Limestone stalactites and stalagmites are formed in caves by the following reaction: Ca2+(aq) + 2HCO3-(aq) --> CaCO3(s) + CO2(g) + H2O(l) . If one mol of CaCO3 forms at 298 K under 1 atm pressure, the reaction performs 2.48 kJ of P - V work, pushing back the atmosphere as the gaseous CO2 forms. At the same time, 38.95 kJ of heat is absorbed from the environment. A.) What is the value of ΔH for this reaction? B.) What is the value of ΔE for this reaction? So I thought that I had to add together the standard enthalpies of formation for each of the products and subtract the standard enthalpies of formation for the reactants. However, I did not see any ΔHf for Ca2+ or HCO3-. Secondly , when I looked at the answers, it said that the answer was ΔH=38.95, ΔE=36.47. I guess I don't understand when we should use the standard enthalpies of formation. Also, do ions made up of a single element have enthalpies of formation? And if I did have all the necessary enthalpies of formation, would my answer for ΔH also be 38.95? Thanks!