Chemical Equilibrium and Redox Processes- Part 1 PDF

Chemical Equilibrium and Redox Processes- PART 1 LU 2 STK2953  Learning Objectives At the end of this learning unit, students should be able to: Define acid and base and their reactions Explain processes involved in hydrologic cycle Distinguish...

Chemical Equilibrium and Redox Processes- PART 1 LU 2 STK2953  Learning Objectives At the end of this learning unit, students should be able to: Define acid and base and their reactions Explain processes involved in hydrologic cycle Distinguish between alkalinity and acidity Explain carbonate system Discuss redox reactions in natural water Explain the processes involved in carbon cycle Describe the reactions involved in the speciation of metals in the aquatic and soil environments. 2 10/21/2024 Add a footer  INTRODUCTION Reactions in natural waters typically involve two key components of physical chemistry: Thermodynamics: This component deals with the energy changes and equilibria associated with chemical reactions. In natural waters, thermodynamic principles help understand the stability of different chemical species, the direction of reactions, and the conditions under which certain reactions occur. For example, thermodynamics can explain the solubility of minerals, the dissolution of gases, and the redox potential of water. Kinetics: This involves the study of reaction rates and the mechanisms by which reactions occur. In the context of natural waters, kinetics helps in understanding how quickly reactions take place, such as the oxidation of organic matter, the decomposition of pollutants, and the formation of complexes. It also considers factors like temperature, pH, and the presence of catalysts that can influence reaction rates. Together, thermodynamics and kinetics provide a comprehensive framework for understanding and predicting the behavior of chemical reactions in aquatic environments. They are essential for assessing water quality, designing treatment processes, and managing aquatic ecosystems effectively. 3 10/21/2024 Add a footer  Introduction Thermodynamics Ionic equilibria Deals with the equilibrium of a system Substances that dissolve in water are called solutes and most of them are ionic in nature. The composition of solutes in the water is controlled mainly by the ionic equilibria. E.g. acid-base reactions, precipitation reactions, complexation and redox reactions. 4  What is Ionic Equilibria? Ionic equilibria refer to the state of balance between the ions in a solution when the rate of formation of ions equals the rate of recombination of ions. In simpler terms, it’s when the number of ions being produced equals the number of ions being consumed, creating a stable system. 5 10/21/2024 Add a footer  Acid-Base Reactions Pure H2O exists as an equilibrium between the acid species, H+ (more correctly expressed as a protonated water molecule, the hydronium ion, H30+) and the hydroxyl radical, OH -. In neutral water the acid concentration equals the hydroxyl concentration and at room temperature they both are present at 10-7 gram equivalents (or moles) per liter. The "p" function is used in chemistry to handle very small numbers. It is the negative logarithm of the number being expressed. Water that has 10-7 gram equivalents per liter of hydrogen ions is said to have a pH of 7. Thus, a neutral solution exhibits a pH of 7. 6 10/21/2024 Add a footer  Acid and Base Acid – chemical species which can DONATE hydrogen ions or protons Base – chemical species which can ACCEPT hydrogen ions or protons 7 10/21/2024 Add a footer  Acid-Base reactions Acid - Base reactions H 2O H+ + OH- pK = -log K ; [H+][OH-] = Kw = 10 -14 pH = -log[H+] Solubility Product Ksp = [A]a[B]b Solubility of Gases in Water Henry’s Law: X = KHPg KH = Henry’s law constant Pg = partial pressure of the gas in air X = mole fraction of the gas dissolved in liquid 8 10/21/2024 LU2 STK2953 9 10/21/2024 Add a footer  Sample Exercise Calculate the values of [H+] and [OH-] in a neutral solution at 25°C. SOLUTION: 10 10/21/2024 Add a footer  Hydrologic cycle The hydrologic cycle plays a fundamental role in the geochemical cycling of element. This cycle illustrate how water within the hydrosphere is being transfer from one location to another as well as transform from one physical state to another. The hydrologic cycle is used to model the storage and movement of water between the biosphere, atmosphere, lithosphere and hydrosphere. http://www.physicalgeography.net/fundamentals/8b.html 11 10/21/2024 Add a footer  Hydrologic cycle The main reservoirs for water on earth are the oceans, lakes, rivers, snow/ice, soil and groundwater. Freshwater is only available in rivers, lakes, snow/ice and the atmosphere and all these only constitute to 0.003% of the water in the hydrosphere. When the water moves from one reservoir to the other, being a universal solvent, it dissolves minerals and other soluble material along its paths. This process when occurs on land, will bring about weathering of the rock material, leaching of substances from soil such as nutrients, salt and minerals. The kinetic energy of the water in rivers (the energy was transform from the potential energy stored in them up in the atmosphere) is able to dislodge soil particles and move them along. This process is known as erosion. Rivers and surface runoffs are the major agents for erosion and transport of continental material to the oceans. The natural water in the river is not pure water but a rather dilute solution of multiple salts as well as suspension. 12 10/21/2024 Add a footer  Hydrologic cycle Evaporation: As water is heated by the sun, its surface molecules become sufficiently energized to break free of the attractive force binding them together, and then evaporate to the atmosphere. Transpiration: Water vapour is also emitted from plant leaves by a process called transpiration. Condensation: As water vapour rises, it cools and eventually condenses, usually on tiny particles of dust in the air. When it condenses it becomes a liquid again or turns directly into a solid (ice, hail or snow). These water particles then collect and form clouds. Precipitation: Precipitation in the form of rain, snow and hail comes from clouds. Clouds move around the world, propelled by air currents. For instance, when they rise over mountain ranges, they cool, becoming so saturated with water that water begins to fall as rain, snow or hail, depending on the temperature of the surrounding air. Runoff: Excessive rain or snowmelt can produce overland flow to creeks and ditches. Runoff is visible flow of water in rivers, creeks and lakes as the water stored in the basin drains out. 13 10/21/2024 Add a footer  Hydrologic cycle Infiltration/Percolation: Some of the precipitation and snow melt moves downwards, percolates or infiltrates through cracks, joints and pores in soil and rocks until it reaches the water table where it becomes groundwater. Groundwater: Subterranean water is held in cracks and pore spaces. Depending on the geology, the groundwater can flow to support streams. It can also be tapped by wells. Some groundwater is very old and may have been there for thousands of years. Humans directly change the dynamics of the water cycle through dams constructed for water storage, and through water withdrawals for industrial, agricultural, or domestic purposes. Climate change is expected to additionally affect water supply Impacts of and demand. human Deforestation activities on Urbanization hydrologic Building dams cycle Overabstraction 14 10/21/2024 Add a footer  AQUATIC CHEMISTRY This Figure summarizes important aspects of aquatic chemistry applied to environmental chemistry. As shown in this figure, a number of chemical phenomena occur in water. Many aquatic chemical processes are influenced by the action of algae and bacteria in water. For example, algal photosynthesis fixes inorganic carbon from HCO3- ion in the form of biomass (represented as CH2O) in a process that also produces carbonate ion, CO32-. Carbonate undergoes an acid-base reaction to produce OH- ion and raise the pH, or it reacts with Ca2+ ion to precipitate solid CaCO3. 15 10/21/2024 Add a footer  Oxidation-reduction reactions in water: Most of the many oxidation-reduction reactions that occur in water are mediated (catalyzed) by bacteria. For example: In the oxygen-deficient (anaerobic) lower layers of a body of water bacteria convert inorganic nitrogen largely to ammonium ion, NH4+. Near the surface: where O2 is available bacteria convert inorganic nitrogen to nitrate ion, NO3-. Metals in water may be bound to organic chelating agents, such as pollutant nitrilotriacetic acid (NTA) or naturally occurring fulvic acids. 16 10/21/2024 Add a footer  Gases are exchanged with the atmosphere, and various solutes are exchanged between water and sediments in bodies of water. Several important characteristics of unpolluted water should be noted. Some of these are: Gas solubility: Oxygen is the most important dissolved gas in water, since it is required to support aquatic life and maintain water quality. Water in equilibrium with air at 25 oC contains 8.3 (mg/L) of dissolved O2. Water alkalinity: is defined as the ability of solutes in water to neutralize added strong acid. Water hardness: is due to the presence of calcium ion, Ca2+, and, to a lesser extent, magnesium ion, Mg2+. 17 10/21/2024 Add a footer  END OF PART 1 18 10/21/2024 Add a footer  Chemical Equilibrium and Redox Processes- PART 2 LU 2 STK2953  Learning Objectives At the end of this learning unit, students should be able to: Define acid and base and their reactions √ Explain processes involved in hydrologic cycle√ Distinguish between alkalinity and acidity Explain carbonate system Discuss redox reactions in natural water Explain the processes involved in carbon cycle Describe the reactions involved in the speciation of metals in the aquatic and soil environments. 2 10/14/2024 Add a footer  ALKALINITY AND ACIDITY Alkalinity: is the capacity of water to accept H+ ions (protons). Alkalinity is important in water treatment and in the chemistry and biology of natural waters. Frequently, the alkalinity of water must be known to calculate the quantities of chemicals to be added in treating the water. Highly alkaline water often has a high pH and generally contains elevated levels of dissolved solids. These characteristics may be detrimental for water to be used in boilers, food processing, and municipal water systems. Alkalinity serves as a pH buffer and reservoir for inorganic carbon, thus helping to determine the ability of water to support algal growth and other aquatic life. It is used by biologists as a measure of water fertility. 3 10/14/2024 Add a footer  The basic species responsible for alkalinity in water are bicarbonate ion, carbonate ion, and hydroxide ion: HCO3- + H+ → CO2 + H2O CO32- + H+ → HCO3- OH- + H+ → H2O Other, usually minor, contributors to alkalinity are ammonia and the conjugate bases of phosphoric, silicic, boric, and organic acids. 4 10/14/2024 Add a footer  The carbonate system The Carbonate System Chemical species - aqueous carbon dioxide CO2(aq) Carbonic acid H2CO3 Bicarbonate ion HCO3- Carbonate ion CO32- CO2(aq) + H2O H2CO3 H+ + HCO3- [H+][HCO3-] [H+][CO32-] K1 = K2 = [CO2(aq) ] [HCO3-] K1= 4.3 x 10-7 K2= 4.8 x 10-11 5 10/14/2024 Add a footer  Carbonate system in natural water Open to the atmosphere H2CO3  H+ + HCO3- K1= 10-6.3 HCO3-  H+ + CO3-2 K2= 10-10.3 CO2(g) + H2O  H2CO3 KH= 10-1.5 H2O  H+ + OH- Kw= 10-14 Combining the equations: a. CO2(g) + H2O  HCO3- + H+ KHK1 = 10-7.8 b. CO2 (g) + H2O  CO3-2 + 2H+ KHK1K2 = 10-18.1 6 10/14/2024 Add a footer  Alkalinity in Water Measure of the ability of water to neutralize H+. Due to the presence of HCO3-, CO32- and OH- at pH = 7.00, the main contributor is HCO3- Natural alkalinity in water, “[alk]” = 1.00 x 10-3 M [OH-] at pH = 7.00 is insignificant (1.0 x 10-7 M) [HCO3-] >> [CO32-] at pH=10.00, [alk] = [HCO3-] + 2[CO32-] + [OH-] = 1.00 x 10-3 M [OH-] = Kw/[H+] = (1.00 x 10-14)/(1.00 x 10-10) = 1.00 x 10-4 M [ CO32 - ] = Ka2[ HCO3-] / [H+] [CO32-] = 2.18 x 10-4 M [HCO3-] = 4.64 x 10-4 M 7 10/14/2024 Add a footer  Effects of Biological Processes on pH and alkalinity Utilization and production of CO2 in natural waters Photosynthesis → CO2 + H2O  “CH2O” + O2  respiration pH < 6.3 CO2 + H2O  “CH2O” + O2 6.3