Question

How do you balance
C2H2 + O2 -> CO2 + H2O??

Is it C2H2 + 5/2 O2 -> 2CO2 + H2O
or 2C2H2 + 5O2 -> 4CO2 + 2H2O ?

Answer 1

All coefficients must be whole numbers. Often, sub-steps will include fractional coefficients, but the final answer must be only whole numbers.

Answer 2

2 C2H2 + 5 O2 --> 4 CO2 + 2 H2O

Answer 3

To answer your question explicitly, your second answer is correct.

Answer 4

that's what i think too, but my teacher put C2H2 + 5/2 O2 -> 2CO2 + H2O

The standard enthalpy of combustion of C2H2 is -1301kJ. the standard enthalpies of formation of CO2 and H2O are -394 and -286kJ, what is delta H f for C2H2??

so in this case, u will still use the 2nd equation?

Answer 5

It shouldn't matter, I don't believe. In either case, you would end up multiplying the enthalpy changes by the coefficients anyway. However, I'm not 100% sure on this.

Let's check.

For example, in this case it would be something like:\[\Delta H_{rxn}^0=\sum r \Delta H_f^0(products)-\sum r \Delta H_f^0(reactants)\]Where r is the coefficients of the compounds. So, using the whole-number reaction equation:\[\Delta H_{rxn}^0= [(4)(-394kJ)+(2)(-286kJ)]-[(2)(-1301kJ)+5(0kJ)]\]\[\Delta H_{rxn}^0=454.kJ\]Now, if we're correct, the reduced reaction equation should produce the same answer. Let's try:\[\Delta H_{rxn}^0=[(2)(-394kJ)+(1)(-286kJ)]-[(1)(-1301kJ)+(5/2)(0kJ)]\]\[H_{rxn}^0=227.kJ\]
Well, based on that, it would seem that the chemical formula obviously does matter.

And I just remembered why this is so.

Answer 6

I believe that the formula for the enthalpy of formation for a compound must be written in such a way that the coefficient of the compound you are trying to find the enthalpy of formation for is 1.

Answer 7

Once again, though, I'm not sure. This should be verified by someone who knows.

Answer 8

ohh ok thank you so much :)
I will check back the notes again!

Answer 9

Are there any more equations involved in this specific problem?

Answer 10

It does not matter which way you solve for it. If you were to use the the delta H rxn of the balanced reaction, you would be getting the enthalpy of formation for 2 moles of C2H2; you can just halve that to get the standard enthalpy of formation which is for 1 mole of the molecule/compound. Solving for it using the reaction with the .5 mol O2 will directly give the you standard enthalpy of formation.

So yeah, Xishem is right. The standard enthalpy of formation is only for 1 mole, but it can solved for via many different ways.

Answer 11

I have many enthalpy ques like that
but yea, i know why
juz ask rogue said, it is 2mol in here, so all i have to do is to divide it to get the answer
cuz all my other questions only have 1 mol of that

but i remember only for this kind of equation can have fraction in the equation, and there is a type of reaction that it must not have fraction of coeeficient , so i messed up those 2..
i need to find out which one is which..

and thanks for both of ur help again!

Answer 12

I apologize for not having a full understanding of the subject. I hope I didn't confuse you further.

Answer 13

no problem :) I understand it better now !