Topic 9 Kinetics and Equilibrium Lesson 41: Equilibrium Introduction Equilibrium is the state of balance between two opposing processes taking place at the same time (simultaneously) and at equal rates. One example of two opposing processes is the freezing of water to ice and the melting of the ice back to water. Since both processes in equilibrium continue to take place, equilibrium is said to be dynamic. In this lesson, you will learn about physical and chemical equilibrium. Equilibrium: Equal Rates, Constant Concentrations Equilibrium can only occur in a closed system in which changes that are taking place are reversible. A closed system is a system in which nothing is allowed in or out. Example of a reversible equilibrium reaction Forward N2 + O2 An equation showing a reversible process at equilibrium always contains a two-headed arrow. 2NO Reverse At equilibrium: Rates of forward and reverse reactions are equal. or Concentrations or amounts of substances remain constant. Physical Equilibrium: Phase and Solution Equilibrium A phase equilibrium occurs in a closed system in which phase changes are occurring. Examples of phase equilibrium are given below. Solid-liquid Equilibrium Example: occurs at or (at 1 atm pressure) melting H2O(s) freezing H2O( ) Rates of melting and freezing are equal. Amounts of ice and water stay constant. H2O(g) Liquid-gas (vapor) Equilibrium Example: occurs at 100oC or 373 K (at 1 atm pressure) evaporation H2O( ) condensation H2O(g) Rates of evaporation and condensation are equal. Amounts of water and vapor stay constant. © 2019 by E3 Scholastic Publishing. All Rights Reserved H2O( ) Solution Equilibrium Solution equilibrium occurs in a closed system in which a substance is dissolving in a liquid . Two types of solution equilibrium are given below. Solid in Liquid Equilibrium: Saturated Solution Equilibrium exists between dissolved and undissolved (crystallized) particles. dissolving NaCl(s) Na+(aq) + Cl-(aq) crystallizing Rate of dissolving of solid is equal to rate of crystallization of ions. Amounts of solid and ions remain constant in the solution. Gas in Liquid Equilibrium: Gaseous solution Equilibrium exists between dissolved gas in the liquid and undissolved gas above the liquid. CO2(g) dissolving undissolving CO2(g) CO2(aq) Rate of dissolving is equal to rate of undissolving of the gas. Amounts of undissolved gas (above liquid) and dissolved CO2(aq) gas (in liquid) remain constant. a closed soda can Chemical Equilibrium Chemical equilibrium is a state of balance between the forward and reverse reactions of a chemical reaction. forward reaction N2(g) + 3H2(g) 2NH3(g) reverse reaction When the above reaction has reached a state of equilibrium: Rates of forward and reverse reactions are equal Concentrations of N2, H2, and NH3 remain constant (stay the same). If a stress is introduced into the reaction while at equilibrium, the reaction will change or shift. A stress is any change in concentration, temperature or pressure to an equilibrium reaction. le states that when a stress is introduced into a reaction at equilibrium, the reaction will change by speeding up in one direction while slowing down in the other direction to bring back or re-establish the reaction to a new equilibrium point. The concentrations of the substances at the new equilibrium point will be different from those of the previous equilibrium point. There will be an increase in the concentration of some substances, and a decrease in others. E3chemistry.com Topic 9 Kinetics and Equilibrium Changing Concentration or Temperature Given the reaction: N2(g) + 3H2(g) forward (exothermic) reverse (endothermic) Increasing any Reactant (Ex. N2) 2NH3(g) + 92 KJ Increasing any Product (Ex. NH3) • Increases forward reaction (shift right) • Increases reverse reaction (shift left) • Increases product concentrations ( NH3, heat) • Increases reactant concentrations ( N2, H2) • Decreases forward reaction • Decreases reverse reaction • Decreases the concentration of all other • Decreases the concentration of all other products ( heat) reactants ( H2) Decreasing any Reactants • same effect as increasing any product. Decreasing any Products • same effect as increasing any reactant . Increasing Heat/Temperature: Favors • In the above reaction, similar effect as increasing any product. Decreasing Heat/Temperature: Favors • In the above reaction, similar effect as increasing any reactant. Reactions Reactions Changing Pressure: Increasing Pressure Favors Side with Fewer Moles When the pressure is changed on an equilibrium reaction, products determine the direction that the reaction will shift. Given the equilibrium reaction: CH4(g) + H2 O(g) number of moles total moles 1 1 forward reverse of gaseous reactants and 3H2(g) + 3 2 moles CO(g) 1 4 moles Decreasing Pressure / Increasing Volume Increasing Pressure / Decreasing Volume • Favors (speeds up) production of substances • Favors (speeds up) production of substances that are on the side of smaller number of moles that are on the side of greater number of moles In the above reaction, increasing pressure: • increases reverse reaction (shift left) • increases CH4 and H2O concentrations • decreases forward reaction • decreases H2 and CO concentrations In the above reaction, decreasing pressure: • increases forward reaction (shift right) • increases H2 and CO concentrations • decreases reverse reaction • decreases CH4 and H2O concentrations In reactions in which the total moles of gases on both sides of the equation are equal, a change in pressure has no effect on the equilibrium reaction. There will be no shift. Adding a Catalyst: No Overall Effect on Equilibrium When a catalyst is added to a reaction at equilibrium: • Rates of both the forward and reverse reactions increase or speed up equally. As a result: • There will be no shift to either direction of the reaction. • There will be no change on equilibrium concentrations. © 201 by E3Scholastic Publishing. All Rights Reserved
