what is the balanced chemical reaction between Calcium Chloride and water in a heat pack?

CaCl₂(s) + aq → Ca²⁺(aq) + 2Cl⁻(aq) ...... ΔH = -ve

Heat pack.....

Solid calcium chloride dissolves in water to form calcium ions and chloride ions and gives off heat in the process. The dissolving process is exothermic. The ions in solution are at a lower energy than the solid salt and water, and so the "extra" energy of the reactants is given off as heat as they form the products.

CaCl2(s) --H2O--> Ca2+(aq) + 2Cl-(aq) ................ ΔH is negative (exothermic)

========== Follow up ===========

You asked about the formation of Ca(OH)2 and HCl. The answer is no, calcium chloride dissolving in water does not produce calcium hydroxide and hydrochloric acid. In fact, if you were to react calcium hydroxide and hydrochloric acid it would PRODUCE calcium chloride as the salt and water.

When calcium chloride is placed in water it simply dissolves, forming calcium ions and chloride ions. Since calcium ion is the cation of a strong base, and chloride ion is the anion of a strong acid, neither Ca2+ nor Cl- will hydrolyze (react with water).