Chapter 13 Chemical Equilibrium

Reversible Reactions REACTANTS react to form products. PRODUCTS then react to form reactants. BOTH reactions occur: forward  aA + bB  cC + dD  reverse

EQUILIBRIUM OCCURS When the amount of REACTANTS becoming PRODUCTS EQUALS the amount of PRODUCTS becoming REACTANTS. CONCENTRATIONS NO LONGER CHANGE! When the forward and reverse reactions rates are equal.

EXAMPLE 2HI (g) ↔ H 2(g) + I 2(g) 2 HI react to form H 2 and I 2 As H 2 and I 2 build up, they react to form HI

EQUILIBRIUM EXPRESSION If we compare the concentrations of all the reactants and products at equilibrium, we discover a relationship: K = [Products] p [Reactants] r –Can be many equilibrium concentrations possible for a given temp, BUT ONLY ONE K –Includes only gases and solutions, no pure solids or liquids

2HI (g) ↔ H 2(g) + I 2(g) Write the equilibrium expression for this reaction. K = [H 2 ][I 2 ] [HI] 2

N 2(g) + H 2(g)  NH 3(g) Write the equation and the equilibrium expression for the synthesis reaction between N 2(g) and H 2(g) to form NH 3(g) NOW….Write the equation and the equilibrium expression for the decomposition of NH 3(g) WHAT DO YOU NOTICE ?

ANOTHER REACTION Write the equation and the equilibrium expression for N 2 O 4(g) decomposing into NO 2 (g) NOW write the equation and the equilibrium expression for 1/2N 2 O 4 forming NO 2(g) WHAT DO YOU NOTICE ?

General rules K = [products] p [reactants] r If you reverse the reaction, you _____the K If you double the reaction, you _____the K If you halve the reaction, you _____ the K

TRY THIS ONE ! Given this equilibrium expression: K = [SO 2 ] 2 [O 2 ] [SO 3 ] 2 Write the balanced equation.

Equilibrium Calculations If you know the equilibrium concentrations of reactants & products, you can solve for K. aA + bB  cC + dD K = [C] c [D] d [A] a [B] b

Sample Problem When solid ammonium chloride is heated, it reaches equilibrium with ammonia and hydrogen chloride. It is found that at equilibrium at 500 o C in a 5.0 L container, there is 2.0 mol NH 3, 2.0 mole HCl, and 1.0 mol NH 4 Cl. Calculate K for this system.

GASES Equilibrium problems can be done using the partial pressures of gases, and solving for Kp Parenthesis are substituted for brackets. See Example 13.4 in text

Kp and Kc A relationship exists between Kc and Kp. Kp= K(RT) Δn R = L atm/mol K T = temp in K  n = sum of the coefficients of the gas product minus sum of the coefficients of the gas reactants

Homogeneous Equilibria –all reactant and products are in the same phase Heterogeneous Equilibria –more than one phase –eliminate pure solids and liquids Check out 13.6 later!

K allow us to predict several things about the reaction: 1)Tendency of the reaction to occur 2)If a given set of concentration represents an equilibrium condition 3)Equilibrium position that will be achieved from a given set of initial conditions.

Tendency of a Reaction to Occur 1.If K>1, mostly products exist, equilibrium lies far to the right Forward Rxn is predominant 2. If K<1, mostly reactant, equilibrium lies far to the left Reverse Rxn is predominant

Reaction Quotient Given some reactants AND some products together in a reaction vessel, which reaction (  or  )will occur to reach equilibrium. Compare Q to K K uses equilibrium concentrations Q uses original concentrations

Q and K Sample Exercise 13.7 If K < Q, reaction shifts  If K > Q, reaction shifts  If K = Q, reaction is AT equilibrium

Calculate Equilibrium Concentrations If given K & original concentrations, you can find equilibrium concentrations using an ICE chart

THE ICE CHART Time for lots of examples and practice! Reactant and Products are listed here! I write my ICE chart underneath the balanced equation. Initial Change Equilibrium

Le Chatelier’s Principle When a chemical system is at equilibrium, and something changes- that change is a “stress”. The equilibrium will adjust to reduce the stress. It will move forward or reverse to reduce the stress. Common stresses are changing concentration, pressure or temperature.

Concentration Adding reactant or product, equilibrium shifts away from the addition. Removing reactant or product, equilibrium shifts towards the subtraction Try 13.13

Pressure Can be changed by 1) adding or removing gaseous species 2) adding an inert gas (no effect) 3) changing the volume Volume change directly related to moles Increase volume, shift to large # of moles Decrease volume, shift to smaller # of moles Try 13.14

Temperature Treat changes in temp like changes in concentration. Treat heat like any other product or reactant. Must know if rxn is endothermic or exothermic Increase heat, shift away from the heat Decrease heat, shift towards heat Try 13.15

NOW WE PRACTICE A LOT!!!!