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Hess's Law Diagram help.

Hello,

I've always struggled with the point of Hess's Law diagrams, I don't understand what the use off the arrows are; I always seem to get the answers right because I just use this formula (Enthalpy change of products - enthalpy change of reactants), but what use are the arrows? and how can I tell if they go up or down or if one goes up and one goes down. Anyone good at explaining this to me, or does anyone have any good wbites which can explain well?

Thanks.
Original post by Paul-Ings
Hello,

I've always struggled with the point of Hess's Law diagrams, I don't understand what the use off the arrows are; I always seem to get the answers right because I just use this formula (Enthalpy change of products - enthalpy change of reactants), but what use are the arrows? and how can I tell if they go up or down or if one goes up and one goes down. Anyone good at explaining this to me, or does anyone have any good wbites which can explain well?

Thanks.


The arrow just shows the direction of the reaction (reactants to products)

If a definition goes in the opposite direction you have to change the sign of the energy change.

For example:

The enthalpy of formation is the energy change when 1 mole of a substance is formed from its constituent elements in their standard states.

A(s) + B(s) --> AB(s) ........... ΔH = -x kJ

If your energy cycle takes you from

AB(s) to A(s) + B(s)

then you have to add the negative of -x = +x kJ
Reply 3
The arrows describe the direction that the relevant enthalpy change is for.
For example, you could have an enthalpy change of 56kJmol^-1, but if you don't know which direction that is, you wouldn't know whether it should be plus or minus. (If following the direction of the arrow, you add the enthalpy change. If going against it, take away the enthalpy change).

It's rather hard to describe ahaha.
If you're still stuck, I'll draw out a couple of diagrams.
Reply 4
Hm, I sort of undertand.. basically the arrows describe if the reaction is endo or exo? But how do you tell if they go up or down? Enthalpy of formation goes UP, combustion down? and which one goes up and down?
Reply 5
Original post by Paul-Ings
Hm, I sort of undertand.. basically the arrows describe if the reaction is endo or exo? But how do you tell if they go up or down? Enthalpy of formation goes UP, combustion down? and which one goes up and down?


Edit: should have said: Formation is up, and combustion down.

The species on the bottom are either the combustion products, or the elements in their standard states. Just think about the word definitions of each - combustion forms products with oxygen, formation is from the elements in their standard states - this tells you that the arrows go towards the combustion products (CO2 and H2O normally) for combustion ie down, and away from the elements for formation ie up.

I do not change the direction of the arrows at all from the above method. If it's exo then the number is negative, if it's endo the number is positive, and I use the positive or negative number in the algebra to solve.
(edited 12 years ago)
Original post by clownfish
You're right, formation is down, combustion is up. The species on the bottom are either the combustion products, or the elements in their standard states.


Formation is not necessarly 'down' ...

There are many so-called 'endothermic' compounds whose enthalpy of formation is positive...
Original post by Paul-Ings
Hm, I sort of undertand.. basically the arrows describe if the reaction is endo or exo? But how do you tell if they go up or down? Enthalpy of formation goes UP, combustion down? and which one goes up and down?


The arrows do not show whether a process is exo or endo, they merely show the direction that you are going. If you wish to go 'against' an arrow you simply reverse the sign of the energy change for that step.
Reply 8
Original post by charco
Formation is not necessarly 'down' ...

There are many so-called 'endothermic' compounds whose enthalpy of formation is positive...


sorry I meant to write formation is up - by up I mean the arrow direction on the page is up (from the elements in their standard states at the bottom of the Hess cycle) to the prodcuts at the top.

I appreciate there are other types of cycles that can be drawn, but for the conventional way it is done in AS level, I have always found this works.

I was not talking about the sign of the arrow, just the traditional way of drawing a triangular Hess cycle.
Reply 9
okay, thanks for helping me understand it further.. ultimately my final question is how do you know in which direction the arrows go?
Reply 10
Original post by Paul-Ings
okay, thanks for helping me understand it further.. ultimately my final question is how do you know in which direction the arrows go?


The direction the arrow points is governed by the direction of the reaction.
Arrow points away from the reactants, towards the products.

For enthalpy level diagrams:
Enthalpy change values are given for the direction the arrow points - and may be positive (endo), or negative (exo).

If you want to reverse the reaction, or go 'against the arrow,' then change the sign of the enthalpy change for the reverse direction.

For Hess Law cycles:
Here you need to sketch a complete cycle to calculate an enthalpy changes indirectly.
You may be given combustion data or formation data.

For combustion data imagine combusting each reactant to form a 'pool' of intermediate compounds.
You could form this same pool by combusting the products of the equation.

So you need to get to this pool from the reactants and then away to the products.
Taking into account the nature of combustion processes you find that

Enthalpy change of reaction= Sum of enthalpies of combustion of reactants- Sum of enthalpies of combustion of products

For formation data a pool of elements can be used to form all the reactants of the reaction.
The same pool can be used to form the products.
Again, you need to get to the pool and then go form the products to obtain the desired enthalpy change.

Taking into account the nature of formation processes you find that

Enthalpy change of reaction= Sum of enthalpies of formation of products - Sum of enthalpies of formation of reactants

If my explanation doesn't make sense you might want to try this link: http://www.knockhardy.org.uk/sci_htm_files/08enth.pdf
(edited 12 years ago)
Reply 11
The direction of reactions becomes quite important at A2 level when you have to interpret, construct and calculate steps in cycles like this:



However, like you, I tended to rely on 'Products -Reactants ' at AS. It's always good to be able to construct the cycles though as it may be required for a question and also allows you to check your working and understand the reactions involved, rather than sticking to a rigid formula which may fall apart if the reaction is reversed or the question is asking for something other than what you may be used to. :smile:
(edited 12 years ago)
It's a real shame people 12 years ago were such *******s and failed to answer the questions. Clearly they're not going to be doing well in life. Time to slit my wrists

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