SME
Grade 10 Science · Problem type
Balancing a single replacement: zinc in acid
Use this for “Balance Zn + HCl → ZnCl₂ + H₂.”
Diagram
Zn + 2 HCl → ZnCl₂ + H₂
Hydrogen: 2 × 1 = 2 before, 1 × 2 = 2 after
Zinc: 1 × 1 = 1 before, 1 × 1 = 1 after
Chlorine: 2 × 1 = 2 before, 1 × 2 = 2 after
Balanced: the same atoms on both sides, only rearranged.
This is an example. Type your own values to start a new problem.
a
Zn
+
b
HCl
→
c
ZnCl₂
+
d
H₂
A coefficient of 1 is not written: 1 O₂ is written O₂.
Formulas
c
=
a
1 = 1
b
=
2
×
c
2 = 2 × 1
d
=
b
2
1
=
2
2
Assumptions
•
Zinc takes the place of hydrogen: the hydrogen leaves as a gas.
•
Each ZnCl₂ needs 2 chlorine atoms, so 2 HCl for each zinc atom.
•
The balanced equation uses the smallest whole numbers, 1, 2, 1 and 1; larger inputs are the same reaction run more times.
•
Zinc chloride is ionic: one Zn²⁺ and two Cl⁻, drawn as ions with no shared bonds.
Step-by-step
WE KNOW
a = 1
FIND
zinc chloride formula units (c), hydrogen chloride molecules (b), hydrogen molecules (d)
Step 1 · Find zinc chloride formula units (c)
Use c = a
Zinc: one ZnCl₂ for each zinc atom.
c
=
1
Step 2 · Find hydrogen chloride molecules (b)
Use b = 2 × c
Chlorine: each ZnCl₂ holds 2, and each HCl brings 1.
b
=
2
×
1
b
=
2
Step 3 · Find hydrogen molecules (d)
Use d = b/2
Hydrogen: the HCl’s hydrogen atoms pair up as H₂.
d
=
2
2
d
=
1
Check
Put the numbers back in. Both sides should match.
1 = 1 ✓
2 = 2 × 1 ✓
1
=
2
2
✓
Topic check
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