Balance by trial
Change the numbers in front. Never the formulae.
The golden rule
Fe + H₂O → Fe₃O₄ + H₂ has 1 oxygen atom on the left and 4 on the right. A student fixes it by changing H₂O to H₂O₄.
Is that allowed?
What this lesson covers
The idea
Formulae are never altered; smallest whole-number coefficients are placed before them, starting with the compound and element having the most atoms, balancing the remaining elements one by one, and finally recounting atoms on both sides.
The golden rule
Fe + H₂O → Fe₃O₄ + H₂ has 1 oxygen atom on the left and 4 on the right. A student fixes it by changing H₂O to H₂O₄.
Is that allowed?
- No. A formula is never changed. We put a number in front instead.
- Yes, as long as the atoms match
- Yes, but only for oxygen
Balance it
Use + and − to put a number (coefficient) in front of a formula. You cannot touch the formulae. The book says: start with the compound that has the most atoms, and in it the element with the most atoms.
The hit-and-trial method
Formulae are never altered. Put the smallest whole-number coefficients in front of them, starting with the compound and element having the most atoms. Balance the other elements one by one, and finally recount the atoms on both sides.
This is called the hit-and-trial method: we make trials with the smallest whole numbers.
- Step | What to do (Fe + H₂O → Fe₃O₄ + H₂)
- I | Draw a box round every formula. Do not change anything inside a box.
- II | List the atoms: Fe 1 | 3, H 2 | 2, O 1 | 4.
- III | Start with Fe₃O₄ and oxygen: put 4 in front of H₂O.
- IV | Hydrogen is now 8 on the left: put 4 in front of H₂.
- V | Iron is 3 on the right: put 3 in front of Fe.
- VI | Recount. All equal: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
Notes
Formulae are never altered; smallest whole-number coefficients are placed before them, starting with the compound and element having the most atoms, balancing the remaining elements one by one, and finally recounting atoms on both sides.
Check yourself
To balance oxygen in Fe + H₂O → Fe₃O₄ + H₂, which is the correct way?
Balance H₂ + O₂ → H₂O. What is the coefficient of H₂O?
Answer: 2
2H₂ + O₂ → 2H₂O: H 4 and 4, O 2 and 2.
A student balances an equation with 2, 4, 2, 4 in front of the formulae. What should she check?
- Change H₂O to H₂O₄. Never change a formula. H₂O₄ is a different substance.
- Write 4 H₂O — correct. Yes! The coefficient 4 goes in front of the whole formula. It makes 4 × 1 = 4 oxygen atoms.
- Write (H₂O)₄. The book says: never write it inside the formula. Put the number in front.
- Whether to change the formulae too. Formulae are never altered.
- Nothing. Any numbers that balance are best. The smallest whole numbers are the right answer.
- Whether all of them can be divided by 2 to get smaller whole numbers — correct. Yes! The book asks for the smallest whole-number coefficients: 1, 2, 1, 2.