9: Relating the Properties of Substances to Structures and Bonding

Question 1

What is the name of the structure of ionic solids?

Answer 1

Giant ionic structure

Question 2

Describe the structure of sodium chloride solid.

Answer 2

Sodium chloride consists of sodium ions and chloride ions that are packed together in a regular arrangement called a lattice. Billions of sodium ions and chloride ions are arranged in this way to make up a giant ionic structure. Ionic bonding extends throughout the whole structure. Each sodium ion is surrounded by 6 chloride ions and each chloride ion is surrounded by 6 sodium ions. They are arranged in a cubic pattern, so sodium chloride crystals are cubic in shape.

Question 3

What are the general properties of ionic compounds?

Answer 3

1. They are hard but brittle
2. They have high melting and boiling points
3. They are usually soluble in water
4. They conduct electricity in molten or aqueous states (they are electrolytes)

Question 4

Why are ionic compounds hard and brittle?

Answer 4

Ionic compounds are hard because the relative motion of ions is restricted by the strong ionic bonds between the ions.
Since an ionic compound forms a lattice of alternating positive and negative ions, when an external force is applied, the layers of ions may move relative to each other. This can bring ions of the same charge close to each other an result in replusion between the ions. As a result, the lattice splits.

Question 5

Why do ionic compounds have high melting and boiling points?

Answer 5

There are strong ionic bonds between the ions with opposite charges. In order to melt and boil ionic compounds, lots of strong ionic bonds between the ions have to be overcome. This takes a lot of heat, so ionic compounds have high melting points and boiling points. All ionic compounds are crystalline solids at rtp.

Question 6

Why are ionic compounds usually soluble in water?

Answer 6

In order for a substance to dissolve in a solvent:
1. the attractive forces between its particles and solvent particles > (can overcome) attractive forces between particles in the pure substance
2. particles in the substance must separarte from each other and become surrounded by the solvent particles.

Since water has partial positive and partial negative charges, the partial positive ends attract the negative ions while the partial negative ends attract the positive ions. The attraction is sufficient to remove the ions from the lattice.

Question 7

Why are ionic compounds usually insoluble in non-aqueous solvents?

Answer 7

The strong ionic bonding within the lattice > the weak van-der Waals’ forces between the ions and the solvent molecules. The solvent cannot pull the ions out of the lattice.

Question 8

What are the conditions that must be met for a substance to conduct electricity?

Answer 8

It must contain mobile charged particles.

Question 9

Why cannot ionic compounds conduct electricity in solid state?

Answer 9

The ions are in fixed positions and not free to move around (not mobile).

Question 10

Why can ionic compounds conduct electricity in molten or aqueous forms?

Answer 10

When an ionic compound is melted, the ions are mobile throughout the liquid. When electricity is passed through the molten compound, cations move towards a negative electrode while anions move towards a positive electrode, allowing the conduction of electricity.
When soluble ionic compounds are dissolved in water, these aqueous solutions can conduct electricity because the ions are free to move around in aqueous solutions.

Question 11

List 3 forms of elements that have giant covalent structures.

Answer 11

Carbon (in the form of diamond and graphite), silicon

Question 12

Give an example of a compound having giant covalent structure.

Answer 12

Silicon dioxide (found in quartz)

Question 13

What are allotropes?

Answer 13

Allotropes are two or more forms of the same element in which the atoms or molecules are arranged in different ways.

Question 14

Describe the structure of diamond.

Answer 14

A diamond has a giant covalent structure. In a diamond, each carbon atom is covalently bonded to 4 neighbouring carbon atoms. All the caron atoms are arranges tetrahedrally. The strong covalent bonds extend in all directions throughout the structure.

Question 15

Describe the structure of graphite.

Answer 15

In graphite, the carbon atoms are arrnaged in flat layers. Each layer contains billions of carbon atoms arranged in hexagons. Each carbon atom is covalently bonded to 3 neighbouring carbon atoms so every layer has a giant covalent structure. Only weak van der Waals’ forces exist between the layers of carbon atoms.

Question 16

Why can graphite conduct electricity?

Answer 16

With three covalent bonds formed between carbon atoms within the layers, an unbonded outermost shell electron is present on each atom. These unbonded electrons become delocalised along the layers of carbon atoms. They can only move within the layer.

Question 17

Describe the structure of silicon dioxide.

Answer 17

Each silicon atom is bonded to 4 oxygen atoms while each oxygen atom is bonded to only 2 silicon atoms. Silicon atoms and oxygen atoms in the whole structure are held together by strong covalent bonds.

Question 18

What is the chemical formula of silicon dioxide and why?

Answer 18

SiO₂: The ratio of atoms in silicon dioxide is two oxygen atoms for every one silicon atom. (Si:O = 1:2)

Question 19

What are the general properties of giant covalent substances and exceptions?

Answer 19

1. They are very hard (exception: graphite)
2. They have high melting points
3. They are insoluble in water or non-aqueous solvents
4. They do not conduct electricity (exception: graphite)

Question 20

Explain why giant covalent substances are hard, thus suggest a usage of diamond and quartz.

Answer 20

In giant covalent structures, strong covalent bonding extends throughout the structure. Relative motion of the atoms is restricted. This makes the substances very hard and useful as abrasives.
Sand paper is coated with grains of quartz.
Diamond is the hardest substance known. It is used for cutting and grinding hard materials such as glass and stone.

Question 21

Why do giant covalent structures have a high melting point?

Answer 21

In order to melt a giant covalent substance, lots of strong covalent bonds between atoms have to be broken. These bonds need a lot of heat to break. Thus, these substances have high melting points.

Question 22

Why are giant covalent substances insoluble in water or non-aqueous solvents?

Answer 22

Weak van der Waals’ forces between their atoms and solvent molecules < (is not strong enough to overcome) the strong covalent bonds that hold the atoms together.

Question 23

Why are giant covalent substances electrical insulators (except for graphite)?

Answer 23

In most giant covalent substances, all the outermost shell electrons are either held by individual atoms or used to form covalent bonds. There are no mobile electrons or ions. Thus, these substances do not consuct electricity.

Question 24

List 4 usages of graphite.

Answer 24

1. It can be used as a solid lubricant since the layers or carbon atoms can slide past each other easily.
2. It can be used to make pencil leads. When a pencil is used on paper, layers of carbon atoms flake off the structure, resulting in pencil marks.
3. It can be used to make brushes in electric motors.
4. It can be used to make electrodes.

Question 25

What is the general structure of simple molecular substances?

Answer 25

Simple molecular structures have strong covalent bands joining their atoms within each molecule. However, only weak van der Waals’ forces exist among the molecules.

Question 26

What are the general properties of simple molecular substances?

Answer 26

1. They are soft if they are solids.
2. They have low melting and boiling points
3. They are often slightly soluble or insoluble in water, but soluble in non-aqueous solvents
4. They do not conduct electricity.

Question 27

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

Answer 27

Only weak van der Waals’ forces among molecules are needed to be overcome when simple molecular substances are melted or boiled. The covalent bonds within each molecule are not broken in the process. Very little heat is needed to overcome the attractive forces among the molecules and separate the molecules. Thus, simple molecular substances have low melting points and boiling points.

Question 28

What is a common property of liquids having simple molecular structure?

Answer 28

They are volatile / they evaporate easily.

Question 29

Explain why iodine is soluble in cyclohexane.

Answer 29

When iodine is added to cyclohexane, weak van der Waals’ forces between molecules of iodine and cyclohexane is about the same as the weak van der Waals’ forces in the pure substances. Thus, the iodine molecules and cyclohexane molecules mix together easily. Dissolving iodine in cyclohexane does not decrease the stability of the two substances.

Question 30

Explain why iodine is only slightly soluble in water.

Answer 30

The strong charged attraction (hydrogen bonds) between water molecules > the weak van der Waals’ forces between iodine molecules and water molecules. Thus, water molecules tend to stick together, rather than allowing iodine molecules to come between them. As a result, iodine molecules and water molecules do not mix easily.

Question 31

Why cannot simple molecular substances conduct electricity?

Answer 31

They do not contain mobile ions or delicalised electrons, so they cannot conduct electricity.

Question 32

What is the general structure of metals?

Answer 32

Metals form giant metalic structures in which a regular three-dimensional arrangement of positive metal ions is surrounded by a “sea” of delocalised electrons. The metal ions are packed closely together.

Question 33

What are common characteristics of metals and what are some exceptions?

Answer 33

1. They are solids at rtp (room temperature & pressure) except for mercury which is a liquid.
2. They are shiny when there is a large and smooth surface.
3. They are usually strong and have a high density. (exception: sodium which is soft)
4. They usually have high melting and boiling points. (exceptions: mercury being liquid and sodium having melting point of below 100C)
5. They are good conductors of heat and electricity.
6. They are malleable (can be pressed) and ductile (can be pulled into wires).

Question 34

Why do metals have high density?

Answer 34

The positive metal ions are very closely packed in the structures of metals.

Question 35

Why do metals generally have high melting points?

Answer 35

In a piece of metal, there are strong attractive forces between their positive metal ions and the “sea” of delocalised electrons. To melt the metal, a lot of heat is needed to overcome the attractive forces. Hence most metals have high melting points.

Question 36

Why does potassium have a lower melting point than sodium?

Answer 36

Potassium has a larger atomic size than sodium, but potassium ions have the same charge as sodium ions. Therefore, potassium has a smaller charge-to-size ratio than that of sodium. Therefore, the attraction force is weaker and potassium has a lower melting point.

Question 37

Why does magnesium have a higher melting point than sodium?

Answer 37

Magnesium ions have a larger charge (2+) than sodium ions, while they are from the same period, meaning that they have similar size. Therefore, magnesium has a larger charge-to-size ratio than that of sodium. In addition, each magnesium atom releases 2 electrons to become a magnesium ion, while sodium atoms only release 1 electron. Magnesium has a higher electron density. Therefore, the attraction force is stronger and magnesium has a higher melting point.

Question 38

Why are metals good conductors of electricity?

Answer 38

Metals conduct electricity due to the delocalised electrons within their structures. When a piece of metal is connected in a circuit with a power supply, the electrons flow towards the positive electrode. At the same time, electrons are fed into the other end of the metal from the negative electrode. The flow of electrons forms an electric current.

Question 39

Why are metals good conductors of heat?

Answer 39

When a piece of metal is heated, the kinetic energy of electrons in the structure increases. The delocalised electrons in the heated region move around faster and conduct the heat rapidly to other parts of the metal.

Question 40

Why are metals malleable and ductile?

Answer 40

When you apply a force to a piece of metal, the layers of metal ions slide over each other to new positions. The delocalised electrons are free to move in the structure, but the ions are still held together in their new positions by the “sea” of electrons. The metal now has a different shape.

Question 41

Why are simple molecular solids soft?

Answer 41

Simple molecular solids are usually soft due to the weak van der Waals’ forces among the molecules. It is easy to break the solids.

Question 42

List two properties of graphite that are different from other giant covalent substances.

Answer 42

1. Only weak van der Waals’ forces exist between the layers of carbon atoms, so the layers can slide past each other easily. Therefore, graphite is soft and feels slippery to touch.
2. Graphite is a conductor of electricity.