Assignment: 
Questions on Topic 4.2: Covalent bonding
This page of questions can be marked as direct student access either for assigning as a test or for students to work on in their own time. If you do not wish to use student access, links to downloadable versions of the questions and, separately the worked answers, can be found at Printable versions of written tasks.
Explain why the bond between two oxygen atoms in oxygen gas is a double bond whereas the bond between two nitrogen atoms in nitrogen gas is a triple bond.
10 lines
Oxygen has the electron configuration 1s22s22p4. When two oxygen atoms combine to form oxygen gas each atom needs to share two more electrons so the bond between the atoms is made from two shared pairs of electrons so that each atom attains the electron configuration a noble gas.
Nitrogen has the electron configuration 1s22s22p3. Each nitrogen atom needs to share three electrons with the other nitrogen atom to form a triple bond (three shared pairs of electrons) between the two atoms so that both achieve a noble gas electron configuration.
Explain why a proton in water, H+(aq) is often written as H3O+(aq).
3 lines
The oxygen atom in water contains two non-bonding pairs of electrons. One of these non-bonding pairs of electrons forms a coordinate bond with the proton (which is extremely small as it has no electrons).
Explain why the carbon to carbon bond in ethyne, C2H2, is stronger and shorter than the carbon to carbon bond in ethene, C2H4.
3 lines
Ethyne contains a triple covalent bond whereas ethene contains a double covalent bond between the carbon atoms. The more electron pairs involved in the bonding the stronger and shorter the bond as the two nuclei are attracted more strongly together.
Explain why there are two different carbon to oxygen bond lengths in a molecule of ethanoic acid.
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Ethanoic acid contains one C=O double bond and one C-O single bond. The double bond is shorter than the single bond.
Use information in Section 8 of the IB chemistry data booklet to explain why a carbon to chlorine bond is polar.
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Chlorine (3.2) is more electronegative than carbon (2.6). This means the shared pair of bonding electrons is closer to the chlorine atom resulting in a polar bond.
Explain why a white precipitate is formed when silver nitrate solution is added to a solution of potassium chloride but not when silver nitrate solution is added to tetrachloromethane.
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The white precipitate is silver chloride formed from the reaction of silver ions with chloride ions.
Ag+(aq) + Cl–(aq) → AgCl(s)
All the chlorine atoms in tetrachloromethane are covalently bonded so no chloride ions are present to precipitate with silver ions.
Carbon dioxide is a linear molecule. It contains two carbon to oxygen double bonds at 180o to each other. Explain why the C=O bonds are polar and yet the molecule is non-polar.
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C=O bonds are polar due to the greater electronegativity of oxygen compared to carbon so the electrons in the C=O bond are drawn closer to the oxygen atom. The carbon dioxide molecule is non-polar as the resultant polarity of the two oppositely opposed dipoles from the two C=O bonds is zero.
Explain why water in a beaker heats up quickly in a microwave oven whereas when the same volume of tetrachloromethane, CCl4(l), is placed in the beaker and the microwave switched on for the same length of time there is no increase in the temperature of the tetrachloromethane.
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Water is a polar molecule as it has a bent shape and polar O-H bonds. These are given energy by the oscillating microwaves as they continually try to line up with the field and so heat up. Tetrachloromethane is made up of non-polar molecule as the resultant polarity of the four symmetrical C-Cl bonds is zero. The molecules are thus unaffected by the microwave radiation.
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