Assign. #13.3 – Le Chatelier’s Principle

Predicting the Direction of Reactions Oftentimes, we examine reactions prior to them achieving equilibrium. How do we know the direction a reaction needs to move in order to achieve equilibrium? We use the reaction quotient (Q) to determine this Q is calculating the equilibrium constant prior to equilibrium: a A + b B c C + d D ⇌

Value of Q and Direction There are three possible results of our Q calculation: Q = K: Reaction is at equilibrium Q > K: Amount of products is too large, reaction needs to shift towards reactants Q < K: Amount of reactants is too large, reaction needs to shift towards products

Class Example ⇌ At 1000 K the value of Kp for the reaction: 2 SO3 (g) 2 SO2 (g) + O2 (g) is 0.338. Calculate the value for Qp and predict the direction in which the reaction proceeds toward equilibrium if the initial partial pressures are PSO3 = 0.16 atm; PSO2 = 0.41 atm; PO2 = 2.5 atm. ⇌

Table Talk ⇌ At 100 oC the equilibrium constant for the reaction: COCl2 (g) CO (g) + Cl2 (g) has the value Kc = 2.19 x 10-10. Are the following mixtures of COCl2, CO, and Cl2 at 100 oC at equilibrium? [COCl2] = 2.00 x 10-3 M; [CO] = 3.3 x 10-6 M; [Cl2] = 6.62 x 10-6 M ⇌

Le Chatelier’s Principle “If a system at equilibrium is disturbed by a change in temperature, pressure, or a component concentration, the system will shift its equilibrium position so as to counteract the effect of the disturbance” When you calculate Q you are doing the math behind Le Chatelier’s Principle

Le Chatelier’s Principle and Concentration If we add more of a product/reactant, then our reaction will shift to the other side in order to compensate for this new amount. Think back to the relationship between Q and K as the reasoning for this.

Le Chatelier’s Principle and Volume/Pressure Changes in pressure can alter the side of the reaction that is favored. If we increase pressure, the reaction will shift to the side that produces less molecules If we decrease pressure, the reaction will shift to the side that produces more molecules

Le Chatelier’s Principle and Temperature When a reaction is endothermic, heat is listed as a reactant. Heat + Reactants  Products When a reaction is exothermic, heat is listed as a product. Reactants  Products + Heat Therefore we can treat heat like a reactant or a product depending where it is listed.

Class Example ⇌ Consider the equilibrium: In which direction will the equilibrium shift when: N2O4 (g) 2 NO2 (g) ΔHo = 58.0 kJ N2O4 is added NO2 is removed The pressure is increased by addition of N2 (g) The volume is increased The temperature is decreased ⇌

PCl5 (g) PCl3 (g) + Cl2 (g) ΔHo = 87.9 kJ Table Talk For the reaction: PCl5 (g) PCl3 (g) + Cl2 (g) ΔHo = 87.9 kJ In which direction will the equilibrium shift when: Cl2 (g) is removed The temperature is decreased The volume of the reaction system is increased PCl3 (g) is added ⇌

Le Chat. and Catalysts A catalyst increases the rate of reaction, but does not impact final concentrations. Therefore, a catalyst does not impact the value of the equilibrium constant