Le Chatelier’s Principle and Equilibrium

EQUILIBRIUM Reversible reactions occur simultaneously in both directions. A system at equilibrium has the forward and reverse REACTIONS occurring at the same RATE The CONCENTRATIONS of the reactants and products are CONSTANT (not equal to each other, but unchanging)

Example of a Reversible Reaction N2O4 (g) + heat 2NO2 (g) Colder temp Warmer temp https://www.youtube.com/watch?v=wlD_ImYQAgQ http://genchem.chem.wisc.edu/demonstrations/Images/13equil/NO2N2O4.jpg

Equilibrium As a reaction proceeds the concentrations of reactants and products change until equilibrium is reached. Once equilibrium is established, the concentration remains constant, unless a stress is applied to change the equilibrium. Equilibrium can only be established for reversible reactions when in a closed system.

Equilibrium N2O4 (g) + heat 2NO2 (g) Chemical equilibrium: when opposing reactions occur at equal rates. N2O4 (g) + heat 2NO2 (g) Phase equilibrium: when both phase changes at the melting point or boiling point for a substance occur at the same rate. Solution Equilibrium: rate of dissolving and recrystallization are equal (saturated solution).

Equilibrium and Keq

Equilibrium Law of Chemical Equilibrium a A + b B  c C + d D Equilibrium Expression The equilibrium expression is a special ratio of the concentrations of the products to the concentrations of the reactants Each concentration is raised to a power corresponding to its coefficient in the balanced equation (Square brackets indicate concentration, K indicates the equilibrium constant) This is based on experimental observations- equilibrium condition could always be described by this special ratio

Equilibrium Expression Write the equilibrium expression for the following reactions: H2 (g) + F2 (g)  2HF(g) N2(g) + 3H2 (g)   2NH3 (g) We place products over reactants and raise each concentration to the power of the coefficient Don’t write 1, it’s understod

Equilibrium Homogeneous equilibrium: when all species are gases. 2 H2 (g) + O2 (g)  2 H2O (g) Heterogeneous Equilibrium: contains multiple phases Experimental results show that equilibrium does not depend on quantities of pure solids or liquids When writing equilibrium expressions for heterogeneous equilibria, do not include pure liquids or solids, only aqueous and gaseous states should be included. Write the equilibrium expression for: PCl5 (s)  PCl3 (l) + Cl2 (g)

Equilibrium expression What does it actually mean? For a reaction at a given temperature, the ratio of the concentration of the products to reactants using the equilibrium expression will always be constant. Meaning, solving for Keq should yield the same results regardless of concentration if equilibrium is established.

Equilibrium Constant Verify the law of chemical equilibrium… NOTE: Concentrations are not EQUAL but they are CONSTANT The reaction of sulfur dioxide with oxygen in the atmosphere to form sulfur trioxide has environmental impact when it combined with water droplets to form sulfuric acid- acid rain The following results were collected for two experiments at 600°C 2 SO2 (g) + O2 (g)  2SO3 (g) Verify the law of chemical equilibrium…

More Equilibrium Constant PCl5 (g)  PCl3 (g) + Cl2 (g) In an experiment, at a temperature where Keq = 8.96 x 10-2, the equilibrium concentrations of PCl5 and PCl3 were found to be 6.70 x 10-3 M and 0.300 M, respectively. Calculate the concentration of Cl2 present at equilibrium.

Equilibrium Constant and SHIFTS IN EQUILIBRIUM Knowing the value of the equilibrium constant helps us do the following: Larger values (larger than 1) of K mean that products are favored and equilibrium lies to the right. Smaller values mean the system contains mostly reactants and the equilibrium lies to the left. If we know the Keq and concentrations of all reactants/products except one we can determine an unknown concentration.

Le Châtelier’s Principle If a stress is applied to a system in equilibrium, the system will shift to relieve the stress. System stresses include increasing or decreasing the following: Concentration of reactants or products Temperature Pressure (on gaseous systems)

STRESS: CONCENTRATION and PRESSURE https://www.youtube.com/watch?v=7zuUV455zFs

Effect of Concentration When increasing a substances' concentration, the reaction will shift in the direction that “uses up” the additional substance to restore equilibrium. (ADD AWAY) When decreasing a substances' concentration the reaction will shift to replace the substance that’s been removed to restore equilibrium. (TAKE TOWARDS) 2 H2 + O2 → 2 H2O  a. Increase the hydrogen concentration 2 H2 + O2 → 2 H2O b. Decrease the hydrogen concentration 2 H2 + O2 → 2 H2O  c. Increase the water concentration

Effect of Temperature and Catalyst https://www.youtube.com/watch?v=XhQ02e gUs5Y

Effect of Temperature Increasing the temperature causes the equilibrium to shift in the direction that absorbs heat. (Endothermic) Decreasing temperature causes the equilibrium to shift in the direction that releases the heat (exothermic) SO2 (g) + O2 (g) 2SO3 (g) + heat a. Increasing the temperature b. Decreasing the temperature

Effect of Pressure Affects gases only. For unequal number of moles of reactants and products: if pressure is increased, the equilibrium will shift to reduce the number of particles. if pressure is decreased, the equilibrium will shift to increase the number of particles. a. increased pressure b. decreased pressure For equal number of moles of reactants and products, no shift occurs. 2NO2 (g) N2O4 (g)

Ex: Effect of Pressure 2NO2 (g) N2O4 (g) Stress: increasing the pressure (decreases volume) Relief: decreasing the number of particles Shift: to the right (side of less molecules) Stress: decrease the pressure (increases volume) Relief: increasing the number of particles Shift: to the left (side of more molecules)