Equilibrium

• Chemical equilibria are important in numerous biological and environmental processes
• Equilibria involving O2 molecules and the protein hemoglobin play a crucial role in the transport and delivery of O2 from our lungs to our muscles
• Similar equilibria involving CO molecules and hemoglobin account for the toxicity of CO
• When a liquid evaporates in a closed container, a constant vapour pressure is established due to an equilibrium where the number of molecules leaving the liquid equals the number returning to the liquid from the vapour
• At equilibrium, the rate of evaporation is equal to the rate of condensation
• Equilibrium can be established for both physical processes and chemical reactions
• The extent of a reaction in equilibrium varies with experimental conditions such as concentrations of reactants and temperature
• Optimization of operational conditions is crucial in industry and laboratory settings to favor equilibrium in the direction of the desired product
• Equilibrium involving physical and chemical processes is discussed in this unit, along with ionic equilibrium in aqueous solutions
• The characteristics of system at equilibrium are better understood by examining physical processes like phase transformations
• Solid-liquid equilibrium involves ice and water in a thermos flask at 273K and atmospheric pressure, where the mass of ice and water remains constant due to equal rates of transfer of molecules between the phases
• Liquid-vapour equilibrium involves the equilibrium vapour pressure of water, where the rate of evaporation equals the rate of condensation at equilibrium
• In the case of single-celled organisms, substances can easily enter the cell due to a short distance, while in multicellular organisms, the distance is larger due to a higher surface area to volume ratio
• Multicellular organisms require specialised exchange surfaces for efficient gas exchange of carbon dioxide and oxygen due to their higher surface area to volume ratio
• The rate of evaporation remains constant when the watch glass is open to the atmosphere, leading to a lower rate of condensation from vapor to liquid state
• Water and water vapor are in equilibrium at atmospheric pressure (1.013 bar) and at 100°C in a closed vessel
• The boiling point of water is 100°C at 1.013 bar pressure, known as the normal boiling point of the liquid
• For solid-vapor equilibrium, solids sublime to vapor phase, reaching equilibrium where the solid sublimes to give vapor and the vapor condenses to give solid
• In dissolution of solids in liquids, a saturated solution is reached when no more solute can be dissolved at a given temperature
• For dissolution of gases in liquids, the concentration of a gas in liquid is proportional to the pressure of the gas above the solvent, governed by Henry's law
• Characteristics of equilibria involving physical processes include occurring in a closed system at a given temperature, both opposing processes occurring at the same rate, and all measurable properties of the system remaining constant
• Chemical reactions reach a state of equilibrium when the rates of the forward and reverse reactions become equal, leading to constant concentrations of reactants and products
• Chemical reactions reach a state of dynamic equilibrium where the rates of forward and reverse reactions are equal, leading to no net change in composition
• Equilibrium can be attained from both sides, whether starting with H2(g) and N2(g) to get NH3(g) or starting with NH3(g) and decomposing it into N2(g) and H2(g)
• The equilibrium mixture in a reversible reaction is related by the equilibrium constant (Kc) equation, where the concentrations of reactants and products are involved
• Experimental studies by chemists Guldberg and Waage led to the proposal of the equilibrium equation, which is also known as the law of mass action
• The law of mass action states that the concentrations in an equilibrium mixture are related by the equilibrium constant expression
• In experiments with gaseous H2 and I2 at 731K, equilibrium was reached from varying initial conditions, showing a constant intensity of the purple color
• The number of moles of dihydrogen reacted equals the number of moles of iodine reacted, which is half the number of moles of HI formed
• Equilibrium concentrations of reactants and products can be related by trying different combinations and expressions
• Equilibrium constant Kc is expressed as the product of concentrations of the reaction products raised to their respective stoichiometric coefficients, divided by the product of concentrations of the reactants raised to their individual stoichiometric coefficients
• The equilibrium constant for a general reaction aA + bB ⇌ cC + dD is written as Kc = [C]c[D]d / [A]a[B]b
• For the reaction 4NH3(g) + 5O2(g) ⇌ 4NO(g) + 6H2O(g, the equilibrium constant Kc is expressed as [NO]4[H2O]6 / [NH3]4[O2]5
• The equilibrium constant for the reverse reaction is the inverse of the equilibrium constant for the reaction in the forward direction
• When changing the stoichiometric coefficients in a chemical equation, the equilibrium constant must reflect that change
• For reactions involving gases, the equilibrium constant is usually expressed in terms of partial pressure, denoted as Kp
• In the example of the reaction H2(g) + I2(g) ⇌ 2HI(g, the equilibrium constant can be expressed as Kp = [HI]2 / [H2][I2] or Kc = [HI]2 / [H2][I2]
• The equilibrium constant for a general reaction aA + bB ⇌ cC + dD is expressed as Kp = ([C]c[D]d) / ([A]a[B]b) = Kc^(∆n), where ∆n is the difference in moles of gaseous products and reactants
• When calculating the value of Kp, pressure should be expressed in bar because the standard state for pressure is 1 bar
• 1 pascal (Pa) = 1 Nm^−2, and 1 bar = 105 Pa