Covalent Bonding Exam Qs

Question 1

State what is meant by a covalent bond (2)

Answer 1

• strong electrostatic attraction (1)
• between two nuclei and a bonding pair of electrons (1)

Question 2

State the type of bond that joins the two AlCl3 molecules together (1)

Answer 2

Dative covalent or coordinate bonds

Question 3

Ammonia and boron trifluoride react to form a compound NH3BF3 which contains a dative covalent bond. Each of the molecules NH3 and BF3 has a different feature of its electronic structure that allows this to happen.

Explain how a dative covalent bond forms (2)

Answer 3

• donation of lone pair or non- bonding pair (1)
• to the BORON atom -> has only 6 outer electrons -> accepts 2 from NITROGEN (1)

Question 4

3 (ii) During the reaction, the bond angles about the nitrogen and boron change. State the new H-N-H and F-B-F angles (2)

Answer 4

• HNH angle approx 109.5 (1)

-FBF angle approx 109.5 (1)

Question 5

State what is meant by the term electronegativity and hence explain the polarity, if any, of the bonds in ClF3 (3)

Answer 5

• electronegativity is the ability of an atom to attract electrons (in a covalent bond) (1)
• F is more electronegative than Cl (1)

-F is negative and chlorine is positive (1)

Question 6

Explain why the melting temperature of silicon oxide is much higher than that of iodine, even though the bonding is covalent (3)

Answer 6

• silicon oxide is a GIANT structure = contains many strong covalent bonds (1)
• iodine has weak intermolecular/ London forces (1)
• MORE ENERGY required to break stronger bonds in silicon oxide -> HIGH MP (1)

Question 7

Explain why both water and carbon dioxide molecules have polar bonds but only water is a polar molecule (4)

Answer 7

(M1) oxygen is more electronegative than hydrogen and carbon
(1)
.
(M2) which results in a polar bond with oxygen 6-so carbon and hydrogen 6+
(1)

• (M3) carbon dioxide is a symmetrical/linear molecule and so the dipole moments/vectors cancel
(1)

(M4) the lone pairs of electrons of oxygen/ the V-shape of the water molecule mean that the dipole moments/ vectors do not cancel

Question 8

Explain, in terms of structure and bonding of each element, the difference between these values (3)

Silicon = MP (K)- 1683
Chlorine= MP (K)- 172

Answer 8

• silicon - giant atomic / giant covalent / giant molecular /
macromolecular and
contains covalent bonds
(1)
.
chlorine - (simple)
molecular / molecules / diatomic / Cl and
contains London forces (1)

Covalent bonds in silicon are stronger than London forces/ intermolecular forces in chlorine or covalent bonds take more energy to break than London forces / intermolecular forces (1)

Question 9

Iodine and diamond are crystalline solids at room temperature.

Explain why diamond has a much higher temperature than iodine (5)

Answer 9

iodine is (simple) molecular

• diamond is a giant (covalent / lattice) structure (with 4 covalent bonds per carbon atom)

• iodine molecules are held together by weak London forces / dispersion forces / van der Waal’s forces /
instantaneous induced dipole-dipole attractions

• carbon atoms in diamond are held together by (strong) covalent bonds

• strong covalent bonds require more energy to break than intermolecular forces

Question 10

Calcium carbonate is thermally decomposed during the manufacture of cement.

Write the equation, inc state symbols, for the thermal decomposition of calcium carbonate (1)

Answer 10

CaCO3 (s) -> CaO (s) + CO2 (g)

Question 11

Name all the types of bonds present in calcium carbonate (1)

Answer 11

Ionic AND covalent

Question 12

Give a reason, in terms of bonding, Calcium carbonate needs a high decomposition temperature is required (1)

Answer 12

Strong bonds within the CARBONATE ION ( C=O)

Question 13

Boron and aluminium are in the same group of the Periodic Table. Both form compounds with chlorine and with fluorine.
Aluminium fluoride and aluminium chloride are both crystalline solids at room temperature. Aluminium fluoride sublimes to form a gas at 1291°C (1564 K), whilst aluminium chloride sublimes at 178°C (451 K).
Use the Pauling electronegativity values in the Data Booklet to explain these differences in sublimation temperature.
(6)

Answer 13

• Aluminium fluoride has a giant structure with strong electrostatic forces of attraction between ions
• aluminium chloride has a molecular structure so weak London forces
• AlCL is covalent
• AlF is polar

-AlF has a electronegativity of 2.5
- AlCl has a electronegativity of 1.5