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The elements sodium to sulfur in Period 3 all react with oxygen to form oxides - AQA - A-Level Chemistry - Question 2 - 2018 - Paper 3
Question 2
The elements sodium to sulfur in Period 3 all react with oxygen to form oxides. 1. Give an equation and two observations made for the reaction that occurs when sodi... show full transcript
Worked Solution & Example Answer:The elements sodium to sulfur in Period 3 all react with oxygen to form oxides - AQA - A-Level Chemistry - Question 2 - 2018 - Paper 3
Step 1
Give an equation and two observations made for the reaction that occurs when sodium is heated in oxygen.
Answer
The equation for the reaction is:
Observation 1: A yellow/orange flame is observed when sodium is heated in oxygen.
Observation 2: White solid (sodium oxide) is formed, which may appear as fumes.
Step 2
Step 3
Explain the increase in melting point from sodium oxide to magnesium oxide.
Answer
The increase in melting point from sodium oxide (1548 K) to magnesium oxide (3125 K) can be attributed to stronger ionic character and greater charge density in magnesium ions compared to sodium ions. Magnesium ion (Mg²⁺) has a higher charge (+2) compared to sodium ion (Na⁺), resulting in stronger electrostatic forces of attraction between the ions in magnesium oxide, leading to a higher melting point.
Step 4
Explain why the melting point of the oxide of silicon is much higher than that of the highest oxide of phosphorus.
Answer
Silicon dioxide (SiO₂) exhibits a giant covalent structure, which means it has a network of covalent bonds that require a significant amount of energy to break. In contrast, phosphorus pentoxide (P₂O₅) has a molecular structure with weaker Van der Waals forces between the molecules, leading to a lower melting point. The structure of SiO₂ is thus much more stable and requires higher energy to break the bonds.
Step 5
Describe a method for determining the melting point of the highest oxide of phosphorus.
Answer
To determine the melting point, a sample of phosphorus pentoxide should be placed in a melting point apparatus. The temperature can be gradually increased until the sample completely melts. As the temperature rises, the point at which the sample changes from solid to liquid (observed via capillary tube) indicates the melting point.
Evaluation of Purity: The observed melting point can be compared with the known melting point of pure phosphorus pentoxide (573 K). A sharp melting point close to the literature value would indicate high purity, while a significant deviation suggests impurities present in the sample.