Structures

Question 1

What are the types of structure in a substance? What are they?

Answer 1

Giant metallic - found in metals, where metallic bonds hold together positive metals via a “sea of delocalised electrons”.

Giant ionic - found in compounds of metals with non-metals. A lattice of positive metal, and negative non-metal ions, held together by electrostatic attractions.

Giant covalent - very large covalent molecules, where atoms are joined by covalent bonds throughout the three dimensional structure.

Molecular covalent - a.k.a “covalent molecular” or “simple covalent” or simple “simple molecular”. Molecular covalent molecules have strong covalent bonds between atoms within a molecule and weak attractions between separate molecules.

Question 2

What is the definition for a melting point?

Answer 2

Melting point: the temperature at which a solid changes into a liquid.

Question 3

What is the definition for a boiling point?

Answer 3

Boiling point: the temperature at which a liquid changes into a gas.

Question 4

What is the definition for electrical conductivity?

Answer 4

Electrical conductivity: the ease with which electricity passes through a substance.

Question 5

What is the definition for thermal conductivity?

Answer 5

Thermal conductivity: the ease with which heat passes through a substance.

Question 6

What is the equation for density?

Answer 6

Density: g/cm³ = Mass (g) / Volume (cm³)

Question 7

What is the definition for hardness?

Answer 7

Hardness: the ability of a substance to resist being scratched.

Question 8

What is the definition for strength?

Answer 8

Strength: how difficult it is to pull apart (tensile) or crush (compressive).

Question 9

What is the definition for flexibility?

Answer 9

Flexibility: the ability of a substance to bend without breaking.

Question 10

What is the definition for malleability?

Answer 10

Malleability: the ability of a substance to be hammered or beaten into flat sheets without breaking.

Question 11

What is the definition for ductility?

Answer 11

Ductility: the ability of a substance to be pulled into long thin wires.

Question 12

What is the definition for solubility in water?

Answer 12

Solubility in water: the maximum mass (g) of a solute that can dissolve in 100g of water at a stated temperature.

Question 13

What is the definition solubility in organic solvents?

Answer 13

Solubility in organic solvents: the maximum mass (g) of solute that can dissolve in 100g of a solvent like tetrachloromethane at a stated temperature.

Question 14

What are giant ionic bonds made up of? What are they?

Answer 14

A compound formed from a metal and non-metal element does so by ionic bonding. For example, when sodium reacts with chlorine, many positive sodium ions and many negative chlorine ions form. These ions are attracted to each other (because of their opposite charges) and a regular three dimensional lattice is formed - this is known as a giant ionic.

Question 15

What are the general properties of a giant ionic structure? Explain.

Answer 15

1. High melting and boiling points: Ionic bonds are very strong and they extend in three dimensions throughout the giant structure. Therefore a lot of heat energy is required to separate the ions and break the lattice, so a high temperature is required.
2. Brittle: bending causes the ions to move into positions where ions of the same charge are opposite each other. This leads to repulsion between layers which then split apart.
3. Electrical insulators when solid: ions held in fixed positions so cannot move to carry a current.
4. Electrical conductors as molten liquids or aqueous solutions: melting the solid or dissolving it in water allows positive and negative ions to move and carry an electric current between electrodes. The ions are charge carriers.
5. Usually soluble in water but insoluble in organic solvents such as tetrachloromethane

Question 16

Give an example of an ionic substance.

Answer 16

NaCl (Sodium Chloride)

Question 17

What happens when you heat NaCl?

Describe what happens if you try and crush some NaCl crystals?

Does NaCl conduct electricity.

When you add a few NaCl crystals to water what happens?

What happens when you add a few NaCl crystals to cyclohexane?

Answer 17

The heat has no effect - to break the strong ionic bonds of NaCl, a large temperature is required.

Ionic structures are very brittle and so the NaCl shatter easily, forming a white powder. When ions are moved slightly they come opposite ions of the same charge, they repel and then the structure shatter.

NaCl does not conduct electricity. The positive and negative ions are fixed in positions in the solid and cannot move, thus they cannot carry an electric current.

NaCl dissolves easily into a colourless solution. The ions in NaCl interact with the water molecules and move apart and occupy spaces between the water molecules in a solution.

NaCl doesn’t dissolve however in cyclohexane. The ions are not able to interact with the molecules of the organic solvent and so it does not dissolve.

Question 18

What are molecular covalent substances made up of?

Answer 18

Molecular covalent substances are made up of small covalent molecules. The forces and attraction between molecules however is weak. The weak attraction holding the molecules together are called Van Der Waal’s forces.

The atoms themselves within each molecule are held together very strongly via strong covalent bonds.

Question 19

What are the general properties of molecular covalent substances?

Answer 19

Very low melting and boiling points: very little heat energy is needed to break the very weak bonds between separate molecules. Hence why many molecular covalent are gases at room temperature. Remember that no covalent bonds are broken when a molecular covalent substance melts - only the Van Der Waal’s forces are broken.

They are brittle and soft if solid because of the weak forces between molecules.

Electrical insulators: their molecules are not charged.

They are insoluble in water or have a very low solubility.

Soluble in organic solvents such as tetrachloromethane, CC1₄.

Question 20

Give two examples of molecular covalent substances which are solids at 20 degrees C.

Answer 20

Iodine and Sulphur.

Question 21

Give three examples of molecular covalent substances which are liquids at 20 degrees C.

Answer 21

Bromine, Water and Ethanol.

Question 22

Give three examples of molecular covalent substances which are gases at 20 degrees C.

Answer 22

Oxygen, Nitrogen and Carbon dioxide.

Question 23

What happens when you heat iodine?

Describe what happens if you try and crush some iodine crystals?

Does iodine conduct electricity.

When you add a few iodine crystals to water what happens?

What happens when you add a few iodine crystals to cyclohexane?

What happens when you add a few iodine crystals to ethanol?

Answer 23

The dark grey solid gives off a purple coloured gas upon heating. This occurs because the neighbouring iodine molecules are only attracted to each other by weak Van Der Waal’s forces which can be easily broken causing iodine to sublime (transition directly from solid to gas).

The dark grey crystals shatter easily upon impact. The weak Van Der Waal’s forces between molecules are broken easily, causing iodine to be very brittle.

Iodine does not conduct electricity because it is made up of electrically neutral molecules which cannot carry a current.

Iodine does not dissolve in water because it has a very low solubility in water.

Iodine is however very soluble in organic solvents like cyclohexane. Iodine is also very soluble in ethanol.

Question 24

What are giant covalent substances?

Answer 24

In giant covalent structures atoms are covalently bonded to other atoms in a regular and repeating way, producing a three dimensional lattice or “giant covalent structure”.

Question 25

What is valency?

Answer 25

Valency is the number of covalent bonds an atom can form.

For example, carbon atoms each have a valency of four, meaning that in a giant covalent structure each carbon atom can form a bond with three OTHER atoms (and including itself this gives four).

Question 26

How is graphite composed in structure?

Answer 26

Graphite is a giant covalent substance, consisting of layers of carbon atoms joined covalently in a hexagonal repeating pattern where each carbon atom forms strong covalent bonds with three surrounding carbon atoms. These flats sheets stack on top of each other and because they are only weakly held layer to layer, they can easily slide over each other.

Question 27

What are the properties of carbon graphite?

Answer 27

1. Very high melting and boiling points. To melt graphite three strong covalent bonds must be broken to free each carbon atom throughout the giant structure. This takes a lot of heat energy so a very high temperature is needed.
2. Hardness: very soft and slippery. Graphite has a layered structure, meaning that one layer is held to another by weak Van Der Waal’s forces, allowing each layer to slide easily over each other.
3. Graphite is a good electrical conductor: carbon is unique among non-metals in this way. The only solids that conduct electricity are metals and carbon (graphite and graphene). The (one) free electron from each carbon atom in graphite are able to move throughout the structure and carry an electric current.
4. Graphite is insoluble in water and organic solvents.
5. Graphite is opaque and black. It does not allow light to pass through it.

Question 28

What the definition of an allotrope?

Answer 28

Allotropes are different crystalline forms of the same element in the same physical state.

Question 29

What allotropes does carbon have? What are they?

Answer 29

Graphite, graphene and diamond are all allotropes of carbon.

Question 30

How is diamond composed in structure?

Answer 30

Diamond is a giant covalent structure made up of carbon atoms. In diamond each carbon atom forms strong covalent bonds with four surrounding carbon atoms in a repeating tetrahedral structure.

The tetrahedral unit is repeated millions and millions of times to form a three dimensional lattice where all the carbon atoms are connected together in one enormous molecule: a giant covalent structure.

Question 31

What are the properties of diamond?

Answer 31

1. Very high melting and boiling points. To melt diamond four strong covalent bonds must be broken to free each carbon atom throughout the giant structure. This takes a lot of heat energy so a very high temperature is needed.
2. Very hard. Diamond is the hardest known material - it can only be scratched by another diamond. The tetrahedral structure makes diamond very rigid. The covalent bonding between the carbon atoms is very strong. Both of these factors makes it very difficult to penetrate into the structure and scratch it.
3. Diamond is a good electrical insulator because there is no free electrons (or lone electrons) in the outer shells of the carbon atoms. Therefore diamond cannot conduct electricity because it has no free electrons to carry a current.
4. Diamond is insoluble in water and organic solvents.
5. It is transparent and colourless: diamond will allow light to pass through it.

Question 32

How is graphene composed?

Answer 32

Graphene consists of a layer of carbon atoms joined covalently in a hexagonal repeating pattern where each carbon atom forms strong covalent bonds with three surrounding carbon atoms. The flat sheet is a single atom thick. The carbon atoms are arranged in hexagons.

Question 33

What are the properties of graphene?

Answer 33

1. Very high melting and boiling points. To melt graphene three strong covalent bonds must be broken to free each carbon atom throughout the giant structure. This takes a lot of heat energy so a very high temperature is needed.
2. Hardness: very strong. Graphene is 100 times stronger than the strongest steel. In graphene each carbon atom forms three strong covalent bonds in the layer of atoms. Graphene is only one atom thick and has a very low density.
3. Graphene is a good electrical conductor: carbon is unique among non-metals in this way. The only solids that conduct electricity are metals and carbon (graphite and graphene). The (one) free electron from each carbon atom in graphene are able to move throughout the structure and carry an electric current.
4. Graphene is insoluble in water and organic solvents.
5. Graphene is the thinnest material possible being only one atom thick and allows light to pass through it.

Question 34

Why do the allotropes of carbon each have different melting points but the same boiling point?

Answer 34

They have different melting points because they have different structures. They have the same boiling point because it is always liquid carbon that is boiling.

Question 35

What happens when you heat carbon graphite?

Describe what happens if you try and scratch carbon graphite?

Does carbon graphite conduct electricity.

When you add a few carbon graphite crystals to water what happens?

What happens when you add a few carbon graphite crystals to cyclohexane?

Answer 35

To melt graphite three strong covalent bonds mush be broken to free each carbon atom. This requires a lot of heat energy and thus a high temperature.

The graphite rubs off onto paper and onto skin. It is a very soft and slippery material. Graphite has a layered structure - layers that can easily slip over each other by breaking the very weak Van Der Waal’s forces that hold the layers together.

Each carbon atom in graphite has a free electron not involved in bonding. These delocalised electrons are free to move throughout the structure and carry an electric charge, The electrons are charge carriers.

Giant covalent structures are too strongly bonded together for the atoms to separate and dissolve in water.

Giant covalent structures are also too strongly bonded together for the atoms to separate and dissolve in any solvent.

Question 36

What happens when you heat sand?

Describe what happens if you try and scratch quartz?

Does quartz conduct electricity.

When you add a few grains of sand to water what happens?

What happens when you add a few grains of sand to cyclohexane?

Answer 36

Heating has no effect because to melt sand many covalent bonds must be broken to free each atom. This requires a lot of heat energy and so a high temperature is needed.

The piece of quartz remains unscratched. The tetrahedral structure of diamond is found in quartz. Meaning that quartz too is very rigid and the covalent bonding is also very strong. It is therefore very difficult to penetrate into the structure and scratch it.

Quartz isn’t a conductor because like diamond quartz is an electrically neutral molecule and has no free electrons to carry a current.

Giant covalent structures are too strongly bonded together for the atoms to separate and dissolve in water. This is the same for any solvent.

Question 37

What are the different uses for graphite, graphene and diamond?

Answer 37

Graphite is used as a lubricant and in pencils.

Diamond is used in cutting tools.

Graphene is used in solar panels and batteries.

Question 38

How are the atoms composed in a giant metallic substance?

Answer 38

In metals the atoms are packed closely together in a regular pattern: a regular lattice. The outer electrons of the metal atoms are lost and leave behind a regular of positive metal ions. The electrons do not stay around their parent atom but move freely between the positive metal ions and throughout the lattice, The free electrons are said to be “delocalised”. The positively charged metal ions are held together by their attraction to the free electrons between them. These forces are called metallic bonds.

Question 39

What are the general properties of metals?

Answer 39

High melting points - most metals change from solid to liquid at a high temperature. It takes a lot of heat energy to break the strong metallic bonds which extend in three dimensions. So a high temperature is necessary.

Good electrical conductors: the free electrons can move through the lattice and carry an electric current.

Good heat conductors - It is the free electrons within the metal that take in heat energy which makes them move faster and spread heat through the lattice.

Malleable - many metals can be bent or hammered into new shapes without breaking.

Ductile - many metals can be drawn into thin wires.

A metallic lustre - shiny. Freshly cut or polished metal surfaces reflect light well.

High strength - most metals have high tensile strength (difficult to pull apart) and high compressive strength (difficult to crush). This is caused by the strong metallic bonds throughout the giant metallic lattice.

Question 40

Why are metals malleable and ductile?

Answer 40

Malleability and ductility are caused by the metal ions being able to slide over each other without the metallic bonds breaking. During the movement of the larger positive metal ions the electrons move to stay between them, so a strong attraction maintained and the structure is held together.

Question 41

What is the definition for an alloy?

Answer 41

An alloy is a mixture of two or more elements, at least one which is a metal, and the resulting mixture has metallic properties.

Question 42

What does adding different ions to metals do to its properties?

Answer 42

The presence of different ions in the metal prevents the layers of metals ions sliding over each other. This makes the alloy harder. For example brass (an alloy of zinc and copper) is harder than copper.

Question 43

How do you calculate the percentage of gold in an alloy?

Answer 43

Percentage of Gold in alloy = (Number of carats/24) X 100

Question 44

What happens to covalent bonds in molecules when the covalent molecular substance changes state?

Answer 44

The covalent bonds in molecules do not break when covalent molecular substances change state.

Question 45

Why do molecular covalent structures not conduct electricity?

Answer 45

Covalent molecular substances do not conduct electricity because the molecules are neutral and there are no charged particles (no ions or electrons) to move and carry a charge (and thus conduct).

Question 46

What is graphene?

Answer 46

A single-atom thick layer of graphite with three strong covalent bonds between each carbon atom, arranged in hexagons.