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Hydrides are compounds containing hydrogen and one other element - Scottish Highers Chemistry - Question 2 - 2023
Question 2
Hydrides are compounds containing hydrogen and one other element. (a) The graph shows the boiling points of group 4 and group 5 hydrides increase going down the gro... show full transcript
Worked Solution & Example Answer:Hydrides are compounds containing hydrogen and one other element - Scottish Highers Chemistry - Question 2 - 2023
Step 1
Explain fully why the boiling points of the group 4 hydrides increase going down the group.
Answer
The boiling points of group 4 hydrides increase down the group due to the presence of London dispersion forces (LDFs), which are the primary type of intermolecular forces in these compounds. As we move down the group, the size of the molecules increases, which leads to more electrons and a greater polarizability. This results in stronger LDFs between the molecules, requiring more energy to overcome these forces during the transition from liquid to gas, hence increasing the boiling point.
Step 2
Step 3
Calculate the enthalpy change, in kJ mol⁻¹, for this reaction using the following information.
Answer
To calculate the enthalpy change for the reaction:
Si(s) + 2H₂(g) → SiH₄(g),
we use Hess's law. First, we note the provided enthalpy changes:
- ΔH of SiH₄(g) formation from its products: -1517 kJ/mol
- ΔH for the formation of SiO₂: -911 kJ/mol
- ΔH for the formation of H₂O: -286 kJ/mol
Combining these gives:
ΔH = (-1517) - (-911) - 2(-286) = -1517 + 911 + 572 = -34 kJ mol⁻¹.
Step 4
Calculate the percentage yield of silicon hydride.
Answer
To calculate the percentage yield of silicon hydride:
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Calculate the theoretical mass of SiH₄ produced: The molar mass of Mg₂Si = 76.7 g, and the molar mass of SiH₄ = 32.1 g. Using the mass of Mg₂Si:
Moles of Mg₂Si = 15.32 g / 76.7 g/mol = 0.2 moles.
The stoichiometry of the reaction shows that 1 mole of Mg₂Si produces 1 mole of SiH₄, thus 0.2 moles of Mg₂Si will produce 0.2 moles of SiH₄.
Theoretical mass of SiH₄ = 0.2 moles × 32.1 g/mol = 6.42 g.
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Now calculate the percentage yield: Percentage yield = (actual yield / theoretical yield) × 100 = (2.56 g / 6.42 g) × 100 ≈ 39.9%.
Step 5
Explain fully why silicon oxide has a much higher melting point than silicon hydride.
Answer
Silicon oxide (SiO₂) has a much higher melting point than silicon hydride (SiH₄) due to the difference in bonding and structure between the two compounds.
Silicon oxide has a covalent network structure, where each silicon atom is covalently bonded to four oxygen atoms in a repeating pattern, forming a strong three-dimensional lattice. This structure requires a significant amount of energy to break the strong covalent bonds, resulting in a high melting point of 1710 °C.
In contrast, silicon hydride consists of discrete molecular entities held together primarily by weak van der Waals forces. These intermolecular forces are easily overcome when heat is applied, leading to a much lower melting point of -185 °C for SiH₄. Therefore, the types of bonds and the structure of silicon oxide contribute to its much higher melting point compared to silicon hydride.