Secondary Chemistry Topic Revision: Chemical Bonding
In Chemical Bonding, there are 3 main parts - Melting Points, Electrical Conductivity and Dot and Cross Diagrams. In this summary, we will be covering the first 2 parts 🙂
Chemical Bonding Part 1: Melting Point of Ionic, Covalent & Metallic Elements/Compounds
|Substance||Ionic compound||Simple covalent molecules||Giant covalent substance||Metals|
Structure the substance adopts.
|Giant ionic lattice||Simple covalent structure||Extensive/ Giant network of covalent bonds||Giant metallic lattice|
Strong or weak attractive forces between atoms or ions etc.
|Strong electrostatic forces of attraction between oppositely charged ions||Weak intermolecular forces of attraction (Van der Waals) between molecules||Strong covalent bonds between atoms||Strong electrostatic forces of attraction between positive ions and sea of delocalised electrons|
The energy required to overcome the attractive forces.
|A lot of energy is required to overcome these strong electrostatic forces of attraction.||Less energy is required to overcome these weak Van der Waals forces between molecules.||A large amount of energy is required to break these strong covalent bonds between atoms.||A large amount of energy is required to overcome these strong electrostatic forces of attraction.|
Linking it back to the question.
|Thus, (name of substance) has a high/low melting/boiling point.|
Common Mistake #1: Since covalent compounds have low melting point, is it right to conclude that covalent bonds are weak?
The answer is no. A common misconception that students have is that the covalent bonds are weak since covalent compounds have low melting point, which is false. Covalent bonds are very strong due to the sharing of electrons between atoms. The reason for the low melting point of covalent compounds is due to the weak intermolecular forces of attraction between molecules, known as Van der Waals forces.
When a question requires an explanation relating to melting or boiling points of substances, answer using SAEC (Structure - Attraction - Energy - Conclusion) format. Below is a table showing a summary on how to tackle a question asking to explain the high melting point of different compounds.
Sometimes, the question may ask to compare the melting points of two different substances. Similarly, SAEC format needs to be used to explain the difference in melting points.
Question: Explain why MgO have a higher melting point than H2O.
A common error #2 that students make would be just stating that MgO is an ionic compound while H2O is a simple covalent molecule, without explaining further.
To obtain full marks, elaborate further;
MgO is an ionic compound with strong electrostatic forces of attraction between Mg2+ and O2- ions in its ionic lattice structure, while H2O is a simple covalent molecule with a simple covalent structure and have weak intermolecular forces of attraction between molecules. As a larger amount of energy is required to overcome the strong electrostatic forces of attraction in MgO than the weak intermolecular forces in H2O, MgO thus has a higher melting point than H2O.
Chemical Bonding Part 2: Electrical Conductivity
|State||Can the substance conduct electricity?|
|Ionic compound||Simple covalent molecules||Giant covalent substance||Metals|
|Liquid or Molten||Yes||No||No||Yes||Yes|
(Dissolved in water)
|Yes||Most covalent compounds are insoluble except for *hydrogen halides. Therefore, hydrogen halides dissolved in water (aqueous hydrogen halide) can conduct electricity.|
*In the table above, most simple covalent molecules are insoluble in water - except for hydrogen halides. Aqueous solutions of hydrogen halides are known as hydrohalic acids (hydrochloric acid, hydrofluoric acid etc).
Note: Pure water does not conduct electricity. However, presence of impurities may affect its electrical conductivity. For example, if the impurity is an ionic salt, then the impure water can conduct electricity. On the other hand, if the impurity is a simple covalent substance that is not a hydrogen halide, then the water is still not able to conduct electricity.
When a question tests the concept of electrical conductivity of substances, the keywords examiners are looking out for are shown below:
mobile charge carriers
mobile charge carriers
Question: Silicon carbide has the same structure as diamond. Predict the electrical conductivity of silicon carbide and explain.
Silicon carbide has a tetrahedral structure where all carbon and silicon atoms use all of its valence electrons for covalent bonding. As there are no free electrons that can act as mobile charge carriers, silicon carbide is not able to conduct electricity.