I am currently investigating the trends in enthalpy change of the reaction of group 2 carbonates with HCl. As per ion polarization theory there should be an upward trend in stability and thus a decrease in heat given off as I go down the group. In my experiment I am seeing this trend in all group 2 carbonates except calcium carbonate. The reaction between HCl Calcium Carbonate is showing a lower temperature rise that of Strontium Carbonate and HCl. I would appreciate it if someone could help me account for this deviation in the predicted trend.