How to Balance Chemical Equations
Balancing is not guesswork, even though it is usually taught that way. Work through the elements in a fixed order and the coefficients fall out. The rule underneath it all is that atoms are never created or destroyed, only rearranged.
The order to work in
- Balance metals first.
- Then non-metals other than hydrogen and oxygen.
- Then hydrogen.
- Then oxygen last, because it usually appears in the most places.
- Count both sides again at the end. Every element, every time.
What you may and may not change
You may only change the coefficients, the big numbers in front of a formula. You may never change a subscript, because that changes the substance itself. Turning H2O into H2O2 balances the equation and also turns water into hydrogen peroxide.
Fractions are allowed, temporarily
If a half coefficient makes the balance work, use it and then multiply the whole equation through to clear it. Most courses want whole numbers in the final answer.
Worked examples
Example 1
Question
Balance: CH4 + O2 gives CO2 + H2O
Answer
CH4 + 2 O2 gives CO2 + 2 H2O
Steps
- 1No metals, so start with carbon: 1 on each side already.
- 2Hydrogen next: 4 on the left in CH4, so we need 2 H2O on the right.
- 3Now count oxygen on the right: 2 in CO2 plus 2 in the two waters, giving 4.
- 4Balance oxygen on the left: 4 atoms means 2 O2.
- 5Final check: C 1 and 1, H 4 and 4, O 4 and 4.
Example 2
Question
Balance: Al + HCl gives AlCl3 + H2
Answer
2 Al + 6 HCl gives 2 AlCl3 + 3 H2
Steps
- 1Metal first: aluminium appears once on each side, so leave it for a moment.
- 2Chlorine: AlCl3 needs 3 Cl, so try 3 HCl, which gives 3 H on the left.
- 3Three hydrogens cannot make whole H2 molecules, so double everything.
- 4That gives 2 Al and 6 HCl on the left, 2 AlCl3 on the right, and 6 H, which is 3 H2.
- 5Final check: Al 2 and 2, Cl 6 and 6, H 6 and 6.
An odd hydrogen count with H2 as a product is the standard signal that the whole equation needs doubling.
Example 3
Question
Balance the combustion of propane: C3H8 + O2 gives CO2 + H2O
Answer
C3H8 + 5 O2 gives 3 CO2 + 4 H2O
Steps
- 1Carbon: 3 on the left, so 3 CO2 on the right.
- 2Hydrogen: 8 on the left, so 4 H2O on the right.
- 3Oxygen on the right: 6 from the CO2 plus 4 from the water, giving 10.
- 4Oxygen on the left: 10 atoms means 5 O2.
- 5Final check: C 3 and 3, H 8 and 8, O 10 and 10.
Common mistakes
Changing a subscript to balance, such as writing O3 instead of O2.
Only coefficients change. Subscripts define the substance and are fixed.
Balancing oxygen early in a combustion equation.
Oxygen appears in the most compounds, so it is easiest last, once everything else is fixed.
Forgetting to multiply a subscript by the coefficient.
2 H2O is 4 hydrogens and 2 oxygens. Coefficient times subscript, for every element.
Doing stoichiometry on an unbalanced equation.
The mole ratio comes from the coefficients, so every calculation after an unbalanced equation is wrong.
Practice it
Work these out first, then open the answer. Stuck halfway? Select the problem and ask Solvecoon for the step you are missing.
1. Balance: N2 + H2 gives NH3
Show answer
N2 + 3 H2 gives 2 NH3.
2. Balance: H2 + O2 gives H2O
Show answer
2 H2 + O2 gives 2 H2O.
3. Balance: Na + Cl2 gives NaCl
Show answer
2 Na + Cl2 gives 2 NaCl.
Questions people ask
- Why can I not change the subscripts?
- Because they identify the compound. Changing them answers a different question about different substances.
- Is there an order that always works?
- Metals, then other non-metals, then hydrogen, then oxygen is reliable for the equations school courses use.
- What if I end up with a fraction?
- Multiply every coefficient through to clear it. Half coefficients are legitimate working, but most mark schemes want whole numbers.
- How do I check I am finished?
- Tally every element on both sides, including the ones you balanced first. They shift when later coefficients change.
Got one in front of you?
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