Zn + 2HCl → ZnCl2 + H2
Zinc and hydrochloric acid — balanced chemical equation, step by step.
This is the classic laboratory method for generating hydrogen gas.
How to balance Zn + HCl = ZnCl2 + H2
In a single-replacement reaction, one element displaces another from a compound. Balancing means choosing coefficients so that every element has the same number of atoms on both sides of the arrow — the Law of Conservation of Mass. Here is how it's done, step by step.
Step 1 — Write the unbalanced equation
Start with the correct formulas for every reactant and product:
Zn + HCl = ZnCl2 + H2
Step 2 — Count the atoms of each element
With no coefficients yet (everything counted once), the atoms do not match. The ✗ marks show which elements are unbalanced:
| Element | Reactants | Products | Equal? |
|---|---|---|---|
| Zn | 1 | 1 | ✓ |
| H | 1 | 2 | ✗ |
| Cl | 1 | 2 | ✗ |
Step 3 — Add the smallest whole-number coefficients
Adjust the coefficients in front of each formula until every element balances. The smallest whole-number coefficients are 1 Zn, 2 HCl, 1 ZnCl2, 1 H2, giving:
Zn + 2HCl = ZnCl2 + H2
For a single-replacement reaction, balance the element being displaced first, then balance the spectator ions, and finish with any free element or hydrogen gas.
Step 4 — Verify the balance
Recount every element. Each one now matches on both sides:
| Element | Reactants | Products | Balanced |
|---|---|---|---|
| Zn | 1 | 1 | ✓ |
| H | 2 | 2 | ✓ |
| Cl | 2 | 2 | ✓ |
All elements are balanced and the coefficients are the smallest whole numbers, so Zn + 2HCl = ZnCl2 + H2 is the correct balanced equation.
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