Balancing chemical equations

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课程笔记

Why balance equations?

Atoms are rearranged in a chemical reaction, not created or destroyed.
The number of atoms of each element must be the same on both sides of the equation.
An unbalanced equation does not obey the Law of Conservation of Mass.

Writing chemical equations

Use chemical symbols and formulae for reactants and products.
Diatomic elements (e.g., O₂, H₂, Cl₂) are written with a subscript 2.
Example: copper + oxygen → copper oxide is written as Cu + O₂ → CuO.

Identifying imbalance

Count atoms of each element on both sides of the arrow.
In Cu + O₂ → CuO: left has 1 Cu, 2 O; right has 1 Cu, 1 O → oxygen is unbalanced.
Only coefficients (numbers in front of formulae) can be changed to balance.

Balancing by adjusting coefficients

Place a coefficient in front of a compound to multiply all atoms in that compound.
Example: 2CuO means 2 Cu atoms and 2 O atoms.
Never change subscripts inside a formula (e.g., CuO to CuO₂) – that changes the substance.

Step-by-step balancing example

Start with the unbalanced equation: Cu + O₂ → CuO.
Balance oxygen: put 2 before CuO → Cu + O₂ → 2CuO.
Now copper is unbalanced (1 left, 2 right). Put 2 before Cu → 2Cu + O₂ → 2CuO.
Check: left 2 Cu, 2 O; right 2 Cu, 2 O → balanced.

Working with polyatomic ions

Treat polyatomic ions (e.g., OH⁻, CO₃²⁻) as a single unit when balancing.
Example: Ca(OH)₂ contains 1 Ca, 2 O, 2 H.
Coefficient multiplies the entire ion group: 2Ca(OH)₂ gives 2 Ca, 4 O, 4 H.

Practice tips

Balance metals first, then nonmetals, then hydrogen and oxygen last.
If an element appears in more than one compound on a side, leave it for later.
Check your work by recounting atoms after each coefficient change.

Bohr model of a copper atom (Cu). Used to understand the number of atoms in compounds like CuO.

Bohr model of an oxygen atom (O). Oxygen forms diatomic molecules (O₂) with two oxygen atoms.

幻灯片

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练习题

免费预览——61 题中的 8 题。注册即可查看全部。

1.How many hydrogen atoms are there in one molecule of H2O?
Easy
- A1
- B2
- C3
- D4
2.How many hydrogen atoms are there in 2H2O?
Easy
- A2
- B4
- C6
- D8
3.How many hydrogen atoms are there in 4C2H4?
Easy
- A4
- B8
- C12
- D16
4.How many hydrogen atoms are there in 2C7H16?
Easy
- A14
- B16
- C28
- D32
5.What is the chemical formula for oxygen?
Easy
- AO
- BO2
- CO3
- D2O
6.In the balanced equation 2Mg + O2 → 2MgO, how many magnesium atoms are on each side?
Easy
- A1
- B2
- C3
- D4
7.Balance the equation: CH4 + ? O2 → CO2 + 2H2O. What number should replace the question mark?
Medium
- A1
- B2
- C3
- D4
8.Balance the equation: C2H4 + 2O2 → ? CO + 2H2O. What number should replace the question mark?
Medium
- A1
- B2
- C3
- D4

更多题目

9.Balance the equation: ? Ag + O2 → 2Ag2O. What number should replace the question mark?
Medium
- A2
- B4
- C6
- D8
10.Balance the equation: 6CO2 + 6H2O → C6H12O6 + ? O2. What number should replace the question mark?
Medium
- A3
- B6
- C9
- D12
11.Balance the equation: ? Na + ? Cl2 → ? NaCl. Which set of numbers correctly balances it?
Medium
- A2, 1, 2
- B1, 2, 1
- C2, 2, 2
- D1, 1, 1
12.Which of the following is a correct word equation for the oxidation of zinc?
Easy
- Azinc + oxygen → zinc oxide
- Bzinc + oxygen → zinc dioxide
- Czinc oxide → zinc + oxygen
- Dzinc + zinc oxide → oxygen
13.In the reaction 2Cu + O2 → 2CuO, how many oxygen atoms are present in total on the right-hand side?
Easy
- A1
- B2
- C3
- D4
14.A student writes the equation: Al + O2 → Al2O3. To balance it, what coefficients should be placed in front of Al, O2, and Al2O3 respectively?
Medium
- A2, 3, 2
- B4, 3, 2
- C2, 1, 2
- D3, 2, 3
15.When balancing the equation N2 + H2 → NH3, the correct set of coefficients is:
Medium
- A1, 3, 2
- B2, 3, 2
- C1, 2, 2
- D2, 1, 2
16.A student balances Fe + O2 → Fe2O3 as 4Fe + 3O2 → 2Fe2O3. How many iron atoms are on the right-hand side?
Medium
- A2
- B4
- C6
- D8
17.In the balanced equation 2H2 + O2 → 2H2O, what is the total number of hydrogen atoms on the left-hand side?
Easy
- A2
- B4
- C1
- D3
18.The equation for the reaction of sodium with chlorine is: ? Na + ? Cl2 → ? NaCl. What set of coefficients balances it?
Medium
- A2, 1, 2
- B1, 1, 2
- C2, 2, 2
- D1, 2, 2
19.Balance the equation: ? C2H6 + ? O2 → ? CO2 + ? H2O. Which set of coefficients is correct?
Hard
- A2, 7, 4, 6
- B1, 3, 2, 3
- C2, 5, 4, 6
- D1, 4, 2, 3
20.In the balanced equation for photosynthesis: 6CO2 + 6H2O → C6H12O6 + ? O2, what number should replace the question mark?
Medium
- A6
- B3
- C12
- D1
21.A student writes the word equation for the oxidation of zinc. Which is correct?
Easy
- Azinc + oxygen → zinc oxide
- Bzinc + oxide → zinc oxide
- Czinc + oxygen → zinc oxygen
- Dzinc + oxide → zinc dioxide
22.Balance the equation: ? Fe + ? H2O → ? Fe3O4 + ? H2. The correct set of coefficients is:
Hard
- A3, 4, 1, 4
- B2, 3, 1, 3
- C3, 4, 1, 3
- D4, 3, 1, 4
23.How many oxygen atoms are in 3Ca(OH)2?
Easy
- A2
- B3
- C6
- D1
24.In the balanced equation 2Mg + O2 → 2MgO, how many oxygen atoms are on the left-hand side?
Easy
- A1
- B2
- C4
- D0
25.In the balanced equation for the oxidation of zinc: ? Zn + O2 → 2ZnO, what number should replace the question mark?
Medium
- A1
- B2
- C3
- D4
26.How many hydrogen atoms are there in one molecule of Ca(OH)2?
Easy
- A1
- B2
- C3
- D4
27.A student balances the equation for the oxidation of magnesium as: 2Mg + O2 → ? MgO. What number should replace the question mark?
Easy
- A1
- B2
- C3
- D4
28.Which of the following is the correct word equation for the oxidation of zinc?
Easy
- Azinc + oxygen → zinc oxide
- Bzinc + oxide → zinc oxygen
- Czinc oxide → zinc + oxygen
- Dzinc + oxygen → zinc dioxide
29.A student writes the following steps to balance the equation Al + O2 → Al2O3. Which step is correct?
Medium
- APlace a 2 in front of Al2O3 to balance oxygen
- BPlace a 3 in front of O2 and a 4 in front of Al
- CPlace a 2 in front of Al and a 3 in front of O2
- DPlace a 2 in front of Al2O3 and a 3 in front of O2
30.Which of the following is the correct word equation for the oxidation of copper?
Easy
- Acopper + oxygen → copper oxide
- Bcopper oxide → copper + oxygen
- Ccopper + copper oxide → oxygen
- Doxygen + copper oxide → copper
31.What is the chemical symbol for copper?
Easy
- ACu
- BCo
- CC
- DCp
32.Oxygen exists as a diatomic molecule. Which of the following represents its formula?
Easy
- AO2
- BO
- CO3
- DO4
33.In the balanced equation 2Cu + O2 → 2CuO, how many copper atoms are on each side?
Easy
- A2 on each side
- B1 on left, 2 on right
- C2 on left, 1 on right
- D0 on left, 2 on right
34.To balance the equation ? Zn + O2 → 2ZnO, what number should replace the question mark?
Easy
- A2
- B1
- C3
- D4
35.What is the total number of hydrogen atoms in 10H2O?
Easy
- A20
- B10
- C2
- D12
36.Which of the following is the correct set of coefficients to balance the equation ? Na + ? Cl2 → ? NaCl?
Medium
- A2, 1, 2
- B1, 1, 1
- C2, 2, 2
- D1, 2, 2
37.In the balanced equation CH4 + ? O2 → CO2 + 2H2O, what number should replace the question mark?
Medium
- A2
- B1
- C3
- D4
38.When balancing chemical equations, which of the following is allowed?
Easy
- AChanging the coefficient in front of a compound
- BChanging the subscript in a formula
- CAdding extra atoms to a compound
- DRemoving atoms from a compound
39.A student writes the word equation for the oxidation of zinc. Which of the following is correct?
Easy
- Azinc + oxygen → zinc oxide
- Bzinc + oxygen → zinc dioxide
- Czinc + oxide → zinc oxide
- Dzinc + oxygen → zinc oxygen
40.Balance the equation: ? Fe + ? H2O → ? Fe3O4 + ? H2. Which set of coefficients is correct?
Hard
- A3, 4, 1, 4
- B3, 4, 1, 2
- C2, 3, 1, 3
- D1, 4, 1, 4
41.A chemical equation is balanced when the number of atoms of each element is the same on both sides.
Easy
True or false?
42.What is the chemical symbol for zinc?
Easy
- AZn
- BZi
- CZc
- DZr
43.When balancing the equation 2Cu + O2 → 2CuO, you can change the formula of copper oxide to CuO2 to balance the oxygen atoms.
Medium
True or false?
44.How many oxygen atoms are in one molecule of carbon dioxide (CO2)?
Easy
- A2
- B1
- C3
- D4
45.Which of the following are allowed when balancing a chemical equation? (Select all that apply)
Medium
- AChanging coefficients
- BChanging subscripts
- CAdding reactants or products
- DRemoving reactants or products
- EMultiplying all coefficients by a common factor
46.Arrange the following steps in the correct order to balance the equation Cu + O2 → CuO.
Medium
- Write the unbalanced equation with symbols.
- Count atoms on each side.
- Add coefficients to balance oxygen.
- Add coefficients to balance copper.
- Check that all atoms are balanced.
47.Match each chemical formula to the number of oxygen atoms in one molecule.
Easy
- H2O
- CO2
- MgO
- 1
- 2
- 1
48.In a balanced chemical equation, the total number of atoms of each element is the same on both sides.
Easy
True or false?
49.A student writes the word equation for the oxidation of zinc as: zinc + oxygen → zinc oxide. Which of the following is the correct balanced chemical equation for this reaction?
Medium
- AZn + O → ZnO
- BZn + O2 → ZnO2
- C2Zn + O2 → 2ZnO
- DZn2 + O2 → 2ZnO
50.Which of the following are allowed steps when balancing a chemical equation? (Select all that apply)
Medium
- AChanging the subscript in a chemical formula
- BAdding a coefficient in front of a compound
- CChanging the chemical symbol of an element
- DMultiplying all coefficients by a common factor
51.Fill in the blank: In a chemical equation, the number in front of a compound is called a _______.
Easy
In a chemical equation, the number in front of a compound is called a ____.
52.Complete the word equation for the oxidation of zinc: zinc + oxygen → ________
Easy
zinc + oxygen → ____
53.How many hydrogen atoms are in one molecule of methane (CH4)?
Easy
54.What is the name of the compound with the formula CuO?
Easy
55.Balance the equation: ? Na + ? Cl2 → ? NaCl. Write the coefficients in order separated by commas.
Medium
____ Na + ____ Cl2 → ____ NaCl
56.How many oxygen atoms are there in total in 3Ca(OH)2?
Medium
57.In the balanced equation 2Cu + O2 → 2CuO, how many copper atoms are present on the right-hand side?
Easy
58.Complete the word equation for the oxidation of zinc:
Easy
zinc + oxygen → ____
59.How many hydrogen atoms are in one molecule of methane (CH4)?
Easy
- A5
- B3
- C6
- D4
60.How many oxygen atoms are there in total in 3Ca(OH)2?
Medium
- A7
- B5
- C6
- D8
61.In the balanced equation 2Cu + O2 → 2CuO, how many copper atoms are present on the right-hand side?
Easy
- A3
- B4
- C2
- D1

复习笔记

## Balancing Chemical Equations - **Law of Conservation of Mass**: Atoms are not created or destroyed in a chemical reaction. The number of each type of atom must be the same on both sides of the equation. - **Word equations** describe reactions using words, e.g., copper + oxygen → copper oxide. - **Symbol equations** use chemical symbols and formulae. Example: Cu + O₂ → CuO (unbalanced). - **Diatomic molecules**: Oxygen exists as O₂, not O. - **Balancing steps**: 1. Count atoms of each element on both sides. 2. Change coefficients (numbers in front of compounds) to balance atoms. Never change subscripts (e.g., CuO to CuO₂) as that changes the substance. 3. Multiply all atoms in a compound by the coefficient. Example: 2H₂O means 4 H atoms and 2 O atoms. - **Balanced example**: 2Cu + O₂ → 2CuO (2 Cu atoms, 2 O atoms each side). ### Practice Balancing: - Oxidation of zinc: 2Zn + O₂ → 2ZnO - Oxidation of magnesium: 2Mg + O₂ → 2MgO - Complete combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O - Incomplete combustion of ethene: C₂H₄ + 2O₂ → 2CO + 2H₂O - Oxidation of silver: 4Ag + O₂ → 2Ag₂O - Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ - Reaction of sodium and chlorine: 2Na + Cl₂ → 2NaCl

Chemical equation: A representation of a chemical reaction using symbols and formulae.
Balancing: Adjusting coefficients to ensure the same number of each atom on both sides of the equation.
Coefficient: The number placed in front of a chemical formula to indicate how many molecules or atoms are involved.
Diatomic molecule: A molecule composed of two atoms, e.g., O₂.
Subscript: A number written below and to the right of an element symbol indicating the number of atoms in a molecule.

闪卡

What is a diatomic molecule?

A molecule composed of two atoms of the same element, e.g., O2.

Why must chemical equations be balanced?

Because atoms are neither created nor destroyed in a chemical reaction; the number of each type of atom must be the same on both sides.

What is the only way to balance a chemical equation?

By changing the number in front of a compound (coefficient), not by changing subscripts within a formula.

In the equation Cu + O2 → CuO, how many oxygen atoms are on each side?

Left: 2 (from O2), Right: 1 (from CuO).

How do you balance Cu + O2 → CuO?

Place a 2 in front of Cu and a 2 in front of CuO: 2Cu + O2 → 2CuO.

How many hydrogen atoms are in H2O?

How many hydrogen atoms are in 2H2O?

4 (2 × 2).

How many hydrogen atoms are in 10H2O?

20 (10 × 2).

How many hydrogen atoms are in CH4?

How many hydrogen atoms are in C2H4?

How many hydrogen atoms are in 4C2H4?

16 (4 × 4).

How many hydrogen atoms are in 2C7H16?

32 (2 × 16).

How many hydrogen atoms are in Ca(OH)2?

Write a word equation for the oxidation of zinc.

zinc + oxygen → zinc oxide

Balance: ___ Zn + O2 → 2ZnO

2 Zn + O2 → 2ZnO

Balance: 2Mg + O2 → ___ MgO

2Mg + O2 → 2MgO

Balance: CH4 + ___ O2 → CO2 + 2H2O

CH4 + 2 O2 → CO2 + 2H2O

Balance: C2H4 + 2O2 → ___ CO + 2H2O

C2H4 + 2O2 → 2CO + 2H2O

Balance: ___ Ag + O2 → 2Ag2O

4 Ag + O2 → 2Ag2O

Balance: 6CO2 + 6H2O → C6H12O6 + ___ O2

6CO2 + 6H2O → C6H12O6 + 6 O2

Balance: ___ Na + ___ Cl2 → ___ NaCl

2 Na + 1 Cl2 → 2 NaCl

What does the coefficient in front of a compound represent?

The number of molecules or formula units of that compound.

If you have 2H2O, how many oxygen atoms do you have?

2 (2 × 1).

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