Zinc reacts with hydrochloric acid to produce zinc chloride and hydrogen gas. The equation for this reaction is: Zn + 2HCl → ZnCl₂ + H₂. The relative atomic mass of zinc is 65. The relative atomic mass of chlorine is 35.5. The relative atomic mass of hydrogen is 1. A student reacts a sample of zinc with excess hydrochloric acid in an open container and measures the mass of the container throughout the reaction. The student notices the total mass of the container decreases during the reaction. Explain why the mass decreases, and explain why the total mass of zinc chloride produced is greater than the mass of zinc used.

AQA GCSE Chemistry (8462) — 4.3.2 Use of amount of substance in relation to masses of pure substances · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

The reaction produces hydrogen gas:
Zn + 2HCl → ZnCl₂ + H₂
The H₂ leaves the open container, so the mass of the system (container + contents) falls by the mass of the gas that escapes.
The zinc chloride that is formed contains the zinc atom from the metal and two chlorine atoms that come from the acid. No atoms are lost or created; the chlorine atoms add mass to the product. Therefore the mass of ZnCl₂ is greater than the mass of the zinc that reacted.

Examiner tips

  • Mention the escape of H₂ gas to explain the mass loss. State that ZnCl₂ contains chlorine atoms from HCl, increasing its mass. Use the phrase "no atoms are lost or made" to show conservation of mass.

Common mistakes

  • Failing to recognise that H₂ gas leaves the system. Confusing the mass of the product with the mass of the reactants. Not explaining that chlorine atoms add to the mass of ZnCl₂.

Mark scheme (4 marks)

  1. Hydrogen gas is released/escapes into the atmosphere/surroundings
  2. The mass of the hydrogen gas is not accounted for / is lost from the system
  3. Zinc chloride contains chlorine atoms (from the hydrochloric acid) as well as zinc atoms
  4. No atoms are lost or made during the reaction (conservation of mass), so the mass of chlorine added to the zinc increases the mass of the product

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