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Use dot and cross diagrams to show the formation of bonds in magnesium chloride - Leaving Cert Chemistry - Question d, e, f, g - 2008
Question d, e, f, g
Use dot and cross diagrams to show the formation of bonds in magnesium chloride. Explain the term intermolecular forces. Use your knowledge of intermolecular force... show full transcript
Worked Solution & Example Answer:Use dot and cross diagrams to show the formation of bonds in magnesium chloride - Leaving Cert Chemistry - Question d, e, f, g - 2008
Step 1
Use dot and cross diagrams to show the formation of bonds in magnesium chloride.
Answer
In magnesium chloride (MgCl₂), magnesium loses two electrons to form a magnesium ion (Mg²⁺), while each chlorine atom gains one electron to form chloride ions (Cl⁻). The dot and cross diagrams would illustrate the transfer of electrons from magnesium to chlorine, showing two dots for magnesium on the left and crosses for the chlorine atoms on the right. This results in the ionic bond formation between Mg²⁺ and Cl⁻, which can be represented as:
Cl Cl
* *
| |
Mg Cl
* *
2+
This illustrates how magnesium forms bonds with two chloride ions.
Step 2
Explain the term intermolecular forces.
Answer
Intermolecular forces are the attractive forces that exist between molecules. These forces can be categorized into several types, including:
- Van der Waals forces: Weak attractions due to temporary dipoles.
- Dipole-dipole interactions: Occur between polar molecules, where positive ends of one molecule are attracted to negative ends of another.
- Hydrogen bonds: A strong type of dipole-dipole interaction occurring when hydrogen is bonded to highly electronegative atoms (like oxygen, nitrogen, or fluorine).
These forces are crucial in determining the physical properties of substances, such as boiling and melting points.
Step 3
Use your knowledge of intermolecular forces to explain why methane has a very low boiling point (b.p. = -164 °C).
Answer
Methane (CH₄) has a very low boiling point primarily due to the weak London dispersion forces that exist between its molecules. Although its molecular mass is slightly lower than that of water, methane is non-polar and lacks strong intermolecular forces like hydrogen bonds, present in water. Consequently, the energy required to overcome these weak dispersion forces in methane is minimal, resulting in a low boiling point.
Step 4
Suggest a reason for this.
Answer
The significant difference in boiling points between methane and water can be attributed to the presence of hydrogen bonds in water. Water molecules exhibit strong intermolecular attractions due to these hydrogen bonds, requiring more energy to break them apart compared to the very weak London dispersion forces in methane. This explains why water has a much higher boiling point than methane.
Step 5
Account for these observations.
Answer
The stream of water is attracted towards the positively charged rod because water is a polar molecule, meaning it has a partial positive charge on one end and a partial negative charge on the other... Because of this polarity, the positive end of water molecules is attracted towards the negatively charged rod, leading to deflection.
Step 6
Explain what would happen in the case of the stream of water if the positively charged rod were replaced by a negatively charged rod.
Answer
If the positively charged rod is replaced by a negatively charged rod, the stream of water would be deflected towards the negatively charged rod instead. This is because the positive end of the water molecules (the hydrogen side) would be attracted to the negative charge of the rod, resulting in the stream bending in that direction. This demonstrates the nature of polar molecules in an electric field.