Dynamic Equilibrium

Objectives Describe chemical equilibrium in terms of equilibrium expressions Use equilibrium constants Describe how various factors affect chemical equilibrium Explain Le Chatelier’s principle

Equilibrium Some chemical systems have little tendency to react, while others go to completion In between these extremes are chemical systems that reach a state of equilibrium with varying amounts of reactants unconsumed For example: N 2 + 3H 2 ⇔ 2NH 3 This reaction reaches equilibrium when fewer than 2 moles of ammonia produced

Law of Chemical Equilibrium At a given temp, ratio of concentrations of reactants and products has a constant value General equation: aA + bB ⇒ cC + dD Equilibrium constant (K eq )

Example Hydrogen and Iodide react to produce Hydrogen Iodide H 2 (g) + I 2 (g) ⇔ 2HI (g) This is known as homogeneous equilibrium = all reactants & products in same physical state Write Equilibrium Expression K eq for this reaction

Example Ammonia gas production: N 2 (g) + 3H 2 (g) ⇔ 2NH 3 (g) Write equilibrium expression for this reaction

Practice Write equilibrium expressions for the following reactions: N 2 O 4 (g) ⇔ 2NO 2 (g) CO (g) + 3H 2 (g) ⇔ CH 4 (g) + H 2 O (g) 2H 2 S (g) ⇔ 2H 2 (g) + S 2 (g)

Heterogeneous Equilibrium Not all reactants and products in same physical state leave out reactants and products in solid or liquid state Just include gas concentration or solute concentration

Heterogeneous Equilibrium Example C (s) + H 2 O (g) ⇔ CO (g) + H 2 (g) Keq = [CO] [H 2 ] [H 2 O]

Why use equilibrium constants? If you know the equilibrium constant for a particular chemical reaction at a particular temperature, you can determine the concentration of one of the reactants or products given the concentration of the remaining reactants and products

Factors Affecting Chemical Equilibrium When manufacturers make products, they want to minimize waste or leftover materials Principles of chemical equilibrium can help determine the conditions that favor the most cost effective and environmentally friendly production of a chemical product

Le Châtelier’s Principle If a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress

How can we apply Le Châtelier’s Principle? For example - we are trying to produce methane using the following reaction: CO (g) + 3H 2 (g) ⇔ CH 4 (g) + H 2 O (g) + heat Unfortunately, using our current manufacturing techniques, at equilibrium we produce only mol of CH 4 - way too low a yield to be cost effective What can we do to increase our yield of methane?

What is a stress? Any kind of a change in a system that upsets equilibrium Le Châtelier’s principle = predict how system will change Going back to CO (g) + 3H 2 (g) ⇔ CH 4 (g) + H 2 O (g) + heat how to stress system to produce more product?

Stresses that affect equilibrium pressure/volume temperature concentration

Changes in concentration CO (g) + 3H 2 (g) ⇔ CH 4 (g) + H 2 O (g) + heat If we increase [CO] or [H 2 ], system will respond by producing more product (shift to the right). If we decrease [CH 4 ] or [H 2 O], system will respond by producing more product. If we increase [CH 4 ] or [H 2 O], system will respond by producing more reactant (shift to the left)

Change in Volume/Pressure N 2 (g) + 3H 2 (g) ⇔ 2NH 3 (g) Increase pressure (reduce volume) → reduce number of moles (shift to the right) Reduce pressure (increase volume) → increase number of moles (shift to the left)

Changes in Temperature CO (g) + 3H 2 (g) ⇔ CH 4 (g) + H 2 O (g) + heat Increase temperature, system will respond in endothermic direction (shift to the left) Decrease temperature, system will respond in exothermic direction (shift to the right)

Practice Problems Use Le Châtelier’s Principle to predict how each of these changes would affect CO (g) + 3H 2 (g) ⇔ CH 4 (g) + H 2 O (g) + heat Increase Temperature Remove Hydrogen gas Increase Volume Increase Pressure Decrease Temperature Remove water vapour

Practice problems How would decreasing the volume of the reaction vessel affect each of these equilibria? 2SO 2 (g) + O 2 (g) ⇔ 2SO 3 (g) H 2 (g) + Cl 2 (g) ⇔ 2HCl (g) 2NOBr (g) ⇔ 2NO (g) + Br 2 (g)