Importance of Chemical Bonding | Essay

Importance of Chemical Bonding | Essay George Brown Chemical bonds are either ionic or covalent. When a metal is present the bond is ionic as an electron is given from the metal to the non-metal, so the two ions are attracted to each other. When a bond is between two non-metal’s then it is covalent meaning that in the outer electron shells of the two atoms bonded electrons share electron to complete their outer electron shell. When two atoms bonded together are not the same, the electrons will not be shared equally as each atom will be positively or negatively charged in relation to each other. This value is measured as the difference of the electronegativity of each atom present. The electronegativity of an atom is the potential for the atom to attract shared electrons towards itself. The difference in electronegativity between ionic bonds is greater than that of polar covalent bonds and greater still of non-polar covalent bond. An example of a non-polar covalent bond is H-I as the difference in electronegativity isn’t very high. An example of a polar covalent bond is H-Cl as the difference in electronegativity is higher than 0.5 and lower than 1.7 and an example of an ionic bond would be Na-Cl as the difference in electronegativity is above 1.7. In figure 1 it can be observed how the electrons are shared in ionic and covalent bonds. This explains the high difference in the resultant electronegativity between ionic and covalent bonds. Metal, as an ion, donates an electron making it positively charged and a non-metal, as an ion, gains an electron making it more negatively charged meaning the attraction between ionic bonds is greater than that of covalent bonds. Ionic bonded substances are more polar than covalent bonded substances thus a better solvent. NaCl has a much high boiling point (around 1413 °c) than for example HCl (around -85c) Sodium Chloride, better known as table salt, is an example of an Ionic bonded substance due to the high difference in electronegativity. Sodium and chlorine as elements are very reactive and thus poisonous to the human body in large quantities if uncontrolled. So the fact that sodium and chloride are ionically bonded helps to control the concentration of sodium and chlorine in the body as it can only be broken down when it needs to be in the liver. Sodium and chlorine is needed In low concentrations for key cell functions in nerve cells for maintaining the â€Å"resting potential of nerve axon cells which is -70 mV† (Hall et al., 2009) maintained by the â€Å"sodium potassium pump†. It is required for the nerve cells in the human body to function. It’s the reason for why you can feel or can react to stimuli. A molecule is always non polar, where the bonds between atoms in the molecule are non-polar an example of this would be H-I. Water is an example of a polar covalent bond as it consists of polar covalent bonds H-O and the molecule is asymmetrical. This means that water is a good solvent allowing it to transport sugars and salts around the body, in blood, as they can be easily dissolved in an aqueous solvent such as water. In figure 2 you can see the relative charge of the molecule in water. CCl4 is example of a molecule of which its atoms are polar bonded together but due to the symmetry of the molecule observed in figure 2 the polarity of the molecules cancels itself out, and it acts as if it’s non-polar in intermolecular interactions The strength between intermolecular bonds is a lot weaker than the strength of intramolecular bonds and the weakest form of intermolecular bond is a bond that uses Van der Waals forces or an instantaneous induced-dipole bond. These are observed when a nearby non-polar molecules experiencing an instantaneous dipole, due to the random nature of electron clouds oscillating on molecules, which induces a dipole of another molecule, it may cause a ripple effect inducing dipoles on nearby non-polar molecules. Larger molecules have a larger electron cloud which means the induced and potential electronegativity of instantaneous dipoles would be stronger. Van der Waals forces is the attraction that occurs between all molecules polar or non-polar, but is the only source of intermolecular attraction between non-polar molecules. Lower boiling points will be observed of substances of similar elements that are polar, dipole-dipole bonds or even stronger hydrogen(dipole-dipole) bonds, than that of n on-polar Van der Waals force bonds, as they are much stronger so it requires more energy(heat) to break them. Figure 3 is an example of an induced dipole bond from an instantaneous dipole. Hydrogen bonding (an example of a strong dipole-dipole bond) is another example of an intermolecular. Standard hydrogen bonding is an intermolecular bond where a hydrogen from one molecule is attached to one of the most electronegative elements; oxygen, nitrogen or fluorine of another molecule. Figure 4 shows a diagram of a hydrogen bond between two water molecules. These bonds help to keep the water molecules together so it requires more energy for the substance to change state from liquid into a gas. It also means when water is a solid (ice) it is less dense than when it is a liquid, which is unusual as usually substances tend to expand when they heat up. Heat is a measure of kinetic energy of a substance, so when substances, molecules or atoms, has more kinetic energy it is more fluid, less packed together (less dense). You can see in figure 4 that each oxygen atom is bonded from a combination of hydrogen (intermolecular) and covalent (intramolecular) bonds to 4 oxygen atoms. Ther e is a lot of free space around these atoms when they are structurally bonded this way, so this is why ice is less dense than water. The structural function of ice is important for all seasonal marine life and some land organisms as it allows ice to float. An example of a more complex intermolecular bond is ionic hydrogen bonding. Potential uses is discussed in the article BIOPHYISCAL (Kaledhonkar et al., 2013). The article states â€Å"Standard hydrogen bonds are of great importance for protein structure and function† but â€Å"Ionic hydrogen bonds often are significantly stronger than standard hydrogen bonds and exhibit unique properties† which allows them to be used in protein folding (polypeptides), modification in the golgi apparatus, enzyme active transport centres and the formation of membranes, processes that are all critical for life. Ionic hydrogen bonding is explained further in an article in chem. rev. (Meot-Ner (Mautner), 2005). The article states that â€Å"ionic hydrogen bonds (IHBs) that form between ions and molecules with bonds strengths of 5-35 kcal/mol, up to a third of the strength of covalent bonds†. Ionic hydrogen bonds are believed to be the strongest intermolecular bond but still only up to a third the strength of a covalent bond. So even the strongest intermolecular bonds are weaker than intramolecular bonds. [Word Count: 1058] References bbc.co.uk, (2014).BBC Higher Bitesize Chemistry Bonding, structures and properties : Revision, Page2. [online] Available at: http://www.bbc.co.uk/bitesize/higher/chemistry/energy/bsp/revision/2/ [Accessed 2 Dec. 2014]. Chemprofessor.com, (2014).Intermolecular Attractions or van der Waals Forces. [online] Available at: http://www.chemprofessor.com/imf.htm [Accessed 2 Dec. 2014]. Hall, A., Hickman, G., Howarth, S., Middlewick, S., Owens, N., Reiss, M., Scott, A. and Wilberforce, N. (2009).Salter-Nuffield Advanced Biology A2 Student Book. London: Edexcel Pearson, p.201. Kaledhonkar, S., Hara, M., Stalcup, T., Xie, A. and Hoff, W. (2013). Strong Ionic Hydrogen Bonding Causes a Spectral Isotope Effect in Photoactive Yellow Protein.Biophysical Journal, 105(11), pp.2577-2585. Meot-Ner (Mautner), M. (2005). The Ionic Hydrogen Bond.Chem. Rev., 105(1), pp.213-284. Physicsofmatter.com, (1998).Hydrogen Bond Disorder in Ice Structures. [online] Available at: http://www.physicsofmatter.com/NotTheBook/Talks/Ice/Ice.html [Accessed 2 Dec. 2014]. Page 1 of 7 Soil Water Contamination: Wheal Jane Incident Soil Water Contamination: Wheal Jane Incident The Wheal Jane incident was a significant mine water discharge event. The incident occurred in 1992, shortly after the mine closure. This report provides a brief description of the mine, the incident itself and the resulting aftermath. Wheal Jane Mine is located near the village of Chacewater, in Cornwall. The mine itself was formed in 1861, after the merging of five smaller mines. The oldest mine workings from the area were thought to date back as far as the 1740s (Cornwall Calling, 2017). The mine had worked many mineral lodes over its lifetime, producing tin, copper, and silver-lead (Cornwall in Focus, 2017). In the years leading up to its closure, the mine was primarily extracting cassiterite, the main source ore of tin, but older workings also produced pyrite and arsenopyrite, with the modern development drives taking the mine to 450 meters below surface level (University of Exeter, 2002). The mines of the Gwennap parish were all interconnected, with Wheal Jane connected to the neighbouring, working mine Mount Wellington, and to the abandoned workings of United Mines. Wheal Jane was an extremely wet mine, requiring dewatering measures in the region of 60,000 m ³ day-1 in the winter months. The pumped water was highly acidic, owing to the dissolved metals from the sulphide mineral deposits. Approximately half of pumped water was treated before being discharged into the Carnon river (Bowen, Dussek, Hamilton, 1998). Wheal Jane Mine had been working, on and off, from this time until its eventual closure in 1991 due to financial difficulties, primarily relating to the low price of tin, following the International Tin Agreement in 1985. Much of the mines infrastructure and equipment was sold off at the time of its closure, with the mines operational dewatering systems being turned off after a government grant subsidising the pumping costs was withdrawn (University of Exeter, 2002). After the mine closure, and the cessation of the government grant, the operational dewatering pumps were switched off, leaving only the tailings dam pumps remaining. With the dewatering measures stopped, the water level rose and filled the expansive voids underground, with the sulphide mineral bearing rock now being leached by the rising ground water. The NRA (National Rivers Authority, now part of the Environment Agency) was concerned of the potential of Acid Mine Drainage (AMD) into the Carnon river and commissioned a survey to determine the potential impact of a mine water discharge, and its likely discharge points and timing. The investigation proved difficult to predict due to the unknown volume of connected, un-surveyed mine workings which would also need to fill before release (Bowen, Dussek, Hamilton, 1998). The NRA continued to monitor the water levels and water quality throughout the year. On November 17th 1991 the mine water levels reached 14.5 m AOD, and a mine water discharge event occurred through Janes adit. The water treatment lagoon onsite was quickly overwhelmed by a flow of approximately 5,000 m ³ per day of AMD at a pH of 2.8 (Bowen, Dussek, Hamilton, 1998). The NRA had contingency plans in place and immediately reacted by adding lime at the head of the adit to raise the pH and precipitate the metals from the mine water. A plug was constructed near the portal of Janes adit and the water pumped out into the tailings dam (University of Exeter, 2002). On January 4th 1992 a technical issue meant that the pumps to the tailings dam were stopped. The mine water levels quickly rose by an estimated 4 meters. The mine water built up and, on January 13th 1992, discharged through the Nangiles adit, which was the second lowest known discharge location after Janes adit. This released an estimated 50,000 m ³ of AMD (pH of 3.1) into the Carnon river over a period of 24 hours, flowing through the Restonguet Creek, Carrick Roads and into the Fal estuary (Bowen, Dussek, Hamilton, 1998). The contaminated water created a highly visual pollution event, as oxidation caused the iron rich water to turn a yellow-brown ochre colour, drawing worldwide media attention and causing much alarm to the community ( CL:AIRE, 2004). The mine water also contained considerable concentrations of heavy metals, most significantly being the presence of over 600 parts per billion Cadmium (University of Exeter, 2002). Following the discharge, new pumps were installed by the owners to pump water from the adit directly into the tailings dam as a short-term solution. Monitoring after the incident proved that the effects of the mine water discharge proved to be short-term only, with the NRA stating that There appears to have been no major adverse effects from the incident on the biota of the estuary. (National Rivers Authority, 1995). However, it was clear after the event that the mine water could not be left unchecked, and thus a water treatment solution was proposed. The NRA proposed both an active and passive treatment system. In 1994 a pilot passive treatment plant (PPTP) was constructed, as a research method for potential long-term treatment options. An active treatment plant was constructed in 2000. The PPTP contains three different treatment streams, all containing aerobic reed beds for removal of Iron and Arsenic, an anaerobic cell for removal of zinc, copper, cadmium and iron by bacterial reduction, and an aerobic rock filter which removes manganese through growth of algae (University of Exeter, 2002). The three streams differ however in the pre-treatment. One stream is first treated with lime to raise the pH, the second is first passed through an anoxic limestone drain, and the third stream involves no pre-treatment at all ( CL:AIRE, 2004). A report into the performance of the PPTP found that it offered inconsistent performance, and water discharge commonly exceeded the permitted water quality guidelines. The PPTP was also only processing 0.6 l/s of contaminated water, The active treatment plant took over from the PPTP in 2000. The process involves the addition of lime to increase the pH, and flocculant to precipitate out the metals in solution. The metal precipitates form a sludge, which is sent to a hold tank, and onto the tailings dam. The treated mine water is discharged out into the Carnon river. This system treats an average of 200 l/s, at a metal removal efficiency of 99.2% ( CL:AIRE, 2004). Whilst there have not been any significant long-term environmental impacts from the incident, it serves as a stark reminder of the potential environmental disaster that can come from mining activities, particularly from historical mines which were not subject to the same environmental regulations that are in place today. References CL:AIRE. (2004). Mine Water Treatment at Wheal Jane Tin Mine, Cornwall. CL:AIRE (Contaminated Land: Applications in Real Environments), 1-4. BBC. (2014, June 3). Pumping the polluted water from mines. Retrieved from BBC News: http://www.bbc.co.uk/news/uk-england-26573994 Bowen, G. G., Dussek, C., Hamilton, R. (1998). Pollution resulting from the abandonment and subsequent flooding of Wheal Jane Mine in Cornwall, UK. London: Geological Society. Cornwall Calling. (2017). Retrieved from Cornwall Calling: http://www.cornwall-calling.co.uk/mines/carnon-valley/wheal-jane.htm Cornwall in Focus. (2017). Cornwall in Focus. Retrieved from http://www.cornwallinfocus.co.uk/mining/whealjane.php National Rivers Authority. (1995). Wheal Jane Mine Water Study. Ashford: Knight Pià ©sold. Retrieved from Environment Data: http://www.environmentdata.org/fedora/repository/ealit:2627/OBJ/20000033.pdf University of Exeter. (2002). The Wheal Jane Incident and water quality. Retrieved from Projects University of Exeter: https://projects.exeter.ac.uk/geomincentre/estuary/Main/jane.htm