S2.2 The covalent model
Practice exam-style IB Chemistry questions for The covalent model, aligned with the syllabus and grouped by topic.
Question 1
What is the best description of a covalent bond?
A.
Electrostatic attraction between oppositely charged ions in a lattice
B.
Electrostatic attraction between a shared electron pair and two positively charged nuclei
C.
Attraction between temporary dipoles in neighbouring molecules
D.
Transfer of valence electrons from a metal atom to a non-metal atom
Question 2
In the H–Cl bond, chlorine is more electronegative than hydrogen. What is the correct bond polarity?
A.
Hδ−–Clδ+
B.
No bond dipole because both atoms are non-metals
C.
H⁺Cl⁻ with complete electron transfer
D.
Hδ+–Clδ−
Question 3
How many lone pairs are present on the central nitrogen atom in the Lewis formula of NH₄⁺?
A.
1
B.
0
C.
4
D.
2
Question 4
For bonds between the same two elements, what happens as bond order increases from single to triple?
A.
Bond length increases and bond enthalpy decreases
B.
Bond length and bond enthalpy both decrease
C.
Bond length decreases and bond enthalpy increases
D.
Bond length and bond enthalpy both increase
Question 5
What intermolecular force can occur between ethanol molecules but not between ethoxyethane molecules?
A.
Dipole-dipole forces
B.
Hydrogen bonding
C.
London dispersion forces
D.
Dipole-induced dipole forces
Question 6
How many σ and π bonds are present in H₂C=CHCl?
A.
4 σ and 2 π
B.
3 σ and 3 π
C.
6 σ and 0 π
D.
5 σ and 1 π
Question 7
In the adduct F₃B←NH₃, what is the electron-pair donor?
A.
The boron atom in BF₃
B.
The hydrogen atoms in NH₃
C.
A fluorine atom in BF₃
D.
The nitrogen atom in NH₃
Question 8
What are the electron domain geometry and molecular geometry around sulfur in SO₂?
A.
Trigonal planar electron domain geometry; trigonal planar molecular geometry
B.
Trigonal planar electron domain geometry; bent molecular geometry
C.
Tetrahedral electron domain geometry; bent molecular geometry
D.
Linear electron domain geometry; linear molecular geometry
Question 9
Which molecule is polar overall?
A.
NH₃
B.
BF₃
C.
CO₂
D.
CH₄
Question 10
What does resonance in ozone, O₃, explain?
A.
One O–O bond is always single and the other is always double
B.
The two O–O bonds are equal and intermediate between single and double bonds
C.
Ozone contains three identical O=O double bonds
D.
Ozone has no π bonding because all electrons are localized
Question 11
Which observation provides physical evidence for delocalization in benzene?
A.
Benzene reacts with bromine by rapid addition in the dark
B.
Benzene is a gas at room temperature
C.
All six C–C bond lengths in benzene are equal
D.
Benzene contains only C–H single bonds
Question 12
What is the hybridization of each carbon atom in ethyne, C₂H₂?
A.
dsp³
B.
sp²
C.
sp
D.
sp³
Question 13
Why are the two C–O bonds in the ethanoate ion, CH₃COO⁻, equal in length?
A.
The carbon atom has sp³ hybridization in the carboxylate group
B.
Both oxygen atoms form only single bonds to carbon
C.
π electrons are delocalized over the O–C–O group
D.
The negative charge is localized on one oxygen atom
Question 14
Oxygen difluoride, OF₂, is a covalent molecule.
State the total number of valence electrons in OF₂.
[1]
Deduce the number of lone pairs on the central oxygen atom.
[1]
State why hydrogen is not normally placed as a central atom in Lewis formulas.
[1]
Question 15
BF₃ and NF₃ both contain polar bonds.
State the molecular geometry of BF₃.
[1]
State the molecular geometry of NF₃.
[1]
Explain why BF₃ is non-polar but NF₃ is polar.
[1]
Question 16
A paper chromatogram is shown for a dye mixture. The solvent front travelled 8.0 cm from the baseline. The centre of spot X travelled 5.2 cm.
Calculate the R_F value of spot X.
[1]
State one condition that must be kept the same when comparing this R_F value with a reference value.
[1]
Explain why the baseline should be drawn in pencil rather than ink.
[1]
Question 17
Ammonia reacts with a hydrogen ion to form ammonium, NH₄⁺.
Identify the Lewis base in this reaction.
[1]
State what is donated by the Lewis base.
[1]
Explain why all four N–H bonds in NH₄⁺ are considered equivalent after formation.
[1]
Question 18
A molecule has the structural formula CH₃C≡N.
State the number of σ bonds in the molecule.
[1]
State the number of π bonds in the molecule.
[1]
Explain why the C≡N bond contains two π bonds.
[1]
Question 19
The visual shows average bond lengths and bond enthalpies for C–C, C=C and C≡C bonds.
| Bond | Bond order | Average length / pm | Average enthalpy / kJ mol⁻¹ |
|---|---|---|---|
| C–C | 1 | 154 | 348 |
| C=C | 2 | 134 | 614 |
| C≡C | 3 | 120 | 839 |
Identify the carbon-carbon bond with the greatest bond enthalpy.
[1]
Describe the relationship between bond order and bond length shown by the data.
[1]
Explain the relationship between bond order and bond enthalpy for these bonds.
[1]
Suggest why comparing C–C and C–O bond enthalpies would be a less direct test of the same trend.
[1]
Question 20
The visual shows a thin-layer chromatogram for a mixture and two pure reference dyes run under identical conditions.
Calculate the R_F value for the spot in the mixture that travelled 3.6 cm when the solvent front travelled 6.0 cm.
[1]
Identify which reference dye matches this spot.
[1]
Explain why the sample spot must be above the solvent level at the start.
[1]
second run uses a more polar mobile phase with the same polar stationary phase. Suggest how the R_F values of polar dyes may change.
[1]
State why R_F values from different solvent systems should not be compared directly.
[1]
Question 21
What is the molecular geometry of SF₄ according to VSEPR?
A.
Square planar
B.
Seesaw
C.
Trigonal bipyramidal
D.
Tetrahedral
Question 22
In a Lewis formula of CO with a triple bond and one lone pair on each atom, what are the formal charges on C and O respectively?
A.
C −1, O +1
B.
C −2, O +2
C.
C 0, O 0
D.
C +1, O −1
Question 23
What are the electron domain geometry and molecular geometry of XeF₄?
A.
Trigonal bipyramidal; seesaw
B.
Square planar; octahedral
C.
Tetrahedral; square planar
D.
Octahedral; square planar
Question 24
Methanal, H₂CO, has the structural formula H₂C=O.
State the number of electron domains around the carbon atom.
[1]
Deduce the electron domain geometry around carbon.
[1]
Explain why the H–C–H bond angle is slightly less than 120°.
[1]
Question 25
Diamond and graphite are allotropes of carbon.
Define the term allotrope.
[1]
Explain why diamond is very hard.
[1]
Explain why graphite conducts electricity along its layers.
[1]
Explain why graphite can be used as a lubricant.
[1]
Question 26
Consider propane, CH₃CH₂CH₃, propanone, CH₃COCH₃, and propan-1-ol, CH₃CH₂CH₂OH.
State the intermolecular force present in all three substances.
[1]
Identify the substance that can form hydrogen bonds between its own molecules.
[1]
Explain why propanone has stronger intermolecular attractions than propane of similar size.
[1]
Predict which of the three has the highest boiling point.
[1]
Question 27
Iodine pentafluoride, IF₅, contains an expanded octet around iodine.
State the number of electron domains around iodine.
[1]
Deduce the electron domain geometry.
[1]
Deduce the molecular geometry.
[1]
State the approximate F–I–F angle between adjacent basal bonds.
[1]
Question 28
O₂ and O₃ absorb different wavelengths of ultraviolet radiation in the atmosphere.
State the bond order of the O–O bonds in ozone.
[1]
Compare this with the bond order in O₂.
[1]
Suggest why different wavelengths are absorbed.
[1]
Question 29
The complex ion [Cu(NH₃)₄]²⁺ contains four ammonia ligands bonded to Cu²⁺.
Define ligand.
[1]
Identify the atom in NH₃ that donates an electron pair to Cu²⁺.
[1]
State the type of bond formed between NH₃ and Cu²⁺.
[1]
Question 30
The visual gives measured bond angles for CH₄, NH₃ and H₂O.
| Molecule | Central atom | Bonding domains | Lone pairs | Bond angle / ° |
|---|---|---|---|---|
| CH₄ | C | 4 | 0 | 109.5 |
| NH₃ | N | 3 | 1 | 107.0 |
| H₂O | O | 2 | 2 | 104.5 |
State the electron domain geometry common to the central atom in all three molecules.
[1]
Identify the molecule with the smallest bond angle.
[1]
Explain the decrease in bond angle from CH₄ to NH₃ to H₂O.
[1]
Suggest one limitation of using VSEPR to interpret the numerical bond angles.
[1]
Question 31
The visual shows test results for three unknown white solids A, B and C: melting behaviour, solubility in water, and conductivity of the aqueous solution when soluble. The possible solids are sucrose, sodium chloride and silicon dioxide.
| Solid | Gentle heating | Solubility in water | Solution conductivity |
|---|---|---|---|
| A | No visible melting | Insoluble | Not tested |
| B | Softens and browns | Soluble | None detected |
| C | No visible melting | Soluble | Conducts well |
Identify the solid most likely to be silicon dioxide.
[1]
Identify the solid most likely to be sodium chloride.
[1]
Explain why a sodium chloride solution conducts but a sucrose solution does not.
[1]
State one safety precaution for the heating test.
[1]
Question 32
The visual shows boiling points and molar masses for several molecular substances, including alkanes and alcohols.
| Substance | Class | Molar mass / g mol⁻¹ | Boiling point / °C |
|---|---|---|---|
| Propane | Alkane | 44.1 | −42.1 |
| Butane | Alkane | 58.1 | −0.5 |
| Pentane | Alkane | 72.2 | 36.1 |
| Hexane | Alkane | 86.2 | 68.7 |
| Methanol | Alcohol | 32.0 | 64.7 |
| Propan-1-ol | Alcohol | 60.1 | 97.2 |
Describe the trend in boiling point down the alkane series shown.
[1]
Explain this trend in terms of London dispersion forces.
[1]
Identify the alcohol that has a higher boiling point than an alkane of similar molar mass.
[1]
Explain why molar mass alone is not sufficient to predict boiling point for all the substances shown.
[1]
Question 33
The visual compares typical single and double C–O bond lengths with measured C–O bond lengths in CO₃²⁻ and CH₃COO⁻.
| Species or bond | C–O bonds compared | Bond length / pm |
|---|---|---|
| Typical C–O single | 1 | 143 |
| Typical C=O double | 1 | 121 |
| CO₃²⁻ ion | 3 equal | 129 |
| CH₃COO⁻ ion | 2 equal | 127 |
State whether the C–O bonds in CO₃²⁻ are closer to single, double or intermediate bond lengths.
[1]
Deduce the bond order of each C–O bond in CO₃²⁻.
[1]
Explain why the two C–O bonds in CH₃COO⁻ are equal.
[1]
Suggest why a single Lewis formula with localized bonds is incomplete for these ions.
[1]
Question 34
The visual shows selected bond angles and structural formulas for ethane, ethene and ethyne.
| Molecule | Structural formula | C angle / ° | Domains around C |
|---|---|---|---|
| Ethane | H₃C–CH₃ | 109.5 | 4 |
| Ethene | H₂C=CH₂ | 120 | 3 |
| Ethyne | HC≡CH | 180 | 2 |
Deduce the hybridization of carbon in ethane.
[1]
Deduce the hybridization of carbon in ethene.
[1]
Deduce the hybridization of carbon in ethyne.
[1]
Explain how the number of electron domains relates to the hybridization in these three molecules.
[1]
Question 35
A white solid is either sucrose, silicon dioxide or sodium chloride. A student tests its properties.
State why a solid sample of sucrose does not conduct electricity.
[1]
State why silicon dioxide has a very high melting point.
[1]
Suggest one test result that would distinguish sodium chloride from sucrose after dissolving in water.
[1]
Suggest why sucrose is much more soluble in water than in hexane.
[1]
Question 36
The carbonate ion, CO₃²⁻, has three equivalent C–O bonds.
State the number of major resonance structures for CO₃²⁻.
[1]
State the bond order of each C–O bond in the resonance hybrid.
[1]
Explain why the three C–O bonds have the same length.
[1]
Question 37
Benzene has a lower enthalpy change of hydrogenation than expected for a hypothetical cyclohexa-1,3,5-triene with three isolated C=C bonds.
State what is meant by resonance energy.
[1]
State the structural feature of benzene responsible for this stabilization.
[1]
Explain why benzene tends to undergo substitution rather than addition reactions.
[1]
Question 38
One Lewis formula for N₂O is :N≡N–O: with one lone pair on the terminal N and three lone pairs on O.
Calculate the formal charge on the terminal nitrogen.
[1]
Calculate the formal charge on the central nitrogen.
[1]
Calculate the formal charge on oxygen.
[1]
State whether the sum of formal charges is consistent with neutral N₂O.
[1]
Question 39
Acetonitrile has the formula CH₃C≡N.
Deduce the hybridization of the carbon atom in the CH₃ group.
[1]
Deduce the hybridization of the carbon atom in the C≡N group.
[1]
State the approximate bond angle around the carbon atom in the C≡N group.
[1]
Explain why the C≡N carbon has this hybridization.
[1]
Question 40
The visual shows enthalpy changes of hydrogenation for cyclohexene and benzene, together with the value predicted for a molecule containing three isolated C=C bonds.
Use the data to state whether benzene is more or less stable than the hypothetical localized structure.
[1]
Explain how the data provide evidence for resonance energy.
[1]
State one physical item of evidence, other than hydrogenation data, that supports delocalization in benzene.
[1]
Suggest why benzene resists addition reactions compared with alkenes.
[1]
Question 41
The visual gives Lewis-domain information for PF₅, ClF₃, BrF₅ and XeF₄.
| Species | Bonding domains / count | Lone pairs / count |
|---|---|---|
| PF₅ | 5 | 0 |
| ClF₃ | 3 | 2 |
| BrF₅ | 5 | 1 |
| XeF₄ | 4 | 2 |
Identify the species with trigonal bipyramidal molecular geometry.
[1]
Deduce the molecular geometry of ClF₃.
[1]
Explain why lone pairs occupy equatorial positions in a trigonal bipyramidal arrangement.
[1]
Identify the species with square planar molecular geometry.
[1]
Question 42
The visual shows three possible Lewis formulas for thiocyanate, SCN⁻, with calculated formal charges on S, C and N.
| Lewis formula | FC on S | FC on C | FC on N |
|---|---|---|---|
| S–C≡N | −1 | 0 | 0 |
| S=C=N | 0 | 0 | −1 |
| S≡C–N | +1 | 0 | −2 |
State the sum of formal charges for each valid structure.
[1]
Identify the preferred Lewis formula using the formal charge data.
[1]
Explain one criterion used to choose the preferred formula.
[1]
If a negative formal charge is unavoidable, state where it is usually preferred.
[1]
State one limitation of formal charge as a model.
[1]
Question 43
Carbon dioxide and water both contain polar covalent bonds.
Deduce the molecular geometry of CO₂ and H₂O.
[1]
Compare and contrast the polarity of CO₂ and H₂O, explaining your answer in terms of bond polarity and molecular geometry.
[1]
Question 44
Ethanol, ethoxyethane and pentane have similar molar masses but different physical properties.
Identify the strongest type of intermolecular force between molecules of each substance.
[1]
Explain the expected differences in boiling point and solubility in water for these substances.
[1]
Question 45
A student separates coloured components of a plant extract by paper chromatography using a non-polar solvent.
Define R_F and state the formula used to calculate it.
[1]
Evaluate how chromatography can be used to identify components in the extract, including limitations of using R_F values.
[1]
Question 46
Diamond, graphite and silicon dioxide are covalent network substances.
Describe the bonding and structure in diamond.
[1]
Discuss how bonding and structure explain the high melting point of silicon dioxide and the electrical conductivity and lubricating properties of graphite.
[1]
Question 47
Nitrate, NO₃⁻, can be represented by resonance structures.
Deduce the number of major resonance structures and the N–O bond order in the resonance hybrid.
[1]
Evaluate how formal charge and resonance are used together to select and interpret Lewis formulas for NO₃⁻.
[1]
Question 48
Benzene is often represented as a hexagon with a circle.
Describe the bonding and geometry around each carbon atom in benzene.
[1]
Discuss physical and chemical evidence for the delocalized model of benzene and relate it to benzene's relative unreactivity in addition reactions.
[1]
Question 49
Sulfur tetrafluoride, SF₄, and xenon tetrafluoride, XeF₄, both contain four bonds to fluorine but have different shapes.
Deduce the number of electron domains around the central atom in each species.
[1]
Compare and contrast the VSEPR shapes of SF₄ and XeF₄, explaining the positions of lone pairs and the effect on bond angles.
[1]
Question 50
Acrylonitrile has the structural formula CH₂=CH–C≡N.
Deduce the hybridization of each of the three carbon atoms, from left to right.
[1]
Analyse the σ and π bonding in acrylonitrile and explain how hybridization relates to the molecular geometry around the carbon atoms.
[1]
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