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Common Chemistry Questions

In this post, we’ll talk about some Common Chemistry Questions. We will explain the key concepts in this post. Not just providing the answer keys.

We will explain the key concepts in this post. Not just providing the answer keys.

Q1

  1. Why does aluminium oxide have a higher melting point than sodium chloride?
  • Aluminium oxide has stronger electrostatic forces of attraction between
    Al3+ and O2− ions compared to the forces between Na+ and Cl− in sodium chloride.
  • More heat energy is needed to break the stronger electrostatic forces of attraction in aluminium oxide, so it has a higher melting point.
    Tip: Ions with higher charges attract each other more strongly, requiring more energy to break the ionic bonds.

Q2

  1. Does C60 have a low or high boiling point?
  • C60 (Buckminsterfullerene) is a simple covalent molecule, not a giant covalent structure like graphite or diamond.
  • It has weak intermolecular forces between molecules, so only a small amount of energy is needed to overcome these forces.
  • Therefore, C60 has a low boiling/melting point.
    Tip: Giant molecular structures have high melting points due to extensive covalent bonding, while simple molecules have lower melting/boiling points because only weak intermolecular forces of attraction are broken.

Q3

  1. How to prepare calcium sulfate from calcium carbonate?
  • Reacting calcium carbonate with sulfuric acid will form calcium sulfate, but the reaction stops quickly because calcium sulfate is insoluble and forms a barrier.
  • To prepare calcium sulfate, use the precipitation method. React calcium carbonate with nitric acid to form soluble calcium nitrate. Then, react this with sulfuric acid to produce calcium sulfate, which can be filtered, wash with distilled water, and dried with filter paper.
    – Tip: Insoluble salts (i.e silver chloride, lead chloride , barium sulfate, calcium sulfate , lead sulfate) can only be prepared by precipitation which requires 2 aqueous reactants!

Q4

  1. Why are hydrogen fuel cells more environmentally friendly than petrol?
  • Hydrogen fuel cells produce only water, which is not harmful.
  • Petrol engines release harmful gases like sulfur dioxide (causing acid rain) and unburnt hydrocarbons (causing smog).
  • However, hydrogen production isn’t completely eco-friendly because it often comes from cracking hydrocarbons which produce huge amount of methane (greenhouse gas)
    – Tip: Hydrogen fuel cells are cleaner because they only produce water, but remember that most hydrogen is made from fossil fuels, which can still release harmful gases like methane. Think about using renewable energy to make hydrogen even greener!

Q5

  1. Electrolysis calculation tips:
  • When the same amount of electricity is used, the same number of moles of electrons are involved.
    {Insert question}
  • Example: To discharge 4 moles of Al3+, 12 moles of electrons are needed. The same amount of electricity will discharge 6 moles of
    Pb2+ because of their different ionic charges.
  • Use ionic equations to work out the moles of electrons needed for each reaction.
    – Tip: Pay attention to the charge on each ion when balancing equations and determining the number of electrons involved.

Answer: Option (B)

Done!

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