6.2.3 Chemical Properties

While the physical properties of transition metals show a high degree of similarity across the period, their chemical properties are more varied. This variation arises from the complex interplay of their 3d and 4s electrons. This section explores the key chemical properties of transition metals: their variable oxidation states and their catalytic activity.

Main Idea

The main idea is that while crossing the first-row transition metals, there is not as much similarity in chemical properties as we found in physical properties. Some important chemical properties of transition metals are their ability to exist in variable oxidation states and their use as catalysts in various chemical reactions.

Core Concept

The core concept is that the chemical behavior of transition metals is largely determined by their electronic configuration, specifically the presence of an incomplete d subshell. The similar energies of the 4s and 3d orbitals allow for the loss or sharing of different numbers of electrons, leading to variable oxidation states. Additionally, the availability of vacant d orbitals and the ability to easily change oxidation states make them excellent catalysts.

Detailed Explanation

The textbook describes two key chemical properties of transition metals.

1. Variable Oxidation States

The common oxidation state for the first row of the d-block elements is +2, obtained by the loss of two 4s electrons to form an M²⁺ ion. The maximum stable oxidation state frequently corresponds to the maximum number of electrons (3d and 4s) available for bonding. For example, manganese, with electronic configuration [Ar]3d⁵4s², can have a maximum oxidation state of +7.

• Trend in Stability: The stability of the +2 oxidation state compared to the +3 oxidation state increases with atomic number from left to right across the first row of the d-block. This is because it becomes increasingly difficult to remove 3d electrons. On the other hand, the stability of the higher oxidation states decreases with atomic number from left to right across the series.
• Explanation Based on Half-Filled Sub-shells: The greater stability of the +2 oxidation state of manganese compared to its +3 oxidation state, and the +3 oxidation state of iron relative to its +2 oxidation state, can be explained on the basis of the stability of the half-filled 3d sub-shell.
• Manganese (Mn): Mn^{2+} is [Ar]3d⁵, which is a stable half-filled configuration. Mn^{3+} would be [Ar]3d⁴, which is less stable. Therefore, Mn^{2+} is more stable than Mn^{3+}.
• Iron (Fe): Fe^{3+} is [Ar]3d⁵, which is a stable half-filled configuration. Fe^{2+} is [Ar]3d⁶. Therefore, Fe^{3+} is more stable than Fe^{2+} in many compounds.

The textbook provides a table showing the oxidation states of the first-row transition metals, illustrating the variety of states that each element can adopt.

2. Catalytic Activity

Transition elements or their compounds can be used as catalysts in various chemical reactions. They show two unique properties responsible for their catalytic activity:

• They can exist in more than one stable oxidation state.
• They have vacant d orbitals capable of forming temporary dative bonds with ligands.

• Variable Oxidation States: First-row transition metals have a unique property to change between different oxidation states, like Fe^{2+} and Fe^{3+} or Cu^+ to Cu^{2+}. They temporarily change their oxidation states by gaining or losing electrons during chemical reactions and help reactants convert to products. At the end of the reaction, they regain their original oxidation state and become ready to play their role as catalysts again.
• Vacant d Orbitals: These metals possess incomplete 3d orbitals in their valence shells. These vacant orbitals can accept electron pairs from ligands (molecules or ions) forming temporary dative bonds between the transition metals and ligands. These dative bonds are responsible for holding reactants on the surface of the transition metal and bringing them closer. Resultantly, the bonds within reactant particles weaken and ultimately break. The product formation takes place on the surface of the transition metal. In this way, transition metals speed up the reaction. After completion of the reaction, the transition metal releases the products and becomes ready to bind to new reactants, continuing its role as a catalyst.

Examples of Catalysis:

• Heterogeneous Catalysis:
• Haber process: Iron is used as a catalyst which adsorbs hydrogen and nitrogen gas on its surface and makes them react faster.
• Contact process: Vanadium pentoxide (V_2O_5) is used as a catalyst involving a reversible change in oxidation state of vanadium from +5 to +4.
• Both of these examples involve reactants and catalysts with different physical states, so they are heterogeneous catalysis.

- Homogeneous Catalysis: - Oxidation of iodide ion by peroxodisulphate(VI) ion: 2I^-_{(aq)} + S_2O_8^{2-}_{(aq)} → 2SO_4^{2-}_{(aq)} + I_{2(aq)} The reaction is slow because the negatively charged ions of reactants repel each other. It can be catalyzed by the addition of d-block metal ions like Fe^{2+}_{(aq)} which change its oxidation states reversibly between +2 and +3: 2Fe^{2+}_{(aq)} + S_2O_8^{2-}_{(aq)} → 2Fe^{3+}_{(aq)} + 2SO_4^{2-}_{(aq)} 2Fe^{3+}_{(aq)} + 2I^-_{(aq)} → 2Fe^{2+}_{(aq)} + I_{2(aq)} Both steps involve a reaction between two oppositely charged ions, making the reactions faster than the uncatalyzed case.

Intermediate Level

The chemical properties of transition metals are defined by their electronic structure. Their variable oxidation states arise from the similar energies of the 4s and 3d orbitals, allowing different numbers of electrons to be lost. The stability of these oxidation states depends on the electronic configuration, with half-filled and fully-filled d subshells being particularly stable. For example, Mn^{2+} (3d⁵) and Fe^{3+} (3d⁵) are stable due to their half-filled d subshells. Their catalytic activity is due to two main factors: the ability to easily change oxidation states (which allows them to temporarily accept or donate electrons) and the presence of vacant d orbitals (which can form dative bonds with reactants, holding them in place and weakening their bonds). This makes them effective catalysts in both heterogeneous (e.g., Haber process) and homogeneous (e.g., oxidation of iodide) reactions.

Important Points

• - Variable Oxidation States: Common +2; maximum +7 (Mn). Stability depends on electron configuration (half-filled/fully-filled d subshells are stable).
• - Stability Trends: +2 vs +3 stability changes across the period.
• - Catalytic Activity: Due to variable oxidation states and vacant d orbitals.
• - Two Key Properties: Ability to change oxidation states; ability to form dative bonds with ligands.
• - Heterogeneous Catalysis: Reactants and catalyst in different phases (e.g., Haber process, Contact process).
• - Homogeneous Catalysis: Reactants and catalyst in the same phase (e.g., oxidation of iodide by peroxodisulfate).

Remember

• - Variable oxidation states: Common +2, Mn to +7.
• - Half-filled (d⁵) and full (d¹⁰) are stable.
• - Catalytic activity: Due to variable oxidation states and vacant d orbitals.
• - Haber process: Iron catalyst (heterogeneous).
• - Contact process: V_2O_5 catalyst (heterogeneous).
• - Homogeneous catalysis: Fe^{2+} in oxidation of iodide.
• - Catalysts speed up reactions and are regenerated.

Common Mistakes

• - Thinking that all transition metals have the same stable oxidation state. The stable oxidation state varies with the element and its electronic configuration.
• - Forgetting the role of vacant d orbitals in catalysis. Both variable oxidation states and vacant d orbitals are important.
• - Confusing homogeneous and heterogeneous catalysis. Homogeneous means same phase; heterogeneous means different phases.
• - Assuming that the maximum oxidation state is always +2. The maximum stable state can be higher (e.g., +7 for Mn).
• - Not recognizing the stability of half-filled and fully-filled d subshells. This is a key factor in determining the stability of different oxidation states.

Quick Revision

The chemical properties of transition metals are characterized by their variable oxidation states and their catalytic activity. The variable oxidation states arise from the similar energies of the 4s and 3d orbitals and the stability of half-filled and fully-filled d subshells. For example, manganese can have oxidation states up to +7. The catalytic activity is due to their ability to change oxidation states and form dative bonds with reactants using vacant d orbitals. Iron is used as a catalyst in the Haber process (heterogeneous), and Fe^{2+} ions catalyze the oxidation of iodide by peroxodisulfate in a homogeneous reaction.

60-Second Revision

Transition metals have variable oxidation states because of the close energies of 4s and 3d orbitals. Common state is +2, but Mn can reach +7. Stability depends on half-filled (d⁵) and full (d¹⁰) subshells. They are also catalysts because they can change oxidation states and form dative bonds with reactants using vacant d orbitals. Examples include Fe in the Haber process and V_2O_5 in the Contact process.

Frequently Asked Questions (FAQs)

Q1. Explain why transition elements form complex ions in terms of vacant d-orbitals that are energetically accessible. Answer: Transition elements form complex ions because their cations have vacant d orbitals that are energetically accessible. These vacant orbitals can accept lone pairs of electrons from ligands, forming coordinate covalent bonds. The small size and high charge density of the transition metal ions also allow them to strongly attract ligands, making the formation of complexes favorable.

Q2. Why do transition metals show variable oxidation states? Answer: Transition metals show variable oxidation states because the energies of the 4s and 3d orbitals are very close. This means that different numbers of electrons can be lost from these orbitals, leading to a variety of oxidation states. For example, manganese can show oxidation states from +2 to +7. The stability of these states depends on the electron configuration, with half-filled and fully filled d subshells being particularly stable.

• Variable oxidation states: Transition metals can easily change between different oxidation states, allowing them to temporarily accept or donate electrons during a reaction.
• Vacant d orbitals: They have vacant d orbitals that can form temporary dative bonds with reactant molecules, holding them in place and weakening their bonds, making the reaction easier.

• Heterogeneous: The Haber process for the synthesis of ammonia uses solid iron as a catalyst, with gaseous nitrogen and hydrogen as reactants.
• Homogeneous: The oxidation of iodide ions by peroxodisulfate ions is catalyzed by Fe^{2+} ions in aqueous solution.

Q5. Explain the stability of the +2 oxidation state of manganese compared to its +3 oxidation state. Answer: The +2 oxidation state of manganese (Mn^{2+}) has the electron configuration [Ar]3d⁵, which is a stable half-filled d subshell. The +3 oxidation state (Mn^{3+}) has the configuration [Ar]3d⁴, which is less stable. Therefore, Mn^{2+} is more stable than Mn^{3+} in many compounds.

Q6. Explain the stability of the +3 oxidation state of iron relative to its +2 oxidation state. Answer: The +3 oxidation state of iron (Fe^{3+}) has the electron configuration [Ar]3d⁵, which is a stable half-filled d subshell. The +2 oxidation state (Fe^{2+}) has the configuration [Ar]3d⁶. Because the half-filled 3d⁵ configuration is more stable, Fe^{3+} is more stable than Fe^{2+} in many compounds.

Q7. What is the role of the catalyst in the Contact process? Answer: In the Contact process, vanadium pentoxide (V_2O_5) is used as a catalyst. It involves a reversible change in the oxidation state of vanadium from +5 to +4. The catalyst provides an alternative reaction pathway with a lower activation energy, increasing the rate of the reaction.

Q8. How does a transition metal catalyst work in a homogeneous reaction? Answer: In a homogeneous reaction, the transition metal ion acts as a catalyst in solution. It often forms an intermediate complex with the reactants, changing its oxidation state temporarily. This provides an alternative pathway with a lower activation energy. After the reaction, the catalyst is regenerated in its original oxidation state.

Q9. What is the maximum oxidation state of manganese and why? Answer: The maximum oxidation state of manganese is +7. Manganese has the electron configuration [Ar]3d⁵4s², giving it a total of 7 valence electrons (5 in 3d and 2 in 4s). All of these can be lost or shared in bonding, so the maximum oxidation state is +7.

Multiple Choice Questions (MCQs)

• Q1. What is the common oxidation state for the first-row d-block elements?
• A. +1
• B. +2
• C. +3
• D. +4
• Correct Answer: B
• Explanation: The common oxidation state for the first-row d-block elements is +2, obtained by the loss of the two 4s electrons.

Q2. Which of the following is a property responsible for the catalytic activity of transition metals? A. They form coloured complexes. B. They have variable oxidation states. C. They have high density. D. They have high melting points. Correct Answer: B Explanation: Variable oxidation states allow transition metals to temporarily accept or donate electrons during a reaction, facilitating catalysis. Vacant d orbitals are also important.

• Q3. Which process uses iron as a catalyst?
• A. Contact process
• B. Haber process
• C. Ostwald process
• D. Solvay process
• Correct Answer: B
• Explanation: The Haber process for the synthesis of ammonia uses iron as a catalyst.

• Q4. What is the maximum oxidation state of manganese?
• A. +2
• B. +4
• C. +7
• D. +6
• Correct Answer: C
• Explanation: Manganese has the electron configuration [Ar]3d⁵4s², giving it 7 valence electrons. It can show a maximum oxidation state of +7.

• Q5. Why is the +2 oxidation state of manganese more stable than its +3 oxidation state?
• A. Because Mn^{2+} has a half-filled d subshell.
• B. Because Mn^{3+} has a half-filled d subshell.
• C. Because Mn^{2+} has a full d subshell.
• D. Because Mn^{3+} has a full d subshell.
• Correct Answer: A
• Explanation: Mn^{2+} has the configuration [Ar]3d⁵, which is a stable half-filled d subshell.

• Q6. Why is the +3 oxidation state of iron more stable than its +2 oxidation state?
• A. Because Fe^{3+} has a half-filled d subshell.
• B. Because Fe^{2+} has a half-filled d subshell.
• C. Because Fe^{3+} has a full d subshell.
• D. Because Fe^{2+} has a full d subshell.
• Correct Answer: A
• Explanation: Fe^{3+} has the configuration [Ar]3d⁵, which is a stable half-filled d subshell.

• Q7. What is the role of the catalyst in the Contact process?
• A. Iron
• B. Vanadium pentoxide
• C. Nickel
• D. Platinum
• Correct Answer: B
• Explanation: The Contact process uses vanadium pentoxide (V_2O_5) as a catalyst.

Q8. Which of the following is a homogeneous catalysis reaction? A. Haber process B. Contact process C. Oxidation of iodide by peroxodisulfate D. Hydrogenation of ethene Correct Answer: C Explanation: The oxidation of iodide by peroxodisulfate is catalyzed by Fe^{2+} ions in aqueous solution. Both the catalyst and reactants are in the same phase.

• Q9. What is the reason for the variable oxidation states of transition metals?
• A. The similar energies of the 4s and 3d orbitals.
• B. The high density of transition metals.
• C. The high melting points of transition metals.
• D. The presence of unpaired electrons.
• Correct Answer: A
• Explanation: The similar energies of the 4s and 3d orbitals allow different numbers of electrons to be lost, leading to variable oxidation states.

• Q10. Which of the following is NOT a property responsible for the catalytic activity of transition metals?
• A. Variable oxidation states
• B. Vacant d orbitals
• C. High density
• D. Ability to form dative bonds
• Correct Answer: C
• Explanation: High density is not a property responsible for catalytic activity. The key properties are variable oxidation states and vacant d orbitals.

Multiple Choice Questions (MCQs)

• Q1. What is the common oxidation state for the first-row d-block elements?
• A. +1
• B. +2
• C. +3
• D. +4
• Correct Answer: B
• Explanation: The common oxidation state for the first-row d-block elements is +2.

• Q2. Which of the following is a property responsible for the catalytic activity of transition metals?
• A. They form coloured complexes.
• B. They have variable oxidation states.
• C. They have high density.
• D. They have high melting points.
• Correct Answer: B
• Explanation: Variable oxidation states allow transition metals to temporarily accept or donate electrons during a reaction.

• Q3. Which process uses iron as a catalyst?
• A. Contact process
• B. Haber process
• C. Ostwald process
• D. Solvay process
• Correct Answer: B
• Explanation: The Haber process for the synthesis of ammonia uses iron as a catalyst.

• Q4. What is the maximum oxidation state of manganese?
• A. +2
• B. +4
• C. +7
• D. +6
• Correct Answer: C
• Explanation: Manganese has 7 valence electrons and can show a maximum oxidation state of +7.

• Q5. Why is the +2 oxidation state of manganese more stable than its +3 oxidation state?
• A. Because Mn^{2+} has a half-filled d subshell.
• B. Because Mn^{3+} has a half-filled d subshell.
• C. Because Mn^{2+} has a full d subshell.
• D. Because Mn^{3+} has a full d subshell.
• Correct Answer: A
• Explanation: Mn^{2+} has the configuration [Ar]3d⁵, which is a stable half-filled d subshell.

• Q6. Why is the +3 oxidation state of iron more stable than its +2 oxidation state?
• A. Because Fe^{3+} has a half-filled d subshell.
• B. Because Fe^{2+} has a half-filled d subshell.
• C. Because Fe^{3+} has a full d subshell.
• D. Because Fe^{2+} has a full d subshell.
• Correct Answer: A
• Explanation: Fe^{3+} has the configuration [Ar]3d⁵, which is a stable half-filled d subshell.

• Q7. What is the catalyst used in the Contact process?
• A. Iron
• B. Vanadium pentoxide
• C. Nickel
• D. Platinum
• Correct Answer: B
• Explanation: The Contact process uses vanadium pentoxide (V_2O_5) as a catalyst.

• Q8. Which of the following is a homogeneous catalysis reaction?
• A. Haber process
• B. Contact process
• C. Oxidation of iodide by peroxodisulfate
• D. Hydrogenation of ethene
• Correct Answer: C
• Explanation: The oxidation of iodide by peroxodisulfate is catalyzed by Fe^{2+} ions in aqueous solution.

• Q9. What is the reason for the variable oxidation states of transition metals?
• A. The similar energies of the 4s and 3d orbitals.
• B. The high density of transition metals.
• C. The high melting points of transition metals.
• D. The presence of unpaired electrons.
• Correct Answer: A
• Explanation: The similar energies of the 4s and 3d orbitals allow different numbers of electrons to be lost.

• Q10. Which of the following is NOT a property responsible for the catalytic activity of transition metals?
• A. Variable oxidation states
• B. Vacant d orbitals
• C. High density
• D. Ability to form dative bonds
• Correct Answer: C
• Explanation: High density is not a property responsible for catalytic activity.