thermochemistry chem conceptual.

Question

i just want to check my understanding of this

Say you got this:

1)A + b=c

A=4.0 grams

and -91.8kj evolve

How do you find change of H ?

I guess you convert A to grams and then shove it under the evolved heat ? ie. change of H= -91.8kj/2 mol=

2)Now if i want to find heat of formation of C I just use the books values and then use algebra ?

Say book values for B=-290 and A=-197

A + b=c change of H= -46kj/mol

[(-197)+(-290)]- x = -46kj/mol

-x=441kj/mol

x=-441

hope you understand this )

Answer 1

I understand you have no idea what you are doing.
first: the 4 grams only has to do with how much (you can convert it to moles).

Second: Hf(of C)=Hreaction-Hf(ofA)-Hf(ofB) all on a per mole basis.

Now if there are coefficients of A or B, you have to multipy those coefficients by the Hf(or A or B).

I would suggest review your text materials, this concept is not difficult. You apparently are stuck on something.

Answer 2

what ?

This is from a test. I don't remember the numbers. The teacher never explained it so im trying to find a method to do it.

In part A we are asked to find the change of heat. Change of heat =standard heat of formation. (H with the little circle)

At this point i don't know what the evolved heat equals. Does it equal the q(heat)of surrondings or reaction?

So to find change of heat i got to find the moles of the limiting reagent. Since the mole ration are the same for the reactants it equals moles of A.

So throw it under the evolved heat to find change H=standard H

B) find x. Plug in the standard heat of formations for the ones i know. Then use algebra ?