Equilibrium

 Equilibrium is a condition where the forward rxn rate equals the reverse rate of reaction ◦ It’s dynamic ◦ The concentrations of both reactants and products remain constant. ◦ Requires a closed system  Popular examples include…soda bottle, gas tank, Haber/Contact process, and stomach digestion.

 How does equilibrium change? By stressing the rxn through temperature, pressure, catalyst, surface area, etc.  The system will compensate by shifting equilibrium in order to relieve that stress.  Consider the reaction... 2 SO 2 + O 2 ↔ 2 SO 3

 Consider the reaction... 2 SO 2 + O 2 ↔ 2 SO 3 ΔH = kJ/mol  What if I increase a reactant concentration?  What if I increase the pressure on the system?  What if I add heat to the system?  What if I add a catalyst to the system?

 A ratio of products to reactants will indicate to which direction the rxn is favoring. Therefore, we use the following general equation. K = [Products] / [Reactants]  The brackets mean molarity, therefore pure solids and liquids are excluded from any expression.  Student Practice: a.) Write a rxn for the decomposition of aqueous Ferric Chloride. b.) Write K eq expression

 Equilibrium expressions do not always involve molarity. Partial pressures may also be used in the equilibrium expression.  What is a partial pressure? It is exactly what it means. The part of the total pressure representing that gas designated by “P” Example: A gas cylinder has a total pressure of 400kPa. Nitrogen has 185kPa, Oxygen has 138kPa, so what is the partial pressure of Argon?  Vapour Pressure is the condition where atmospheric pressure is equal to the boiling point. In other words, the pressure of a vapor when a liquid or solid becomes a gas. 

 The Haber process was developed by Fritz Haber who was looking to produce Ammonia on an industrial scale. N 2 + 3H 2 ↔ 2 NH 3 ΔH = -92 kJ/mol  This relatively simple process requires 200 atm, a temperature of 450°C, and an Iron catalyst.  The equilibrium position is critical to the processing plant in order to maximize efficiency. What factors affecting reaction rate can influence the reaction?

 K eq represents the equilibrium constant at a specific temperature and pressure. Later, we will calculate Q which is the reaction quotient that we compare to K to determine which direction the reaction will proceed.  When we complete the K eq calculation, we assume that all concentrations are at equilibrium. So, last year in Chemistry it was an easy cross multiplication problem.  However, most practical applications of an equilibrium calculation actually have initial reactant concentrations.  Therefore, we set up an “ICE” diagram. I = Initial, C = Change, and E = At equilibrium

 Example: At 500°C, the Haber process has a K p = 1.45x An equilibrium mixture of these gases shows the partial pressure of Nitrogen to be 0.43atm and Hydrogen to be 0.95atm. What is the equilibrium pressure of Ammonia? N 2 + 3H 2 ↔ 2 NH 3 [NH 3 ] 2 [x] 2 K eq = = = 1.45 x [N 2 ] [H 2 ] 3 [0.43][0.95] 3 Answer: Ammonia = ??? atm

 The last example had information that was all at equilibrium conditions so K=P/R was used, here is a problem that has initial concentrations or pressures.  Example: At 500°C for the synthesis of hydrochloric acid has initial concentrations of 0.3M for hydrogen and 0.5M for chlorine. If the equilibrium concentration for the acid is 0.46M, what is the equilibrium concentration for each reactant? H 2 + Cl 2 ↔ 2 HCl Initial Zero Change -x -x +2x Equilibrium0.3-x0.5-x 0.46 Solution: Since the equilibrium concentration is equal to 2x, we do 2x = 0.46, therefore, x = We plug this into the equilibrium equations for the reactants to solve the problem.

 Last Example: The decomposition of hydrogen iodide gas has a K eq = 84.2 at 850°C. If the initial concentration of HI is 0.75, what is the concentration of each species in the reaction at equilibrium? 2HI↔H 2 + I 2 Initial 0.75 Zero Zero Change -2x +x +x Equilibrium0.75-2x+x +x 84.2 [x][x] = [0.75-2x] 2 Do you remember the quadratic equation? Good, now quit complaining and solve for “x” and plug it into your ICE chart to find the concentrations at equilibrium.

120 mol of hydrogen gas are mixed with 40mol of nitrogen gas then pressurized. The mixture of gases at a constant pressure over an iron catalyst at 9 million deg until the mixture reaches equilibrium. The total volume of the mixture is 1.0 dm 3. Only 20% of the reactants are converted to products. a.) Write an equation for the Haber process (ΔH = -92 kJ/mol) b.) How many moles of Nitrogen and Hydrogen remain at equilibrium? c.) How many moles of Ammonia are formed? d.) Write the equilibrium expression. e.) Calculate the equilibrium constant, including units. f.) Does this reaction favor reactants or products? g.) What will happen to K when P goes up? h.) What will happen to K when T goes up?

a.) N 2 + 3H 2 ↔ 2 NH 3 (ΔH = -92 kJ/mol) b.) Since the word “remain”, 0.8(120) = 96 moles of Hydrogen 0.8(40) = 32 moles of Nitrogen c.) Since a 1:2 ratio exists between N 2 :NH 3, 32=2x x=16 moles 3:2 ratio exists between H 2 :NH 3, 96/3=2x x=16 moles d.) Write it yourself…  e.) (16) 2 / [(96) 3 (32)] = 9.04 x10 -6 mol -2 dm 6 f.) This reaction favors reactants since the K is so small. g.) What will happen to K when P goes up? K goes up h.) What will happen to K when T goes up? Since the reaction is exothermic, heat is on the products side and adding heat would shift the reaction left, so K goes down.

 From your first year in chemistry, you should know that Arrhenius developed the simple fact that all acids have Hydrogen and all bases have Hydroxide.  However, Brönsted-Lowry came later to expand on this definition to include metal oxides and functional groups such as carbonates and bicarbonates.  Newsflash! Bases in the UK are known as an alkali.  Amphoteric substances act as both an acid and a base where water is the perfect example.  Buffers are solutions created to resist the change in pH. As you recall, >7 is base and <7 is an acid.

What does dissociation mean? The hydrogen ion (also called the proton) may dissociate completely or partially which explains the main difference between a strong and a weak acid. Class Examples: Just to thoroughly confuse the scientific community, Americans call H 3 O + the Hydronium ion where as the UK calls it the Hydroxonium ion. What is an OH group called? (3 names)

 Do you remember electrochemistry where that concept had the “agents”. Well, acid/base chemistry has a similar concept. What does the word “conjugate” mean? We must follow the H/OH path to find the answer. NH 3 + H 2 O ↔ NH OH - Base Acid Conj. Acid Conj. Base Student Practice: Write a reaction for Acetic acid and water

 1.) If Ammonia were to combine with a proton, what type of bonding would that be? What bond angle would the resultant molecule have?  2.) What is the name given to the species in #1? How would you classify this molecule?  3.) Is saliva normally acidic / basic / neutral? Why?  4.) Sponge Bob Squarepants places a Sodium lozenge into the mouth of his best friend, __________, write a chemical rxn to illustrate what is going on.  5.) Assuming water is a pure liquid, write the equilibrium expression for the reaction in #4.

 Questions you can count on… 1.) Don’t just know a definition, be able to apply it. Ex: Electrolyte, Anode, Electrolysis 2.) Know what changes to make in equilibrium for both the Haber and Contact process. 3.) How does K c differ from K p ? 4.) Make a neutralization reaction and indicate the conjugate acid/base pair. 5.) Does Sicilian pizza actually come from Sicily?