In order to properly balance an equation, we need to make sure that the same amount of elements on the reactants side matches the number of elements on the products side, we can do that by increasing the number in front of each molecule, the so called stoichiometric coefficient. In the reaction from the question we can properly balance by adding the following stoichiometric coefficients
3. Zn + 2 HCl -> ZnCl2 + H2
This will be a single displacement reaction, since Zn switched places with H2
4. Na2SO4 + SrCl2 -> 2 NaCl + SrSO4
In this case we have a Double displacement reaction, where we have two elements switching places and forming new compounds
5. S8 + 24 F2 -> 8 SF6
This is a Synthesis reaction, where two elements join together and form a new compound
6. 4 Fe + 3 O2 -> 2 Fe2O3
Again we have a Synthesis reaction
7. 2 C4H10 + 13 O2 -> 8 CO2 + 10 H2O
Here we have a Combustion reaction, this reaction mostly occur between hydrocarbons and oxygen gas, forming carbon dioxide and water as products
8. 2 MgO -> 2 Mg + O2
This is a Decomposition reaction, where one compound is decomposed into two or more elements