Electronegativity and bond polarityAQA A-Level Chemistry: Subtopic test

1. 1
   Hydrogen chloride, HCl, is a gas that dissolves in water to form hydrochloric acid. The two atoms in an HCl molecule share one pair of electrons, and chlorine is more electronegative than hydrogen.

   (a)

   Which pair of partial charges is correct for a molecule of HCl?

   [1 mark]

   • AHδ⁻ and Clδ⁺
   • BBoth atoms carry δ⁺
   • CBoth atoms carry δ⁻
   • DHδ⁺ and Clδ⁻

   (b)

   Which statement defines electronegativity?

   [1 mark]

   • AThe energy released when an atom gains an electron
   • BThe power of an atom to attract the pair of electrons in a covalent bond
   • CThe energy needed to remove an electron from an atom
   • DThe tendency of an atom to lose electrons to form a positive ion

   (c)

   Explain why the bond in HCl is polar.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Carbon dioxide, CO₂, and sulfur dioxide, SO₂, both contain polar bonds between oxygen and a central non-metal atom. Oxygen is more electronegative than both carbon and sulfur. A CO₂ molecule is linear, whereas an SO₂ molecule is bent.

   (a)

   Which statement about carbon dioxide is correct?

   [1 mark]

   • AIt has a permanent dipole because the C=O bonds are polar
   • BIt is non-polar because the C=O bonds are non-polar
   • CIt has polar bonds, but the dipoles cancel so the molecule has no permanent dipole
   • DIt has a permanent dipole because it contains two oxygen atoms

   (b)

   Which of these molecules has a permanent dipole?

   [1 mark]

   • ASO₂
   • BCCl₄
   • CBF₃
   • DXeF₄

   (c)

   Explain why SO₂ has a permanent dipole but CO₂ does not.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Chlorine is found in several molecules, including chlorine gas, Cl₂, tetrachloromethane, CCl₄, and trichloromethane, CHCl₃. Pauling electronegativity values are 2.5 for carbon, 2.1 for hydrogen and 3.0 for chlorine.

   (a)

   Explain why the bond in Cl₂ is non-polar but the C–Cl bond in CCl₄ is polar, stating the partial charge on each atom in C–Cl.

   [3 marks]

   (b)

   Explain why CCl₄ has no permanent dipole but CHCl₃ does.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student is given four molecules: BF₃, NH₃, HF and CH₄. Pauling electronegativity values are 2.1 for hydrogen, 2.0 for boron, 2.5 for carbon, 3.0 for nitrogen and 4.0 for fluorine.

   (a)

   Deduce which of these four molecules have a permanent dipole, using bond polarity and shape to justify each answer.

   [6 marks]

   (b)

   Use the electronegativity values to put the bonds B–F, N–H and C–H in order of increasing polarity, showing the partial charges on each, and explain why the bond in F₂ is non-polar.

   [6 marks]

   Total for question 4: 12 marks

End of questions