Chemical Equilibrium SC4. Obtain, evaluate, and communicate information about how to refine the design of a chemical system by applying engineering principles to manipulate the factors that affect a chemical reaction. d. Refine the design of a chemical system by altering the conditions that would change forward and reverse reaction rates and the amount of products at equilibrium. (Clarification statement: Emphasis is on the application of Le Chatelier’s principle.)

A + B ↔ C A + B C Reversible Reactions Reversible reactions bounce back and forth from forming products to reforming reactants Reversible reactions occur simultaneously in both directions Reversible reactions can be written as follows: A + B ↔ C A + B C

Chemical Equilibrium At chemical equilibrium there is no net change in the actual amounts of the components of the system. Although the rates of the forward & reverse reactions are equal at chemical equilibrium, the concentrations of the components on both sides of the equation are not necessarily the same. In fact they can be dramatically different.

Consider a set of escalators as being like the double arrows in a dynamic equilibrium. The # of people using the up escalator must be the same as the # of people using the down escalator for equilibrium to have been established However, the # of people upstairs do not have to equal the # of people downstairs Just the transfer between floors must be consistent

Which Direction is Favored? The equilibrium position of a reaction is given by the concentrations of the system’s components at equilibrium The equilibrium position indicates whether the components on the left or right side of a reversible reaction are at a higher concentration.

Which Direction is Favored? If A reacts to give B and the mixture at equilibrium contains more of B – say 1% of A vs. 99% of B –the formation of B is said to be favored. On the other hand, if the mixture contains 99% of A and 1% of B at equilibrium then the formation of A is favored.

Which Direction is Favored? Reverse direction is favored! Forward direction is favored! A B 99% 1%

Reversibility vs. Reality In principle, almost all reactions are reversible to some extent under the right conditions In practice, one set of components is often so favored at equilibrium that the other set cannot be detected. If one set of components has established equilibrium by converting mostly into products, the reaction has gone to completion When no products can be detected, you can say there is no reaction

Reversibility vs. Reality Reversible reactions occupy a middle ground between the theoretical extremes of irreversibility and no reaction. The addition of a catalyst will speed up forward and reverse reactions equally by reducing the energy needed to activate the reaction in both forward and reverse directions. Does not effect the amount of reactants and products present at equilibrium; it simply decreases the time it takes to establish equilibrium

Manipulating the Equilibrium… There is a principle that can be studied to govern changes in equilibrium – Le Châtelier’s Principle Le Châtelier’s Principle states that “If a stress is applied to a system in dynamic equilibrium, the system changes to relieve the stress.” Stresses are changes in temperature, pressure, concentration of reactants,or concentration of products

Concentration & Equilibrium Adjusting the concentrations of either reactants or products can have dramatic impact on the equilibrium If we add more of reactant A to a system at equilibrium the system will strive to reestablish equilibrium at a new equilibrium position. The reaction will push to use up the extra A and generate more C [A]↑, rxn will shift toward products

Concentration & Equilibrium Adjusting the concentrations of either reactants or products can have dramatic impact on the equilibrium If we add more of product C to a system at equilibrium the system will strive to reestablish equilibrium at a new equilibrium position. The reaction will push to use up the extra C and generate more A and B [C]↑, rxn will shift toward reactants

Temperature & Equilibrium The impact of temperature changes on an equilibrium is dependent on if the process is endothermic or exothermic Exothermic processes produce energy (feel hot) Energy is a product If T↑, the equilibrium shifts left

Temperature & Equilibrium The impact of temperature changes on an equilibrium is dependent on if the process is endothermic or exothermic Endothermic processes use energy as a reactant (feel cold) Energy is a reactant If T↑, the equilibrium shifts right

Pressure & Equilibrium If A, B, and C are all gases, then the equil they establish is pressure dependent When the pressure is increased, the system relieves the pressure by favoring the direction that produces fewer gas molecules. Pressure is # of particles dependent, the more particles the higher the pressure Fewer gas molecules will exert less pressure. So, more product is formed, which overall reduces the pressure, this is a shift right

Pressure & Equilibrium Conversely, a decrease in pressure will favor the rxn that produces the most molecules So we have a shift to the left P↑, this equilibrium shifts right If P↓, this equilibrium shifts left