Problem 15
Question
The correct order of the solubility of alkaline-earth metal sulphates in water is : (a) \(\mathrm{Mg}>\mathrm{Ca}>\mathrm{Sr}>\mathrm{Ba}\) (b) \(\mathrm{Mg}>\mathrm{Sr}>\mathrm{Ca}>\mathrm{Ba}\) (c) \(\mathrm{Mg}<\mathrm{Ca}<\mathrm{Sr}<\mathrm{Ba}\) (d) \(\mathrm{Mg}<\mathrm{Sr}<\mathrm{Ca}<\mathrm{Ba}\)
Step-by-Step Solution
Verified Answer
The correct order is (a) Mg > Ca > Sr > Ba.
1Step 1: Understand the Alkaline Earth Metals
The alkaline earth metals are located in Group 2 of the periodic table and include magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). These metals form sulphates which have varying solubility in water.
2Step 2: Analyze the Trends in Solubility
When moving down the group in the periodic table, the size of the metal cations increases and the lattice energy of their compounds typically decreases. This generally leads to a decrease in the solubility of sulphates from magnesium sulphate to barium sulphate.
3Step 3: Predict Solubility Order
Based on the trend where the lattice energy decreases faster than the hydration energy, the solubility of sulphates decreases as we move from magnesium to barium. Therefore, the order of solubility is: MgSO₄ > CaSO₄ > SrSO₄ > BaSO₄.
4Step 4: Match Prediction with Options
Compare the predicted order from the previous step with the given options:
- Option (a) matches the predicted order
- Option (b) is incorrect due to the Sr > Ca ordering
- Option (c) is incorrect because it reverses the solubility order
- Option (d) is incorrect due to inconsistent ordering.
Key Concepts
Alkaline Earth MetalsPeriodic Table TrendsSulphates Solubility
Alkaline Earth Metals
Alkaline earth metals are a group of elements in the periodic table. They are located in Group 2 and include magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). These metals have two valence electrons, which make them highly reactive, though they are less reactive than the alkali metals in Group 1.
These elements are known for forming compounds with significant earth crust presence, like sulphates and carbonates. Due to similar physical and chemical properties, understanding their trends can be insightful for predicting and explaining reactions. One interesting property of alkaline earth metals is their tendency to form water-insoluble compounds like sulphates, which vary in solubility across the group.
These elements are known for forming compounds with significant earth crust presence, like sulphates and carbonates. Due to similar physical and chemical properties, understanding their trends can be insightful for predicting and explaining reactions. One interesting property of alkaline earth metals is their tendency to form water-insoluble compounds like sulphates, which vary in solubility across the group.
Periodic Table Trends
The position of an element within the periodic table reveals a lot about its characteristics and behavior. As we move down Group 2 from magnesium to barium, some noticeable trends arise.
First, atomic size increases. This is because as you move down the group, additional electron shells are added, thereby increasing the size of the atoms.
Second, electronegativity decreases. The ability of an atom to attract electrons weakens, as elements get larger and the additional shielding lowers the effective nuclear charge felt by the valence electrons.
Third, ionization energy decreases. The larger size and additional electron shells mean that removing an electron is easier, requiring less energy.
These trends are crucial in understanding how these metals interact with other substances, especially in forming compounds like sulphates.
First, atomic size increases. This is because as you move down the group, additional electron shells are added, thereby increasing the size of the atoms.
Second, electronegativity decreases. The ability of an atom to attract electrons weakens, as elements get larger and the additional shielding lowers the effective nuclear charge felt by the valence electrons.
Third, ionization energy decreases. The larger size and additional electron shells mean that removing an electron is easier, requiring less energy.
These trends are crucial in understanding how these metals interact with other substances, especially in forming compounds like sulphates.
Sulphates Solubility
Understanding the solubility of sulphates formed by alkaline earth metals can be intriguing. Solubility is influenced by two main factors: lattice energy and hydration energy.
Lattice energy is the energy required to separate the ions of the compound into gaseous ions. As the size of the metal ion increases, lattice energy decreases because the separation between charges becomes larger, thus releasing less energy.
Hydration energy is the energy released when ions interact with water molecules. Since larger ions have a lower charge density, they interact less effectively with water, reducing hydration energy as we move down Group 2.
Lattice energy is the energy required to separate the ions of the compound into gaseous ions. As the size of the metal ion increases, lattice energy decreases because the separation between charges becomes larger, thus releasing less energy.
Hydration energy is the energy released when ions interact with water molecules. Since larger ions have a lower charge density, they interact less effectively with water, reducing hydration energy as we move down Group 2.
- In the case of alkaline earth metal sulphates, the decrease in lattice energy occurs faster than the decrease in hydration energy.
- This results in a decrease in the sulphates' solubility from magnesium sulphate (\( \text{MgSO}_4 \)) to barium sulphate (\( \text{BaSO}_4 \)).
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