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alevel chem help please??

How do you know which way the arrows in Hess cycles should be facing?
For example how would i know if the arrows should go from the elements to the reactants or from the reactants to the elements and then to the products?
Thank you so very much in advance!!

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Reply 1
Original post by grey_cloud
How do you know which way the arrows in Hess cycles should be facing?
For example how would i know if the arrows should go from the elements to the reactants or from the reactants to the elements and then to the products?
Thank you so very much in advance!!

Arrow facing depends on whether sth is endothermic (up) or exothermic (down). I think the question tells you about enthalpy change of atomisation or hydration or ionisation energy etc. therefore correspondingly yiu have to jnow your equation first. But it always go from reactants to products.
Original post by grey_cloud
How do you know which way the arrows in Hess cycles should be facing?
For example how would i know if the arrows should go from the elements to the reactants or from the reactants to the elements and then to the products?
Thank you so very much in advance!!

Generally it helps to know definitions of different kinds of enthalpy changes when making that decision.

Suppose we are required to make a Hess cycle in order to calculate the enthalpy of solution. For this, you will be given the lattice enthalpy of dissociation and the hydration enthalpies of the ions.

By definition, the lattice dissociation enthalpy is the enthalpy change when 1.00 mol of a solid ionic lattice is broken up into gaseous ions, so the relevant arrow should point from the solid ionic compound to the gaseous ions.

Similarly, hydration enthalpies involve 1.00 mol of gaseous ions dissolving in water (to form an infinitely dilute solution) to give aqueous ions. As such, this arrow will point from the gaseous ions to the aqueous ions.
Original post by CoalL
Arrow facing depends on whether sth is endothermic (up) or exothermic (down). I think the question tells you about enthalpy change of atomisation or hydration or ionisation energy etc. therefore correspondingly yiu have to jnow your equation first. But it always go from reactants to products.

In Born-Haber cycles, yes.

In most other Hess cycles, it doesn’t actually matter all that much.
Reply 4
Right, i was thinking about born haber.
I think it makes more sense when you know your equations that corresponds to your enthalpy change. Arrows go in the direction determined by the initial equation you know about based on your enthalpy change of __ provided
Reply 5
Original post by TypicalNerd
Generally it helps to know definitions of different kinds of enthalpy changes when making that decision.
Suppose we are required to make a Hess cycle in order to calculate the enthalpy of solution. For this, you will be given the lattice enthalpy of dissociation and the hydration enthalpies of the ions.
By definition, the lattice dissociation enthalpy is the enthalpy change when 1.00 mol of a solid ionic lattice is broken up into gaseous ions, so the relevant arrow should point from the solid ionic compound to the gaseous ions.
Similarly, hydration enthalpies involve 1.00 mol of gaseous ions dissolving in water (to form an infinitely dilute solution) to give aqueous ions. As such, this arrow will point from the gaseous ions to the aqueous ions.

When my teacher was covering this cover she didn't mention any of this
We learnt the symbols+ definitions for Standard enthalpy of combustion, formation, reaction and neutralisation
I might have to ask her to go over lattice dissociation enthalpy and hydration enthalpies
But thank you anyways
Reply 6
Original post by CoalL
My teacher did say though, arrows face up for formation and arrows face down for combustion

I noticed this pattern in practice questions so this makes sense to me
Is there a pattern like this for neutralisation and reaction?
(edited 1 month ago)
Reply 7
When exactly do you need to use hess cycles in questions? Why can't you just do the bond enthalpies of reactants- bond enthalpies of products?
Reply 8
What exam board do you do? For ocr a they normally only involve formation and combustion
Reply 9
Ohh that makes sense, I do ocr a
Reply 10
Original post by grey_cloud
When exactly do you need to use hess cycles in questions? Why can't you just do the bond enthalpies of reactants- bond enthalpies of products?


I mean obviously you can do that if the data of the bond enthalpies of the bonds in a compound are given. But you use hess cycles, when that isnt given and enthalpy change of xxx is given .
Original post by CoalL
What exam board do you do? For ocr a they normally only involve formation and combustion

They also do solution enthalpy cycles in the second year of A level (these are very common for OCR A), and also Hess cycles can be used with mean bond enthalpies (though they tend not to be needed in these cases)
Original post by CoalL
I mean obviously you can do that if the data of the bond enthalpies of the bonds in a compound are given. But you use hess cycles, when that isnt given and enthalpy change of xxx is given .

and is it always reactants- products or is it sometimes the other way round. if it is the latter then how do you know which way round it is?
Reply 13
Yeah so it really depends on the information that you’re given in the question.
Reply 14
Original post by grey_cloud
and is it always reactants- products or is it sometimes the other way round. if it is the latter then how do you know which way round it is?


ALWAYS reactants minus products for enthalpy

Product minus reactants is only for entropy change.
Am I right in guessing entropy change is second year or a later topic?
Reply 16
Original post by grey_cloud
Am I right in guessing entropy change is second year or a later topic?


Yep!
Thank you so much, that was so helpful!
Reply 18
Original post by grey_cloud
Thank you so much, that was so helpful!


No worries
Original post by CoalL
ALWAYS reactants minus products for enthalpy
Product minus reactants is only for entropy change.

I don’t believe so. It depends on whether you have combustion or formation enthalpy data

For combustion enthalpies and bond enthalpies, it’s reactants - products, but with formation enthalpies it’s products - reactants

Though your statement about entropies is correct, since they behave identically to formation enthalpies
(edited 1 month ago)

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