Question

Calculate ΔHrxn for the reaction
CH4(g)+4Cl2(g)→CCl4(g)+4HCl(g)

Given these reactions and their ΔH′s .
C(s)+2H2(g)→CH4(g)ΔH=−74.6kJ

C(s)+2Cl2(g)→CCl4(g)ΔH=−95.7kJ

H2(g)+Cl2(g)→2HCl(g)ΔH=−184.6kJ

ΔHrxn =

Answer 1

his can be done using Hess' Law
reverse equation 1 to get CH4 ---> C + 2H2 ... ∆H = + 74.6 kJ
keep equation 2 as is to get C + 2Cl2 ---> CCl4 ... ∆H = - 95.7 kJ
keep equation 3 but multiply by 2 to get 2H2 + 2Cl2 ---> 4HCl ... ∆H = -184.6 x 2 = -369.2 kJ
add the three equations and cancel what appears on both sides to get ...
CH4 + C + 2Cl2 + 2H2 + 2 Cl2 ---> C + 2H2 + CCl4 + 4 HCl and canceling you get...
CH4 + 2Cl2 + 2Cl2 ---> CCl4 + 4HCl
CH4 + 4Cl2 ---> CCl4 + 4HCl which is the target equation
Add the ∆H's: +74.6 + (-95.7) + (-369.2) = -390.3 kJ