Balancing Chemical Equations Worksheet for 8th-grade science, aligned with Tennessee state standards.

Let's go through the Balancing Chemical Equations Worksheet step by step and solve each part with clear explanations.

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Part 1: Identify Subscripts and Coefficients

Instructions:
1. Draw a circle around each subscript.
2. Draw a square around each coefficient.

We are given:

- H₂O
- 5Cl₂
- 2Mg
- 3H₂O₂

#### Step-by-step breakdown:

| Expression | Subscripts (circle) | Coefficients (square) |
|-----------|----------------------|------------------------|
| H₂O | Circle around 2 (in H₂), and 1 (implied in O, but usually not written) → only 2 is circled | No coefficient → none squared |
| 5Cl₂ | Circle around 2 (in Cl₂) | Square around 5 |
| 2Mg | No subscript (Mg is atomic) → no circle | Square around 2 |
| 3H₂O₂ | Circle around 2 (in H₂), 2 (in O₂) | Square around 3 |

> ✔ So:
>
> - H₂O: Circle the "2" in H₂
> - 5Cl₂: Square the "5", circle the "2" in Cl₂
> - 2Mg: Square the "2"
> - 3H₂O₂: Square the "3", circle both "2"s (in H₂ and O₂)

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Part 2: List Elements and Count Atoms

Instructions:
1. List chemical symbols of each element.
2. Give number of atoms of each element.

#### 1. HCl
- Elements: H, Cl
- Atoms:
- H: 1
- Cl: 1

#### 2. CO₂
- Elements: C, O
- Atoms:
- C: 1
- O: 2 (because of subscript 2)

#### 3. Na₂SO₄
- Elements: Na, S, O
- Atoms:
- Na: 2 (subscript 2)
- S: 1
- O: 4 (subscript 4)

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Part 3: Balance the Chemical Equations

We must balance each equation so that the number of atoms of each element is equal on both sides (following the Law of Conservation of Mass).

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#### 1. ___ Cu₂O + ___ C → ___ Cu + ___ CO₂

Step 1: Count atoms on each side

Unbalanced:
- Left: Cu₂O → 2 Cu, 1 O; C → 1 C
- Right: Cu → 1 Cu; CO₂ → 1 C, 2 O

So we have:
- Left: Cu = 2, O = 1, C = 1
- Right: Cu = 1, C = 1, O = 2 → Not balanced

Step 2: Balance oxygen first (appears in two compounds)

- On left: 1 O from Cu₂O
- On right: 2 O from CO₂

To get 2 O on left, need 2 Cu₂O → gives 4 Cu and 2 O
→ Now left: 4 Cu, 2 O, 1 C

Right: 1 Cu, 1 C, 2 O → still unbalanced

Now add coefficients:

Try:
- 2 Cu₂O → gives 4 Cu, 2 O
- Need 1 CO₂ to match 2 O → gives 1 C, 2 O
- But now we have 4 Cu on left, only 1 Cu on right → need 4 Cu on right
- So put 4 Cu on right
- C: 1 on left, 1 on right → OK

So far:
- 2 Cu₂O + 1 C → 4 Cu + 1 CO₂

Check atoms:
- Left: Cu = 4, O = 2, C = 1
- Right: Cu = 4, C = 1, O = 2 → Balanced!

✔ Final answer:
> 2 Cu₂O + 1 C → 4 Cu + 1 CO₂

But we can write without "1":
> 2 Cu₂O + C → 4 Cu + CO₂

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#### 2. ___ H₂O₂ → ___ H₂O + ___ O₂

This is the decomposition of hydrogen peroxide.

Unbalanced:
- Left: H₂O₂ → 2 H, 2 O
- Right: H₂O → 2 H, 1 O; O₂ → 2 O

Total right: H = 2, O = 1 + 2 = 3 → not balanced

We need even number of O atoms.

Try balancing O:

- Each H₂O₂ has 2 O → total O = 2 per molecule
- O₂ has 2 O → good for pairing
- But H₂O has only 1 O → so we need even number of H₂O molecules?

Let’s try:

Suppose we use 2 H₂O₂:
- Left: H = 4, O = 4

Now right:
- Let’s make 2 H₂O → gives 4 H, 2 O
- Then O₂ → needs 2 more O → so 1 O₂

So:
- 2 H₂O₂ → 2 H₂O + 1 O₂

Check:
- Left: H = 4, O = 4
- Right: H = 4 (from 2 H₂O), O = 2 (from H₂O) + 2 (from O₂) = 4 → Balanced!

✔ Final answer:
> 2 H₂O₂ → 2 H₂O + O₂

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#### 3. ___ Al + ___ Fe₃N₂ → ___ AlN + ___ Fe

Unbalanced:
- Left: Al, Fe₃N₂ → 3 Fe, 2 N
- Right: AlN → 1 Al, 1 N; Fe → 1 Fe

We need to balance:
- N: 2 on left → need 2 AlN on right → gives 2 N and 2 Al
- Fe: 3 on left → need 3 Fe on right
- Al: 2 on right → need 2 Al on left

So:
- 2 Al + 1 Fe₃N₂ → 2 AlN + 3 Fe

Check:
- Left: Al = 2, Fe = 3, N = 2
- Right: Al = 2 (from 2 AlN), N = 2 (from 2 AlN), Fe = 3 → Balanced!

✔ Final answer:
> 2 Al + Fe₃N₂ → 2 AlN + 3 Fe

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✔ Final Answers Summary

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Part 1: Subscripts & Coefficients

- H₂O → Circle: 2 (in H₂)
- 5Cl₂ → Square: 5, Circle: 2 (in Cl₂)
- 2Mg → Square: 2
- 3H₂O₂ → Square: 3, Circles: 2 (in H₂), 2 (in O₂)

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Part 2: Element Symbols and Atom Counts

| Compound | Elements | Number of Atoms |
|---------|----------|------------------|
| HCl | H, Cl | H: 1, Cl: 1 |
| CO₂ | C, O | C: 1, O: 2 |
| Na₂SO₄ | Na, S, O | Na: 2, S: 1, O: 4 |

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Part 3: Balanced Equations

1. 2 Cu₂O + C → 4 Cu + CO₂
2. 2 H₂O₂ → 2 H₂O + O₂
3. 2 Al + Fe₃N₂ → 2 AlN + 3 Fe

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🧠 Key Concepts Recap

- Subscript: small number after element symbol (e.g., H₂ → "2" is subscript). It tells how many atoms of that element are in one molecule.
- Coefficient: number in front of a compound (e.g., 3H₂O → "3" is coefficient). It multiplies the entire formula.
- Balancing equations: Adjust coefficients so that atoms of each element are equal on both sides. Never change subscripts!
- Law of Conservation of Mass: Matter cannot be created or destroyed → mass of reactants = mass of products.

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