Magnesium reacts with hydrochloric acid according to the equation: Mg + 2HCl → MgCl₂ + H₂. Explain why the mass of the products always equals the mass of the reactants in this reaction, and state what happens to the total number of atoms during the reaction.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Magnesium ribbon is added to excess hydrochloric acid in a sealed flask. The flask is weighed before and after the reaction. The total mass does not change.
Model answer (5 marks)
The law of conservation of mass states that no atoms are lost or made during a chemical reaction. In the reaction Mg + 2HCl → MgCl₂ + H₂ the atoms are simply rearranged; the same atoms are present before and after the reaction. Because the total number of atoms is the same on both sides of the equation, the total mass of the products equals the total mass of the reactants. The total number of atoms does not change – it stays the same during the reaction. The equation is balanced, showing equal numbers of each type of atom on both sides (1 Mg, 2 H, 2 Cl on each side).
Examiner tips
- Use the phrase "law of conservation of mass" to show understanding of the principle.
- Show that the equation is balanced and explain that this means the same atoms are present on both sides.
- Explain that no atoms are lost or created, so mass is conserved.
- Mention that the total number of atoms remains unchanged.
Mark scheme (5 marks)
- The law of conservation of mass states that no atoms are lost or made during a chemical reaction
- The atoms are simply rearranged / the same atoms are present before and after the reaction
- Because the total number of atoms is the same on both sides of the equation, the total mass of products equals the total mass of reactants
- The total number of atoms does not change / stays the same during the reaction
- The equation is balanced, showing equal numbers of each type of atom on both sides (e.g. 1 Mg, 2 H, 2 Cl on each side)
Key terms in this question
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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