AQA A-level Chemistry: Transition Metals - Coloured Ions

What Causes The d-orbital To Split Into Two?

When ligands bond with the central metal ion.

What Happens \`When The d-orbital Splits?

Some of the orbitals gain energy; the d-orbital then splits into two and an energy gap ΔE is created between the “ground state” and the “excited state”.

How Do Electrons Rise From The Ground State To The Excited State?

By absorbing light energy; electrons can rise from the lowest energy level G.S.

Energy needed to jump = ΔE (Energy gap).

The Size Of The Energy Gap (ΔE) Is Dependent On:

1. The central metal ion + it’s oxidation state
2. The type of ligand
3. The coordination number/complex shape

Why Is Zinc Not Considered A Transition Metal With Relation To D-Orbital Splitting?

Zinc + its ion have full 3d-subshells; therefore if the d-orbital were to split, the electrons in the ground state wouldn’t be able to rise to the higher energy level as it’s already occupied.

The Equation For Calculating ΔE And Its Units.

ΔE = hv = hc/λ

The Equation For Calculating The Energy Gap Is ΔE = hv = hc/λ. What Is ΔE And Its Units?

ΔE = Change In Energy (J)

The Equation For Calculating The Energy Gap Is ΔE = hv = hc/λ. What Is h And Its Units?

h = Planck’s Constant (6.63 x 10^-34 J)

The Equation For Calculating The Energy Gap Is ΔE = hv = hc/λ. What Is v And Its Units?

v = Frequency of light (Hz)

The Equation For Calculating The Energy Gap Is ΔE = hv = hc/λ. What Is c And Its Units?

c = Speed Of Light (3.00 x 10^8 ms-1)

The Equation For Calculating The Energy Gap Is ΔE = hv = hc/λ. What Is λ And Its Units?

λ = Wavelength Of Light Absorbed (m)

What Does The Frequency Of Visible Light Absorbed Depend On?

The size of ΔE (Energy gap).

The ………… The Energy Gap ΔE, The ……………… The Frequency Of Light Absorbed.

The __larger__ the energy gap ΔE, the __higher__ the frequency of light absorbed.

Why Are Transition Metal Complexes Coloured?

After the d-orbital splits, when the electrons in the ground state absorb light energy they jump to the excited state; the frequencies of light not absorbed are all reflected or transmitted, which is what causes the coloured compounds.

How Can We Predict What Colour Will Be Transmitted/Reflected?

By knowing the frequency of light absorbed and then knowing the complementary colour of the absorbed light.

E.g.: If frequencies that produce red light are absorbed, then light blue is reflected/observed.

Why Are Zinc And Scandium Colourless/White?

Zn + Sc are both not transition elements; their complexes either have full or empty d-subshells; this means no electrons can migrate to the higher energy level; no frequencies absorbed - no frequencies/all frequencies reflected, therefore they appear colourless/white.

Consider:

\[Co(H2O)6\]2+(aq) + 6NH3(aq) —> \[Co(NH3)6\]2+(aq) + 6H2O(l)

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\[Co(H2O)6\]2+ is pink in colour and octahedral.

\[Co(NH3)6\]2+ is straw/yellow in colour and octahedral.

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Why Did The Colour Of The Cobalt Complex Change?

Because the type of ligand changed from H2O to NH3.

Consider:

\[Cu(H2O)6\]2+(aq) + 4Cl-(aq) —> \[CuCl4\]2-(aq) + 6H2O(l)

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\[Cu(H2O)6\]2+ is pale blue in colour and octahedral.

\[CuCl4\]2- is yellow in colour and tetrahedral.

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Why Did The Colour Of The Copper Complex Change?

Because the coordination number changed from 6 to 4, and alongside it the shape from octahedral to tetrahedral. The ligand type also changed.

Consider:

\[Fe(H2O)6\]2+(aq)—> \[Fe(H2O)6\]3+(aq)

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\[Fe(H2O)6\]2+ is pale green in colour and octahedral.

\[Fe(NH3)6\]3+ is yellow in colour and octahedral.

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Why Did The Colour Of The Iron Complex Change?

Though the ligands and coordination number remained the same, the oxidation state changed.

How Can The Concentration Of Metal Ions In Solution Be \`Measured?

Using colorimetry.

How Does A Colorimeter Work?

It measures the absorbance of light by a coloured sample.

The more concentrated the sample, the darker the colour, the more light absorbed.

Outline How The Concentration Of A Coloured Sample Would Be Measured Using A Colorimeter.

1. Calorimeter set to 0 by measuring the absorbance of a blank sample.
2. White light is filtered into a narrow range of frequencies — a monochromatic light is produced.
3. The monochromatic light passes through the sample held in a cuvette; some light is absorbed.
4. Light not absorbed travels to the detector.
5. The detector then measured the level of absorbance by comparing into the absorbance in the blank sample.

Why Is The Choice Of Filter In Colorimetry Important?

Because the colour produced from the filter must be absorbed by the metal ion solution.

What Is The Blank Sample That Is Used When Calibrating A Colorimeter To Zero?

A blank sample, typically the solvent that’s being used to dissolve the metal ions. The solvent is normally water.

The Sample Being Tested In Colorimetry Is Usually Held In A Cuvette. The Cuvette Has Two Frosted Sides And Two Clear Sides. Through Which Side Is The Light Shined Through, And Why?

The light is directed through the clear sides.

The frosted sides are where the cuvette is picked up/handled from, as the oil on our hands + fingerprints + other dirt can affect the light absorption of the sample.

Before Using A Colorimeter To Find A Metal Ion’s Concentration In Solution, A Calibration Graph Must Be Made. What Is A Calibration Graph Meant For?

To reference/use for comparison to find the concentration of the metal ion in the solution.

Outline How A Calibration Graph For Colorimetry Would Be Made.

1. Plot the calibration curve using a range of different known concentrations of the T.M ion solution. Measure the absorption for each one and plot the results.
2. The samples from 1 are made by diluting different concentrations of the metal ion using a pipette. The same metal ion and solvent must be used.
3. Then, test the sample with the unknown concentration by measuring the absorbance.
4. The calibration graph is then used to find the concentration of the metal ion in the solution.

Why Is Pipette Used In Colorimetry?

To ensure accurate volumes are used.