Chapter 15 Chemical Equilibrium

Equilibrium - Condition where opposing processes occur at the same time. - Processes may be physical changes or chemical changes

Equilibrium Expression Concentrations are in molarity (M) Often stated as “products over reactants”

[H 2 ][I 2 ][HI] 16.47E E E E E

Equilibrium Expression Partial pressures are in atmospheres (atm)

What is the difference between K c and K p K p = K c only when the change in the number of moles of gas in balanced equation is zero

What is the difference between K c and K p

Homogeneous equilibrium - An equilibrium that involves reactants and products in a single phase. Heterogeneous equilibrium - An equilibrium involving reactants and products in more than one phase. Solid components or liquid solvent components are usually ignored in writing equilibrium expressions.

Reaction Quotient K c is at equilibrium (at given temp, K c =const.) Q is at any point in the rxn (Q can be any value)

If Q>K, the rxn will shift to the left If Q<K, the rxn will shift to the right If Q=K, the rxn is at equilibrium

Calculating Concentrations with Equilibrium Constants 1.Write equation and equilibrium expression 2.Create table of initial concentrations 3.Indicate a change of a component by “x” 4.Change other variables using stoichiometry 5.Add initial concentrations with changes to yield equilibrium concentrations 6.Substitute into equilibrium expression 7.Use algebra to solve for “x” 8.Use solution of “x” to find equil. concentrations

Le Chatelier's Principle If a system in equilibrium is disturbed, the system will adjust to reestablish equilibrium. Ways to disturb equilibrium: 1.Add or subtract chemical components 2.Increase or decrease temperature 3.Increase or decrease pressure (adjust volume) (adding a catalyst will not disturb equilibrium)

1.Add or subtract chemical component –Use reaction quotient (Q) to determine how the reaction will shift. 2.Effects of temperature on equilibrium –If rxn is exothermic, an ↑ in temp will ↓ products. –If rxn is endothermic, an ↑ in temp. will ↑ products. 3.Effects of pressure on equilibrium –If the pressure is increased by decreasing the volume of a reaction mixture, the reaction shifts in the direction of fewer moles of gas.

Effects of catalysts on equilibrium ‐ Catalysts have NO effect on equilibrium. ‐ Catalysts only affect speed of reaching equilibrium