11-3 LeChatelier’s Principle: Shifts in Equilibrium (Section 17.7) And you 7zuUV455zFs

Introduction: Chemists can control the position (a set of concentrations or pressures) of a reaction to “favor” or maximize the amount of product. Chemists shift the equilibrium in one direction or the other using Le Chatelier’s Principle: If a system at equilibrium is subjected to a stress, the system will shift in the direction that relieves the stress.

3 variables to discuss: Temperature: affects Keq value Concentration: doesn’t affect Keq value Pressure: is only applicable to gases and doesn’t affect Keq value

Example: change the concentration Pink Ion + Cl - ↔ Blue Ion + H 2 O What would happen to the equilibrium if additional Cl - is added? 1. ________ collisions between Pink Ion and Cl Forward reaction rate ____________. 3. ________ Blue ion produced and concentrations of Pink Ion and Cl - decrease. 4. Reach a new equilibrium position. (but K value stays the same!!!! more increases more

This new set of concentrations (new “position”), with more product than reactants, “________________” the product ; we say that the equilibrium has shifted to the right. To clarify, the concentration of the blue ion goes up and the concentration of the pink ion decreases. But once again, the value of K stays the same if the temperature remains the same. favors

Meaning of “Shifting” an Equilibrium Consider the reaction of bromothymol blue indicator (Ind - ) with acid: Ind - + H + ↔ HInd Blue(B) Colorless Yellow(Y) Using 6 molecules: BBBBBY ↔ YYYYYB What would you get with equal numbers of Y and B molecules?? Green!!!! Why can we not have all six molecules either B or Y? It never runs all the way to completion!!!!!

Ind - + H + ↔ HInd blue yellow Adding acid will _____________ the [H + ] (recall: acid = H + donor). Increased H + will cause the reaction to shift __________, reducing the concentrations of the reactants and increasing the concentration of the products. We would predict a __________ color. Note: shifting right has reduced the #B and increased the #Y! increase right yellow

Ind - + H + ↔ HInd blue yellow Adding base (OH - ): the hydroxide ion (OH - ) reacts with H + to form ________, thereby reducing the [H + ]. The equilibrium shifts to the __________, to make more H +. We would predict a _____________ color. How would you draw a green color with 6 molecules: BBBYYY H2OH2O left blue

11-4 The Haber Process for Producing Ammonia Effect of Change in Temperature: (changes K value) N 2 (g) + 3 H 2 (g) ↔ 2 NH 3 (g) + 92 kJ The above reaction is ________thermic, which is why we write heat as a. Chemists trying to produce more ammonia (NH 3 ) gas would the reaction to shift the equilibrium to the exo product cool right

Effect of Change in Pressure: doesn’t affect Keq value!!!! N 2 (g) + 3 H 2 (g) ↔ 2 NH 3 (g) Compare the number of molecules (moles) in the GAS PHASE ONLY! Looking at the equation, how many molecules (moles) of gas reactants are present? ______ How many molecules (moles) of gas products are present? ______ The greater the number of molecules of a gas, the ____________ the pressure and vice versa. 4 2 greater

N 2 (g) + 3 H 2 (g) ↔ 2 NH 3 (g) Chemists trying to produce more ammonia (NH 3 ) gas would ___________ the pressure to shift the equilibrium to the __________. Note: If the volume of a container of this reaction is reduced, then the pressure will ____________ and the reaction will shift to the __________. right increase right

Effect of Change in Concentration: N 2 (g) + 3 H 2 (g) ↔ 2 NH 3 (g) Add more reactants:reaction equilibrium would shift __________ to reduce the extra reactants. Add more products: reaction equilibrium would shift __________ to reduce the extra products. Remove either reactant:reaction would shift __________ to produce more of the reactant. right Left

N 2 (g) + 3 H 2 (g) ↔ 2 NH 3 (g) Chemists trying to produce more ammonia (NH 3 ) gas would _________ the product ammonia to shift the equilibrium to the __________. remove right

N 2 (g) + 3 H 2 (g) ↔ 2 NH 3 (g) What if more N 2 is added 1)Initially, the [N 2 ] increases, since more was added. However, as it reacts with H 2, the concentration decreases slightly. 2) [H 2 ] decreases as it reacts with the added N 2. 3) The product NH 3 increases in concentration as the 2 products react. See how both reactants concentrations have decreased from the time of addition and the product has increase? This is what we call “shifting” to the right.

Le Chatelier’s Demo with Temperature: 2 NO 2 (g) ↔ N 2 O 4 (g) + heat Brown Colorless As written, this equation is an ________thermic reaction, since heat is being released. Cooling the reaction would shift the reaction to the __________ to produce more heat. Prediction: the brown color would turn (darker or lighter); it can’t reach colorless! exo right

2 NO 2 (g) ↔ N 2 O 4 (g) + heat Brown Colorless Heating the reaction would shift the equilibrium to the __________ to reduce the added heat. We would predict that the brown color would turn (darker or lighter). left