Would the enthalpy change be different if solid sodium hydroxide pellets were added to hydrochloric acid instead of adding hydrochloric acid to sodium hydroxide solution?

Answer 1

Yes, the reaction involving the solid is actually an individual step in the equation of the reaction between the solutions. If you were to add the change in enthalpy of the reaction with the solid NaOh to the change in enthalpy of the other step in the reaction (that's adding water and the NaOh pellets) you would find the sum equivalent to the change in enthalpy of the reaction involving the two solutions (this is supported by Hess's law). I suggest that you consider Hess's law for more information.

Answer 2

No, the enthalpy change would be the same regardless of the order of mixing, as it is a state function. The enthalpy change for the neutralization reaction between hydrochloric acid and sodium hydroxide does not depend on the physical form in which the reactants are added.