Question

Enthalpy question?

Answer 1

No, it's not correct. Does carbonic acid dissolve in water (could it ever be a solid?), or does it decompose in water? Furthermore, is the second reaction balanced? (No, it is not. Where is carbon?)

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The corrected reactions are:

`"CaO" (s) + 2"HCl"(aq)->"CaCl"_2(aq)+"H"_2"O"(l), " " DeltaH=-194.2 " kJ"`

`"CaO"(s)+"CO"_2(g)->"Ca"color(red)"C""O"_3(s)," " DeltaH=-178.4 " kJ"`

You should get `(1) - (2)`:

`cancel("CaO" (s)) + 2"HCl"(aq)->"CaCl"_2(aq)+"H"_2"O"(l)`
`"CaCO"_3(s) -> cancel("CaO"(s))+"CO"_2(g)`
`"----------------------------------------------------------------------"`
`"CaCO"_3(s) + 2"HCl"(aq) -> "CaCl"_2(aq) + "H"_2"O"(l) + "CO"_2(g)`

which is due to correcting and then reversing the second reaction mechanistic step.

The enthalpy of reaction then comes from the additive property of state functions:

`DeltaH_1 + DeltaH_2 = DeltaH_(rxn)`

And the second step was reversed, thus reversing the sign of its `DeltaH`.

`color(blue)(DeltaH_"rxn") = -"194.2 kJ" + (-(-"178.4 kJ"))`

`= color(blue)(-"15.8 kJ")`