02 1.2 Direct combination reactions
A direct combination reaction is a reaction in which two or more substances join to form one new, more complex substance.
General pattern: A + B → AB
Think of two separate objects snapping together. Typical examples:
- Metal + oxygen → metal oxide
- Non-metal + oxygen → non-metal oxide
- Metal + non-metal → salt
Activity: reaction of metals with oxygen. Using tongs, a student heats three samples in a Bunsen flame for the same time: a sanded magnesium ribbon, a piece of steel wool (iron) and a copper wire.
Observations and conclusions:
- Ignites quickly and burns with a dazzling white flame, leaving a white powder
- Magnesium oxide, MgO
- 2Mg + O₂ → 2MgO
- Glows and burns with bright sparks
- Magnetic iron oxide, Fe₃O₄
- 3Fe + 2O₂ → Fe₃O₄
- No visible flame; a black layer forms on the surface
- Copper(II) oxide, CuO
- 2Cu + O₂ → 2CuO
- Conclusion: most metals combine with oxygen when heated in air, but they do so with very different vigour.
- Magnesium reacts most readily, iron less so, and copper least.
- So magnesium is more reactive than iron, and iron is more reactive than copper.
- Safety note. Burning magnesium gives out very intense light and ultraviolet radiation that can injure the retina.
- Never stare at it; wear safety goggles.
Notice how each equation is balanced. In 2Mg + O₂ → 2MgO there are two Mg atoms and two O atoms on each side. In 3Fe + 2O₂ → Fe₃O₄ there are three Fe and four O atoms on each side. In 2Cu + O₂ → 2CuO there are two Cu and two O on each side. The oxygen molecule O₂ always contains two atoms, so we must produce an even number of oxygen atoms on the left.
Other direct combination examples you can understand with the same pattern: 2Na + Cl₂ → 2NaCl (a metal and a non-metal give a salt), and Fe + S → FeS (heating iron and sulphur). Also, magnesium combines directly with bromine: Mg + Br₂ → MgBr₂.