Describe three characteristic chemical properties of transition elements that are not shown by Group II elements. When NH₃(aq) is added to a green solution containing Ni²⁺(aq) ions, a grey-green precipitate is formed. This precipitate dissolves in an excess of NH₃(aq) to give a blue-violet solution. Suggest an explanation for these observations, showing your reasoning and including equations for the reactions you describe. Dimethylglyoxime, DMG, is a useful reagent for the quantitative estimation of nickel. It forms an insoluble salt with nickel ions according to the following equation. Ni²⁺(aq) + C₄H₆N₂O₂ (DMG) ⟶ Ni(C₄H₆N₂O₂)₂(s) + 2H⁺(aq) The mass of nickel coin was 3.40g was dissolved in nitric acid and an excess of DMG was added. The precipitated Ni-DMG was filtered off, washed and dried. Its mass was 4.00g. Calculate the % of nickel in the coin.

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Describe three characteristic chemical properties of transition elements that are not shown by Group II elements - CIE - A-Level Chemistry - Question 2 - 2010 - Paper 1

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Describe three characteristic chemical properties of transition elements that are not shown by Group II elements. When NH₃(aq) is added to a green solution containi... show full transcript

Worked Solution & Example Answer:Describe three characteristic chemical properties of transition elements that are not shown by Group II elements - CIE - A-Level Chemistry - Question 2 - 2010 - Paper 1

Step 1

Describe three characteristic chemical properties of transition elements that are not shown by Group II elements.

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Colored Ions/Compounds: Transition metals form colored solutions and compounds due to the presence of partially filled d-orbitals, allowing electron transitions between energy levels.
Variable Oxidation States: Transition elements can exhibit multiple oxidation states, unlike Group II elements which typically display only a +2 oxidation state.
Formation of Complexes: Transition metals readily form complex ions by coordinating with ligands, which can alter their reactivity and solubility.

Step 2

When NH₃(aq) is added to a green solution containing Ni²⁺(aq) ions, suggest an explanation for these observations, showing your reasoning and including equations for the reactions you describe.

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The initial green solution contains Ni(OH)₂, indicating the presence of nickel(II) hydroxide. When excess NH₃ is added, a ligand exchange occurs that transforms the nickel(II) ion:

ightarrow ext{Ni(OH)}_2(s)$$ The formation of a grey-green precipitate is noted. In excess NH₃, the precipitate dissolves: $$ ext{Ni(OH)}_2(s) + 6 ext{NH}_3(aq) ightarrow ext{Ni(NH}_3)_6^{2+}(aq) + 2 ext{OH}^-(aq)$$ This results in a blue-violet solution due to the formation of the complex ion.

Step 3

Calculate the % of nickel in the coin.

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Given:

Molar mass of Ni = 58.7 g/mol
Molar mass of DMG = 172.7 g/mol
Mass of Ni-DMG = 4.00 g

First, calculate moles of Ni-DMG:

$n(Ni-DMG) = \frac{4.00}{172.7} = 0.0232 \text{ mol}$

The number of moles of nickel ( ext{Ni}) will be the same as the moles of Ni-DMG formed:

$n(Ni) = 0.0232 \text{ mol}$

Now, calculate the mass of nickel in the Ni-DMG:

$\text{mass}(Ni) = 0.0232 \text{ mol} \times 58.7 \text{ g/mol} = 1.36 \text{ g}$

Finally, calculate the percentage of nickel in the coin:

$\text{percentage} = \frac{1.36}{3.40} \times 100 \approx 40.0\%$

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Describe three characteristic chemical properties of transition elements that are not shown by Group II elements - CIE - A-Level Chemistry - Question 2 - 2010 - Paper 1

Worked Solution & Example Answer:Describe three characteristic chemical properties of transition elements that are not shown by Group II elements - CIE - A-Level Chemistry - Question 2 - 2010 - Paper 1

Describe three characteristic chemical properties of transition elements that are not shown by Group II elements.

When NH₃(aq) is added to a green solution containing Ni²⁺(aq) ions, suggest an explanation for these observations, showing your reasoning and including equations for the reactions you describe.

Calculate the % of nickel in the coin.

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