Balancing Equations by Inspection
A balanced chemical equation uses coefficients so that each element has the same number of atoms on both sides. It is a numerical record of atom conservation.
1. What may and may not change
Change coefficients
A coefficient before a formula multiplies every atom in that formula. It changes the number of formula entities and the reaction ratio. A coefficient of 1 is normally omitted.
Never change subscripts
Subscripts define each substance. Changing H2O to H2O2 replaces water with a different compound and does not balance the original reaction.
2. A systematic six-step method
- Write the skeleton equation. Confirm correct formulas for every reactant and product.
- Build an atom inventory. Count each element on the left and right with all current coefficients treated as 1.
- Choose a starting element. Begin with an element that occurs in one formula on each side; complex substances are often useful starting points.
- Adjust one coefficient. Use a common multiple to make that element count equal.
- Recount everything. A changed coefficient affects every element in its formula, so continue element by element.
- Reduce and verify. Use the smallest whole-number coefficients and confirm every element count.
3. Core worked pattern
Start with (H_2+O_2\rightarrow H_2O). Oxygen is 2 on the left and 1 on the right, so put 2 before water: (H_2+O_2\rightarrow2H_2O). This creates 4 H on the right, so put 2 before H2:
\(2H_2+O_2\rightarrow2H_2O\)
| Element | Left | Right |
|---|---|---|
| H | (2\cdot2=4) | (2\cdot2=4) |
| O | 2 | (2\cdot1=2) |