Really confused on finding delta h formation

[spoog74]

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okay, i get the whole hess's law when they just simply give you 2-3 reactions and ask you to find another one based on those.

Now, there are some questions that ask you GIVEN THE REACTIONS AND THERMODYNAMIC DATA BELOW calculate the heat of formation for BLANK


For example;

They give you

C6H5OH + 7 O2 >>> 6 CO2 + 3H2O (729.8)
C+ O2 >>> CO2 (94.4)
2H2 + O2 >>> 2H20 (136.8)

Now this is the question; calculate heat of formation for C6H5OH

NOW this is where my confusion comes in;

When they ask for heat of formation dont think want the C6H5OH to be on the PRODUCTS side? So shouldnt you be looking for 6 CO2 + 3H2O >>> C6H5OH + 7 O2 (-729.8 ) ? IS that the wrong way of thinking of it? I assume so because when i do it that way the answer comes out wrong.


I ALWAYS get these problems wrong because i dont know how to set it up.

 

[gtk06]

okay, i get the whole hess's law when they just simply give you 2-3 reactions and ask you to find another one based on those.

Now, there are some questions that ask you GIVEN THE REACTIONS AND THERMODYNAMIC DATA BELOW calculate the heat of formation for BLANK


For example;

They give you

C6H5OH + 7 O2 >>> 6 CO2 + 3H2O (729.8)
C+ O2 >>> CO2 (94.4)
2H2 + O2 >>> 2H20 (136.8)


i think you would make c6h5oh a variable x and solve for it.
so its like
729.8=products -reactants(C6H5OH)
try that correct me if im wrong

 

[annie12]

read the quesiton wrong haha sorry

 

[AlbinoPolarBear]

C6H5OH + 7 O2 >>> 6 CO2 + 3H2O (729.8) [eq1]
C+ O2 >>> CO2 (94.4) [eq2]
2H2 + O2 >>> 2H20 (136.8) [eq3]

Now this is the question; calculate heat of formation for C6H5OH

Flipping the equation is just the first step, you have to find out what the formation is from it's elements. Long solution, but I hope you can follow along.

1. Flip first equation to get C6H5OH on product side
6CO2 + 3H2O -> C6H5OH + 7O2 (-729.8) [eq4]

2. Multiply second equation by 6 to cancel out the CO2
6C + 6O2 -> 6CO2 (94.4X6) [eq5]

3. Add equations 4 and 5
6C + 3H2O --> C6H5OH + O2 (-729.8 + 94.4X6) [eq6]

4. Take 3rd equation and multiply by 3/2 to elminate H2O
3H2 + 1.5O2 --> 3H2O (1.5X136.8) [eq7]

5. Add equation 6 and 7
6C + 3H2 + 0.5O2 -> C6H5OH (-729.8 + 94.4X6+1.5X136.8)

Answer should be 41.8

Hope that helps.

 

[toothhornet88]

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