Can someone help me attempt to understand this question?

Question

Given the information:

A + B -> 2D DeltaH = 704.7 kJ Delta S = 304.0 J/K

C -> D DeltaH = 419.0 kJ Delta S = -205.0 J/K

Calculate DeltaG at 298 K for the reaction

A + B -> 2C

DeltaG = (blank) kJ

Answer 1

This problem makes use of Hess' Law (look it up and read about it for more information).

The target equation is A + B ==> 2C

Given:

eq.1: A + B ==> 2D ... ∆H = 704.7 kJ; ∆S = 304 J/K

eq.2: C ==> D .. ∆H = 419.0 kJ; ∆S = -205.0 J/K

For each of the above equations, we can calculate a value for ∆G using the equation

∆G = ∆H - T∆S

For eq.1: A + B ==> 2D

∆G = 704.7 kJ - 298K(0.304 kJ/K) = 704.7 kJ - 90.6 kJ = 614.1 kJ

For eq.2: C ==> D

∆G = 419 kJ - 298K(-0.205 kJ/K) = 419 kJ + 61.1 kJ = 480.1 kJ

Procedure to find ∆G for A + B ==> 2C

copy eq.1 to get A + B on the left side: A + B ==> 2D ... ∆G = 614.1 kJ

reverse eq.2 and multiply by 2 to get 2C on the right: 2D ==> 2C .. ∆G = -480.1x2 = -960.2 kJ

Add the equations and combine / cancel like terms to get...

A + B + 2D ==> 2D + 2C

A + B ==> 2C = TARGET EQUATION

∆G = 614.1 kJ - 960.2 kJ

∆G = -346.1 kJ