Chemistry – Module 4 – Energy
- Living organisms make compounds which are important sources of energy.
- Outline the role of photosynthesis in transforming light energy to chemical energy and recall the raw materials for this purpose.
Photosynthesis is the process by light energy is trapped by the chlorophyll in the plants leaves and is used to transform the raw materials of carbon dioxide and water into glucose and oxygen. Photosynthesis occurs in all green plants and is the source of life of all organisms. Without photosynthesis, no organism would be able to use the sun’s energy, thus everything would perish.
Light
Carbon Dioxide + Water Glucose + Oxygen
Chlorophyll
Photosynthesis is an endothermic reaction as it absorbs energy from sunlight
It is the process by which light energy is converted into chemical energy which is stored as glucose. It occurs only in green plants.
When the above chemical reaction occurs, 2830kJ of energy is absorbed per mole of glucose formed. Thus the overall role of photosynthesis is to capture the light energy of the sun and transform it into chemical energy which is stored in plants.
- outline the role of the production of high energy carbohydrates from carbon dioxide as the important step in the stabilisation of the sun‘s energy in a form that can be used by animals as well as plants.
Carbohydrates are compound of hydrogen, carbon and oxygen. Common carbohydrates include glucose, starch and cellulose.
Photosynthesis is a complex multi-step reaction brought about by the chlorophyll in the leaves plants. Thus the energy that is captured by the plant is chemically stored in glucose.
Carbohydrates in plants are the energy source of animals.In cellular respiration, the stored chemical energy is made available to the organism through the following equation:
Glucose + Oxygen Carbon Dioxide + Water + Energy
The amount of energy released during respiration is the same as was absorbed during photosynthesis, namely 2830kJ per mole of glucose. Some of this energy is used for daily activities whereas the majority is dissipated as heat. A small portion of this energy is transformed into protein and fat (or lipids).
Carbohydrates are considered to be high energy compounds because when they react chemically as in respiration they release large amount of energy.
Note: Virtually all the solar energy trapped by plants ends up as heat in the environment to be re-radiated into space.
Note: The longer the food chain, the more inefficient it is.
All forms of life on Earth are dependant upon sunlight for their supply of energy; without the sun there would no life as we know it. Production of carbohydrates by photosynthesis is the main way in which solar energy is collected for use by plant.
- Identify the photosynthetic origins of the chemical energy in coal, petroleum and gas.
Plants harvest solar energy to live and grow, and animals live eating plants. Normally, when plants and animals die, they are decomposed and they are converted back into carbon dioxide, water and nutrients which are released back into the environment, so completing the materials and energy cycles that are part of the living world.
However in certain locations, instead of being fully decomposed to carbon dioxide and water, some plant and animal material was only partially decomposed and remained stored in the Earth as energy-rich compounds. These are known as fossil fuels. They were mainly formed due to intense pressure and extremely high temperatures for millions of years.
Energy rich compounds are those that release large amounts of energy when they undergo chemical reactions. The stored energy is known as chemical energy. By burning fossil fuels we are able to release the stored chemical energy within them.The common fossil fuels are coal, crude oil and natural gas.
Living matter is mainly made up of compounds of carbon. Therefore fossil fuels were naturally synthesized by geological processes acting upon compounds of carbon. Thus it is not surprising when we see that fossil fuels themselves are compounds of carbon.
Dead Plants and Animals
Underground Temperature and Pressure for millions of years
Without Bacterial ActionWith Bacterial Action
Coal (Solid)Crude Oil (liquid)
Natural Gas (Gas)
The origin of chemical energy in fossil fuels in form the sun. Living organisms obtain energy directly or indirectly from the sun via photosynthesis.
- process and present information from secondary sources on the range of compounds found in either coal, petroleum or natural gas and on the location of deposits of the selected fossil fuel in Australia
Fuel / Major Australian Resources
Black coal / BowenBasin in Qld There is very little coal in the
SydneyBasin in NSW western half of the continent.
Brown coal / LatrobeValley in Victoria
Natural Gas / GippslandBasin – Victoria
CooperBasin – South Australia
Crude Oil / Bass Strait in Victoria (will be exhausted in a few decades).
Coal is a particularly important fossil fuel NSW as we use it to generate most of our electricity. Burning coal produced steam which drives the turbines to make electricity. Coal is found in every state in Australia, but Nsw, Vic and Qld are our main producers.
Coal is a rot consisting mainly of carbon (50-98%), some hydrogen (3-13%) and oxygen. Coal can also contain small amount of other elements such as sulfur and nitrogen. Australian coal generally has low sulfur content and most of Australia’s coal is found in 5 basins – Sydney, Bowen, Clarence-Morton, Surat ad Gippsland.
- There is a wide variety of carbon compounds.
- Identify the position of carbon in the periodic table and describe its electron configuration.
The element Carbon is located in group 4 of the periodic table. It is also located in period 2 and has atomic number of 6. Carbon’s electron configuration is 2,4 indicating the presence of four valence electrons. Carbon is a non-metal but is able to conduct electricity when in the form of graphite.
- Describe the structure of the diamond and graphite allotropes and account for their physical properties in terms of bonding.
When an element exists as more than one crystalline form, those forms are known as allotropes. Allotropes are forms of the one elements (in the same physical state) which have significantly different physical properties (such as density, hardness, electrical conductivity and colour). There are eight main allotropes of carbon:
- Diamond
- graphite
- lonsdaleite
- single-walled carbon nanotube (also known as buckytube)
- Buckminsterfullerene (also known as buckyball)
- C540
- C70
- Amorphous carbon
Diamond:
Diamond has a three-dimensional crystal structure, which consists of an infinite array of carbon atoms, each of which forms a structure in which each of the bonds makes equal angles with its neighbours. The ends of the bonds are connected, and the structure formed is that of a tetrahedron. Every carbon atom is covalently bonded at the four corners of the tetrahedron to four other carbon atoms (by single bonds).
This arrangement of layers of carbon atoms explains some of diamonds properties. It is extremely difficult to destroy such an arrangement of covalent bonds especially when the covalent bonds are extended throughout the lattice. This means that a extreme amount of energy is required to break the bonds between the carbon atoms in diamond’s three dimensional lattical structure; thus giving diamond extreme hardness, high melting and boiling points, reduced chemical reactivity (since the electrons are tightly bound within the covalent bonds, they are unable to move or be transferred to other elements thus minimizing reactivity), non-conductor of electricity (no free electrons).
Furthermore due to the covalent network bonding present in diamond, it is transparent and high light reflective index. This is simply because the atoms are arranged in an orderly fashion (as can be seen in the image on the previous page), this gives diamond its transparency as light is able to pass between the space between the particles.
Below is a summary of the properties of Diamond:
Property / Explanation in terms of bondingHard / Three-dimensional lattical structure (the tetrahedral arrangement of the carbon atoms arranged systematically in layers that are not flat) and strong covalent bonds between the carbon atoms that extend across the lattice, give strong intermolecular bonds which are extremely difficult to break ensuring hardness of diamond.
High M.P. and B.P. / It requires huge amounts of energy to rupture the strong intermolecular bonds between the carbon atoms due to its covalent network structure in which strong covalent bonds are extended throughout the lattice, the melting points and boiling points must be high.
Transparency / The carbon atoms are arranged in orderly fashion throughout the entire crystal. This gives the diamond its transparency as light is able to pass between the atoms giving the diamond its colourless appearance and high light-refractive index making it extremely attractive.
Non-conductor of electricity / Diamond is a covalent network substance, this means that the bonds between the carbon atoms are covalent bonds. This means that the valence electrons of the carbon atoms are not free to move and since there are no mobile electron there is no conduction of electricity.
Minimal chemical reactivity / In order for substances to be reactive they must be able to transfer their electrons easily. In diamond, the electrons are tightly bound due to the covalent bonds, therefore they are unable to move or be transferred to other elements, giving diamond a very low reactivity.
High Density / The structure of the diamond shows that the atoms are tightly bound in a strong three dimensional lattical structure. Also the six-membered rings are stacked on top of one another, giving diamond a high density (3.5g/mL)
Insoluble in all solvents / Diamond is a very hard and extremely unreactive substance. Due to this unreactive state it has because of its covalent network structure (as there are no free electrons), it is insoluble in all solvents. This simply means that no substance will be able to chemically react with diamond when the solvent is an aqueous state.
Excellent Thermal conductor
/ All carbon atoms in diamond are strongly bonded via covalent bonds. The diamond crystal has a symmetric cubic structure. The atoms in diamond are precisely aligned. Thus diamond is known as an ideal crystal. Atoms in the crystal lattices in solids vibrate. These are called the atomic vibrations which allow for thermal conduction in solids. In an ideal crystal, the lattices are aligned so that they don’t interact with each other. Therefore an ideal crystal conducts heat better than a non-ideal crystal. Diamond being an ideal crystal is a good thermal conductor.
Graphite:
The structure of graphite is significantly different to the structure of diamond and thus its physical and chemical properties are different as well. Graphite is also a covalent network solid (covalent lattice) like diamond but in this case each carbon atom is only three other carbon atoms (in diamond it was bound to four). This forms a planar structure as shown in the figure above.
Each ring consists of six carbon atoms which is also evident from the diagram. Since each carbon atom only has three other carbon atoms attached to it, it must mean that one electron is not covalently bonded (i.e it is free). These extra valence electrons form a sea of delocalised electrons similar to that in metals. It is the presence of the sea of delocalised electrons which makes graphite an electrical conductor (since the electrons can move when influence by an applied voltage similar to that in metals). However, electricity is only conducted along the plane of layers, graphite does not conduct electricity at 90 degrees to the plane. This is simply because the sea of delocalised electrons are only able to move across the planes and not jump from one plane to another.
It can also be seen that the two-dimensional lattices are packed one upon the other as shown in the figure above. Since, there are only weak intermolecular forces between the layers, they can easily slide across one another, and this explains the slippery-ness of graphite and its good lubricating characteristics.
Another phenomenon when it comes to graphite is that every second layer is stacked identically upon each other. “The crystal structure of graphite amounts to a parallel stacking of layers of carbon atoms. Within each layer the carbon atoms lie in fused hexagonal rings that extend infinitely in two dimensions. The stacking pattern of the layers is ABABA...; that is, each layer separates two identically oriented layers.”[1]
Below is a Summary of the properties of graphite:
Property / Explanation in terms of BondingSlippery
(Good Lubricant)
/ As previosuly explains, the carbon atoms in graphite are connected in hexagonal rings which connect to form a layer. These layers are then piled one on top of the other. The forces that hold these layers together are known as the van der Waals forces. These forces are extremely weak and the layers are seperated by a large distance. Due to these two factors the layers can slip over each other easy giving graphite it’s slippery nature and making it a good lubricant.
Extremely soft substance / The structure of graphite explains why it an extremely soft substance . This is because despite having strong covalent bonds between carbon atoms in each layer,the forces between layers are extremely weak (Van de Waals forces). This allows layers of carbon to slide over each other in graphite making the substance very soft and greasy.
Medium Density
(2.3 g/mL)
/ Graphite’s density is less than that of diamond. This is due to the structural layout. The layers are seperated by large distances due to the weak van der Waals forces which are unable to tightly bind the layers together. Due to this the carbon atoms are more spread out, reducing graphite’s density.
High M.P. and B.P.
/ Graphite can be considered as a covalent network substance despite no bonding in the vertical direction. The carbon atoms are connected via strong covalent bonds which extend throughout the horizontal lattice. These intermolecular bonds are hard to break and thus more energy is required to break them. Consequently, the melting and boiling points of graphite are high.
Good Electrical Conductor
/ In graphite, each carbon atom only has three other carbon atoms bound to it via single bonds. Therefore it must mean that one electron is not covalently bound (i.e it is free). These extra valence electrons form a sea of delocalised electrons similar to that in metals. It is the presence of the sea of delocalised electrons which makes graphite an electrical conductor (since the electrons can move when influence by an applied voltage similar to that in metals). However, electricity is only conducted along the plane of layers, graphite does not conduct electricity at 90 degrees to the plane. This is simply because the sea of delocalised electrons are only able to move across the planes and not jump from one plane to another.
Note:
“Effect of heat: it is the most stable allotrope of carbon. At a temperature of 2500 degree Celsius, it can be transformed into diamond. At about 700 degree Celsius it burns in pure oxygen forming carbon dioxide.
Chemical activity: it is slightly more reactive than diamond. This is because the reactants are able to penetrate between the hexagonal layers of carbon atoms in graphite. It is unaffected by ordinary solvents, dilute acids, or fused alkalis. However, chromic acid oxidises it to carbon dioxide.”[2]
- Process and present information from secondary sources on the uses of diamond and graphite and relate their uses to their physical properties
Uses of Diamond
Jewellery / Lustrous, High Light Refractive Index, Scratch resistant, Hard, Transparent
Industrial purposes like cutting tools / Hardest known substance on Earth which means it can cut through any substance, high melting point also allows it to be used in hot environments.
Heat sinks / Highest thermal conductivity of any substance which allows it to quickly extract heat from sensitive areas eg. Computer chips have a diamond layer that is able to quickly remove heat from the area. Also, high melting point.
Abrasives / Hard, Scratch resistant
Wear resistant parts / Resistant to corrosion, low chemical reactivity
Low friction microbearings / These are needed in extremely small mechanical devices. Diamond bearings are used when extreme abrasion resistance and durability are essential.
Diamond windows / transparent, very durable and resistant to heat and abrasion, hard (security)
Diamond Speaker Domes / Very stiff material (hard), also rapid vibrations will not cause deformation, therefore it enhances the performance of high quality speakers.
Uses of Graphite
Use / Property(ies) related to use“Lead” Pencils / Soft and slippery nature, layers can easily be separated
Refractory crucibles / High melting and boiling points, when mixed with other substances it can become extremely hard.
Electrodes / Good electrical conductivity, high melting point
Polishes and paints / Soft, slippery nature, metallic luster
Lubricant in machines / Slippery nature, high melting points, greasy nature since layers can be easily separated.
Electrotypes for printing / Good electrical conductivity, high melting point, soft nature so can be made into a fine powder that is still able to induce an electrical current.
Dry cell batteries / Good electrical conductivity, high melting point
- Identify that carbon can form single, double or triple covalent bonds with other carbon atoms.
Carbon atoms are able to form single, double or triple covalent bonds with other carbon atoms.