The Covalent Model (SL IB Chemistry)

For each of the molecules below, draw the Lewis (electron dot) structure and use the valence shell electron pair repulsion theory (VSEPR) to predict the shape of each molecule.

Oxygen difluoride (OF₂), phosphorus trifluoride, (PF₃) and boron trichloride, (BCl₃).

Crystalline ionic compounds do not conduct electricity. 

State and explain in which states ionic compounds conduct electricity.

The melting point of sodium chloride, NaCl, is 801° C. 

Explain, with reference to structure and bonding, why sodium chloride melts at such a high temperature.

We can use electronegativity values to deduce whether a compound is likely to be ionic or covalent.

Use Section 9 of the Data Booklet to state and explain whether each of the following compounds are ionic or covalent:

   ICl

   SrCl₂

   RbI

   HI

Diimide, N₂H₂, is a useful reagent in organic synthesis and can be made by the thermal decomposition of azodicarboxylic acid.

N(COOH)₂ (g) → N₂H₂ (g) + 2CO₂ (g) 

Another useful compound of nitrogen is hydrazine, N₂H₄. 

Draw Lewis (electron dot) structures for diimide and hydrazine.

Deduce the molecular geometry of diimide and estimate its H-N-N bond angle.

List, with an explanation, the three compounds in order of increasing carbon to oxygen bond length (shortest first). 

               H₃COCH₃              CO              CO₂       

Use Section 9 of the Data Booklet to predict which bond in each of the following pairs is more polar:

i)         C–H or C–Cl

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ii)        Si–Li or Si–Cl

[1]

Three types of covalent bonds are present in the molecules in the following equation. 

            2C₂H₂ (g) + 5O₂ (g) → 4CO₂ (g) + 2H₂O (l) 

Identify one bond in these molecules that is correctly described by the following: 

i)       A polar single bond. 

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ii)       A non-polar double bond. 

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iii)      A non-polar triple bond.

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Explain which of the bonds in part (a) is the shortest.

Table 1 shows the carbon-carbon bond enthalpy values for three different hydrocarbons. 

Table 1 

Explain the difference in carbon-carbon bond enthalpy values for the three hydrocarbons.

We can use electronegativity values to deduce whether a compound is likely to be pure covalent (non-polar) or polar covalent. 

Use Section 9 of the Data Booklet to state and explain whether each of the following covalent compounds is polar or non-polar:

H₂       HCl       CO

Ammonia, NH₃, is a chemical that is key in the manufacture of certain fertilisers and cleaning products.

An ammonia molecule will react with an H⁺ ion, to form the ammonium ion, NH₄⁺.

Draw a Lewis (electron dot) diagram to show the bonding in the ammonium ion and name the type of bond formed between the ammonia molecule and the hydrogen ion. 

Lewis (electron dot diagrams) are used to show the electron arrangement in the valence shells of covalently bonded molecules.

Draw Lewis diagrams for the following molecules:

i)         Hydrogen cyanide.

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ii)        Carbon dioxide.

[1]

iii)        Boron trifluoride.

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Benzene is an aromatic hydrocarbon which is often drawn as Figure 1.

Discuss the physical evidence that justifies this structure for benzene.

Figure 1

Benzene and cyclohexene are both unsaturated molecules, but cyclohexene reacts with bromine water and benzene does not.

Table 1 below shows the enthalpy changes for the hydrogenation of cyclohexene, benzene, and the theoretical molecule 1,3,5-cyclohexatriene.

Table 1

The equations for the hydrogenation reactions are:

Cyclohexene               C₆H₁₀ + H₂ ⭢ C₆H₁₂

Benzene                      C₆H₆ + 3H₂ ⭢ C₆H₁₂

An unknown aromatic compound has the molecular formula C₈H₈O₂.

Deduce the structural formula of two isomers of this compound which contain an ester group.

Phosphorus tribromide and sulfur tetrafluoride are two colourless compounds which both react with water to form toxic products. 

Deduce the Lewis (electron dot) structure of both molecules.

Predict the shapes of the two molecules of phosphorus tribromide and sulfur tetrafluoride

Explain why both phosphorus tribromide and sulfur tetrafluoride are polar.

Compounds containing two different halogen atoms bonded together are called interhalogen compounds. They are interesting because they contain halogen atoms in unusual oxidation states. One such compound is BrF₃. 

Deduce the electron domain geometry and molecular geometry of BrF₃.

Give the approximate bond angle(s) and a valid Lewis (electron dot) structure for BrF₃. 

Explain why bromine trifluoride, BrF₃ has its lone pairs of electrons located in equatorial positions. 

Based on the type of intermolecular force present, explain why butan-1-ol has a higher boiling point than butanal.

Ethane, C₂H₆, and disilane, Si₂H₆, are both hydrides of Group 4 elements with similar structures but different chemical properties.

Explain why disilane has a higher boiling point than ethane.

Put the following molecules in order of increasing boiling point and explain your choice:

Based on the type of intermolecular force present, explain the difference in solubility in water between ethane and ethanol.

The melting points of some Group 1 elements are listed in Table 1.

Table 1

Predict, with a reason, the melting point of Rb.

Explain why ammonia, NH₃, is a gas at room temperature. 

Phosphine (IUPAC name phosphane) is a hydride of phosphorus, with the formula PH₃. Phosphine has a much greater molar mass than ammonia.

Explain why phosphine has a significantly lower boiling point than ammonia.

Identify the type of interaction that must be overcome when liquid hydrazine, N₂H₄, vaporizes. Suggest, with a reason, whether hydrazine has a lower or higher boiling point than diimide, N₂H₂.

Carbonation is the process of increasing the concentration of carbonate ions in water to produce carbonated drinks.

Identify the hybridisation of the central carbon atom.

Explain, with the use of diagrams, how there are three valid structures for the carbonate ion.

Describe the distribution of pi (π) electrons and explain how this can account for the structure and stability of the carbonate ion, CO₃^2–.

Draw the structure of silicon dioxide and state the type of bonding present.

Describe the similarities and differences you would expect in the properties of silicon and diamond.

The boiling point of diamond is 3550 ℃, but for carbon dioxide it is -78.5 ℃. Both are covalent substances. 

Explain this difference with reference to structure and bonding.

Silicon dioxide has a similar name to carbon dioxide, but its boiling point is 2230 ℃. 

Briefly outline the reason for this difference.

In 1996 the Nobel prize in Chemistry was awarded for the discovery of a new carbon allotrope, known as fullerenes. 

Outline the structure of buckminsterfullerene.

Like carbon dioxide, graphite is also a covalent substance, but it is a solid at room  temperature. Graphite has a melting point of around 3600 ^oC.           

Describe the structure and bonding of graphite and explain why it has such a high melting point.

Graphite is made purely of carbon, a non-metal, yet it conducts electricity.
Diamond, which is also made purely of carbon, cannot conduct electricity. 

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[1]

Graphite is soft and so is used as a lubricant, whereas diamond is hard and so is used in many cutting tools. Both are giant covalent structures.

Explain this difference with reference to structure and bonding.

The Valence Shell Electron Pair Repulsion Theory (VSEPR) is used to predict the shapes of many chemical molecules.

Describe the main features of the VSEPR theory for predicting shapes of molecules.

State and explain the bond angle F-O-F in OF₂.

Deduce whether each of the three molecules oxygen difluoride, OF₂, phosphorus trifluoride, PF₃, and boron trichloride, BCl₃, are polar or non-polar.

Give a reason in each case.

Predict and explain the shapes and bond angles of the following molecules:

i)        BF₃ 

[2]

ii)       NBr₃ 

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Ethene, C₂H₄, and hydrazine, N₂H₄, are hydrides of adjacent elements in the periodic table. 

State and explain the H一C一H bond angle in ethene and the H一N一H bond angle in hydrazine.

Hydrazine can be oxidised to form diimide, which is a useful compound used in organic synthesis. 

Deduce the molecular geometry of diimide, N₂H₂, and estimate its H–N–N bond angle.

Explain whether ethene and hydrazine are polar or non-polar.

Hydrazine forms a cation with an ethane-like structure called hydrazinediium,  N₂H₆²⁺.

Predict the value of the H–N–H bond angle in N₂H₆²⁺.

Draw the resonance structures for the following ions:

i)        Methanoate, HCOO^-.

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ii)       Nitrate(III), NO₂^–.

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Deduce the resonance structures of the carbonate ion, giving the shape and the oxygen-carbon-oxygen bond angle.

In December 2010, researchers in Sweden announced the synthesis of N,N–dinitronitramide, N(NO₂)₃. They speculated that this compound, more commonly called trinitramide, may have significant potential as an environmentally friendly rocket fuel oxidant. 

Deduce the N–N–N bond angle in trinitramide and explain your reasoning.

Predict, with an explanation, the polarity of the trinitramide molecule.

Silver chloride, AgCl, is a chloride compound that has uses in photography films as well as having antiseptic properties. 

Silver chloride has a high melting point and a structure similar to sodium fluoride.

Explain why, with reference to structure and bonding, why silver chloride has such a high melting point.

Cyanide is a fast-acting chemical, which can be found in various forms and can have toxic effects on the body.

Draw the Lewis structure for a CN^- ion. 

Show the outer electrons only.

Aluminium chloride, $\text{A}{\text{l}}_{\text{2}}\text{C}{\text{l}}_{\text{6}}$, does not conduct electricity when molten but aluminium oxide, $\text{A}{\text{l}}_{\text{2}}{\text{O}}_{\text{3}}$, does.

Explain this in terms of the structure and bonding of the two compounds.

State why magnesium and oxygen form an ionic compound while carbon and oxygen form a covalent compound.

Explain why the melting point of phosphorus(V) oxide is lower than that of sodium oxide in terms of their bonding and structure.

N, N–dinitronitramide N(NO₂)₃, also known as trinitramide, has been identified as a potentially more environmentally friendly rocket fuel oxidant.

Using Section 11 of the data booklet, outline how the length of the bond between nitrogen atoms in trinitramide compares with the bond between nitrogen atoms in nitrogen gas, N₂.

Draw a diagram to show the resonance structure in a molecule of benzene. 

The energy change for hydrogenation of cyclohexene is -120 kJ mol^-1. However, when benzene undergoes hydrogenation, the energy change is 152 kJ mol^-1 less than expected. 

Use this data to explain the relative stabilities of benzene and the theoretical cyclohexa-1,3,5-triene molecule. 

Silicon can form silicon tetrachloride SiCl₄ and also silicon hexachloride, SiCl₆^2-.

Deduce which, if any, of SiCl₄ and SiCl₆^2-, are polar molecules and explain your choice.

Deduce the number of carbons with a tetrahedral geometry in both the monomer, isoprene, and the repeating unit of the polymer, polyisoprene.

One interhalogen compound is IF₅.

[1]

[1]

[2]

Iodine can also form the triiodide ion, I₃^-.

Explain why methanol is soluble in water.

Methanol, ethanol and propan-1-ol are all primary alcohols. Describe and explain the trend in their melting points shown below.

These longer primary alcohols have the following melting points:

Describe and explain this trend.

Predict, with a reason, whether ethanol or ethane-1,2-diol will have the higher melting point?

C₂H₆, C₄H₁₀ and C₃H₈ are alkanes.

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Pentane can exist as isomers, including pentane, CH₃CH₂CH₂CH₂CH₃ and 2,2-dimethylpropane, CH₃C(CH₃)₂CH₃.

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There are two isomers possible with the molecular formula C₂H₆O.

[2]

[2]

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Ibuprofen is a common non-steroidal anti-inflammatory drug (NSAID). It contains a benzene ring and a carboxylic acid at the end of one of the branches.

Deduce the number of resonance structures possible in the deprotonated form of ibuprofen.

A simple amide is HCONH₂.

Draw the Lewis (electron dot) structure for this molecule.

Predict and explain the bond angle around the C and N atoms.

Predict the molecular geometry and the electron domain geometry around the C and N in HCONH₂.

State, with a reason, whether HCONH₂ is a polar molecule.

Tetrafluoroethene, C₂F₄, and tetrafluorohydrazine, N₂F₄, are fluorides of adjacent elements in the Periodic Table.

Draw the Lewis (electron dot) structures for C₂F₄ and N₂F₄ showing all valance electrons.

Predict and explain the F-C-F bond angle in tetrafluoroethene and the F-N-F bond angle in tetrafluorohydrazine.

Tetrafluorohydrazine is a polar molecule but tetrafluoroethene is not.

Explain the difference in molecular polarity.

Draw the Lewis (electron dot) structure of the carbonate ion, CO₃^2-.

Deduce the number of possible resonance structures for the carbonate ion, CO₃^2-, and draw two of them.

Discuss how the bonding in the carbonate ion, CO₃^2-, evidences the presence of the resonance structures.

Organic molecules can also show resonance. The methanoate ion, HCOO^-, shows similar resonance forms to the carbonate ion, CO₃^2-.

The corresponding organic acid, methanoic acid, also has resonance structures.

Draw another resonance structure of methanoic acid.

Some of the physical and structural properties of diamond and graphite are shown below:

Suggest why the melting point of graphite is higher than that of diamond, using the information in the table.

Graphene has the structure of a single layer of graphite. 

Suggest, giving a reason, the electron mobility of graphene compared to graphite.

Graphite is a layered giant structure, containing London dispersion forces between the layers, whereas diamond has covalent bonds across all planes.

Describe and explain, based on structure and bonding, the differences expected when each of graphite and diamond are moved across a paper surface.