Covalent, dative & bond strength
Covalent, dative & bond strength
Covalent bonds and dot-and-cross diagrams
🔓 CRACK IT
In a covalent bond, two atoms share a pair of electrons. The shared electrons are attracted to both positive nuclei, holding the atoms together.
A single bond contains one shared pair, a double bond two and a triple bond three. In a dot-and-cross diagram, put every shared pair in the overlap between the atoms.
A lone pair is not shared in a bond. Show these electrons outside the overlap, and use dots and crosses consistently to show their atom of origin. Count all the outer-shell electrons before finishing.
- Covalent bond
- The strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms.
| Bond | Shared pairs | Line model |
|---|---|---|
| Single | 1 | H–H |
| Double | 2 | O=O |
| Triple | 3 | N≡N |
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
(a) State the precise OCR A definition of a covalent bond. [3 marks]
(b) State the number of shared electron pairs in H2, O2 and N2. [2 marks]
Total: 5 marks
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(a) A covalent bond is the strong electrostatic attraction [1] between a shared pair of electrons [1] and the nuclei of the bonded atoms [1]. [3]
(b) H2 has one shared pair; O2 has two; N2 has three [2]. [2]
Draw complete dot-and-cross diagrams for HCl, O2 and N2. Show all shared pairs and all lone pairs. [7 marks]
Total: 7 marks
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HCl: one shared dot-and-cross pair and three lone pairs on Cl [2]. O2: two shared pairs and two lone pairs on each O [2]. N2: three shared pairs and one lone pair on each N [3]. [7]
Carbon dioxide, CO2, contains two carbon-oxygen double bonds.
(a) State the total number of shared electron pairs in one CO2 molecule. [1 mark]
(b) Draw a complete dot-and-cross diagram for CO2. Show electron origin and all lone pairs. [6 marks]
Total: 7 marks
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(a) Two C=O double bonds contain four shared electron pairs in total [1]. [1]
(b) Carbon is central with two shared pairs to each oxygen [2]; electron origin is shown consistently in all four shared pairs [2]; each oxygen has two lone pairs and carbon has no lone pairs [2]. [6]
Dative bonds: donor, acceptor and arrow
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A covalent bond contains a shared pair of electrons. Usually, each atom supplies one electron. In a dative covalent bond, one atom supplies both.
The atom supplying the pair is the donor. The species receiving it is the acceptor. Draw the arrow from the donor's lone pair to the acceptor.
For NH₃ reacting with H⁺, nitrogen supplies its lone pair to the hydrogen ion. Once NH₄⁺ has formed, all four N-H bonds are equivalent. The origin of the electrons does not make one of those bonds different afterwards.
- Dative covalent bond
- A shared pair of electrons in which both electrons were supplied by the same atom.
| Role | NH3 + H+ → NH4+ |
|---|---|
| Donor | Nitrogen in NH3 supplies its lone pair. |
| Acceptor | H+ accepts the pair. |
| After formation | All four N–H bonds are equivalent. |
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
(a) State what is meant by a dative covalent bond. [2 marks]
(b) For NH3 + H+ → NH4+, name the electron-pair donor and acceptor and state the arrow direction. [2 marks]
(c) Draw the ammonium ion, showing the origin of the electron pair in the newly formed bond, square brackets and overall charge. [2 marks]
Total: 6 marks
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(a) A dative covalent bond is a shared pair of electrons [1] in which both electrons were supplied by the same atom [1]. [2]
(b) NH3 is the electron-pair donor and H+ is the acceptor [1]; the arrow begins at the N lone pair and points towards H+ [1]. [2]
(c) NH4+ is enclosed in square brackets with an overall + charge [1]; the new N-H shared pair contains two electrons originating from N [1]. [2]
A water molecule reacts with H+ to form H3O+.
(a) Identify the electron-pair donor and acceptor and state the direction of the dative-bond arrow. [3 marks]
(b) Draw the H3O+ ion, showing the origin of the electron pair in the dative bond, brackets and charge. [3 marks]
Total: 6 marks
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(a) H2O is the donor [1]; H+ is the acceptor [1]; the arrow starts at an oxygen lone pair and points to H+ [1]. [3]
(b) H3O+ is bracketed and has an overall + charge [1]; the newly formed O-H shared pair contains two electrons from O [1]; the remaining outer electrons are complete and consistently shown [1]. [3]
A student writes calcium nitrate as CaNO3 and ammonium sulfate as NH4SO4.
- Correct both formulae and explain the charge balance in each compound. [4]
The same student draws the following diagram for NH3 + H+ → NH4+.
(b) Identify two errors in the diagram. [2]
(c) Draw a corrected model diagram. [2]
Total: 8 marks
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(a) Calcium nitrate is Ca(NO3)2: one Ca2+ requires two NO3- ions [2]. Ammonium sulfate is (NH4)2SO4: two NH4+ ions balance one SO42- ion [2]. [4]
(b) The arrow is reversed; it must begin at the nitrogen lone pair and point to H+ [1]. The product must be enclosed in square brackets with an overall + charge [1]. [2]
(c) Correct model: NH3 donates its lone pair to H+ and forms bracketed NH4+ [2]. [2]
Average bond enthalpy and bond strength
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Bond strength tells you how hard it is to separate the bonded atoms. A larger average bond enthalpy means more energy is needed to break the bond, so the bond is stronger.
Use the values supplied to make a comparison. Do not confuse the strength of a covalent bond inside a molecule with the attractions between separate molecules.
At this stage, use the comparison. Formal definitions and bond-enthalpy calculations are developed in Energetics.
Compare two bonds
Bond A has average bond enthalpy 347 kJ mol-1; bond B has 612 kJ mol-1.
State which bond is stronger and explain your choice using the bond enthalpies.
Worked answer
612 is the larger value, so more energy is required to break a mole of B bonds.
🧪 TEST IT
Work on paper. Attempt each question before opening its answer. These are original CrackingChem questions, written in OCR A style.
(a) State the precise definition of metallic bonding. [2 marks]
(b) Bond X has an average bond enthalpy of 436 kJ mol-1 and bond Y has 598 kJ mol-1. Identify the stronger bond and justify your choice. [1 mark]
Total: 3 marks
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(a) Metallic bonding is the strong electrostatic attraction [1] between positive metal ions and delocalised electrons [1]. [2]
(b) Bond Y is stronger because it has the larger average bond enthalpy, so more energy is needed to break it [1]. [1]
Average bond enthalpy data are shown: C–C 347 kJ mol-1; H–H 436 kJ mol-1; C=C 612 kJ mol-1; N≡N 945 kJ mol-1.
(a) Identify the strongest bond and explain your choice. [2 marks]
(b) Use the data to compare the strengths of a C–C bond and a C=C bond. [2 marks]
Total: 4 marks
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(a) N≡N is strongest because 945 kJ mol-1 is the largest average bond enthalpy [1], so it requires the most energy to break [1]. [2]
(b) C=C is stronger than C-C [1] because 612 kJ mol-1 is greater than 347 kJ mol-1 [1]. [2]