I think you're not clearly seeing the reaction that you want to get when you add the two given reactions together. From message 1, this desired reaction is
Li+ (g) + F�?/SUP> (g) ––�?gt; Li (g) + F (g)
Note that you must have Li+ ion on the left, Li+ (g) with a +1 charge, not Li (g). Also, there shouldn't be any F+ (g) in any of the reactions, but I think that was just a typo.
And when you reverse a reaction, you must change the sign of its ΔH.
Here are some examples:
Original reaction:
A + B ––�?gt; C ΔH = �?00 kJ/mol
Reversing this reaction gives:
C ––�?gt; A + B ΔH = +100 kJ/mol
The sign of ΔH was changed from negative to positive.
If we multiply the original reaction through by 2 like this:
2A + 2B ––�?gt; 2C ΔH = 2(�?00 kJ/mol) = �?00 kJ/mol
ΔH is multiplied by 2 also.
Or if we divide the original reaction through by 2,
1/2 A + 1/2 B ––�?gt; 1/2 C ΔH = 1/2(�?00 kJ/mol) = �?0 kJ/mol
we divide ΔH by 2 as well.
Reversing the original reaction and multiplying through by 2 gives:
2C ––�?gt; 2A + 2B ΔH = 2(+100 kJ/mol) = +200 kJ/mol
The sign of ΔH was reversed and ΔH was multiplied by 2 also.
When you add two reactions together, add their ΔHs together also:
A + B ––�?gt; C + D ΔH = �?00 kJ/mol
D + E ––�?gt; B + F ΔH = +250 kJ/mol
A + E ––�?gt; C + F ΔH = �?00 kJ/mol + +250 kJ/mol = +150 kJ/mol
Terms have to be on opposite sides of the reactions being added together in order to "cancel." B and D cancel out here.
OK... try again!
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