Chemical equations are representations of chemical reactions that show reactants on the left and products on the right, separated by an arrow.
A balanced chemical equation follows the law of conservation of mass – This law says that the number of atoms of each element must be the same on both sides of the equation.
In many exam questions, the right side of the equation may already be balanced and your task is to determine the correct coefficients for the left side only
Method for Balancing Equations: #
- Count atoms of each element on both sides
- Add coefficients (whole numbers) in front of the formulas to make the number of atoms equal on both sides
- Start with the most complex compound or the element that appears in the fewest compounds
- Balance each element one at a time
- Double-check that all elements are balanced
Simple Example: Hydrogen + Oxygen → Water #
Step 1: Write the correct formulas
H2 + O2 → H2O
Step 2: Count atoms on each side
- Left side: 2 H atoms and 2 O atoms
- Right side: 2 H atoms and 1 O atom
- Oxygen is not balanced!
Step 3: Add coefficients to balance
Add coefficient 2 in front of H2O to balance oxygen:
H2 + O2 → 2H2O
Step 4: Recount atoms
- Left side: 2 H atoms and 2 O atoms
- Right side: 4 H atoms and 2 O atoms
- Hydrogen is now unbalanced!
Step 5: Continue balancing
Add coefficient 2 in front of H2 to balance hydrogen:
2H2 + O2 → 2H2O
Step 6: Final check
- Left side: 4 H atoms and 2 O atoms
- Right side: 4 H atoms and 2 O atoms
- The equation is now balanced!Metal Oxide Reactions
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Metal Oxide Reactions #
1. Formation of Metal Oxides: #
Metal + Oxygen → Metal Oxide
Example: 4Fe + 3O2 → 2Fe2O3
2. Conversion Between Metal Oxides: #
Metal Oxide + Oxygen → Higher Metal Oxide
Example: 4FeO + O2 → 2Fe2O3
3. Reduction of Metal Oxides: #
Metal Oxide + Reducing Agent → Metal + Other Products
Example: Fe2O3 + 3CO → 2Fe + 3CO2
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Common Metal Oxide Reactions #
Examples of Metal Oxide Reactions: #
6CoO + O2 → 2Co3O4 #
4FeO + O2 → 2Fe2O3
2Cu + O2 → 2CuO #
2Cu2O + O2 → 4CuO #
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What is Combustion? #
Combustion is a type of chemical reaction where a substance reacts very quickly with oxygen, usually producing heat, light, and new compounds.
General Combustion Equation: #
Petrol + Oxygen → Carbon Dioxide + Water + Energy
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Examples of Combustion: #
Methane (CH4) – Burning in Oxygen
CH4 + 2O2 → CO2 + 2H2O
Propane (C3H8) – Burning in Oxygen (Propage is the gas used in homes)
C3H8 + 5O2 → 3CO2 + 4H2O
Butane (C4H10) – Burning in Oxygen (Butane is a gas used in camping stoves)
2C4H10 + 13O2 → 8CO2 + 10H2O
Combustion of Other Compounds #
Hydrogen Sulfide – Burning in Oxygen #
2H2S + 3O2 → 2SO2 + 2H2O
Carbon Disulfide – Burning in Oxygen
CS2 + 3O2 → CO2 + 2SO2
Ammonia Combustion – Burning in Oxygen #
4NH3 + 5O2 → 4NO + 6H2O
Important Things to Remember: #
- Law of Conservation of Mass: Atoms are neither created nor destroyed in chemical reactions
- Balancing Steps: Write formulas → Count atoms → Add coefficients → Check
- Combustion: Usually results in these 2 products CO2 and H2O (carbon dioxide and water)
- Coefficients vs. Subscripts:
- Coefficients (the large numbers in front of formula): Change the number of molecules
- Subscripts (the small numbers in a formula): are the number of atoms in a molecule
- Never change subscripts when balancing equations
Test Questions #
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When cobalt(II) oxide, CoO, is heated in air an oxide with the formula Co3O4 is formed.
Balance the equation for this reaction by filling in the blanks on the left side only.
__CoO + __O2 → 2Co3O4
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Hydrogen sulfide burns in air to produce sulfur dioxide and water.
Complete the chemical equation for this reaction by filling in the blanks on the left side only.
__H2S + __O2 → 2H2O + 2SO2
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Phosphorus reacts with chlorine to form phosphorus(V) chloride.
Balance the equation for this reaction by filling in the blanks on the left side only.
__P + __Cl2 → 2PCl5
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Zinc reacts with phosphorus to form zinc phosphide.
Balance the equation for this reaction by filling in the blanks on the left side only.
__Zn + __P → Zn3P2
Balance these equations
Balancing Chemical Equations – Answers #
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Question 1: __CoO + __O2 → 2Co3O4
Answer: 6CoO + O2 → 2Co3O4
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Question 2: __H2S + __O2 → 2H2O + 2SO2
Answer: 2H2S + 3O2 → 2H2O + 2SO2
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Question 3: __P + __Cl2 → 2PCl5
Answer: 2P + 5Cl2 → 2PCl5
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Question 4: __Zn + __P → Zn3P2
Answer: 3Zn + 2P → Zn3P2
