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Chemical Equilibrium AUTHOR: WESLEY TUNG Technical Things A chemical equation of a reaction uses mole ratios to describe the production of products from reactants. ◦ In 2𝐻2 + 𝑂2 → 2𝐻2 𝑂, hydrogen and oxygen are reactants, and water is a product. ◦ This equation has a 2:1:1 mole ratio. 1 mole contains 6.022 × 1023 molecules/atoms. ◦ This is also known as Avogadro's number. Intro to Equilibrium History ◦ In 1803, Claude Louis Berthelot, a Savoyard-French chemist, discovered some chemical reactions were reversible. Definition of Equilibrium ◦ The state in a chemical reaction in which both products and reactants are present in concentrations that have no further tendency to change over time. Two types of chemical equilibrium exist: ◦ Static Equilibrium ◦ The reaction is irreversible. ◦ Dynamic Equilibrium ◦ The reaction is reversible. Reaction Quotient The reaction quotient is a constant that tell us the ratio of products to reactants at a specific state in the reaction. ◦ For a reaction 𝑎𝐴 + 𝑏𝐵 ↔ 𝑐𝐶 + 𝑑𝐷 , the reaction quotient (𝑄𝑐 ) is calculated by: [𝐶]𝑐 [𝐷]𝑑 ◦ 𝑎 𝑏 [𝐴] 𝐵 = 𝑄𝑐 ◦ Brackets stand for concentration in molarity. ◦ Molarity (M) = 𝑚𝑜𝑙𝑒𝑠 𝑜𝑓 𝑠𝑢𝑏𝑠𝑡𝑎𝑛𝑐𝑒 𝐿𝑖𝑡𝑒𝑟𝑠 ◦ Only gaseous and aqueous compounds are included in the equation. Equilibrium Constant The equilibrium constant is the reaction quotient value for a system when it is in it’s equilibrium state. Every system of equilibrium has a unique equilibrium constant (K) that is determined experimentally initially. ◦ To do this, you must let the reaction run to its equilibrium state, then figure out the concentrations of each component. ◦ This constant allows us to determine whether the same system under different conditions is at equilibrium. ◦ When 𝑄𝑐 = 𝐾 the system is at equilibrium. ◦ When 𝑄𝑐 ≠ 𝐾, the system is not at equilibrium and will produce more products or reactants to achieve equilibrium. Le Chatelier's Principle History ◦ Developed by Henry-Louis Le Chatelier in 1884 to predict the change in disturbed equilibrium systems to re-achieve equilibrium. ◦ In general: ◦ Increasing the concentration of reactants causes more products to be made (Shift right). ◦ Increasing the concentration of products causes more reactants to be made (Shift left). ◦ Decreasing the volume but keeping everything else constant would shift the reaction towards the side of the reaction that produces the least total amount of moles. ◦ Increasing the volume but keeping everything constant would shift the reaction towards the side of the reaction that produces the most amount of moles. Le Chatelier’s Principle Temperature is unique ◦ It doesn’t modify the reactant quotient like volume and concentration but modifies the equilibrium constant. ◦ The shift depends on whether the reaction is exothermic or endothermic: ◦ Endothermic: Heat is a reactant and the reaction shifts right if you increase the temperature. ◦ Exothermic: Heat is a product and the reaction shifts left if you increase the temperature.