In A-Level Chemistry, understanding chemical bonding is crucial for mastering various concepts in organic, inorganic, and physical chemistry. One important type of bond is the dative covalent bond, also known as a coordinate bond. This bond plays a vital role in many chemical reactions and molecular structures.
This topic will explain dative covalent bonding, provide examples, and offer A-Level Chemistry questions with detailed explanations to help students strengthen their understanding.
What is a Dative Covalent Bond?
Definition
A dative covalent bond is a type of covalent bond in which both electrons in the shared pair come from the same atom. Unlike a regular covalent bond, where each atom donates one electron, in a dative bond, one atom provides the entire electron pair while the other atom accepts it.
Key Characteristics
- One atom acts as the electron donor (lone pair provider).
- The other atom acts as the electron acceptor.
- The resulting bond behaves like a normal covalent bond once formed.
Representation
Dative covalent bonds are often represented using an arrow (→) pointing from the donor atom to the acceptor atom in chemical diagrams.
Examples of Dative Covalent Bonds
1. Ammonium Ion (NH₄⁺)
When ammonia (NH₃) reacts with a proton (H⁺), the nitrogen atom donates a lone pair to form a dative covalent bond:
text{NH₃} + text{H⁺} → text{NH₄⁺}
- Nitrogen (N) donates a lone pair.
- Hydrogen ion (H⁺) accepts the electrons.
- The bond between N and H⁺ is a dative covalent bond.
2. Aluminium Chloride Dimer (Al₂Cl₆)
Aluminium chloride exists as a dimer (Al₂Cl₆) rather than a simple AlCl₃ molecule because aluminium in AlCl₃ lacks a full octet.
text{AlCl₃} + text{AlCl₃} → text{Al₂Cl₆}
- Chlorine (Cl) donates a lone pair.
- Aluminium (Al) accepts the electrons to complete its octet.
- This forms a dative bond between Al and Cl.
3. Carbon Monoxide (CO)
In CO, oxygen (O) donates a lone pair to carbon © to form a triple bond, with one of them being dative.
text{C} equiv text{O}
- Oxygen donates a lone pair to carbon.
- The molecule remains neutral but has a strong bond due to the dative interaction.
A-Level Chemistry Questions on Dative Covalent Bonds
Question 1: Identifying Dative Covalent Bonds
Which of the following species contains a dative covalent bond?
A) NH₃
B) H₂O
C) NH₄⁺
D) CH₄
Answer: C) NH₄⁺
- NH₄⁺ contains a dative bond where nitrogen donates a lone pair to H⁺.
- NH₃, H₂O, and CH₄ contain only regular covalent bonds.
Question 2: Explaining Formation of Dative Bonds
Explain why aluminium chloride (AlCl₃) forms Al₂Cl₆ in the solid state instead of existing as simple AlCl₃ molecules.
Answer:
- Aluminium in AlCl₃ has only six valence electrons, making it electron-deficient.
- To achieve stability, two AlCl₃ molecules pair up, forming dative covalent bonds between aluminium and chlorine atoms.
- This creates the dimer Al₂Cl₆, stabilizing the structure.
Question 3: Drawing Lewis Structures
Draw the Lewis structure of the ammonium ion (NH₄⁺) and indicate the dative covalent bond.
Answer:
- Nitrogen in NH₃ has a lone pair that it donates to an H⁺ ion.
- The structure of NH₄⁺ is:
H
|
H—N→H
|
H
- The arrow (→) represents the dative covalent bond.
Question 4: Comparing Bond Strength
Is a dative covalent bond stronger, weaker, or the same as a normal covalent bond? Explain.
Answer:
- A dative covalent bond is chemically identical to a normal covalent bond once formed.
- The only difference is how the bond originates (both electrons from one atom).
- Therefore, in terms of strength, a dative covalent bond is the same as a normal covalent bond.
Question 5: Application in Complex Ions
Which of the following complex ions contains a dative covalent bond?
A) Cl₂
B) H₂O
C) [Cu(NH₃)₄]²⁺
D) O₂
Answer: C) [Cu(NH₃)₄]²⁺
- In this complex ion, each NH₃ molecule donates a lone pair to Cu²⁺, forming four dative covalent bonds.
- This is common in transition metal complexes.
Summary of Key Concepts
| Concept | Explanation |
|---|---|
| Definition | Dative covalent bond occurs when both electrons in a bond come from the same atom. |
| Notation | Represented with an arrow (→) from the donor to the acceptor. |
| Examples | NH₄⁺, Al₂Cl₆, CO, and transition metal complexes. |
| Strength | Once formed, it is equivalent to a normal covalent bond. |
| Importance in Chemistry | Seen in acids, bases, coordination compounds, and molecular structures. |
Dative covalent bonds are an important part of A-Level Chemistry, appearing in topics such as acid-base reactions, coordination chemistry, and molecular bonding. Understanding their formation, representation, and role in chemical compounds helps students apply concepts in exams and real-world chemistry.
By practicing A-Level Chemistry questions on dative covalent bonds, students can strengthen their understanding and improve their problem-solving skills.