New 21st Century Chemistry
Suggested answers to in-text activities and unit-end exercises
Topic 6 Unit 24
In-text activities
Checkpoint (page 38)
1 a) P–H non-polar
δ+ δ–
b) H–F
δ– δ+
c) O–N
d) F–F non-polar
δ– δ+
e) N–H
f) C–I non-polar
2 In NH3, the bond pairs of electrons are attracted towards the nitrogen atom to a greater extent as nitrogen is more electronegative than hydrogen.
The bond pairs of electrons repel each other to a greater extent and thus the H–N–H bond angle is greater.
In NF3, the bond pairs of electrons are closer to the fluorine atom as fluorine is more electronegative than nitrogen.
The bond pairs of electrons repel each other to a less extent and thus the F–N–F bond angle is smaller.
Checkpoint (page 44)
Molecular formula / Electron diagram / Shape of molecule / Polar bond / Polar molecule?H2O / / V-shaped / O–H / yes
BeH2 / / linear / Be–H / no
CH2F2 / / tetrahedral / C–F / yes
Cl2O / / V-shaped / Cl–O / yes
2 a)
b) In trichloromethane, each C–Cl bond is polar.
The trichloromethane molecule is tetrahedral in shape.
The individual C–Cl bond dipole moments reinforce each other.
The molecule has a net dipole moment and it is polar.
When a positively charged rod is brought close to the jet of trichloromethane, negative ends of the molecules are attracted towards the rod.
When a negatively rod is brought close to the jet of trichloromethane, positive ends of the molecules are attracted towards the rod.
Checkpoint (page 52)
1
Substance / Type of attractionsInstantaneous
dipole-induced
dipole attractions / Permanent
dipole-permanent
dipole attractions
a) Bromine / ✔
b) Liquid sulphur dioxide / ✔ / ✔
c) Methane / ✔
2 The boiling point of H2S is higher than that of SiH4.
The boiling point of a compound depends on the strength of its intermolecular attractions.
H2S is a polar substance. There are permanent dipole-permanent dipole attractions and instantaneous dipole-induced dipole attractions between H2S molecules.
SiH4 is a non-polar substance. There are only instantaneous dipole-induced dipole attractions between SiH4 molecules.
More heat is needed to separate the H2S molecules during boiling.
Checkpoint (page 54)
The boiling point of a compound depends on the strength of its intermolecular attractions.
The intermolecular attractions in the carbon compounds are van der Waals’ forces.
The number of electrons in the molecule / the molecular mass increases from methane to propane.
Hence the strength of van der Waals’ forces also increases from methane to propane. This suggests that the boiling points should increase from methane to propane in accordance with the data.
Discussion (page 60)
Without hydrogen bonding, the boiling point of H2O would be around –68 °C while that of HF would be around –88 °C.
HF molecules can form an average of one hydrogen bond per molecule while H2O molecules can form an average of two hydrogen bonds per molecule.
Hence the difference between the actual boiling point and estimated boiling point without hydrogen bonding for H2O is much greater than that for HF.
Checkpoint (page 62)
1 a)
Liquid / Type of attractions /Instantaneous
dipole-induced
dipole attractions / Permanent
dipole-permanent
dipole attractions / Hydrogen
bonds /
a) / ✔
b) / ✔ / ✔ / ✔
c) / ✔ / ✔
b) The boiling point of a compound depends on the strength of its intermolecular attractions.
The boiling point of is the lowest.
Only weak instantaneous dipole-induced dipole attractions exist between the molecules.
The boiling point of is the highest.
Hydrogen bonds exist between the molecules.
2
(For the sake of clarity, interaction is not shown at every –OH group.)
Checkpoint (page 69)
1 (a) and (c)
2 a)
Compound / Type of attractionsInstantaneous dipole-induced
dipole attractions / Permanent dipole-permanent
dipole attractions / Hydrogen bonds
i) NH3 / ✔ / ✔ / ✔
ii) CH3OH / ✔ / ✔ / ✔
iii) / ✔ / ✔
iv) / ✔
b)
Library Search & Presentation (page71)
Proteins
Biological functions of proteins
Proteins are large molecules that occur in every living organism. They are of many types and have many biological functions. The following table lists some biological functions of proteins.
Some biological functions of proteinsType / Function / Example
Enzymes / catalyze biological processes / pepsin
Hormones / regulate body processes / insulin
Storage proteins / store nutrients / ferritin
Transport proteins / transport oxygen and other substances through the body / hemoglobin
Structural proteins / form an organism’s structure / collagen
Protective proteins / help fight infection / antibodies
Contractile proteins / form muscles / actin, myosin
Toxic proteins / serve as a defense for the plant or animal / snake venoms
Amino acids making up the body’s proteins
All proteins are made up of many amino acid units linked together in a long chain. Twenty different amino acids are used to make the body’s proteins.
An amino acid contains both an amino group and a carboxyl group.
The following table lists some amino acids essential to living organisms.
Name / Abbreviations / Structure(shaded portion is the R group of the amino acid) /
Alanine / Ala /
Aspartic acid / Asp /
Cysteine / Cys /
Glycine / Gly /
Histidine / His /
Serine / Ser /
Valine / Val /
Peptide link formation
Two amino acids can undergo a condensation reaction to form a dipeptide. A water molecule is eliminated between the –NH2 group of one amino acid and the –COOH group of the other. The amino acid units are linked by a peptide link.
From a dipeptide, a tripeptide can be made by adding another amino acid molecule. Peptides that contain many amino acid units are polypeptides. Proteins are polypeptides consisting of one or more polypeptide chains.
Levels of protein structure
Chemists usually speak about four levels of structure when describing proteins.
Primary structure
The primary structure specifies the unique amino acid sequence of the polypeptide chain.
Secondary structure
Most polypeptide chains fold in such a way that the segments of the chain orient into regular patterns, called secondary structures. There are two common kinds of patterns: the α-helix and the β-pleated sheet.
In the α-helix, the polypeptide chain is coiled tightly in the fashion of a spring. The helix is stabilized by hydrogen bonds between the N–H group of one amino acid unit and the C=O group on the 4th amino acid unit away from it.
The following figure shows the α-helical secondary structure of keratin, a fibrous protein found in wool, hair, fingernails and feathers.
The following figure shows the β-pleated sheet secondary structure found in fibroin, the fibrous protein found in milk. A polypeptide chain doubles back on itself after a hairpin bend. The two sections of the chain on either side of the bend line up in a parallel arrangement held together by hydrogen bonds.
Tertiary structure
Secondary protein structures result primarily from hydrogen bonding between peptide links along the protein backbone, but higher levels of structure result primarily from interactions of R groups in the protein.
The tertiary structure of a protein is its three-dimensional shape that arises from further foldings of its polypeptide chains, foldings superimposed on the coils of the α-helices.
Various forces are involved in stabilizing tertiary structures, including van der Waals’ forces, ionic linkages, hydrogen bonds and disulphide bridges (refer to the following figure for details).
Quaternary structure
A protein molecule may be made up of more than one polypeptide chain. The overall arrangement of the polypeptide chains is called the quaternary structure. A variety of interactions including hydrogen bonding hold the various chains into a particular geometry.
There are two major categories of proteins with quaternary structure — fibrous and globular.
Collagen is a fibrous protein in tendons and muscles, consisting of intertwining polypeptide chains.
Globular proteins are mostly clumped into a shape of a ball. For example, the hemoglobin molecule consists of four separate polypeptide chains or subunits. These subunits are held together by van der Waals’ forces and ionic forces.
DNA
Functions of DNA
The nucleic acids are informational molecules because their primary structure contains a code or set of directions by which they can duplicate themselves and guide the synthesis of proteins. There are two types of nucleic acids which are polymers found in all living cells. Deoxyribonucleic acid (DNA) is found mainly in the nucleus of the cell, while ribonucleic acid (RNA) is found mainly in the cytoplasm of the cell.
Coded in an organism’s DNA is all the information that determines the nature of the organism and all the directions that are needed for producing the thousands of different proteins required by the organism.
Structure of nucleic acids
Nucleic acids are polymers made up of nucleotide units linked together to form a long chain. Each nucleotide is composed of a nucleoside plus phosphoric acid, and each nucleoside is composed of a sugar plus an amine base.
The sugar in DNA is 2-deoxyribose.
Four different cyclic amine bases occur in DNA: adenine, guanine, cytosine and thymine.
In DNA, the cyclic amine base is bonded to C1’ of the sugar, and the phosphoric acid is bonded to the C5’ sugar position. The following figure shows the general structures of a nucleoside and a nucleotide.
Nucleotides join together in nuclei acids by forming a phosphate ester bond between the phosphate group at the 5’ end of one nucleotide and the hydroxyl group on the sugar component at the 3’ end of another nucleotide.
This makes the nuclei acid a long unbranched chain with a backbone of sugar and phosphate units with bases protruding from the chains at regular intervals.
The following diagram shows a segment of one DNA chain. See how the phosphate ester groups link the 3’- and 5’- OH groups of the sugar units.
Base pairing in DNA: the Watson-Crick Model
According to the model, DNA consists of two polynucleotide strands coiled around each other in a double helix. The two strands run in opposite directions and are held together by hydrogen bonds between pairs of bases. Adenine (A) and thymine (T) form two strong hydrogen bonds to each other, but not to guanine (G) or cytosine (C); G and C form three strong hydrogen bonds to each other, but not to A or T.
The specific base pairing also means that the two chains of DNA are complementary. Wherever adenine appears in one chain, thymine must appear opposite it in the other; wherever cytosine appears in one chain, guanine must appear in the other (refer to the following diagram of the DNA double helix showing complementary base pairing).
Replication of DNA
Just prior to cell division the double strand of DNA begins to unwind. Complementary strands are formed along each chain. Each chain acts as a template for the formation of its complement. When unwinding and duplication are complete, there are two identical DNA molecules where only one had existed before. These two molecules can then be passed on, one to each daughter cell.
Unit-end exercises (pages 75-84)
Answers for the HKCEE (Paper 1) and HKALE questions are not provided.
1
2 a) The electronegativity of an element represents the power of an atom of that element to attract a bonding pair of electrons towards itself in a molecule.
b) δ– δ+ δ+ δ– δ– δ+ δ– δ+
C–H C=O O–H N–H
c) CH4
3 a)
b) The electronegativity values of carbon and chlorine determine where the partial charges are placed on the molecule.
c) Yes
Each C–Cl bond is polar.
Because of its tetrahedral shape, the individual C–Cl bond dipole moments reinforce each other.
Hence the whole molecule has a net dipole moment.
4 a)
Substance / Boiling point (K) / Type(s) of intermolecular forcesPropane / 229 / instantaneous dipole-induced dipole attractions
Methanol / 338 / instantaneous dipole-induced dipole attractions, permanent dipole-permanent dipole attractions, hydrogen bonds
b) The boiling point of a compound depends on the strength of its intermolecular attractions.
Hydrogen bonds exist between methanol molecules, in addition to permanent dipole-permanent dipole attractions and instantaneous dipole-induced dipole attractions.
There are only weak instantaneous dipole-induced dipole attractions between propane molecules.
Hence the boiling point of methanol is higher than that of propane.
5 The volatility of a halogen depends on the strength of its intermolecular attractions.
Van der Waals’ forces exist between halogen molecules.
The number of electrons in the molecule increases from chlorine to iodine. Hence the strength of van der Waals’ forces also increases from chlorine to iodine.
Thus the trend in volatility of the three halogens is chlorine > bromine > iodine.
6 B Van der waals’ forces exist in methane and neon.
The stronger van der Waals’ forces in methane (due to greater molecular surface area allowing greater contact between molecules) account for its higher boiling point as more heat is required to separate its molecules during boiling.
7 C Although the H–Cl bond is polar, the chlorine atom is quite large and its lone pairs of electrons are not very accessible to a hydrogen atom.
Hence there is no strong attraction between the hydrogen atom and the lone pair on the chlorine atom of another HCl molecule.
A HCl molecule will not form a hydrogen bond with another HCl molecule.
8 D X is non-polar. It does not mix with water due to the difference in the strength of intermolecular attractions between water molecules and those between molecules of X.
Thus, X is insoluble in water.
Y is soluble in water because hydrogen bonds can form between molecules of Y and water molecules.
The water solubility of Z is higher than that of Y.
Each molecule of Z has two –OH groups that can take part in hydrogen bonding while each molecule of Y has only one –OH group that can take part in hydrogen bonding.
9 D (1) In a BF3 molecule, each B–F bond is polar.
A BF3 molecule has a trigonal planar shape. The three identical bond dipole moments cancel one another out exactly.
So a BF3 molecule is non-polar.