Would you expect a B-F bond to be polar or non-polar? Justify your answer

Polar

Large electronegativity difference

Would you expect a BF₃ molecule to be polar or non-polar? Justify your answer

Non-polar

symmetry

Predict the shape of a molecule of PH₃. Explain you prediction

Pyramidal

three bond pairs and one lone pair (on P)

Neither BF₃ nor PH₃ is very soluble in water. Explain why

Non-polar / little hydrogen bonding / little intermolecular forces

Predict the type of bonding that occurs in the compound bromine monochloride (BrCl)

Pure covalent / non-polar covalent / slightly polar covalent

Which of the two atoms in one of the O-F bonds in a molecule of the compounds OF₂ has a partial negative charge? Justify your answer

F (Fluorine)

F is more electronegative than O

Why is a BeCl₂ molecule linear in shape but a H₂O molecule is not?

2 bond pairs no lone pairs in beryllium //

2 lone pairs and 2 bond pairs in water (H₂O)

What is the total number of sigma bonds in a benzene molecule?

12

Explain how the weak intermolecular forces that occur in hydrogen gas (H₂) arise

Temporary dipoles between molecules due to movement of electrons

Explain why nitrogen is chemically inert

Very strong triple bond / N≡N //

non-polar / high degree of symmetry / no dipole movement / no unpaired electrons

Define electronegativity

The relative attraction of an atom for a shared pair of electrons in a covalent bond

Account for the decreasing trend in electronegativity values down Group 17 of the periodic table.

Atomic radius increasing //

more screening /

Predict the shape of a molecule of

i) silicon tetrachloride (SiCl₄),

ii) sulphur dichloride (SCl₂),

iii) phosphorous trichloride (PCl₃)

i) tetrahedral

ii) v-shaped / bent / angular

iii) pyramidal

Which one of these three compounds (SiCl₄,SCl₂,PCl₃)

iv) is non-polar,

v) has bonds that have the least degree of polarity?

iv) silicon tetrachloride (SiCl₄)

v) sulphur dichloride (SCl₂)

Write the chemical formulae for:

i) silicon chloride

ii) manganese(III) oxide.

i) SiCl₄

ii) Mn₂O₃

What shapes are possible for a molecule of the formula QX₂?

Linear //

v-shaped

Define bond energy

Average amount of energy required in KJ, to break 1 mole of covalent bonds of the same type, all species being in the gaseous state.

Take the average bond energies for the C-C single bond and the C≡C triple bond as 348 and 839 kj mol^-1 respectively.

Why is the average bond energy for the C≡C triple bond less than three times the average bond energy for the C-C single bond?

C-C is a sigma bond and C≡C is a sigma and 2 pi bonds //

sigma is stronger than pi / pi weaker than sigma

Why are NH₃ molecules not trigonal planar like BF₃ molecules?

Lone pair present in ammonia / 4 electron pairs present in ammonia / trigonal planar possible if only three bond pairs

Explain why both CH₄ and BF₃ are non-polar molecules.

Centres of negative and positive charge coincide /

high degree of symmetry in arrangement of bonds /

only bond pairs present / no lone pairs present

Give two possible shapes of a covalent molecule with the general formula QX₃

Trigonal planar //

pyramidal

Under what circumstances do ionic compounds conduct electricity?

In aqueous solution / dissolved in water //

molten (liquid state, melted)

Explain why, although carbon and selenium have the same electronegativity value, CH₄ is almost completely insoluble in water and H₂Se is slightly soluble in water

Methane is non-polar / centres of negative and positive charge coincide in methane /

hydrogen selenide is polar / centres of negative and positive charge do not coincide in hydrogen selenide

Use electronegativity values to predict the type of bonding in oxygen difluoride (OF₂)

Slightly polar covalent

State and account for the shape of the OF₂ molecule

V-shaped

4 electron pairs of which 2 lone pairs /

2 bond pairs and 2 lone pairs

Select, giving your reasons, which of these angles is the most probable value for the bond angle in oxygen difluoride.

180.0^o 109.5^o 120.0^o 103.0^o

103.0^o

2 lone pairs push 2 bond pairs closer together (so bond angle is less than 109.5^o)

or

180.0^o corresponds to exactly 2 bond pairs

120.0^o corresponds to exactly 3 bond pairs

109.5^o corresponds to exactly 4 bond pairs

[Note: lone pair lone pair repulsion > lone pair bond pair repulsion > bond pair bond pair repulsion]

What is the valency of carbon in tetrachloromethane?

4

Identify, in ammonia, the type of

i) intramolecular bonding,

ii) intermolecular forces, present

i) polar (covalent)

ii) hydrogen bonds (dipole-dipole, van der Waals)

Use electron pair repulsion theory to predict the shape of a molecule of boron trifluoride (BF₃)

Trigonal planar

How many (i) sigma bonds, (ii) pi bonds, result from sharing of the valence electrons between the atoms in a molecule of nitrogen?

i) 1

ii) 2

Give the reason why a molecule with polar bonds can be non-polar

Centres of positive and negative charge coincide / symmetrical distribution of bonds

Explain the following in terms of intramolecular bonding or intermolecular forces or both.

When a charged rod is held close to a thin stream of water flowing from a burette, the stream of water is deflected.

Charge on rod attracts // opposite charge on polar dipole of water molecule

Why do ionic substances conduct electricity when molten or dissolved in water but not in the solid state?

molten/dissolved: ions free to move //

solid: ions not free to move / ions locked (fixed) in position

Hydrogen bonding occurs between ammonia molecules. What are hydrogen bonds?

Intermolecular forces between molecules when hydrogen is bonded to a small electronegative atom (N, O, or F)