chemical bonding & structures

Question 1

Definition of ionic bonds

Answer 1

Ionic bonds are strong electrostatic forces of attraction between oppositely changed ions, metals and non metals

Question 2

Structure of ionic compounds

Answer 2

• In the solid state, the actual structure consists of a continuously repeating three-dimensional lattice with an uncountable large number of formula unit of positive and negative ions
• ions are closely packed, arranged in an orderly manner and held in fixed positions by strong elect static forces of attraction between oppositely-changed ions
• the structure formed by ionic compounds is known as giant ionic structure
• the electrostatic attraction of each ion affects all the other ions around it. It acts equally in all directions

Question 3

Structure of sodium chloride

Answer 3

• Sodium chloride has a giant ionic structure
• in the structure of sodium chloride, the ratio of sodium ions to chloride ions is 1 : 1
• each sodium ion is surrounded by 6 chloride ions
• Each chloride ion is surrounded by 6 sodium ions

Question 4

Physical properties of ionic compounds - high melting point

Answer 4

• Ionic compounds generally have high melting points
• they generally exist as solid at room temperature and pressure
• explanation: ionic compounds, such as sodium chloride, have giant ionic structure. Large amount of energy is required to overcome the strong electrostatic forces of attraction between oppositely-changed ions

Question 5

Physical properties of ionic compounds - conductor of electricity in molten and aqueous states

Answer 5

• In the solid state, ionic compounds cannot conduct electricity
• ionic compound conduct electricity only when in molten state or aqueous state
• molten hate is a state when a solid substance has melted. Aqueous state to a state when a substance has dissolved in water
• explanation: ionic compounds, such as sodium chloride, have giant ionic structure.in the solid state, the oppositely charged ions are held in a fixed position by strong electrostatic forces of attraction between oppositely-changed ions. Thus, ions are not free-moving and hence cannot conduct electricity.in the molten or aqueous state, the strong electrostatic forces of attraction between oppositely changed ions have been overcome. The ions are free-moving and hence can conduct electricity.

Question 6

Physical properties of ionic compounds - soluble in water but insoluble in organic solvent

Answer 6

• Ionic compounds are generally soluble in water but insoluble in organic solvent.
• examples of organic solvent are ethanol, trichloromethane, turpentine, and benzene
• examples of ionic compounds that and insoluble in water: silver chloride, barium sulfate

Question 7

Uses of ionic compounds - high melting point

Answer 7

• Used as refractory materials
• refracting materials are heat resistant , the melting point of magnesium oxide in 2852°C. It is used to line the inner surface of a high temperature furnace

Question 8

Covalent bonding

Answer 8

• covalent bonds are generally formed between non-metal atoms by the sharing of electrons

Question 9

Electronegativity

Answer 9

• Electronegativity refers to the ability of an atom to attract a shared pair of electrons towards itself in a covalent bond
• electronegativity increases across a period
• electronegativity decreases down a group
• fluorine is the most electronegative atom

Question 10

Non-polar and polar covalent bonds

Answer 10

• if the two atoms involved in covalent bond are either the same or have similar electronegativity, a non-polar covalent bond results. The two atoms have equal pull on the shared pair of electrons.
  -If the two atoms involved in covalent bonding have different electronegativity values, the pair of shared electrons are not equally shared between the two atoms and polar covalent bond results ( if atom B is more electronegative than A,the bonding electrons are nearer to B in the bond )
• in a polar covalent bond, the more electronegative atom acquires a partial negative charge and the less electronegative a tem acquires a partial positive charge

Question 11

Structure of simple molecular substances

Answer 11

• Simple molecular substances consist of many simple molecules
• within each molecule, the atoms are held together by strong covalent bonds
• however, the molecules are held together by weak intermolecular forces of attraction
• the structure of simple molecular substances is known as simple molecular structure
• examples: water, methane, carbon dioxide, ammonia, iodine.

Question 12

Properties of simple molecular substances - low melting point and boiling point

Answer 12

• Simple molecular substances generally have low melting point and boiling point
• they generally exist as liquids or gases at room temperature
• explanation: methane example. Methane has a simple molecular structure. Small amount of energy is required to overcome the weak intermolecular forces of attraction. Therefore, methane has a low melting and boiling point

Question 13

Properties of simple molecular structure - non-conductor of electricity in any state

Answer 13

• Simple molecular substances generally do not conduct electricity in any state
• Explanation: using methane. Methane has a simple molecular structure. There are no free moving valence electrons available to conduct electricity

Question 14

Properties of simple molecular structure - insoluble in water but soluble in organic solvents

Answer 14

• Simple molecular substances are generally insoluble in water but soluble in organic solvents

Question 15

Uses of simple molecular substances

Answer 15

Volatile - uses in perfume and flavorings, in room deodorants, insect repellents

Question 16

Structure of giant molecular substances

Answer 16

• Besides forming simple molecules, atoms can form a network of strong covalent bonds throughout the structure
• this will result in the formation of a giant molecular structure
• examples: diamond, graphite, silicon dioxide

Question 17

Allotropes of Carbon

Answer 17

• Allotropes are different forms of the same element. Allotropes may have different properties such as hardness and electrical conductivity
• two well-known allotropes of carbon are diamond and graphite

Question 18

Structure of diamond

Answer 18

• diamond has a giant molecular structure
• in the structure of diamond, each Carbon atom is covalently bonded to four other Carbon atoms in a tetrahedral arrangement

Question 19

Properties of diamond - high melting point

Answer 19

• Diamond has very high melting point, about 3550°C
• explanation: diamond has a giant molecular structure. A large amount of energy is required to break strong covalent bonds between carbon atoms, therefore, diamond has a high melting point

Question 20

Properties of diamond - diamond does not conduct electricity

Answer 20

• Explanation: diamond has a giant molecular structure. All the four valence electrons of each carbon atom are used for covalent bonding, so there are no free-moving valence electrons present to conduct electricity

Question 21

Properties of diamond - solubility (in water and organic solvents)

Answer 21

• Diamond is insoluble in water and organic solvents

Question 22

Properties of diamond - diamond is hard

Answer 22

• All the carbon atoms are hold by strong covalent bonds throughout the structure, hence diamond is hard
• due to its hardness, diamond is often used as drill tips and polishing tools

Question 23

Uses of diamond

Answer 23

• hard: tips of cutting, grinding, and polishing tools

Question 24

Structure of graphite

Answer 24

• Graphite has a giant molecular layered structure
• graphite is made up of many layers of hexagonal rings of carbon atoms
• each carbon atom forms strong covalent bonds with three other carbon atoms, in hexagonal arrangement in layers
• the carbon atoms are held together by strong covalent bonds but the layers are held together by weak intermolecular forces of attraction

Question 25

Properties of graphite - graphite has high melting point

Answer 25

• Graphite has very melting point, about 3650°C
• explanation: graphite has a giant molecular layered structure. A large amount of energy is required to break the strong covalent bonds between the carbon atoms. Therefore, graphite has a high melting point.

Question 26

Properties of graphite - graphite can conduct electricity

Answer 26

• Explanation: each carbon atom uses only three out of four valence electrons in bonding. The valence electrons not used in bonding are free-moving to conduct electricity

Question 27

Properties of graphite - solubility of graphite

Answer 27

• Graphite is insoluble in water and organic solvents

Question 28

Properties of graphite - graphite is soft and slippery

Answer 28

• Graphite has a giant molecular layered structure
• the layers of carbon atoms in graphite are held together by weak intermolecular forces of attraction. The layers of atoms are able to slide over one another when a force is applied

Question 29

Uses of graphite

Answer 29

• Soft and slippery: in pencil lead. Since it is soft, the layers of carbon atoms slide off the pencil onto the paper easily. As a soft lubricant to reduce friction in machinery.
• conductor of electricity: in brushes for electric motors and as inert electrodes

Question 30

Structure of silicon

Answer 30

• Giant molecular structure
• high melting point
• non-conductor of electricity
• hard solid
• insoluble in water and organic solvent

Question 31

Structure of silicon dioxide

Answer 31

• Giant molecular structure
• high melting point
• non-conductor of electricity
• hard solid
• insoluble in water and organic solvent

Question 32

Structure of macromolecules

Answer 32

• Covalent substance
• polymers are a class of substances that can be classified as macromolecules
• a polymer or macromolecule consists of many simple covalent molecules or simple molecular substances joined together into chains of much larger molecules (strong covalent bonds between non-metal atoms within each large molecule )
• examples (natural): silk, wool, starch, rubber
• examples (man-made): poly, nylon, polyester
• the huge variety of polymers result in a wide range of physical properties such as hardness and flexibility, allowing them to be made into many different products

Question 33

Properties of polymer/macromolecules - melting and boiling point

Answer 33

• Most macromolecules are solid at room temperature and pressure due to their large molecule size, weak intermolecular forces of attraction are strong enough for it to be solid at rtp
• due to the range of sizes of macromolecules, they do not have a fixed melting or boiling point
• instead, they typically soften over a range of low temperatures when heated as the weak intermolecular forces of attraction between the large molecules are overcome by molecular vibrations with higher kinetic energy

Question 34

Properties of polymer/macromolecules - solubility

Answer 34

• Most macromolecules are insoluble in water but soluble in organic solvents

Question 35

Properties of polymer/macromolecules - electrical conductivity

Answer 35

• most macromolecules are not able to conduct electricity in any states as they do not have mobile ions or electrons

Question 36

Structure of pure metals

Answer 36

• In any pure metal, metal atoms are closely packed together in the solid state to form giant metallic structure
• all metal atoms are of the same size and regularly arranged in layers within the metallic lattice.
• the metal atoms in the giant metallic structure lose their valence electrons to become positive ions
• the valence elections are delocalised and can move freely between the metal ions resulting in a ‘sea’ of free-moving valence elections
• metallic bond is the strong electrostatic forces of attraction between the positive metal ions and the ‘sea’ of free-moving valence electrons

Question 37

Structure of alloys

Answer 37

• Alloys are a type of solid-solid mixture with one or more other elements
• In alloys, the regular arrangement of metal atoms is disrupted by the addition of other elements with atoms of different sizes which are randomly distributed
• this disrupts the orderly arrangement of atoms in pure metals and hence there is a difference in properties between alloys and pure metal

Question 38

Properties of metals and alloys - metals have high melting point

Answer 38

• most metals have high melting point
• explanation: metal has a giant metallic structure. A large amount of energy is needed to overcome the strong electrostatic forces of attraction between the positive metal ions and the ‘sea’ of free-moving valence electrons (exception: mercury)
• alloys melt over a range of temperatures

Question 39

Properties of metals and alloys - pure metals and alloys can conduct electricity in all states

Answer 39

• Explanation: Metal has a giant metallic structure. There is a ‘sea’ of free-moving valence electrons present to conduct electricity.

Question 40

Properties of metals and alloys - solubility of metals and alloys

Answer 40

• Metals and alloys are insoluble in water and organic solvents

Question 41

Properties of metals and alloys - ductility and malleability

Answer 41

• Ductile: metals can be stretched into wires without breaking
• malleable: metals can be hammered into different shapes
• pure metals are ductile and malleable while alloys are harder and stronger

Question 42

Explanation for why pure metals are malleable and ductile

Answer 42

• Metals have giant metallic structure
• metal ions are arranged regularly. When a force is applied, the layer of metal ions can slide over one another easily. Therefore, metals are malleable and ductile

Question 43

Explanation for why alloys are harder and stronger than pure metals

Answer 43

• In an alloy, the metal atoms and atoms of the added element are of different sizes
• as this disrupts the orderly arrangement of atoms in pure metals, it is much harder for the atoms to slide over each other when a force is applied.
• hence, alloys are harder and stronger than the pure metals they are made from, they tend to be less malleable and ductile

Question 44

uses of metals and alloys

Answer 44

• metals are good conductor of electricity, ductile: metals are used in electric wires and in electric goods. the most common metal used is copper.
• high melting and boiling points of metals: tungsten metal has a high melting point of 3410°C. it is used to make the filaments of light bulbs.
• increased hardness and strength of alloys: bronze and brass alloys are used in making more durable items (eg bells). steel is used to make items that can withstand heavy loads (eg warehouse shelves).