DM ghl: D-block electronic configurations; transition metal catalysis Flashcards
Indicate the orbital blocks on the periodic table.
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Draw a diagram showing the energies of electron sub-shells from n = 1 to n = 4.
State the elements for which this diagram is correct.
Correct up to Ni in period 4, after which 4s sub-shell has higher energy than 3d.
Note: energy gaps should decrease with increasing principal quantum numbers, as below.
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What defines d-block elements?
Elements for which the last-placed electron is in a d-orbital.
Not outermost shell; 4s already filled since lower energy.
What is the electronic configuration of argon?
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1s22s22p63s23p6
Give the electronic configuration of all of the elements in period 4 of the d-block, in terms of the electronic configuration of argon.
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Give the electronic configuration of chromium, and explain why it is is not [Ar]3d44s2, as might be expected.
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[Ar]3d54s1.
This is an energetically favourable arrangement since it avoids repulsion of two electrons in 4s orbital.
Give the electronic configuration of copper, and explain why it is is not [Ar]3d94s2, as might be expected.
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[Ar]3d104s1
This is an energetically favourable arrangement, since 2 electrons in the fifth 3d orbital experience less repulsion than 2 electrons in the 4s orbital.
Draw a diagram showing the arrangement of electrons in the ground state of elements in the first row of the d-block.
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What is a transition metal?
A d-block element which forms one or more stable ions with incompletely filled d-orbitals.
From where are electrons first lost when transition metals form simple ions?
The 4s sub-shell (and then the 3d sub-shell).
When empty, 3d has more energy than 4s, but this reverses when electrons are populated (quantum physics…)
Which elements are transition metals?
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Ti to Cu (not Sc or Zn).
All trans metals are d-block elements but not all d-block elements are trans metals.
Use the electronic configurations of the common ions of zinc and scandium to demonstrate why they are not transition metals.
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- Zn [Ar] 3d104s2 →* Zn2+ [Ar] 3d10
- Sc [Ar] 3d14s2 →* Sc3+ [Ar]
Neither form stable ions with incompletely filled d-orbitals, so they are not transition metals.
Why do transition metals exist in variable oxidation states?
There are several stable arrangements of their 3d and 4s electrons.
- What are the common oxidation states of iron?
- Write their electronic configurations.
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Fe [Ar] 3d64s2
Oxidation state = +2: Fe(II) or Fe2+ [Ar] 3d6
Oxidation state = +3: Fe(III) or Fe3+ [Ar] 3d5
With reference to electronic configuration, suggest and explain why Fe(II) is spontaneously oxidised to Fe(III).
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- Fe2+ has 2 electrons in its 5th d-orbital, causing repulsion
- Fe3+ has only one electron in each d-orbital, so experiences less repulsion
- Fe3+ has the more energetically favourable configuration so is stabler
- What are the common oxidation states of copper?
- Write their electronic configurations.
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Cu [Ar] 3d104s1
Oxidation state = +1: Cu(I) or Cu+ [Ar] 3d10
Oxidation state = +2: Cu(II) or Cu2+ [Ar] 3d9
What is the full electronic configuration of vanadium(III)?
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V [Ar] 3d34s2
V3+ 1s22s22p63s23p63d2
Why do the transition metals tend to have higher oxidation states only in combination with other elements?
- Higher oxidation states mean removal of more electrons
- Makes ions smaller + more charged
- High charge density
- More reactive
- More likely to occur in molecules
Suggest why manganese(II) is more stable than manganese(III).
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Mn2+ [Ar] 3d5
Mn3+ [Ar] 3d4
3d5 electronic arrangement is more stable than 3d4
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C
What is a catalyst?
A chemical which increases the rate of a reaction by providing an alternative pathway with a lower activation enthalpy, but remains chemically unchanged.
What is heterogeneous catalysis?
Catalysis in which the catalyst and reactants are in different physical states.
What is homogeneous catalysis?
Catalysis in which the catalyst and reactants are in the same physical state.
Why are 3d transition metals good:
- Heterogeneous catalysts?
- Homogeneous catalysts?
Heterogeneous: They have 3d + 4s electrons available to form weak bonds with reactants by adsorption.
Homogeneous: They exist in variable oxidation states, since there are several stable arrangements of their 4s + 3d electrons. This allows them to undergo redox.
Describe the typical action of transition metals as heterogeneous catalysts.
- Gaseous/liquid reactant(s) are adsorbed onto solid metal surface by forming weak bonds with its 3d + 4s electrons
- Weakens + breaks bonds in reactants
- New bonds form
- Weak bonds break; products are released
How does homogeneous catalysis usually occur for transition metal catalysts?
- Aqueous phase
- Metal ion + reactant(s) → intermediate → product(s)
- Write an overall equation for the reactions below.
- Suggest what is acting as a catalyst, and justify your suggestion.
- How could this hypothesis be tested?
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- S2O82-(aq) + 2I-(aq) → 2SO42-(aq) + I2(aq)
- Fe2+ since it is reformed (it is a reactant and final product)
- Attempt reaction with and without Fe2+ present. If reaction is slower or does not occur without Fe2+, it acts as a catalyst.
You could record initial [Fe2+], then measure final [Fe2+] by manganate(VII) titration. If [Fe2+] were unchanged, you’d have proved it isn’t used up, but not that it’s involved in the reaction.
An example of homogeneous catalysis is the reaction between 2,3-dihydroxybutanedioate ions and hydrogen peroxide, catalysed by Co(II) ions, which are pink in aqueous solution.
The reactant mixture begins pink, then turns green and back to pink. Suggest why.
- Mixture initially contains Co2+, which is pink
- Co2+ reduces H2O2 and is oxidised to intermediate containing Co3+, which is green
- Co2+ is reformed in products, so pink is seen again
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An example of homogeneous catalysis is the exothermic reaction between 2,3-dihydroxybutanedioate ions and hydrogen peroxide, catalysed by Co(II) ions.
Draw an enthalpy profile, showing both the uncatalysed and catalysed reaction.
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