add and subtracting negative number in a chemistry equation?

Question

What causes one of these equation to have a postive answer and one to have a negative answer.
Here are the variables.

HF(g) -271, Cl(g) 122, HCl(g) -92, F(g) 79

Here are the equations

1. HF(g) + Cl(g) (-) HCl(g) +F(g) =
Answer -136 mol


2. Cl(g) + HF(g) (-) HCl(g) + F(g) =
Answer +136 mol


the book using says this is because reversing the direction of a reaction alters the sign of the enthalpy change.

I just like to see the actually math and know a laymen's explantion why these to equaitons are different?

Thank to all that answer.

Answer 1

Balanced equation: 2HF + 2Cl -> 2HCl + F2

Add the reactants...
(-271 +122)

Subtract the products...
(-(-92) - 79) = (92 - 79)

Get the answer...
= -136

Reverse: 2HCl + F2 -> 2HF + 2Cl
Add the reactants...
(-92 + 79)

Subtract the products...
(-(-271) - 122) = (271 - 122)

Get the answer...
= +136

Answer 2

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