Chromyl chloride, CrO2Cl2, can be prepared by heating a mixture of potassium dichromate(VI) and potassium chloride with concentrated sulfuric acid. K2Cr2O7 + 4 KCl + 3 H2SO4 → 2 CrO2Cl2 + 3 K2SO4 + 3 H2O Use the following data to complete the Hess' Law cycle and calculate the enthalpy change of the reaction, ΔHf: | compound | enthalpy change of formation, ΔHf / kJ mol⁻¹ | |-----------------|-----------------------------------------------| | K2Cr2O7 | -2061 | | KCl | -437 | | H2SO4 | -814 | | CrO2Cl2 | -580 | | K2SO4 | -1438 | | H2O | -286 | ΔHf = ............................ kJ mol⁻¹ [2]

A-Level CIE Chemistry 6.2 Transition Metals (A Level only): Chromyl chloride, CrO2Cl2, can b

Chromyl chloride, CrO2Cl2, can be prepared by heating a mixture of potassium dichromate(VI) and potassium chloride with concentrated sulfuric acid - CIE - A-Level Chemistry - Question 8 - 2016 - Paper 1

Question 8

Chromyl chloride, CrO2Cl2, can be prepared by heating a mixture of potassium dichromate(VI) and potassium chloride with concentrated sulfuric acid. K2Cr2O7 + 4 KCl ... show full transcript

Worked Solution & Example Answer:Chromyl chloride, CrO2Cl2, can be prepared by heating a mixture of potassium dichromate(VI) and potassium chloride with concentrated sulfuric acid - CIE - A-Level Chemistry - Question 8 - 2016 - Paper 1

Step 1

Calculate ΔHf using Hess's Law

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Answer

To find the enthalpy change ΔHf for the reaction:

Identify the correct enthalpy values from the given data:
- K2Cr2O7: -2061 kJ/mol
- KCl: -437 kJ/mol (times 4)
- H2SO4: -814 kJ/mol (times 3)
- CrO2Cl2: -580 kJ/mol (times 2)
- K2SO4: -1438 kJ/mol (times 3)
- H2O: -286 kJ/mol (times 3)
Apply Hess's Law:

Using the equation:
$ΔH_f = [2 imes (-580) + 3 imes (-286) + 3 imes (-1438)] - [(-2061) + 4 imes (-437) + 3 imes (-814)]$
Calculate the left side:

For 2 CrO2Cl2: 2 × -580 = -1160
For 3 K2SO4: 3 × -1438 = -4314
For 3 H2O: 3 × -286 = -858

Sum: -1160 - 4314 - 858 = -6332

Calculate the right side:
- For K2Cr2O7: -2061
- For 4 KCl: 4 × -437 = -1748
- For 3 H2SO4: 3 × -814 = -2442
Sum: -2061 - 1748 - 2442 = -6251
Calculate ΔHf:
$ΔH_f = -6332 - (-6251) = -6332 + 6251 = -81 kJ/mol$

Step 2

Suggest the type of isomerism shown by these complex ions.

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Answer

The type of isomerism shown by these complex ions is cis/trans or geometrical isomerism.

Step 3

Explain why these two complex ions - are coloured

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Answer

In a complex, the d-orbitals are split into two energy levels. The color is due to the absorption of light in the visible region, which promotes an electron to a higher orbital.

Step 4

have different colours.

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Answer

The d-d energy gap is different for the two complexes, hence they absorb different wavelengths of light, resulting in different colors.

Chromyl chloride, CrO2Cl2, can be prepared by heating a mixture of potassium dichromate(VI) and potassium chloride with concentrated sulfuric acid - CIE - A-Level Chemistry - Question 8 - 2016 - Paper 1

Worked Solution & Example Answer:Chromyl chloride, CrO2Cl2, can be prepared by heating a mixture of potassium dichromate(VI) and potassium chloride with concentrated sulfuric acid - CIE - A-Level Chemistry - Question 8 - 2016 - Paper 1

Calculate ΔHf using Hess's Law

Suggest the type of isomerism shown by these complex ions.

Explain why these two complex ions - are coloured

have different colours.

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