Answer:
(A) Sr (s) + (1/2) O2 (g) ---> SrO (s) dH = -592 kJ
(B) SrO (s) + CO2 (g) ---> SrCO 3 (s) dH = -234 kJ
(C) C (graphite) + O2 (g) ---> CO 2 (g) dH = -394 kJ
to get : Sr (s) + (3/2) O2 (g) + C (graphite) ---> SrCO 3 (s)
we use 1 equation (B) to provide the product , SrCO 3 (s)
we use 1 equation (A) to provide the Sr as a starting material
we use 1 equation (C) to provide the C as a starting material
(we are not concerned with providing the necessary O2, since it is provided from more than 1 source
combining 1 (A) & 1(B) & 1 (C) gives:
Sr + (1/2) O2 & SrO (s) + CO2 (g) & C (graphite) + O2 (g) ---> SrO (s) & SrCO 3 (s) & CO 2
we notice that
SrO (s) --> cancels out .--> SrO (s)
CO2 (g) --> cancels out .--> CO2 (g)
leavng only your desired equation
Sr (s) + (3/2) O2 (g) + C (graphite)--> SrCO 3 (s)
Hess's law says that if adding 1 (A) & 1(B) & 1 (C) gives you the equation you wish,
the combining the dH's from 1 (A) & 1(B) & 1 (C) gives you the dH you wish
-592 kJ & -234 kJ & -394 kJ = -1220 kJ
your answer is
dH = -1220 kJ
