Fusion reactions are analogous to reaction 4 (2 H2O(l) → 2H2(g) + O2(g)), involving the combining of lighter elements into a heavier one, while fission reactions are similar to reaction 5 (2Mg(s) + O2(g) → 2MgO(s)), where a heavier element splits into lighter ones.
Fusion reactions involve the merging or combining of lighter atomic nuclei to form a heavier nucleus. In the provided reactions, reaction 4 (2 H2O(l) → 2H2(g) + O2(g)) exemplifies a fusion-like process where water molecules combine to produce hydrogen and oxygen gases. This is akin to the fusion reactions occurring in stars where hydrogen atoms fuse to form helium, releasing energy in the process.
On the other hand, fission reactions entail the splitting of a heavier atomic nucleus into lighter nuclei. In reaction 5 (2Mg(s) + O2(g) → 2MgO(s)), magnesium combines with oxygen to form magnesium oxide, representing a fission-like process where a compound breaks down into its constituent elements.
It's important to note that while these reactions share conceptual similarities with fusion and fission processes, they are not true representations of nuclear fusion or fission, which involve changes at the atomic or nuclear level and typically release substantial amounts of energy. The given chemical reactions showcase analogous principles on a molecular scale.