A student reacts zinc with excess hydrochloric acid in a sealed container. The equation for the reaction is: Zn + 2HCl → ZnCl₂ + H₂. The student notices that 0.5 mol of hydrogen gas is produced. Avogadro's law states that equal amounts in moles of gases occupy the same volume under the same conditions of temperature and pressure. The molar volume of a gas at room temperature and pressure is 24 dm³/mol. Explain how the student can use Avogadro's law to determine the volume of hydrogen gas produced, and describe what would happen to the volume if the same experiment were repeated at a higher temperature but the same pressure.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Zinc reacts with hydrochloric acid according to the equation: Zn + 2HCl → ZnCl₂ + H₂. In an experiment carried out at room temperature and pressure, 0.5 mol of hydrogen gas is produced. The molar volume of any gas at room temperature and pressure is 24 dm³/mol.
Model answer (4 marks)
Avogadro's law states that equal amounts in moles of gases occupy the same volume under the same conditions of temperature and pressure, so 1 mol of any gas occupies 24 dm³ at room temperature and pressure.
The volume of 0.5 mol of hydrogen gas at room temperature and pressure is 0.5 × 24 dm³ = 12 dm³.
If the same experiment were repeated at a higher temperature but the same pressure, the gas particles would have more kinetic energy and move faster, so they would spread out more. Therefore the volume of hydrogen gas produced would be greater (larger) at the higher temperature.
The volume of 0.5 mol of hydrogen gas at room temperature and pressure is 0.5 × 24 dm³ = 12 dm³.
If the same experiment were repeated at a higher temperature but the same pressure, the gas particles would have more kinetic energy and move faster, so they would spread out more. Therefore the volume of hydrogen gas produced would be greater (larger) at the higher temperature.
Examiner tips
- Show the calculation 0.5×24=12 dm³ to earn the volume mark.
- Explain the temperature effect using kinetic energy/particle motion to gain the final point.
Common mistakes
- Using the wrong molar volume (e.g. 22.4 dm³) instead of 24 dm³.
- Failing to state that the volume increases with temperature at constant pressure.
Mark scheme (4 marks)
- Avogadro's law states that equal amounts in moles of gases occupy the same volume under the same conditions of temperature and pressure, so 1 mol of any gas occupies 24 dm³ at room temperature and pressure.
- The volume of 0.5 mol of hydrogen gas at room temperature and pressure is 12 dm³ (0.5 × 24 dm³/mol).
- At a higher temperature (same pressure), the gas particles have more energy and move faster, so they spread further apart / the gas expands.
- Therefore the volume of hydrogen gas produced would be greater (larger) at the higher temperature.
Key terms in this question
Avogadro's law · molar volume · mole
Related
- All AQA GCSE Chemistry (8462) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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