Equilibrium

This is usually Question #1 on FR write equilibrium expressions convert between K P and K c eq. constants calculate eq. constants calculate concentrations from lab data apply LeChatelier’s principle of eq. systems

What is Equilibrium? occurs when a reaction can occurs in both the forward and reverse direction at the same time when we look at equilibrium, it appears that nothing is changing, but at the atomic level it is bidirectional arrow is used between the reactants and products

Equilibrium Constant always [product]/[reactant] each [] is raised to the power of the coefficient in the balanced reaction can be expressed in molarity (K c ) or pressure (K p ) To convert btwn K c and K p use: K p = K c (RT)  n

What’s included? When writing eq. expressions, only (aq) and (g) are included Pure liquids and solids are NOT included!!! Write the eq. constant expressions for the following reaction: NaCl(s)Na + (aq) + Cl - (aq)

What does it mean? the size of K is kind of a ratio of products to reactants K can tell you if there will be more products or reactants once the reaction reaches eq. Large K means that products are favored and the rxn “lies to the right” Small K means that reactants are favored and the rxn “lies to the left” When you write a rxn in reverse, the new K is the reciprocal of the old K

Finding K Say “hello” to RICE – R = reaction – I = initial concentrations – C = change in concentrations – E = equilibrium concentrations

Example Initially 0.40 mol of nitrogen and 0.96 mol of hydrogen are placed in a 2.0 L container at constant temperature. The mixture is allowed to react and at equilibrium, the molar concentration of ammonia is found to be 0.14 M. Calculate the eq. constant, K c, for the reaction.

What does it all mean? Let me introduce you to Q: – reaction quotient expression – used to indicate where a reaction is RIGHT NOW, not necessarily at equilibrium – calculated the same way as K comparing Q to K will let you know what a reaction will do to reach equilibrium…

Q vs. K if K = Q, the system is at equilibrium If K < Q, the system will go to the left (towards reactants) to achieve equilibrium If K > Q, the system will go to the right (towards products) to achieve equilibrium (notice that if K & Q are in alpha order, the sign points in the direction of the shift)

Example You are investigating the decomposition of dinitrogen tetraoxide to nitrogen dioxide: N 2 O 4 (g) NO 2 (g), at 100 o C K c = What direction will the reaction proceed if [N 2 O 4 ] = 1.5 M & [NO 2 ] = 0.75 M?

LeChatelier’s Principle states that if a change is applied to a system at equilibrium, the system will shift to counteract that change Changes that affect equilibrium include: – change in concentration of reactants or products – change in volume (gases only) – change in temperature Catalysts do NOT affect equilibrium

 concentration adding reactant shifts rxn towards products (“shift right”) adding product shifts rxn towards reactants (“shift left”) removing reactant shifts rxn left removing product shifts rxn right  [] will NOT change K

Example You have the following net ionic reaction: NaCl(s) Na + (aq) + Cl - (aq) How will adding concentrated HCl to the above solution affect the equilibrium? If the solution was heated and some of the water was driven off, how would that affect the equilibrium?

 V only affects rxn with gases in them decreasing volume shifts rxn towards the side with fewer moles of gases increasing volume shifts rxn towards the side with more moles of gases  V will NOT change K

Example Consider the following reaction: 2H 2 (g) + O 2 (g) 2H 2 O(g) If the size of the reaction vessel is halved, which direction will the reaction shift? If you wanted to produce more oxygen, what should you do to the container?

 T think of heat as a reactant (in endothermic rxns) or a product (in exothermic rxns) If temp is increased, rxn will shift away from the heat if temp is decreased, rxn will shift towards the heat  T WILL change K

Catalysts & K The presence of a catalyst does not change K or cause the reaction the shift in any way, it only increases the rate at which equilibrium is established.