Consider the intermediate chemical reactions.
ca(s)+ co2(g) + o2(g) → caco(s)
ah, = -812.8kj
ah, = -1,269.8 kj
2ca(s)+ o2(g) → 2cao(s)
the final overall chemical equation is cao(s) + co2(g) → caco3(s). when the enthalpy of this overall chemical equation is
calculated, the enthalpy of the second intermediate equation

is halved and has its sign changed.
is halved.
has its sign changed.
is unchanged.