Chemistry · Rung 7 of 24

Reactions and balancing

In a reaction not a single atom appears or disappears: they are rearranged. Balancing follows from that — count atoms on both sides and put whatever numbers are needed in front, the coefficients, never touching the subscripts, because changing a subscript changes the substance. The least painful order: first the elements that appear in only one compound on each side, then hydrogen, and oxygen last. Five types cover almost everything you meet: synthesis (A + B → AB), decomposition (AB → A + B), single displacement (A + BC → AC + B), double displacement (AB + CD → AD + CB) and combustion, which with a hydrocarbon and oxygen always gives CO₂ and H₂O.

Where this sits

Comes after
Formulas and names
Opens up
The mole and the sums, Carbon chemistry, The carbon cycle, Reactions that give and take heat

Exercises from this rung

Generated by the app, from easiest to hardest

  1. When balancing an equation, what may you change?

    • Only the coefficients in front
    • You may add new substances
    • Coefficients and subscripts
    • Only the subscripts

    Why Coefficients say how many molecules there are and you may set them as needed. Subscripts say what each molecule is: changing H₂O to H₂O₂ does not balance anything, it swaps water for hydrogen peroxide, a different substance with different properties. Balancing is counting, not rewriting the chemistry.

    easier

  2. A hydrocarbon burns with plenty of oxygen. What always comes out?

    • Carbon dioxide and water
    • Carbon and water
    • Carbon dioxide only
    • Carbon monoxide and water

    Why A hydrocarbon has only carbon and hydrogen: with enough oxygen, every carbon ends as CO₂ and every two hydrogens as H₂O. That makes combustion the easiest reaction to balance, because you know the products before you start. Short of oxygen it changes: you get carbon monoxide and soot, which is why a badly ventilated heater is dangerous.

    in between

  3. To balance H₂ + O₂ → H₂O, someone writes H₂ + O₂ → H₂O₂. What did they get wrong?

    • They changed the substance: H₂O₂ is hydrogen peroxide, not water
    • They forgot a coefficient
    • Nothing, it is a valid way to balance
    • They put the coefficient in the wrong place

    Why The equation adds up on paper but no longer describes the reaction anyone wanted: hydrogen peroxide is a different compound that disinfects, bleaches and decomposes on its own. Balancing means saying how many molecules of each there are, never changing which ones they are. The correct equation is 2 H₂ + O₂ → 2 H₂O.

    harder