O-Level Chemistry: Question on Chemical Bondings


In the last blog post, we went through a common exam-based question on Chemical Bondings and revised  briefly on:

  • Types of Bondings
  • Types of Structures & its properties

Today, we shall look at a few more exam-based Chemistry MCQ questions on this topic:

Question 1:

Which of the following is most likely to be the melting point of an ionic solid?

A. -182oC

B. 0oC

C. 114oC

D. 943oC

Suggested Solutions:

Ionic compounds tend to have high melting point and boiling point. They tend to exist as solids at room temperature and pressure.

Strong electrostatic forces of attraction holds the positive ions and the negative ions together in the giant ionic/crystal lattice structure.

As such, a large amount of energy is required to overcome these strong forces.

As such, answer in (Option D).

 

Question 2:

Which one of the following substances;

(i) is an element?

(ii) also forms crystals composed of small molecules?

A. Copper

B. Graphite

C. Carbon dioxide

D. Iodine

Suggested Solutions:

Option A: Copper is an element BUT it exists as positive metal ions surrounded by ‘sea of delocalised valence electrons’.

Option B: Graphite is an element BUT it exists as giant molecular structure of carbon atoms held together by strong covalent bonds.

Option C: Carbon dioxide exists as simple molecular structure (i.e. small molecules) BUT it is a compound.

Option D: Iodine is an element and it exists as a diatomic molecule (i.e. simple molecular structure). At room temperature, iodine is a solid so that diatomic molecules are packed very closely together in a regular pattern, to form “crystals”. DO NOT be tricked to think that iodine is an ionic compound due to the word “crystals” which is usually associated with ionic compound. It could also mean that simple molecular structures are packed closely together in a regular pattern when they are in solid state. You can refer to Kinetic Particle Theory to learn more on 3 states of matter.

As such, answer is (Option D).

PS: Hmm, just realised this is quite a tricky question. Going to test my students in our O-Level Pure Chemistry Tuition Classes on the same question and see how they fare for it :)

I am sure you have learned something useful today.

Feel free to share it with your friends who need help in Basic Chemistry.

Cheers! :)

O-Level Chemistry: Questions on Chemical Bondings


Many students do not know the importance on this fundamental topic known as Chemical Bondings which involves:

3 Types of Bondings:

  • Ionic Bonding
  • Covalent Bonding
  • Metallic Bonding

And from the key concepts of the three types of bonding above, students will be further asked on the Types of Structures which includes:

  • Giant Ionic/Crystal Lattice Structure (due to Ionic Bonding)
  • Simple Molecular Structure (due to Covalent Bonding)
  • Giant Molecular Structure (due to Covalent Bonding)
  • Giant Metallic Lattice Structure (due to Metallic Bonding)

You can do a search for “Chemical Bondings” on the top right hand side to look at the numerous discussions on this topic.

Today, i would like to share with you an  exam-based questions that you might be asked in Singapore GCE O-Level Pure Chemistry Examination.

Question 1:

Which of the following substance conducts electricity by the movement of free ions?

A. Copper

B. Molten aluminium oxide

C. Solid potassium chloride

D. Graphite

Suggested Solutions:

Option A: Copper is a metal and it conducts electricity due to the presence of ‘sea of delocalised valence electrons’ which are mobile. Students would need to know about Giant Metallic Lattice Structure to understand its structural properties.

Option B: Aluminium oxide is made up of aluminium ions and oxide ions which are mobile when it is in the molten state. Students would need to know about Giant Ionic/Crystal Lattice Structure to understand its structural properties.

Option C: Potassium chloride is made up of potassium ions and chloride ions BUT the ions are not mobile in the solid state because they are fixed in positions in the Giant Ionic/Crystal Lattice Structure.

Option D: Graphite does conduct electricity but it is due to unbonded delocalised valence electrons in the hexagonal layers of carbon atoms. It is not due to mobile ions! Students would need to know about Giant Molecular Structures to understand their structural properties.

As such, answer is (Option B).

For Chemical Bonding, it is important that students understand the key concepts of each type of bondings, their structural properties and the use of the correct keywords. Examiners will look at the keywords required to see if you understand your concepts. I always remind students in my O-Level Pure Chemistry Tuition Classes in Singapore.

I am sure you have learned something useful today.

Feel free to share it with your friends who need help in Basic Chemistry.

Cheers! :)

Chemistry Phenomenon: Methane, an Air Pollutant is Flammable


O-Level Chemistry Tuition Class - Methane

Source: Yahoo Singapore News on 28th Jan 2014

KABOOM! Yes, methane is highly flammable!

Balanced Chemical Equation for combustion: CH4(g) + 2O2(g) –> CO2(g) + H2O(g)

Besides that, methane, CH4 is also listed as one of the air pollutants (other common pollutants are: carbon monoxide, sulfur dioxide, oxides of nitrogen, unburnt hydrocarbons and ozone) in our atmosphere.

It is colourless and odourless gas, which is produced when plant and animal matter decay.

Sheep and cows give off methane due to the digestion of food so we will have high concentration of this gas in farming areas.

Methane is also produced from the decay of rubbish in landfills.

Scientists are studying closely the concentration level of methane in our atmosphere because it is a potent greenhouse gas which causes global warming and their levels has doubled over the last 150 years because of human activities like fossil fuel use and intensive farming.

The possible consequences of global warming are:

  • Unusual weather conditions such as warm spells, droughts, unexpected storms, floods and tsunamis
  • A decrease in world-wide crop yields
  • Melting of large quantities of ice in north and south poles which will cause sea levels to rise and flood low-lying countries such as Netherlands and Singapore

Note:

Air pollution is tested in the GCE O-Level Chemistry Examination in Singapore and is very common. You can find out more information from the two recommended textbooks in Singapore, namely: Marshall Cavendish Education’s Chemistry Matters and Pearson’s All About Chemistry. Refer to the topic called Atmosphere or Air.

Hope you find the above useful and informative.

PS: Feel free to share it with your friends. Keep Sharing; Keep Learning!