Reaction Quotient Le Chatlier’s Principle K vs. Q.

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1 Reaction Quotient Le Chatlier’s Principle K vs. Q

2 Reaction Quotient (Q) measures the relative amount of products and reactants present during a reaction at a particular point in time Reaction quotient aids in figuring out which direction a reaction is likely to proceed K eq vs. Q K describes a reaction at equilibrium Q describes a reaction not at equilibrium

3 Just like calculating K! Calculating Q

4 Compare Q vs. K If Q>K, reaction favors the reactants causing the system to shift to the LEFT. If Q<K, reaction favors the products. causing the system will shift to the RIGHT. If Q=K, then the reaction is already at equilibrium.

5 Example Find the value of Q and determine which side of the reaction is favored.Given K eq =0.5 HCl (g) + NaOH (aq) ⇌ NaCl (aq)+ H 2 O (l) [HCl]= 3.2 M[NaOH]= 4.3 M[NaCl]=6 M Q = 0.436 … Q is less than Keq so the reaction shifts RIGHT, favors the products.

6 Why does all this shifting occur? Le Chatlier’s Principle states that any change to a system (stress) at equilibrium will force the system to compensate for that change (shift). Concentration Pressure Temperature Catalyst

7 Concentration Stress Increasing [M] of A equilibrium will move in such a way as to counteract the change equilibrium will move so that the concentration of A decreases equilibrium moves to the right (green arrow ) Decreasing [M] of D equilibrium will move in such a way as to counteract the change equilibrium will move so that the concentration of D increases.. to replace what was lost equilibrium moves to the right (green arrow )

8 Temperature Stress Increasing Temperature equilibrium will move in such a way as to counteract the change equilibrium will shift to reduce temperature the reverse reaction absorbs heat, so shift to left Decreasing Temperature equilibrium will shift to increase temperature the forward reaction releases heat, so shift to right

9 Temperature Summary Increasing the temperature of a system in dynamic equilibrium favors the endothermic reaction. The system counteracts the change by absorbing the extra heat. Decreasing the temperature of a system in dynamic equilibrium favors the exothermic reaction. The system counteracts the change by producing more heat.

10 Pressure Stress (gases only) Increasing Pressure equilibrium will move so that the pressure is reduced Shift to side producing fewer molecules Decreasing Pressure equilibrium will move so that the pressure is increased Shift to side producing more molecules

11 Catalyst Stress Adding a catalyst makes absolutely no difference to the position of equilibrium Catalysts increase the rate of both the forward and reverse reactions. Equilibrium is achieved faster, but the equilibrium composition remains unaltered.