Balancing Chemical Equations — Chemistry 练习题
A chemical equation is balanced when each element has the same number of atoms on both sides of the arrow. This reflects the law of conservation of mass: atoms are never created or destroyed in a chemical reaction, only rearranged.
To balance an equation you may only change coefficients, the whole numbers placed in front of each formula. You may never change subscripts, because that changes the substance itself — and are completely different chemicals.
A reliable strategy is to balance elements one at a time, saving elements that appear as pure substances (like ) for last. When a polyatomic ion such as or appears unchanged on both sides, treat it as a single intact unit to simplify the atom counting.
Finish by recounting every atom on both sides and reducing the coefficients to the lowest whole-number ratio. Fractions can help mid-problem (for example ), but multiply through so the final answer uses whole numbers.
练习的技能
- Counting atoms of each element on both sides of an equation
- Balancing equations by adjusting coefficients only
- Distinguishing coefficients from subscripts
- Keeping polyatomic ions intact while balancing
- Connecting balanced equations to conservation of mass
练习题：Balancing Chemical Equations
说明： 仔细解答每一道题。清晰地展示所有解题过程。在指定位置或按指示在另一张纸上写出最终答案。
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1.Balance the following equation: _____ Fe + _____ O₂ → _____ Fe₂O₃. Write the coefficients in the blanks above.
Balancing Chemical Equations — 练习题（续）
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2.Balance the following equation: _____ C₃H₈ + _____ O₂ → _____ CO₂ + _____ H₂O. Write the coefficients in the blanks above.
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3.Balance the following equation: _____ Al + _____ HCl → _____ AlCl₃ + _____ H₂. Write the coefficients in the blanks above.
Balancing Chemical Equations — 练习题（续）
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4.Balance the following equation: _____ N₂ + _____ H₂ → _____ NH₃. Write the coefficients in the blanks above.
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5.Balance the following equation: _____ KClO₃ → _____ KCl + _____ O₂. Write the coefficients in the blanks above.
Balancing Chemical Equations — 练习题（续）
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6.Balance the following equation: _____ C₂H₆ + _____ O₂ → _____ CO₂ + _____ H₂O. Write the coefficients in the blanks above.
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7.Balance the following equation: _____ Na + _____ H₂O → _____ NaOH + _____ H₂. Write the coefficients in the blanks above.
Balancing Chemical Equations — 练习题（续）
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8.Which of the following is the correctly balanced equation for the reaction of methane with oxygen? Circle the letter of the correct answer.
- A. CH₄ + O₂ → CO₂ + H₂O
- B. CH₄ + 2O₂ → CO₂ + 2H₂O
- C. 2CH₄ + O₂ → 2CO₂ + 2H₂O
- D. CH₄ + O₂ → CO₂ + 2H₂O
Balancing Chemical Equations — 练习题（续）
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9.Balance the following equation: _____ Zn + _____ AgNO₃ → _____ Zn(NO₃)₂ + _____ Ag. Write the coefficients in the blanks above.
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10.Balance the following equation: _____ Ca(OH)₂ + _____ H₃PO₄ → _____ Ca₃(PO₄)₂ + _____ H₂O. Write the coefficients in the blanks above.
参考答案
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1.
4Fe + 3O₂ → 2Fe₂O₃最终答案： 4, 3, 2
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2.
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O最终答案： 1, 5, 3, 4
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3.
2Al + 6HCl → 2AlCl₃ + 3H₂最终答案： 2, 6, 2, 3
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4.
N₂ + 3H₂ → 2NH₃最终答案： 1, 3, 2
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5.
2KClO₃ → 2KCl + 3O₂最终答案： 2, 2, 3
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6.
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O最终答案： 2, 7, 4, 6
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7.
2Na + 2H₂O → 2NaOH + H₂最终答案： 2, 2, 2, 1
参考答案（续）
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8.
CH₄ + 2O₂ → CO₂ + 2H₂O. Check: C: 1=1, H: 4=4, O: 4=4最终答案： B
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9.
Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag最终答案： 1, 2, 1, 2
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10.
3Ca(OH)₂ + 2H₃PO₄ → Ca₃(PO₄)₂ + 6H₂O. Check: Ca: 3=3, O: 6+8=14, 8+6=14, H: 6+6=12, 12=12, P: 2=2最终答案： 3, 2, 1, 6
需要避免的常见错误
- Changing subscripts instead of adding coefficients
- Forgetting a coefficient multiplies every atom in the formula, including inside parentheses
- Splitting apart a polyatomic ion that stays together on both sides
- Leaving coefficients in a ratio that is not reduced to lowest whole numbers
- Not recounting all atoms after the final coefficient change
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最后更新：2026