Reverse the first reaction:
Now add the two reactions together. Note which intermediates get excluded and you will get your net reaction and the required enthalpy change.
Question from my grade 11 advanced chemistry class
The enthalpy change for the formation of two wolfram bromides are shown below.
Calculate the standard enthalpy change for the following reaction.
It's just a matter of logic really. Also, sticking that wouldn't balance the equation at all since we haven't compensated for the carbons and hydrogens!
Notice that there's 7 carbons and 14 hydrogens on the left. We can stick a 7 in front of carbon dioxide to balance the carbons (it's our only source!) and stick a 7 in front of our dihydrogen monoxide (since this is our only source as well).
Now, let's compensate for the oxygens. On the right hand side, we would have 14 oxygens from and 7 oxygens from giving us a total of 21 oxygens.
This suggests sticking in in front of on the left hand side and we'll be done.
If you don't like fractions, multiply both sides by 2 and you'll get integer values.