The balanced chemical equation for the reaction Na2O + H2O -> 2NaOH is:

Na2O + H2O -> 2NaOH

The balanced chemical equation for the reaction 2NaOH + SO3 -> Na2SO4 + H2O is:

2NaOH + SO3 -> Na2SO4 + H2O

Therefore, the overall balanced chemical equation for the reaction Na2O + SO3 -> Na2SO4 is:

2Na2O + 2SO3 -> 2Na2SO4