Environmental Issues (Introduction And Effects)

Effects Of Oxidation Reactions In Everyday Life (Corrosion, Rancidity)

Corrosion:

Definition: Corrosion is the process of gradual destruction of metals by their reaction with the surrounding environment, usually through oxidation. It is an electrochemical process.

Corrosion of Iron (Rusting):

• Mechanism: Rusting of iron involves an electrochemical cell formation on the surface of iron.
  • Anode: Iron gets oxidized to ferrous ions ($Fe^{2+}$).

    $Fe(s) \rightarrow Fe^{2+}(aq) + 2e^-$

  • Cathode: Oxygen from the atmosphere dissolved in water gets reduced to hydroxide ions ($OH^-$).

    $O_2(g) + 2H_2O(l) + 4e^- \rightarrow 4OH^-(aq)$

  • The ferrous ions ($Fe^{2+}$) are further oxidized by atmospheric oxygen to ferric ions ($Fe^{3+}$).
  • $4Fe^{2+}(aq) + O_2(g) + 4H^+(aq) \rightarrow 4Fe^{3+}(aq) + 2H_2O(l)$
  • Ferric ions react with water to form hydrated ferric oxide ($Fe_2O_3 \cdot nH_2O$), which is rust.
  • $2Fe^{3+}(aq) + 6H_2O(l) \rightarrow Fe_2O_3 \cdot nH_2O(s) + (6-2n)H_2O$
• Factors Accelerating Rusting: Presence of moisture, oxygen, electrolytes (like salts, acids), and higher temperature.
• Prevention of Rusting:
• Painting or Greasing: Forms a barrier against moisture and oxygen.
• Alloying: Making stainless steel by adding chromium and nickel.
• Galvanization: Coating iron with a layer of zinc, which provides sacrificial protection.
• Electroplating: Coating iron with a less reactive metal like tin or nickel.
• Sacrificial Protection: Connecting iron with a more electropositive metal like magnesium or zinc. The more electropositive metal corrodes preferentially.

Corrosion of Other Metals: Similar electrochemical processes occur for other metals, leading to tarnishing of silver (formation of $Ag_2S$) or the formation of a protective oxide layer on aluminium.

Rancidity:

Definition: Rancidity is the process where food substances containing fats and oils, when left for some time, develop an unpleasant smell and taste due to the oxidation or hydrolysis of fats and oils.

• Oxidation of Fats: Fats and oils get oxidized by atmospheric oxygen, especially at the double bonds, forming aldehydes and ketones, which have foul smells. This process is often accelerated by light and heat.
• Hydrolysis of Fats: In the presence of moisture, fats can undergo hydrolysis to form fatty acids and glycerol. The fatty acids released can have unpleasant odours (e.g., butyric acid in butter).
• Prevention of Rancidity:
• Storing food in airtight containers: Minimizes contact with oxygen.
• Storing food in refrigerators: Low temperatures slow down the rate of oxidation and hydrolysis.
• Adding antioxidants: Antioxidants like BHT (Butylated Hydroxytoluene) and BHA (Butylated Hydroxyanisole) are added to fatty foods to prevent oxidation.
• Storing food in opaque containers: Light can catalyze the oxidation process.
• Nitrogen Packaging: Packing food in nitrogen gas displaces oxygen.

Corrosion (from Metals And Non-metals)

Corrosion:

Definition: Corrosion is the process of gradual degradation of a metal due to its reaction with the atmosphere (oxygen, moisture) or chemicals. It is essentially an electrochemical process where the metal is oxidized.

Electrochemical Mechanism of Rusting of Iron:

1. Anodic Area: A small part of the iron surface acts as the anode, where iron is oxidized to ferrous ions.

   $Fe(s) \rightarrow Fe^{2+}(aq) + 2e^-$

2. Cathodic Area: Another part of the surface acts as the cathode. In neutral or acidic solutions with dissolved oxygen, oxygen is reduced to hydroxide ions.

   In neutral/weakly acidic medium: $O_2(g) + 2H_2O(l) + 4e^- \rightarrow 4OH^-(aq)$

   In acidic medium: $O_2(g) + 4H^+(aq) + 4e^- \rightarrow 2H_2O(l)$

3. Formation of Rust: The ferrous ions ($Fe^{2+}$) migrate towards the cathode region and are further oxidized to ferric ions ($Fe^{3+}$) by dissolved oxygen.

   $4Fe^{2+}(aq) + O_2(g) + 4H^+(aq) \rightarrow 4Fe^{3+}(aq) + 2H_2O(l)$

   These ferric ions react with hydroxide ions to form hydrated ferric oxide ($Fe_2O_3 \cdot nH_2O$), which is the reddish-brown rust.

   $2Fe^{3+}(aq) + 6H_2O(l) \rightarrow Fe_2O_3 \cdot nH_2O + (6-2n)H_2O$

Factors Affecting Corrosion Rate:

• Nature of Metal: Reactivity of the metal (metals higher in the electrochemical series corrode more readily).
• Nature of Atmosphere: Presence of oxygen, moisture, $CO_2$, $SO_2$, $H_2S$, etc.
• Presence of Electrolytes: Salts dissolved in water increase the conductivity of the electrolyte, accelerating corrosion.
• Presence of Impurities: Impurities can set up electrochemical cells.
• pH of Water: Acidic solutions increase the corrosion rate.

Methods to Prevent Corrosion:

1. Barrier Protection: Coating the metal surface with a protective layer that prevents contact with the environment.
   • Painting: Applying paint forms a barrier.
   • Oiling/Greasing: Applied to machinery parts.
   • Galvanization: Coating with zinc. Zinc is more reactive than iron and corrodes preferentially, providing sacrificial protection.
   • Electroplating: Coating with a less reactive metal like tin or nickel.
   • Anodizing: Forming a protective oxide layer on metals like aluminium.
2. Sacrificial Protection: Attaching a more reactive metal (e.g., Mg, Zn) to the metal to be protected (e.g., Fe). The more reactive metal acts as the anode and corrodes preferentially.
3. Alloying: Mixing the metal with other elements to form alloys that are more resistant to corrosion (e.g., Stainless steel, which contains iron, chromium, nickel, and carbon).

Carcinogenicity And Toxicity (from Hydrocarbons)

Some hydrocarbons and their derivatives can be harmful to living organisms, acting as carcinogens (causing cancer) or toxins (causing poisonous effects).

Carcinogenicity:

• Definition: The ability of a substance to cause cancer.
• Hydrocarbons and Cancer: Many aromatic hydrocarbons, particularly polycyclic aromatic hydrocarbons (PAHs), are known carcinogens. These are formed during incomplete combustion of organic matter, such as in fuels, tobacco smoke, and exhaust fumes.
• Examples:
  • Benzene ($C_6H_6$): A known human carcinogen, linked to leukaemia. Its use is restricted, especially in gasoline.
  • Polycyclic Aromatic Hydrocarbons (PAHs): Compounds like Benzo[a]pyrene (found in tobacco smoke, coal tar, charcoal-grilled foods) are potent carcinogens.
• Mechanism: Carcinogenic hydrocarbons often undergo metabolic activation in the body to form reactive intermediates that can bind to DNA, causing mutations and initiating cancer.

Toxicity:

• Definition: The degree to which a substance can damage an organism.
• Hydrocarbons and Toxicity:
• Alkanes: Shorter chain alkanes (like methane, ethane) are gases and can displace oxygen, leading to asphyxiation in confined spaces. Longer chain alkanes are generally less toxic but can cause skin irritation and have narcotic effects at high concentrations.
• Alkenes and Alkynes: Ethene can cause premature ripening of fruits. Ethyne (acetylene) is flammable and can be explosive under pressure.
• Aromatic Hydrocarbons: Toluene and Xylene are solvents and can cause dizziness, headache, and nervous system effects.
• Halogenated Hydrocarbons: Many halogenated hydrocarbons are toxic. For example, carbon tetrachloride ($CCl_4$) is hepatotoxic (damages the liver) and nephrotoxic (damages the kidneys) and also depletes the ozone layer. Chloroform ($CHCl_3$) is also toxic and a suspected carcinogen.

Environmental Pollution (Introduction from Environmental Chemistry)

Environmental pollution refers to the introduction of contaminants into the natural environment that cause adverse change. These contaminants can be chemicals, energy, or waste materials that exceed the environment's capacity to neutralize them.

Types of Pollution:

• Air Pollution: The contamination of the atmosphere by harmful gases, particulate matter, and biological substances.
• Sources: Combustion of fossil fuels, industrial emissions, vehicular exhaust, agricultural activities.
• Pollutants: $SO_2$, $NO_x$, $CO$, $CO_2$, hydrocarbons, particulate matter, CFCs.
• Effects: Acid rain, respiratory diseases, global warming, ozone depletion.
• Water Pollution: The contamination of water bodies (lakes, rivers, oceans, groundwater) by harmful substances.
• Sources: Industrial discharge, sewage, agricultural runoff (pesticides, fertilizers), oil spills.
• Pollutants: Pathogens, toxic chemicals, heavy metals, excess nutrients, oil, plastics.
• Effects: Eutrophication, diseases (e.g., cholera, typhoid), harm to aquatic life, contamination of drinking water.
• Soil Pollution: The contamination of land by harmful substances.
• Sources: Improper disposal of solid waste, industrial waste, agricultural chemicals (pesticides, herbicides), deforestation.
• Pollutants: Plastics, heavy metals, pesticides, industrial chemicals.
• Effects: Reduced soil fertility, contamination of food crops, harm to soil organisms.
• Noise Pollution: Unwanted or excessive sound that can have detrimental effects on human health and the environment.
• Sources: Traffic, construction, industrial machinery, loud music.
• Effects: Hearing loss, stress, sleep disturbances, impact on wildlife.
• Radioactive Pollution: Contamination of the environment by radioactive substances.
• Sources: Nuclear power plants, nuclear waste disposal, nuclear weapons testing.
• Effects: Cancer, genetic mutations, long-term health and environmental damage.

Greenhouse Effect and Global Warming:

• Certain gases in the atmosphere (greenhouse gases) trap heat, maintaining the Earth's temperature.
• Increased concentrations of greenhouse gases, primarily due to human activities like burning fossil fuels ($CO_2$) and deforestation, enhance this effect, leading to global warming and climate change.

Ozone Depletion:

• Chlorofluorocarbons (CFCs) and other halogenated compounds are released into the atmosphere and deplete the stratospheric ozone layer, which protects life from harmful UV radiation.

Acid Rain:

• Pollutants like sulfur dioxide ($SO_2$) and nitrogen oxides ($NO_x$) from burning fossil fuels react with water in the atmosphere to form sulfuric acid ($H_2SO_4$) and nitric acid ($HNO_3$).
• These acids fall to the Earth as acid rain, damaging ecosystems, buildings, and human health.