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Questions and Answers
What is the characteristic property of transition metals?
Which ligands can participate in a ligand substitution reaction without changing the coordination number?
What occurs when Cl- ligands substitute NH3 or H2O ligands in a complex?
What defines the coordination number of a complex?
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What is the coordination number when complexes are formed with only Cl- ligands?
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Which of the following is a characteristic of bidentate ligands?
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Why are transition metals good catalysts?
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Which multidentate ligand forms six coordinate bonds during a ligand substitution reaction?
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What is a common characteristic when transition metals form complexes?
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What happens to copper complexes when NH3 is in excess?
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What effect does carbon monoxide have on the haemoglobin complex?
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What is the main advantage of using bidentate or multidentate ligands in ligand substitution?
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Which coordination number is common for platinum complexes?
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What is typically true regarding the enthalpy change (∆H) for ligand substitution reactions?
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Which of the following statements regarding the chelate effect is accurate?
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What is the role of the lone electron pairs in multidentate ligands?
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What colour does the ion with an oxidation state of +3 exhibit?
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What causes the color observed in transition metal ions?
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Which conditions are required for ions to be reduced?
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What is the role of Tollens Reagent in chemical reactions?
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Which equation correctly relates frequency, wavelength, and Planck's constant?
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What defines a heterogeneous catalyst?
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In colorimetry, what does a calibration graph represent?
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Which statement is true regarding catalysts?
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What is one key property of transition metals related to their chemical behavior?
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What phrase can help remember the colours of vanadium ions?
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When light is shone on a substance, what happens to certain wavelengths?
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Which of the following is a characteristic of a catalyst?
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What is the result of the oxidation of vanadium by zinc in acidic solution?
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Which factor does NOT influence the color of transition metal ions?
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Which reaction type is characterized by a change in the oxidation state of reactants?
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What happens to electrons in the d-orbital when they absorb energy?
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What role does the vanadium oxide catalyst play in the Contact Process?
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What happens to vanadium during the reaction it catalyzes?
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Which of the following best describes how a solid catalyst increases the reaction rate?
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What is the effect of catalyst poisoning on heterogeneous catalysts?
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What characterizes homogeneous catalysts?
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How do impurities affect the use of a catalyst in a reaction?
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What is the main benefit of using a catalyst in chemical reactions?
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Which of the following transition metals is noted for being an effective catalyst due to its cost and efficiency?
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Study Notes
Transition Metal Properties
- Transition metals are elements in the d-block, with partially filled d orbitals
- They lose electrons to form positive ions, with s-orbital electrons being removed first
- Transition metals have very similar physical properties:
- Atomic radii
- High densities
- High melting and boiling points
- Transition metals have special chemical properties:
- Can form complexes
- Form colored ions
- Exhibit multiple oxidation states
- Are good catalysts
Complexes
- Complexes are formed from transition metals
- Consists of a central metal ion, surrounded by ligands
- Example: [Cu(H2O)6]2+
- Can be represented in formulas using square brackets
Ligands
- Ligands are molecules or ions with a lone electron pair
- This electron pair can form a coordinate bond to the central metal ion
- Common ligands include:
- Cl-
- H2O
- NH3
Coordination Number
- The coordination number is equal to the number of coordinate bonds around the central metal ion
- Silver complexes have a coordination number of 2
- Platinum complexes commonly have a coordination number of 4
Ligand Substitution Reactions
- NH3 and H2O ligands are similar in size and uncharged
- Ligands can be exchanged without a change in coordination number via a ligand substitution reaction, e.g.:
- [Cu(H2O)6]2+ + 4NH3 <=> [Cu(NH3)4(H2O)2]2+ + 4H2O
- With copper complexes and excess NH3, this substitution is incomplete
- The complex formed has a combination of water and ammonia ligands
- e.g. [Cu(NH3)4(H2O)2]2+ produces a deep blue solution
- The Cl- ligand is much larger than NH3 and, H2O ligands
- Substitution with this ligand results in a change in coordination number, e.g.:
- [Cu(H2O)6]2+ + 4Cl- <=> [CuCl4]2- + 6H2O
- Complexes with just Cl- ligands always have a coordination number of four, producing a tetrahedral shaped complex
Bidentate Ligands
- Bidentate ligands can form two coordinate bonds to the central metal ion
- They have two lone electron pairs
- Common examples include:
- Ethanedioate ions: A lone electron pair on two of the oxygen atoms
- Ethane-1,2-diamine: A lone electron pair on each of the nitrogen atoms
- Replacing two unidentate ligands with a bidentate ligand results in a change in coordination number from 6 to 4
- Example:
- [Cu(H2O)6]2+ + 2en <=> [Cu(en)2(H2O)2]2+ + 4H2O
Multidentate Ligands
- Multidentate ligands can form more than six coordinate bonds
- They have multiple lone electron pairs
- EDTA (ethylenediaminetetraacetic acid) is a common multidentate ligand
- It forms six coordinate bonds in a ligand substitution reaction
- Example: [Cu(H2O)6]2+ + EDTA4- <=> [Cu(EDTA)]2- + 6H2O
- It forms six coordinate bonds in a ligand substitution reaction
- Haem, a component of haemoglobin, is another multidentate ligand
- Six coordinate bonds form around a central Fe2+ ion
- Shape and structure allows it to transport oxygen around the body
- Carbon monoxide is toxic to humans
- It replaces oxygen in the haem complex, thus preventing it from being transported around the body
The Chelate Effect
- A positive entropy change in a ligand substitution reaction is favourable
- This means that a more stable complex will be formed
- It is favourable to have more moles on the right-hand side of the reaction
- This is achieved by substituting unidentate ligands with bidentate or multidentate ligands, known as the chelate effect
- The greater the entropy change, the more negative the free energy change (∆G) will be
- This means that the reaction is more favourable
Enthalpy Change
- The enthalpy change (∆H) for ligand substitution reactions is very small
- This is because the bonds being formed are very similar to the bonds that were broken
- Therefore, the overall enthalpy change is near zero
Coloured Ions
- Transition metal ions can be identified by their colour
- The colour depends on:
- Coordination number of the complex
- Type of ligand bonded to the ion
- Oxidation state
- The observed colour arises because of how substances absorb and reflect light
- When white light shines on a substance, some wavelengths of light are absorbed
- The remaining wavelengths are reflected and transmitted to the human eye
- These wavelengths correspond to a specific color, which is then observed
- Electrons in the d-orbital exist in a series of energy states
- When an electron is given energy, it moves from a ground state (n=1) to a higher energy state called an excited state
- The change in energy (∆E) between these states corresponds to a wavelength and frequency of light
- This can be calculated using the equation:
- v = frequency in hertz
- λ = wavelength in meters
- h = Planck’s constant, 6.63x10-34
Colorimetry
- Colorimetry is an analytical technique that uses the absorption of visible light to determine the concentration of colored ions
- This is done by measuring absorbance
- A calibration graph is produced by measuring solutions of known concentrations
- This graph can then be used to calibrate the colorimeter
- This can then be used to determine the concentration of an unknown substance
Variable Oxidation States
- Transition metals can exhibit variable oxidation states
- Vanadium has four possible oxidation states (+5 to +2), each of which produces a different coloured compound
- These different species can be produced from the oxidation of vanadium by zinc in acidic solution
- Example:
- Oxidation State | Ion Formula | Ion Colour
- +5 | VO2+ | Yellow
- +4 | VO2+ | Blue
- +3 | V3+ | Green
- +2 | V2+ | Violet
- These colours can be remembered by the phrase "You Better Get Vanadium"
- Example:
- The pH of the reaction conditions determines whether a transition metal is oxidized or reduced
- For ions to be reduced (oxidation state = more negative), acidic conditions are required
- For ions to be oxidized (oxidation state = more positive), they react with water to produce OH- ions, therefore requiring alkaline conditions
- Tollens reagent is a silver complex, [Ag(NH3)2]+
- It is reduced by aldehydes to form silver atoms, seen as a silver mirror
- This is used as a test for aldehydes
- Redox titrations are one of the longer calculation questions on the A-Level exam
- Multiple high tariff marks are often awarded
- Redox reactions involved are limited to three main reactions
- It is useful to learn the reactions and reacting molar ratios
Catalysts
- A catalyst is a substance that speeds up the rate of a reaction without being used up in the reaction
- Transition metals make good catalysts because of their variable oxidation states
- Electrons are transferred to produce a reactive intermediate, speeding up the reaction rate
- There are two main types of catalyst: heterogeneous and homogeneous
Heterogeneous Catalysts
- Heterogeneous catalysts are in a different phase or state to the species in the reaction
- e.g. the Haber Process uses a solid iron catalyst to speed up the reaction between hydrogen and nitrogen gases
- Transition metals make good catalysts due to their variable oxidation states
- Electrons are transferred to produce a reactive intermediate, speeding up the reaction rate
- Example: The Contact Process uses a vanadium oxide catalyst to speed up the conversion of sulfur dioxide to sulfur trioxide
- Vanadium is reduced from +5 to +4
- Vanadium is then reformed in its original oxidation state
- This shows that it has acted as a catalyst for the reaction
- Adsorption:
- Solid catalysts work by adsorbing molecules onto an active site on the surface of the catalyst
- Active sites increase the proximity of molecules and weaken the covalent bonds in the molecules
- This causes reactions to occur more easily, increasing the rate
- Example:
- The strength of adsorption depends on the type of catalyst
- This can affect how much it affects the rate of the reaction
- Iron, Cobalt and Nickel make the best catalysts as they are relatively cheap and increase the rate of reaction the most out of all the period 4 transition metals
Catalyst Poisoning
- Heterogeneous catalysts can be poisoned by impurities
- These impurities block the active sites and prevent adsorption
- Therefore the bonds of the molecules remain strong and the catalyst has no effect on the rate of the reaction
- Reaction impurities can lead to an increase in chemical production costs in industry
- The catalyst has to be replaced or cleaned regularly
- Example:
- Sulfur impurities poison the solid iron catalyst used in the Haber Process
Homogeneous Catalysts
- Homogeneous catalysts are in the same phase as the species in the reaction
- Example: the reaction between S2O82- ions and I- ions where Fe2+ has to be used as a catalyst
- Without a catalyst, these negative ions would naturally repel each other and never react
- The catalyst works by combining with the reactants to produce a reactive intermediate
- This changes the reaction path
- The enthalpy change for the formation of the intermediate is much lower than the original reaction
- This lowers the activation energy, making the reaction more feasible
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Description
Test your knowledge on the properties and behaviors of transition metals, including their formation of complexes and ligands. This quiz covers key concepts such as coordination numbers and the unique characteristics of these elements. Perfect for chemistry students looking to reinforce their understanding of transition metals.
