Ch.9 Structures and properties of substances

Question 1

What is the structure of a substance?

Answer 1

The structure of a substance is a description of what its consitituent particles are, and how they are arranged and pack together. In chemistry, it is important to study the structure of a substance since we may predict and explain properties of the substance from its structure.

Question 2

What is a giant metallic structure?

Answer 2

A giant metallic structure is a three-dimentional giant lattice of metal ions surrounded by a ‘sea’ of delocalized electrons.
(refer to testbook for the drawings)

Question 3

What are the properties of metals?

Answer 3

1. Metals are good conductors of electricity
2. Metals are good conductors of heat
3. Most metals are solids with high melting points
4. Most metals have high densities
5. Metals are malleable (can be rolled into sheets) and ductile (can be pulled into wires)

Question 4

Why are metals good conductors of electricity?

Answer 4

In a piece of metal, delocalized electrons move freely and randomly in all directions. When both ends of the metal piece are connected to the terminals of a battery, the delocalized electrons flow in one direction only towards the positive terminal of the battery. At the same time, an equal number of electrons flow into the other end of the metal from the negative terminal of the battery. This makes a complete circuit and this explains why metals can conduct electricity.

Question 5

Why are metals good conductors of heat?

Answer 5

When one end of a metal piece is heated, the delocalized electrons there gain heat energy and move faster. They transfer the heat energy to the whole piece of metal reapidly.

Question 6

Why are most metals solids with high melting points?

Answer 6

To melt a metal, a lot of energy is required to break the metallic bonds between the metal ions in a giant metallic structure. As the metallic bonds in most metals are strong, most metals have high melting points.

Question 7

Why do most metals have high densities?

Answer 7

The metal ions in a giant metallic structure pack closely together. Hence, metals generally have high densities.

Question 8

Why are metals malleable and ductile?

Answer 8

In a giant metallic structure, the metal ions pack in layers and are held together by metallic bonds. When we apply a force on a piece of metal, the layers of metal ions can slide over one another. Since the non-directional metallic bonds still hold the metal ions together, the ions settle into new positions and the piece of metal takes on a new shape.

Question 9

What is giant ionic structure?

Answer 9

An ionic compound usually forms when metal combines with non-metal. It consists of cations and anions held together by ionic bonds. The ions pack regularly to form a continuous, three-dimensiaonl giant ionic structure.

Question 10

What is the structure of sodium chloride?

Answer 10

Sodium chloride has a giant ionic structure. In this structure, cations (Na+) and anions (Cl-) are held togeter by ionic bonds. They pack regularly so that each ion is surrounded by six ions of the opposite charge. (refer to tb for figure). The ions are arranged to form a cubic structure. It should be noted that the structure contains equal numbers of Na+ and Cl-.

Question 11

What is the structure of caesium chloride?

Answer 11

Caesium chloride (CsCl) also has a giant ionic structure, which is different from that of sodium chloride. In the structure of caesium chloride, cations (Cs+) and anions (Cl-) are held together by ionic bonds. Each caesium ion is surrounded by eight chloride ions and each chloride ion is in turn surrounded by eight caesium ions. (refer to tb for the figure)

Question 12

What are the properties of ionic compounds?

Answer 12

1. Conduct electricity in molten state or in aqueous solution
2. Usually have high melting points and boiling points
3. They are hard but brittle
4. Many of them are soluble in water, but all are insoluble in organic solvents

Question 13

Why do ionic compounds conduct electricity in molten state or in aqueous solution?

Answer 13

Ionic compounds are non-conductors of electricity in the solid state. This is because in the solid state, the ions present are not mobile. When the ionic compounds are in the molten state or in aqueous solution, the ions become mobile and the ionic compounds can conduct electricity. Ionic compounds are regarded as electrolytes.

Question 14

Why do ionic compounds usually have high melting points and boiling points?

Answer 14

A lot of energy is required to break the strong ionic bonds between ions during melting and boiling.

Question 15

Why are ionic compounds hard but brittle?

Answer 15

The oppositely charged ions are held together by strong ionic bonds which make the ionic compounds hard.
However, when under a great force, the relative movement of the ions bring the ions of the same charge close to each other. This will result in repulsion and the crystal will break. Therefore, ionic compounds are brittle.

Question 16

Why are many ionic compounds soluble in water, but all are insoluble in organic solvents?

Answer 16

Many ionic compounds interact strongly with water, so they are soluble in water. Attraction exists between the ions in the ionic compounds and water molecules. This attraction causes the ions to move away from the cyrstal and go into the water. Then water molecules surround the ions. The ions are said to be hydrated.

No such attraction exists between the ions in sodium chloride and molecules of organic solvents. Thus, sodium chloride is insoluble in organic solvents.

Question 17

What are simple molecular structures?

Answer 17

Most non-metal elements and covalent compounds are composed of simple, discrete molecules. These substances have a simple molecular structure. They may be solids, liquids or gases under room conditions.

The atoms in a molecule are bonded together by strong covalent bonds. However, each molecule is attracted to neighbouring molecules by weak intermolecular forces (e.g. van der Waals’ forces). These intermolecular forces are much weaker than ionic bond, covalent bond and metallic bond.

Question 18

What is the strcture of carbon dioxide?

Answer 18

Each carbon dioxide molecule consists of one carbon atom and two oxygen atoms covalently bonded together. Under room conditions, carbon dioxide is a gas. When carbon dioxide gas is cooled to temperatures below -78.5C at 1atm, it changes to a solid called dry ice directly without going through the liquid state.

Question 19

What is the structure of iodine?

Answer 19

Iodine is a solid under rom conditions. In an iodine crystal, iodine molecules pack together regularly and are held together by weak van der walls’ forces. They are still discrete molecules.

Question 20

What are the properties of substances with a simple molecular structure?

Answer 20

1. They are usually non-conductors of electricity
2. They have low melting points and boiling points
3. Solid with a simple molecular structure are usually brittle
4. They are usually slightly soluble or insoluble in water, but soluble in organic solvents.

Question 21

Why are substances with a simple molecular structure usually non-conductors of electricity?

Answer 21

Most simple molecular substances do not conduct electricity in their solid, liquid and gas states because they do not contain mobile ions or delocalized electrons.

However, some of them conduct electricity when in aqueous solutions. When such substances dissolve in water, mobile ions form. Examples include hydrogen chloride and ammonia. These substances can be regarded as electrolytes.

Question 22

Why do substances with simple molecular structure have low melting points and boiling points?

Answer 22

Under room conditions, simple molecular substances are gases, volatile liquids, or solids with low melting points. They have low melting points and boiling points because only a small amount of energy is needed to overcome the weak intermolecular forces between molecules. Thus the molecules can be separated quite easily.

In general, the larger the molecular size, the stronger will be the van der Waals’ forces between molecules.

Question 23

Why are solids with a simple molecular structure usually brittle?

Answer 23

As the molecules are held together by weak intermolecular forces, solids with a simple molecular structure are usually brittle.

Question 24

Why are substances with a simple molecular structure usually slightly soluble or insoluble in water, but soluble in organic solvents?

Answer 24

The attraction between water molecules is quite strong. The weak attraction between the molecules of simple molecular structure and water molecules is not strong enough to overcome the attraction between the water molecules. Thus, most simple molecular structures are only slightly soluble or insoluble in water.

In organic solvents, the molecules are held by weak attraction. Thus the molecules of simple molecular structures and molecules of the organic solvent can mix together more readily.

Question 25

What are giant covalent structures?

Answer 25

In some non-metal elements and covalent compounds, the atoms are joined by strong covalent bonds to form a three-dimensional giant network, called a giant covalent structure. Covalent bonds extend throughout the whole structure. There are no discrete molecules.

Question 26

What are the structure and properties of diamond?

Answer 26

Diamond is one form of carbon. It has a giant covalent structure. Each carbon atom is covalently bonded to four other carbon atoms, forming a three-dimensional giant network.

To break the structure, a large number of strong covalent bonds between carbon atoms must be broken
-> extreme hardness
-> very high melting point
Cannot conduct electricity because all the outermost shell electrons on each carbon have been used to form single covalent bonds. There are no delocalized electrons.

Question 27

What is the structure of graphite?

Answer 27

Graphite is another form of carbon. It also has a giant covalent structure. In graphite, the carbon atoms are arranged in flat, parallel layers. Each layer is a two-dimensional network of carbon atoms and contains many six-membered carbon rings.

In graphite, each carbon atom is covalently bonded to only three other carbon atoms in its layer, and one outer electron of each carbon atom is delocalized. These electrons are free to move from one six-membered carbon ring to the next within a layer.

Question 28

What are the properties of graphite?

Answer 28

1. conduct electricity
   (because it contains delocalized electrons)
2. very high melting point
   (because melting requires the breaking of many strong covalent bonds between carbon atoms within the layers.)
3. soft and easy to cleave
   (because the layers are held together by weak vander Waals’ forces and can be separated quite easily)
4. lubricating property
   (because the layers are held together by weak vander Waals’ forces, can easily slide over each other)

Question 29

What are the structure and properties of quartz?

Answer 29

Quartz is a crystalline form of silicon dioxide (SiO2), which has a giant covalent structure.
In the structure of silicon dioxide, each silicon atom is covalently bonded to four oxygen atoms. Each oxygen atom is bonded to two silicon atoms. Silicon and oxygen atoms are held together by covalent bonds throughout the whole structure.

Silicon dioide has a very high melting point & does not conduct electricity

Question 30

What are the properties of substances with a giant covalent structure?

Answer 30

1. solids with very high melting points
   (∵ a lot of energy is required to break the large number of strong covalent bonds)
2. all (except graphite) are hard
   (∵ a network of strong covalent bonds)
3. insoluble in any solvent
   (∵ atoms are held together by strong covalent bonds)
4. all (except graphite) are non-conductors of electricity
   (∵ no delocalized electrons)