I decided to write about a few common olevels chemistry questions that I want my students to take note. Please note that I am explaining the key concepts in this post. Not just providing the answer keys. 🙂
- Why does aluminium oxide has a higher melting point than sodium chloride?
This is because Mg2+ and O2- ions stronger electrostatic forces of attraction compared to Na+ and Cl- ions. A bigger amount of heat energy is required the electrostatic forces of attraction and hence it has a higher melting point.
- Does C60 have a low or high boiling point?
A few of my students have mistaken C60 having giant covalent structures. C60 is actually a simple covalent molecule. For giant covalent structures ( i.e graphite or diamond), the structures extend ‘forever’. C60 structure do not extend extensively. The molecules are actually made up of 60 carbon atoms joined together by strong covalent bonds with weak intermolecular forces of attraction. A small amount of energy is required to overcome these weak forces of attraction and hence, it has a low boiling or melting point.
- How to prepare calcium sulfate from calcium carbonate?
When calcium carbonate react with sulfuric acid, calcium sulfate and water will be produced. (Remember neutralization reaction?). However, calcium sulfate is insoluble according to sulfates solubility table. As a result, a non-porous insoluble layer of calcium sulfate will be formed on calcium carbonate, inhibiting further reaction. The reaction will stop after a while.
Always identify if the salt preparation is for soluble or insoluble salt. Calcium sulfate is insoluble hence precipitation method should be used. For precipitation, both reagents must be soluble. So, let excess calcium carbonate react with nitric acid to obtain calcium nitrate which is soluble. The calcium nitrate can be used to react with sulfuric acid to prepare calcium sulfate using precipitation method. Remember to include wash the precipitate with distilled water and pat dry between filter paper. 🙂
- Explain why hydrogen fuel cells are more environmentally friendly than normal automobile petrol fuel cell?
Please note for this question, student is not suppose to include carbon monoxide and nitrogen monoxide as the waste gases from the combustion of petrol. This is because these 2 gases will usually be eliminated by the catalytic converters to nitrogen gas and carbon dioxide gas. Some unburnt hydrocarbons and sulfur dioxide will be waste gases from automobile fuel emissions. SO2 causes acid rain that will corrode building and destroy crops. Unburnt hydrocarbons causes photochemical smog and respiratory issues. The product of hydrogen fuel cell is water which have minimal harmful effect to the environment.
However, do note that hydrogen fuel cell is not 100% environmental friendly because hydrogen gas needed comes mainly from cracking of hydrocarbons. The extraction of petroleum emit harmful S02 gas too.
- A fundamental electrolysis calculation question
Whenever the question mention same amount of electricity or same circuit, the moles of electrons is the same.
First calculate the number of moles of electrons. For the above question, to discharge 4 moles of Al3+ ions, 12 moles of electrons are needed. The same amount of electricity will have the same number of moles of electrons. Write ionic equations for the 4 options above and you will realize 12 moles of electrons will discharge 6 moles of Pb2+ ions because of the mole ratio from the ionic equation. 🙂