Which of the following equations involving sulfur compounds represents a synthesis reaction?

1. \(\ce{2ZnS(s) + 3O2(g) -> 2ZnO(s) + 2SO2(g)}\)
2. \(\ce{SO2(g) + O2(g) -> 2SO3(g)}\)
3. \(\ce{SO2(g) -> S(s) + O2(g)}\)
4. \(\ce{H2SO4(aq) -> SO3(g) + H2O(l)}\)