International Baccalaureate Chemistry International Baccalaureate Chemistry Topic 7 – Chemical Equilibrium

Equilibrium In general, equilibrium is the state in which the rate of the forward process/reaction equals the rate of the reverse process/reaction.

Physical Equilibria 1. Liquid - Vapor Equilibria The following equation represents Bromine liquid with Bromine gas in equilibrium. The following equation represents Bromine liquid with Bromine gas in equilibrium. Br (l) → Br (g) At equilibrium, the At equilibrium, the rate of evaporation equals the rate of condensation. NOTE: This does not mean that the number of molecules in each state is the same!

Physical Equilibria Solute – Solution Equilibria The following equation represents sodium chloride in with its in a The following equation represents solid sodium chloride in equilibrium with its ions in a saturated solution: NaCl (s) ↔ Na + (aq) + Cl - (aq) At equilibrium, the At equilibrium, the rate of dissolving equals the rate of crystalization. NOTE: This does not mean that the number of molecules in each state is the same!

What Determines the Equilibrium Point of a Substance? Nature of the reactants a. Type of Liquid (in liquid – vapor equilibria) b. Type of Solid (in solute – solution equilibria) Temperature a. In liquid – vapor equilibria more particles will be found in the gas phase with an. a. In liquid – vapor equilibria more particles will be found in the gas phase with an increase in temperature. b. In a solute-solution equilibria, will be found in the phase with an. b. In a solute-solution equilibria, more particles will be found in the dissolved phase with an increase in temperature. In a physical equilibrium, the system must be closed!

Reversible Reactions chemical are and Many chemical reactions are reversible and never go to completion. Consider the following reaction: (Haber Process) Consider the following reaction: (Haber Process) N 2 + 3H 2 ↔ 2NH 3 If a certain quantity of each reactant is placed in a closed container, initially, N 2 and H 2 are the only species present in the container. If a certain quantity of each reactant is placed in a closed container, initially, N 2 and H 2 are the only species present in the container. because their. N 2 and H 2 will begin to react at their maximum rate because their concentrations are at a maximum. Forward Reaction → maximum rate Reverse Reaction → no rate

N 2 + H 2 NH 3 As time passes, the will start to because (used up). As time passes, the forward reaction will start to decrease because [N 2 ] and [H 2 ] are decreasing (used up). As this happens, an opposing change begins to occur. As this happens, an opposing change begins to occur. into N 2 and H 2 through the. NH 3 begins to decompose into N 2 and H 2 through the reverse reaction. The of the will (as the [NH 3 ] is increasing). The rate of the reverse reaction will steadily increase (as the [NH 3 ] is increasing)., the becomes to the. Eventually, the forward reaction rate becomes equal to the reverse reaction rate. The is said to be in a The system is said to be in a state of Dynamic Equilibrium.

Chemical Equilibrium

of a System in a State Characteristics of a System in a State of Equilibrium can be approached from. Equilibrium can be approached from both directions. The of the reaction the of the reaction and the of The rate of the forward reaction equals the rate of the reverse reaction and the concentrations of all reactants and products remains constant (NOT EQUAL!) The. The system must be closed. (although microscopically there is lots going on!) Macroscopic properties remain constant (although microscopically there is lots going on!)

The Equilibrium Constant Expression For all chemicals, For all chemicals, K c = [Products] [Reactants] Equilibrium ConstantEquilibrium Expression For the reaction aA + bB ↔ cC + dD, The Equilibrium Constant expression is: Where: a,b,c,d are the coefficients from the chemical equation K c = [Products] = [C] c [D] d [Reactants] [A] a [B] b

Characteristics The coefficients from the equation become a power in the equilibrium expression. The coefficients from the equation become a power in the equilibrium expression. in the equilibrium expression since solids and liquids cannot be expressed as concentrations. Only gasses and aqueous solutions are involved in the equilibrium expression since solids and liquids cannot be expressed as concentrations. NOTE: NOTE: Solids cannot be compressed so their density cannot be changed. Solids cannot be compressed so their density cannot be changed. Therefore, the molar concentrations cannot be changed. Therefore, the molar concentrations cannot be changed. Liquids cannot be compressed either so that they have a constant density and molar concentration. Liquids cannot be compressed either so that they have a constant density and molar concentration. If there is another liquid present that can dilute the first liquid, then the liquid is not “pure” and can have its concentration changed by dilution. If there is another liquid present that can dilute the first liquid, then the liquid is not “pure” and can have its concentration changed by dilution.

Practice: Write K c Expressions for the Following Reactions N 2(g) + 3H 2(g) ↔ 2NH 3(g) Cu (s) + 2AgNO 3(aq) ↔ 2Ag (s) + Cu(NO 3 ) 2(aq) P 4(s) + 5O 2(g) ↔ P 4 O 10(s) Br 2(l) + H 2(g) ↔ 2HBr (g) CH 3 COCH 3(l) + Cl 2(g) ↔ CH 3 COCH 2 Cl (l) + HCl (g) 1. K c = [NH 3 ] 2 2. K c = [Cu(NO 3 ) 2 ] 3. K c = 1 [N 2 ] [H 2 ] 3 [AgNO 3 ] 2 [O 2 ] 5 4. K c = [HBr] 2 5. K c = [HCl] [H 2 ] [Cl 2 ]

K c Values Whenever (K c > > 1): Whenever K c is large (K c > > 1): are and the reaction goes Products are favored and the reaction goes almost to completion. K c = [Products]K c = [ BIG # ] > > 1 [Reactants] [ small # ] Whenever (K c < < 1): Whenever K c is small (K c < < 1): are and the reaction. Reactants are favored and the reaction hardly proceeds. K c = [Products] K c = [Products] K c = [ small # ] < < 1 [Reactants] [Reactants] [BIG #] Note: The is. Note: The value of K c is affected only by temperature.

Changes in Equilibrium will in a it has been disturbed or. Changes will occur in a closed system if it has been disturbed or stressed. To explain these changes, we use To explain these changes, we use which states: Le Chatelier’s Principle which states: “Whenever a, it will applied.” “Whenever a stress is applied to a system at equilibrium, it will shift to relieve that stress applied.”

Types of Stresses That Can Be Placed On a System at Equilibrium Change in Concentration Change in Temperature Change in Volume or Pressure Addition of a Catalyst Affect Equil. Position Does Not Affect Equil. Position (does not cause a shift)

Change in Concentration Only affects substances that are (g) or (aq) Only affects substances that are (g) or (aq) Or if two liquids are present (l) Or if two liquids are present (l) Example 1: Example 1: Fe 3+ (aq) + SCN - (aq) ↔ FeSCN 2+ (aq) If we add more [Fe 3+ ] to the reaction mixture, the reaction will shift forward to use up what is added. If we add more [Fe 3+ ] to the reaction mixture, the reaction will shift forward to use up what is added. The result will result in a decrease of [SCN - ] and an increase in [FeSCN 2+ ] product produced. The result will result in a decrease of [SCN - ] and an increase in [FeSCN 2+ ] product produced. Conclusion: Increasing the concentration of a reactant favors the forward reaction. Conclusion: Increasing the concentration of a reactant favors the forward reaction. Eventually, a new equilibrium point will be reached and no further change in concentration will occur. Eventually, a new equilibrium point will be reached and no further change in concentration will occur. The same rules apply if a concentration is decreased. The same rules apply if a concentration is decreased. To offset the stress, more of what is removed must be produced. To offset the stress, more of what is removed must be produced.

Change in Concentration Example 2: N 2(g) + H 2(g) ↔ NH 3(g) Example 2: N 2(g) + H 2(g) ↔ NH 3(g) If the [NH 3 ] is decreased, the system will shift forward in an attempt to replace ammonia that was removed. If the [NH 3 ] is decreased, the system will shift forward in an attempt to replace ammonia that was removed. As a result, N 2 and H 2 decrease. As a result, N 2 and H 2 decrease. Example 3: Identify the shift (forward or reverse) that occurs for the following reaction and indicate the results of the shift and identify the remaining reactants. Example 3: Identify the shift (forward or reverse) that occurs for the following reaction and indicate the results of the shift and identify the remaining reactants. 2A (s) + 3B (g) ↔ 2E (g) + 4D (g) 1. [B] is increased 2. [E] is increased 3. Amount of A is increased 4. [D] is decreased

Changes in Temperature At equilibrium, the temperature is constant. At equilibrium, the temperature is constant. If the reaction vessel is cooled, the reaction will shift to produce heat. If the reaction vessel is cooled, the reaction will shift to produce heat. If the reaction vessel is heated, the reaction will shift to use up heat. If the reaction vessel is heated, the reaction will shift to use up heat.

Example 1: Endothermic Reaction Example 1: Endothermic Reaction Co(H 2 O) 6 2+ (aq) + 4Cl - + energy ↔ CoCl 4 2- (aq) + 6H 2 O (l) Co(H 2 O) 6 2+ (aq) + 4Cl - + energy ↔ CoCl 4 2- (aq) + 6H 2 O (l) If we disturb the system by increasing the temperature, both the forward and reverse reaction rates increase, however, in this reaction the forward will increase more than the reverse. Why? If we disturb the system by increasing the temperature, both the forward and reverse reaction rates increase, however, in this reaction the forward will increase more than the reverse. Why? Forward reaction – uses heat (cools) Forward reaction – uses heat (cools) Reverse reaction – gives off heat (warms) Reverse reaction – gives off heat (warms) Results: Results: [Co(H 2 O) 6 2+ ] and [Cl - ] Decrease [Co(H 2 O) 6 2+ ] and [Cl - ] Decrease [CoCl 4 2- ] increases [CoCl 4 2- ] increases The moles of water will also increase The moles of water will also increase Changes in Temperature

Example 2: Exothermic Reaction Example 2: Exothermic Reaction 3H 2(g) + N 2(g) ↔ 2NH 3(g) + 91 kJ 3H 2(g) + N 2(g) ↔ 2NH 3(g) + 91 kJ If we disturb the system by increasing the temperature, both the forward and reverse reaction rates increase, however, in this reaction the reverse reaction will increase more than the forward. If we disturb the system by increasing the temperature, both the forward and reverse reaction rates increase, however, in this reaction the reverse reaction will increase more than the forward. Forward reaction – gives off heat (warms) Forward reaction – gives off heat (warms) Reverse reaction – uses heat (cools) Reverse reaction – uses heat (cools) Results Results [H 2 ] and [N 2 ] increase [H 2 ] and [N 2 ] increase [NH 3 ] Decreases [NH 3 ] Decreases Changes in Temperature

Generally, a temperature increase will Generally, a temperature increase will favor endothermic reactions Not favor exothermic reactions.

Change in Volume or Pressure Only gasses are affected by pressure/volume changes. Only gasses are affected by pressure/volume changes. Recall: Recall: If the volume of a container is decreased (pressure increased), all concentrations increase. If the volume of a container is decreased (pressure increased), all concentrations increase. This will increase both the forward and reverse reactions, but the reaction will try to offset the increase in pressure. This will increase both the forward and reverse reactions, but the reaction will try to offset the increase in pressure. To decrease the pressure, the reaction will favor the reaction which produces fewer gas particles. To decrease the pressure, the reaction will favor the reaction which produces fewer gas particles.

Example 1: Example 1: 3H 2(g) + N 2(g) ↔ 2NH 3(g) + 91 kJ 3H 2(g) + N 2(g) ↔ 2NH 3(g) + 91 kJ If the volume of the container is decreased, pressure is increased. If the volume of the container is decreased, pressure is increased. 4 moles of gas (3H 2 and 1N 2 ) vs. 2 moles of gas (2NH 3 ) 4 moles of gas (3H 2 and 1N 2 ) vs. 2 moles of gas (2NH 3 ) Results Results Forward reaction is favored since the reaction is producing fewer gas molecules and alleviates the pressure. Forward reaction is favored since the reaction is producing fewer gas molecules and alleviates the pressure. [H 2 ] and [N 2 ] decrease [H 2 ] and [N 2 ] decrease [NH 3 ] increase [NH 3 ] increase What happens to the same reaction if the volume is increased? What happens to the same reaction if the volume is increased? Change in Volume or Pressure

Example 2: Example 2: H 2(g) + I 2(g) ↔ 2HI (g) H 2(g) + I 2(g) ↔ 2HI (g) If you were to either increase or decrease the pressure of the system, it would not make a difference in the reaction rate as it cannot get rid of stress in either direction (2 moles of gas on either side) If you were to either increase or decrease the pressure of the system, it would not make a difference in the reaction rate as it cannot get rid of stress in either direction (2 moles of gas on either side) Example 3: Example 3: NH 3(g) + HCl (g) ↔ NH 4 Cl (g) NH 3(g) + HCl (g) ↔ NH 4 Cl (g) If the pressure is decreased, which reaction is favored and what are the results? If the pressure is decreased, which reaction is favored and what are the results? Change in Volume or Pressure

Example 4: Example 4: 2A (g) + B (g) ↔ 3D (s) + C (g) 2A (g) + B (g) ↔ 3D (s) + C (g) Stress: Increase the pressure by decreasing the volume: Stress: Increase the pressure by decreasing the volume: Reaction favored: _______________ Reaction favored: _______________ [A] will ________________________ [A] will ________________________ [B] will ________________________ [B] will ________________________ [C] will ________________________ [C] will ________________________ Amount of [D] will _______________ Amount of [D] will _______________ Change in Volume or Pressure

Addition of a Catalyst A catalyst will increase both forward and reverse rate equally. A catalyst will increase both forward and reverse rate equally. Concentrations of all substances remain constant. Concentrations of all substances remain constant. Catalysts do not affect the position of equilibrium. Catalysts do not affect the position of equilibrium.

Summary Change Effect on Equilibrium Change in Kc? Increase concentration Shifts to opposite side No Decrease concentration Shifts to same side No Increase pressure Shifts to side with least moles of gas No Decrease pressure Shifts to side with most moles of gas No Increase temperature Shifts in endothermic direction Yes Decrease temperature Shifts in exothermic direction Yes Add a catalyst No change No

Haber Process/Contact Process Using the resources available to you, describe and explain the application of equilibrium and kinetics concepts to the Haber process and Contact process. Using the resources available to you, describe and explain the application of equilibrium and kinetics concepts to the Haber process and Contact process. Start with the IB text and supplement with other texts and/or internet to make a complete set of notes. Start with the IB text and supplement with other texts and/or internet to make a complete set of notes.