Thermochemestry Test Corrections - Hess's Law

[StarTurtle]

Hey there everyone.

My Chemistry grade is not so hot right now, and this test basically decides my final grade. ^^; So I really want to get it right, so if you could help me out a little, it would be greatly appriciated.

 

So, here's the thing: I got very sick last week and missed four classes. This means I missed almost all of the lessons for thermochem. Because of this, I have no clue what I'm doing.

 

My current and biggest issue is something about Hess's Law. Anyone think they can help me with it?

 

Mainly, I don't know how I'm allowed to cancle things out and how I'm not allowed to. If someone could explain that to me, or if I could just enter in my problem and have some of you guys look at it, it would probably help a whole bunch.

[hermanntrude]

generally when summing equations, you can cancel out any compound which appears on BOTH sides of the equation after the addition is performed. It's just like algebra.

 

if 2x + 2 + z = y + 2 + z

 

then 2x = y

 

Also, for instance if you have 3 water molecules on the right hand side and 2 on the left, you can cancel two water molecules from each side, leaving one on the right hand side and none on the left.

 

it's easier to see what will cancel if you add the equations first, which gives you a long, untidy-looking equation, and then examine THAT for any possible cancellations.

 

for instance:

 

1) H2O + CH3COOH --> CH3COO- + H3O+

+ 2) H3O+ + OH- --> 2H2O

 

total =H2O + H3O+ + OH- + CH3COOH --> CH3COO- + H3O+ + 2H2O

 

now you can see there are two water molecules on the RHS, one of which will cancel with the one on the LHS, and also there is an H3O+ on both sides, so they cancel too:

 

total = OH- + CH3COOH --> CH3COO- + H2O

[DavyJonesLoquet]

Hermann your a chemistry nut!

 

Your total is partially correct since H20 + H30 does not = OH but OH2 + CH3COOH3--> Base