Chemistry Practice Test: Balancing Chemical Equations
- Balancing Equations
- 10 questions
- 14 minutes
This test covers balancing equations with the smallest whole-number coefficients and identifying reaction types. Tip: change only the coefficients in front of formulas, never the subscripts inside them.
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Question 1
Balance this equation with the smallest whole numbers: ___Al + ___O2 → ___Al2O3. What is the coefficient of Al?
Correct answer: C
The balanced equation is 4Al + 3O2 → 2Al2O3: 4 Al and 6 O on each side. Writing 2 for Al only works if O2 gets the fraction 3/2.
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Question 2
Balance this combustion: ___C2H6 + ___O2 → ___CO2 + ___H2O. With the smallest whole numbers, what is the coefficient of O2?
Correct answer: A
The balanced equation is 2C2H6 + 7O2 → 4CO2 + 6H2O. The right side has 8 + 6 = 14 oxygen atoms, which is 7 O2 molecules.
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Question 3
Balance the equation ___Fe + ___Cl2 → ___FeCl3. What is the sum of all the coefficients?
Correct answer: D
The balanced equation is 2Fe + 3Cl2 → 2FeCl3, so the sum is 2 + 3 + 2 = 7. The sum 5 comes from writing Fe + 3Cl2 → FeCl3, which has 6 Cl atoms on the left and only 3 on the right.
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Question 4
Why must a chemical equation be balanced?
Correct answer: B
The law of conservation of mass means every atom on the left must appear on the right. The number of molecules can change, as in 2H2 + O2 → 2H2O, where 3 molecules become 2.
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Question 5
Which equation is correctly balanced?
Correct answer: B
In 2H2O2 → 2H2O + O2 there are 4 H and 4 O on each side. The KClO3 equation has 6 O on the left but only 4 on the right; it needs 3O2.
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Question 6
What type of reaction is 2KClO3 → 2KCl + 3O2?
Correct answer: C
One compound breaks apart into two or more simpler substances, which is a decomposition reaction. Synthesis is the reverse, where simpler substances combine into one product.
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Question 7
What type of reaction is Zn + CuSO4 → ZnSO4 + Cu?
Correct answer: A
An element (zinc) takes the place of another element (copper) in a compound. In a double replacement reaction, two compounds swap partners instead.
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Question 8
Balance the equation ___Pb(NO3)2 + ___KI → ___PbI2 + ___KNO3. What is the coefficient of KI?
Correct answer: D
The balanced equation is Pb(NO3)2 + 2KI → PbI2 + 2KNO3. PbI2 needs two iodide ions, and the two nitrate groups end up in two KNO3.
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Question 9
Balance the equation ___NH3 + ___O2 → ___NO + ___H2O. With the smallest whole numbers, what is the coefficient of O2?
Correct answer: C
The balanced equation is 4NH3 + 5O2 → 4NO + 6H2O. The right side has 4 + 6 = 10 oxygen atoms, which is 5 O2 molecules.
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Question 10
To balance H2 + O2 → H2O, a student writes H2 + O2 → H2O2. What is wrong with this?
Correct answer: A
H2O2 is hydrogen peroxide, not water, so the equation now describes a different reaction. The correct way is to change coefficients: 2H2 + O2 → 2H2O.
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