Classification of bonding
Nowadays there are many different types of materials in everyday use in society and industry, all of which have been specifically designed and manufactured for their intended purpose.
We have come a long way from stone and wooden tools
An understanding of the underlying structure of the materials and its consequences in terms of property has been essential in this process.
From the previous sections on ionic, covalent and metallic bonding we have seen how the structure influences the physical property of the material.
However, the concepts of bonding can get blurred around the edges, as covalent compounds can exhibit a degree of ionic behaviour in terms of polarity, while ionic compounds can exhibit a degree of covalent character. We see that the classification of matter becomes more of a gradual change from one bonding type to another with a commensurate gradual change of property.
This gives rise to the term "bonding continuum", where boundaries between the major structural types are not well defined.
Napoleon's army and the Russian campaign of 1812
There is a theory that degradation of the properties of tin due to the cold, contributed to the defeat of Napoleon's army in the winter of 1812.
The story goes that the army uniforms were held together by buttons made of tin. Under normal European conditions this was just fine and the buttons did their job.
However, in the harsh Russian winter the low temperatures caused the tin to change from the metal allotrope to a non-metal allotrope (alpha tin), with disasterous consequences on its properties.
The buttons lost their hardness and became soft and crumbly. They fell apart, as did the soldiers' uniforms.
While this may just be an apocryphal "urban legend", it highlights the link between structure and property in terms of materials.
The van Arkel Ketelaar bonding triangle
The bonding triangle shows us how to predict the type of bonding within a structure by referring to the electronegativities of the bonded elements.
^ top
Mixtures and composites
Mixtures
A mixture is a combination of two or more substances that are physically combined but not chemically bonded. Each substance in a mixture retains its own properties.
Mixtures may be:
- 1. Homogeneous
- 2. Heterogeneous
Homogeneous Mixtures are mixtures where the components are evenly distributed, and appear to be the same throughout. Examples include salt water and air.
Heterogeneous Mixtures are mixtures where the components are not evenly distributed, and you can see the different parts. Examples include a salad and a mixture of sand and iron filings.
Properties of Mixtures:
- Components can be separated by physical methods (e.g., filtration, distillation).
- The composition can vary (e.g., you can add more sugar to tea to make it sweeter).
Uses of Mixtures:
- **Air:** Essential for breathing, contains oxygen, nitrogen, and other gases.
- **Salt Water:** Used in cooking and preserving food.
- **Alloys:** Mixtures of metals, like steel (iron and carbon), are used in construction and manufacturing.
Composites
A composite is a material made from two or more different substances that, when combined, produce a material with characteristics different from the individual components. They are engineered to have better properties.
Examples of Composites:
- 1. Fiberglass
- 2. Carbon Fibre
Fibreglass is made from plastic reinforced with glass fibers. It is strong and lightweight. Carbon Fibre is made from carbon filaments (fibres) reinforced with plastic. It is very strong and light, often used in sports equipment and aerospace.
Properties of Composites:
- Enhanced strength and durability.
- Often lighter than pure metals.
- Resistant to corrosion and wear.
Uses of Composites:
Composites are used in building materials like concrete reinforced with steel rebar.
Automotive and Aerospace: Used to make lightweight and strong parts for cars and airplanes.
Sports Equipment: Used in making bicycles, tennis rackets, and golf clubs for better performance.
In summary, mixtures are combinations of substances that retain their own properties and can be separated physically, while composites are engineered materials that combine substances to produce new properties, making them useful in a wide range of applications.
^ top
Worked examples
Q271-01 Select the substance
with the higher boiling point in each of the following pairs. Explain your reasoning:
- C2H6 and C3H8
- CH3CH2OH and CH3OCH3
Answer
|
In the first pair of compounds there are no dipoles so the intermolecular
forces are due to dispersion attractions only. The larger of the
two molecules has the greater intermolecular force. Therefore C3H8
has the higher boiling point.
In the second pair of compounds their relative masses are the same,
but the alcohol, CH3CH2OH, has hydrogen bonding
due to the OH group, whereas the ether (methoxymethane) has only weak
dipole - dipole interactions. The alcohol, CH3CH2OH,
has the higher boiling point.
|
Q271-02 Which substance has
the highest boiling point?
- CH4
- He
- HF
- Cl2
Answer
- CH4 is simple molecular, non-polar, very low boiling
point.
- He is atomic, non-polar, very low boiling point.
- HF is simple molecular, but very polar and the hydrogen can hydrogen
boind to fluorine atoms on neighbouring molecules. HF
has the highest boiling point.
- CH4 is simple molecular, non-polar, low boiling point,
but higher than methane due to its higher relative mass (stronger
dispersion interactions).
|
Q271-03 The very high heat
of vaporization of H
2O is mainly a result of
- dispersion forces
- covalent bonds
- inter-ionic attractions
- hydrogen bonding
Answer
|
The enthalpy of vaporisation is the amount of energy needed to turn
one mole of a substance from liquid into vapour. It is a measure of
the strength of the intermolecular forces. Water's high enthalpy of
vaporisation is due to the strengthof intermolecular forces caused
by hydrogen bonding.
|
Q271-04 Explain the following
statement: "Cis-butenedioic acid has a lower melting point than its isomer
trans-butenedioic acid."
Answer
|
The lower melting point of the cis-butenedioic acid is indicative
of the weaker intermolecular forces compared to the trans isomer.
To understand why it is necessary to look at the structure of each
isomer.
|
The cis isomer is able to form intramolecular hydrogen bonds
diminishing the possibility of intermolecular hydrogen bonding.
In the trans isomer, all of the hydrogen bonding is intermolecular.
There are more intermolecular forces in the trans isomer, so
it has higher boiling and melting points.
|
 |
|
Q271-05 Arrange the following
compounds in order of increasing boiling point and explain your choice by reference
to the intermolecular forces involved: Bromoethene, chloroethene, ethene
Answer
|
Ethene, CH2=CH2, is a non-polar simple molecular
substance with a low boiling point. However, both bromoethene, CH2=CHBr
and chloroethene, CH2=CHCl are polar substances with similar
polarity. Bromoethene has a much larger relative mass than chloroethene
and so larger dispersion forces. This is the deciding factor,
leaving the order of increasing boiling point as ethene
< chloroethene < bromoethene.
|
Q271-06 Aluminium fluoride
AlF
3 is a solid up to temperatures of 1250ºC, whereas nitrogen
trifluoride is a gas above -129ºC. Describe the bonding and structure in
samples of each of these substances.
Answer
|
Aluminium fluoride has a giant ionic structure, whereas nitrogen
trifluoride has a simple molecular structure.
|
|
|
|
Aluminium fluoride - a giant ionic lattice
|
nitrogen trifluoride - a simple molecular
substance
|
|
Q271-07 Which statements are
generally true about the melting points of substances?
- I - Melting points are higher for compounds containing ions than for compounds
containing molecules.
- II - A compound with a low melting point is less volatile than a compound
with a higher melting point.
- III - The melting point of a compound is decreases by the presence of impurities.
- I only
- I and III only
- II and III only
- I, II and III
Answer
|
I - Melting points are higher for compounds containing ions than
for compounds containing molecules - true
II - A compound with a low melting point is less volatile than a compound
with a higher melting point - false
III - The melting point of a compound is decreases by the presence
of impurities - true
Correct response I and III only
|
Q271-08 CH
3COCH
3
is the first member of the ketone homologous series. Draw the full structural
formula of the next member of the homologous series and predict how its melting
point compares with that of CH
3COCH
3
Answer
|
Members of homologous series differ by one -CH2- unit.
The first member of the ketones (alkanones) is propanone, thus the
second member is butanone, CH3COCH2CH3.
|
|
Both molecules have dipole - dipole interactions due to the
same functional group, so their effects may be discounted.
However, butanone has a larger relative mass than propanone
and stronger dispersion forces.
Its melting point is higher.
|
|
Butanone
|
|
Q271-09 The compounds A, B
and C have approximately the same molar mass:
| A |
B |
C |
| C4H10 |
CH3CH2CH2OH |
CH3OCH2CH3 |
When the compounds are arranged in order of increasing boiling points (lowest
boiling point first), the correct order is:
- A, C, B
- A, B, C
- B, C, A
- C, B, A
Answer
|
The three compounds belong to different homologous series, alkanes,
alcohols and ethers. As they have the same relative mass we can discount
the effect of dispersion forces. Alkanes are non-polar and have
very low melting and boiling points. Ethers are weakly polar and have
higher boiling points than alkanes. Alcohols have extensive hydrogen
bonding and the highest boiling point of the three groups. The correct
order of increasing boiling point is A,
C, B.
|
|
|
|
|
butane
|
propan-1-ol
|
methoxyethane
|
|
Q271-10 Identify the strongest
type of intermolecular force present in each of the compounds propan-1-ol, propanal
and propanoic acid. List these compounds in decreasing order of boiling point.
Answer
|
The three compounds belong to different homologous series, alcohols,
aldehydes (alkanals) and carboxylic acids (alkanoic acids). Although
all three compounds have different relative masses, they are not extremely
different and the effect of the functional groups is greater than
that of dispersion forces. In propanoic acid there is extensive
hydrogen bonding and so it has the highest boiling point. The next
highest boiling point is in propan-1-ol which also has hydrogen bonding.
The aldehyde has only weaker dipole-dipole interactions.
|
|
|
|
|
propanal
|
propan-1-ol
|
propanoic acid
|
|
^ top
