Curriculum

Course: Science — Second Preparatory

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Curriculum

Science — Second Preparatory

Unit 1: Matter and Energy

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Unit 2: Matter and Chemical Reactions

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Unit 3: Energy Flow in Photosynthesis and Respiration

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Unit 4: Geological Processes

0/4

Text lesson

The Chemical Equation

Science — Second Preparatory

The Chemical Equation

Worked examples: balancing and mass calculations

Worked examples: balancing and mass calculations

Comparison aspect	Main idea	Keywords
Example 1	Balance CH₄ + O₂ → CO₂ + H₂O	Balance
Solution	there is 1 carbon on each side. There are 4 hydrogen atoms on the left so we place 2 in front of H₂O to get 2H₂O and now there are 4 hydrogen atoms on each side. There are 4 oxygen atoms on the right (2 + 2) so we place 2 in front of O₂. The balanced equation is CH₄ + 2O₂ → CO₂ + 2H₂O	carbonsidehydrogenatoms
Check the masses	16 + 2 × 32 = 80 and 44 + 2 × 18 = 80. The two masses are equal	equal
Example 2	Balance C₃H₈ + O₂ → CO₂ + H₂O	BalanceC₃H₈
Solution	there are 3 carbon atoms so we place 3 in front of CO₂. There are 8 hydrogen atoms so we place 4 in front of H₂O. There are 3 × 2 + 4 = 10 oxygen atoms on the right so we place 5 in front of O₂. The balanced equation is C₃H₈ + 5O₂ → 3CO₂ + 4H₂O	carbonatomsplacefront

Comparison aspect	Main idea	Keywords
Example 3: mass calculation.	If 12 g of magnesium burn completely according to 2Mg + O₂ → 2MgO what mass of magnesium oxide is formed? (Relative atomic masses: Mg = 24 O = 16)	magnesiumburncompletelyaccording
Solution	the mass of 2Mg is 48 g and the mass of 2MgO is 2 × 40 = 80 g. So 48 g of magnesium gives 80 g of magnesium oxide	mass2MgOmagnesiumgives
•	Mass of MgO = 12 × 80 ÷ 48 = 20 g	Mass
•	The oxygen that reacted is 8 g and this agrees with the law of conservation of mass: 12 + 8 = 20	oxygenreactedagreesconservation
Example 4: balancing an iron reaction.	Balance Fe + O₂ → Fe₂O₃	BalanceFe₂O₃
Solution	place 2 in front of Fe₂O₃ so that there are 2 iron atoms on the right then place 4 in front of Fe. There are 3 × 2 = 6 oxygen atoms on the right so we place 3 in front of O₂. The equation is 4Fe + 3O₂ → 2Fe₂O₃. Check: 4 × 56 + 3 × 32 = 320 and 2 × 160 = 320	placefrontFe₂O₃iron

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