[malc7067]

Hi all.

I was wondering if anyone knew of an easy method of calculating the reaction enthalpies using hess's law. It seems to me that this is really trial and error, rather than actual working out, so I was wondering if there really is a good method to use to solve these.... For e.g. (this is a random question i found off the internet...) p.s. I can actually do this problem, but it takes a long time trying to find the right amounts, this wouldnt be too good in an exam...

As an alternative to combustion, coal gas can undergo a process called methanation.

3H2 (g) + CO (g)       -->       CH4 (g) + H2O (g)

Determine the standard enthalpy change for this methanation reaction using the following chemical equations and the value for the standard enthalpy changes.

2H2 (g) + O2 (g)      -->      2 H2O (g)                                deltaH = -483.6 kJ
2C (s) + O2 (g)        -->      2CO (g)                                   deltaH = -221.0 kJ
CH4 (g) + 2O2 (g)   -->       CO2 (g) + 2 H2O (g)                 deltaH = -802.7 kJ
C (s) + O2 (g)         -->       CO2 (g)                                   deltaH = -393.5 kJ

[enahs]

I find it is typically easier to start backwards.

At the bottom of the work area of your page draw a line and write the final reaction. Then directly above that line write the equation in the proper direction with the most unique final product.

I.E., for this one, like the following.



**************************************************************





CO2 (g) + 2 H2O (g)   ----->     CH4 (g)    +   2 O2 (g)              + 802.7 kJ
_______________________________________________________________
3 H2 (g) + CO (g)       ----->      CH4 (g)    +   H2O  (g)


Then from there just work backwards until you use ever reaction and cancel out everything except what is below the line.
Such as this example, it is pretty easy. You know you have to have CH4 on the right (final product), so the one above the line has to be that. You now know you need more H2O on the right, and ther is only one other reaction with H2O in it, so you know which direction it goes too. From there you only have two reactions left and it is pretty easy.

Then all you have to do is decide what to multiply them by to cancel out properly and you are done.

In short, start backwards and on the last line put the reaction with the most unique final product you want, and start canceling backwards from there.

Note: It does not matter what line you put the reactions (you can put the one with the final product at the very top if you wish), as long as they all cancel properly.

[malc7067]

Seems like a very good technique, I'll have to try it out...


Thanks

[malc7067]

I tried this technique just then and it works !!! I solved one with three equations in no time at all! Thanks alot.

[Borek]

Note: it is still trial and error

[enahs]

Note: it is still trial and error

It is guided trial and error, where you eliminate certain choices so you have less trail and error to work with!