First slide
Hess law of constant summation
Question

 ΔHf of BaCO3(s), CO2(g) , HCl(aq) and H2O(l) respectively are –1216, – 393, – 176 and – 286 KJ/mol. Then ΔHf of BaCl2(aq) from BaCO3(s)+ 2HCl(aq) → BaCl2(aq)+ CO2(g)+ H2O(l), ΔH = – QKJ

Easy
Solution

\large {\Delta _r}H\; = \;\sum {{\Delta _p}H\; - \;\sum {{\Delta _R}H} }
\large - Q = {\Delta _f}{H_{(BaC{l_2})}} + ( - 393) + ( - 286) - [ - 1216 + 2( - 176)]
\large - Q\; = \;{\Delta _f}{H_{(BaC{l_2})}}\; + \;889\
\large {\Delta _f}{H_{(BaC{l_2})}}\; = \; - (889 + Q)KJ

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