602) = -1345
but using the enthalpy of formation question how would ik i have to divide -602 by 2 and multiply -92 by 2 like im supposed to from the equation? pls help me
how would i do this question without using the hess law diagrams, would i use the enthalpy of formation=products-reactants the markscheme shows ΔH + 963 = –75 – 432 OR ΔH + 963 = – 507 (M1) ΔH = –75 – 432 – 963 (M1 and M2) ΔH = –1470 (kJ mol–1) Award 1 mark for + 1470
how would i do this question without using the hess law diagrams, would i use the enthalpy of formation=products-reactants the markscheme shows ΔH + 963 = –75 – 432 OR ΔH + 963 = – 507 (M1) ΔH = –75 – 432 – 963 (M1 and M2) ΔH = –1470 (kJ mol–1) Award 1 mark for + 1470
but i dont understand this markscheme
Whenever you are given enthalpy data, the only way about to answering the question is to construct a Hess's law cycle, there is no other way around. You are familiar with the definition of Hess's law right?
We use -963 instead of +963 because we are interested in the opposite reaction going, which means this is an exothermic process as opposed to the reaction (+963) given in question which is endothermic.
As for what the MS is trying to say, think of it like this: if I wanted to go from Birmingham to Manchester, but couldn't go directly. Then I could go from Birmingham to Nottingham, then from Nottingham to Manchester. If the displacement from Birmingham to Nottingham is X, then the displacement from Nottingham to Birmingham is -X (going in opposite direction). Hope this analogy is of some help.
Whenever you are given enthalpy data, the only way about to answering the question is to construct a Hess's law cycle, there is no other way around. You are familiar with the definition of Hess's law right?
We use -963 instead of +963 because we are interested in the opposite reaction going, which means this is an exothermic process as opposed to the reaction (+963) given in question which is endothermic.
As for what the MS is trying to say, think of it like this: if I wanted to go from Birmingham to Manchester, but couldn't go directly. Then I could go from Birmingham to Nottingham, then from Nottingham to Manchester. If the displacement from Birmingham to Nottingham is X, then the displacement from Nottingham to Birmingham is -X (going in opposite direction). Hope this analogy is of some help.
wow thank u so much, so is there no way i can use the formula for these hess law types of questions, my teacher taught that u could use either of them, so i have to use the diagram in this case? also how do u know which route or way to go, which enthalpy change are we trying to find out? im guessing its the one that says enthalpychange in the diagram?