Hey there,

I‘ve got some questions about Hess law or to be specific I’ve got some exercises I’m not getting the right answers:

1. C2H2 is being burned and 1.3 *10^3 KJ/mol energy is relieved . H(CO2)= -394KJ/mol and H(H20)= -286 KJ/mol.The binding enthalpy for C2H2 is asked. How do I calculate that?

 

2. Si can be gained if SiF4 reacts with Al. Thereby AlF4 and Si is being produced. H(SiF4) = -1550 KJ/mol and H(AlF3)= -1301 KJ/mol. What would the enthalpy be?

 

I tried to do this two questions for hours but I’m always receiving the wrong answers. I would be thankful if someone could help me.

Thanks a lot!

 

greetings 

Can you show your attempts? 

2 hours ago, Trychem said:

1. C2H2 is being burned and 1.3 *10^3 KJ/mol energy is relieved . H(CO2)= -394KJ/mol and H(H20)= -286 KJ/mol.The binding enthalpy for C2H2 is asked. How do I calculate that?

You start by writing down the reaction equations making sure you include the states at start and finish.
This is because state change involves energy change.
This is likely where you are missing something.

 

So let us see you equation cycle, as hypervalent iodine says.

 

States don't really matter here, just the reaction itself and relevant stoichiometric coefficients. 

Did you first write a balanced equation of this form (with the right coefficients)?

\[   C_2H_2 + O_2 -->   CO_2 +  H_2O  \]