It's a law because the theory is just a statement
Aluminium reacts with sulphuric acid
Add the reltive atomic mass of Ca and O which are 40 and 16 respectively after addition we get molecular mass of CaO which is 40+16=56 now divide atomic mass of O by molecular mass of CaO and multiply it with 100 as show n 16/56 *100=28.57%
Enthalpy is the change in energy. A cold pack will react endothermically and enthalpy will be positive and a heat pack will have negative enthalpy. You cannot determine which has more change in enthalpy unless you measure the temperature change. Eg. a heat pack went from 25c to 35c will have greater change in enthalpy then a cold pack going from 25 to 20c because the net temperature difference is greater
