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Energy Changes, calculations explanation help please!

I've been doing past papers in chem, my weak points are ionic equations, parts of alcohols and ALL of ENERGY CHANGES calculations etc, i just don't understand the topic. It's all the Hess's Law calculations and that. That is stopping me from getting the grade i want, anyone care to explain this topic to me and show me how to these calculations? Will deffinately reppp!! :smile:

NOTE: I know how to calculate energy evolved via that Q = mc/\T formula.

Thanks for those patient enough :p:

Reply 1

charco

Study Forum Helper

You understand (or accept) the law of conservation of energy?
Well that is basically Hess' law. However you go from one situation to another situation the energy change must always be the same. And if this were not the case then energy could be manufactured by going from A to B and then reversing the process to go from B to A + energy. Clearly this cannot happen.

OK so far?

Right then the chemistrs of the 19th century recognised all of this and went on to measure the energy changes for some simple situations such as combustion. Pretty easy to do. You set fire to something and measure the heat change in the surroundings. In crude experiments this could be a beaker of water and in more accurate experiments this is a bomb calorimeter. But it's all the same.

But scientists being what they are also wanted to know some hypothetical changes such as the energy change when a substance is formed from its elements in their states under standard conditions. Now nobody knew how much chemical potential energy an element contained so the boffins got together and decided that as it was impossible to determine they may as well use all of the elements as the base line zero. i.e. to say that the chemical energy needed to form an element in its standard state is zero and measure everything from there. (At first glance this may seem a bit strange but when you consider that we do it all the time with centigrade or height above sea level or even the length of the metre then you soon realise that almost everything is measured from some reference point)

OK back to energy.

So if you invent a hypothetical path for the formation of a substance using things that you can measure in the lab then you can work out the energy change even though you cannot actually do the experiment. And that's where all of the questions that are pose at A level or IB come from.

Take the enthalpy of formation of say ethene CH2CH2 - its impossible to measure directly. But take a look at the hypothetical equation for its formation:
2C + 2H2 --> C2H4

the enthalpy of combustion of carbon is easy to finad experimentally
C + O2 --> CO2
as is that of hydrogen
2H2 + O2 --> 2H2O
and so is that of ethene
CH2CH2 + 3O2 --> 2CO2 + 2H2O

Now to form 2CO2 is twice the enthalpy of combustion of carbon
to form 2H2O is twice the enthalpy of combustion of hydrogen

So we can calculate how much energy it would take to form the products

To turn CH2CH2 back to its elements is the reverse of its enthalpy of formation (CH2CH2 --> 2C + 2H2)

So our combustion equation can be equated to

Enthalpy of combustion (CH2CH2) = -(enthalpy of formation of CH2CH2) + enthalpy of formation of the products.

and by rearrangement you can get the enthalpy of formation of CH2CH2

Although this may seem rather long winded, it is the same principle that is always used. And as I said the reason is historical - this is actually how the data values that we have in data booklets were originally obtained (an are still being refined).

Reply 2

NeedHelp!

Hess's Law is fairly easy, you know the deinfition yeah about whatever route is taken the enthalpy changes will be the same, an example:

Reacting Carbon and Hydrogen to form Methane gas:

Formation of CH4 : ?
Combustion C : -393
Combustion H2: -286
Combustion CH4: -890

C(s) + 2H2 (g) ----> CH4(g) (This is the enthaly change of formation - so you want to make one mole of methane)

Well this can be done in two ways, you need to work out the enthaly change to get from the C and 2H2 to Methane. So what can happen is the Carbon and hydrogen can become oxidised to form CO2 and H20, yeah?

This is basically going from C to CO2 + H20 to Methane.

To go from Carbon to CO2 and H20 is named delta2 for the moment, to work out delta 2 you do the combustion energy of carbon and the combustion energy of hydrogen (-393) + 2(-286) = -965kJmol-1

delta3 is going FROM methane to H20 and CO2, and that is -890, however u want to go from CO2 +H20 TO methane, so u change the -890 to +890...

so finally to work out the energy change of formation from C + 2h2 --> methane is:

detlah2 + 890 = deltah1
-965 + 890 = -75

so its -75kJmol-1

this wuld be a lot easier if i had a diagram, but the important thing is to work out exactly whats reacting and to label the different routes so when u come to working it out u dont get confused, cos the calculating is fairly easy :smile:

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Ive just learnt this so its fresh in my head, lol, once you learn the rules you can apply it to any situation (of course if you're with OCR they change everything in an exam :rolleyes:

lol)

the top post exlpained it better than me, obviously had some notes right next to him :P

Reply 3

Doji

thank you charco for that historic explanation, i understood some of it lol, and NeedHelp, i do nuffield, i just don't get the hess cycle calculations and stuff sometimes...but both of you have given alot of effort, +rep on your way.
If anyone else also thinks they can give a gd explanation please dnt be shy :p: