Chemical Equilibrium Essential Question: To what extent are chemical reactions reversible?

What is dynamic equilibrium? Chemical Equilibrium What is dynamic equilibrium? Expressions for equilibrium constants, Keq; Factors that affect equilibrium; Le Chatelier’s Principle

What is Equilibrium?

This is not Equilibrium?

Chemical Equilibrium in Nature: (The formation of stalagmites and Stalactites)

Chemical Equilibrium Consider the following reactions: CaCO3(s) + CO2(aq) + H2O(l)  Ca2+(aq) + 2HCO3-(aq) ..(1) and Ca2+(aq) + 2HCO3-(aq)  CaCO3(s) + CO2(aq) + H2O(l) ..(2) Reaction (2) is the reverse of reaction (1). At equilibrium the two opposing reactions occur at the same rate. A + B → C+ D Forward Reaction C + D → A + B Reverse Reaction A + B C+ D Reaction can be both at the same time simultaneously and can reach a chemical equilibrium (state of balance) *How do we know is something is at Equilibrium?

Key Idea #1 At equilibrium, the rate of the forward reaction is equal to the rate of the reverse reaction.

Key Idea # 2 At equilibrium, the concentrations of reactants and products are constant.

“Dynamic” implies movement/motion For a given reaction, there will be the same relative concentration of reactants and products at equilibrium regardless of the initial concentration.

Equilibrium Constant What information does an equilibrium constant give us? [ R ] [ P ] = constant xA + xB mC + nD Forward Reaction Rate = Reverse Reaction Rate =

Expression for Equilibrium Constant Consider the following equilibrium system: wA + xB ⇄ yC + zD Kc = The numerical value of Kc is calculated using the concentrations of reactants and products that exist at equilibrium.

Expressions for Equilibrium Constants Examples: N2(g) + 3H2(g) ⇄ 2NH3(g); Kc = PCl5(g) ⇄ PCl3(g) + Cl2(g); Kc = CH4(g) + H2(g) ⇄ CO(g) + 3H2(g); Kc =

Equilibrium Expression For every equilibrium reaction, there is an equilibrium constant, K. and an equilibrium expression For the general reaction: aA + bB  cC + dD The equilibrium expression is K = [C]c[D]d [A]a[B]b 2NH3 (g)  N2 (g) + 3H2 (g) K = [N2] [H2]3 [NH3]2

Le Chatelier’s Principle Says simply that when a system at equilibrium is stressed, the equil. will shift in a direction that relieves that stress & to reach a new equil. position

How does a system react to stress? Write this slide down! Apply this idea & you’ll be able to do all of the LeChatlier’s problems Equilibrium will shift in order to use up what has been added Equilibrium will shift in order to produce what has been removed

Equilibrium is a RATE balance. Anything that can affect one reaction more than another can throw the equilibrium off.

Steps of Le Chatellier’s Principle Stress: adding or removing reactant, adding or removing a product, changing the temperature or pressure. 2) Shift: the equilibrium will shift AWAY from what is added and TOWARDS what is removed. (undo the stress to relieved the stress) 3) Change in Concentration: The concentration of what is being shifted towards INCREASES, the concentration of what is being shifted away from DECREASES.

Examples: If a reactant is added: The number of collisions between reactant particles increases. The rate of the forward reaction increases (FORWARD SHIFT) If a product is added: The number of collisions between product particles increases. The rate of the reverse reacting increases (REVERSE SHIFT)

If a reactant is removed: The number of collisions between reactant particles decreases. the rate of forward reaction decreases (REVERSE SHIFT) If a product is removed: The number of collisions between product particles decreases. the rate of reverse reaction decreases (FORWARD SHIFT)

N2 + 3H2 ↔ 2NH3 + Heat If N2 is added What reaction will the equilibrium shift? _____ What will be the resulting effect on the concentration of NH3 _________ What will be the resulting effect on the concentration of H2 _____________

If the temperature is increased: The equilibrium will shift in the endothermic direction (converting KE to PE) the endothermic direction is away from the side with heat. If the temperature is decreased The equilibrium will shift in the exothermic direction releasing PE to keep up the lost KE Exothermic direction is toward the side with heat.

N2 + 3H2 ↔ 2NH3 + Heat If temperature is increase the shift will be away from the heat. If temperature is decrease the shift will be towards the heat.

For Gaseous Equilibria… N2(g) + 3H2(g)-> 2NH3(g) Remember that 1 mole of gas will occupy a space of 22.4 L Increasing pressure will shift equilibrium in direction that produces fewer moles of gas, to occupy less volume. Decreasing pressure will shift in direction that will produce more moles of gas, it will expand.

Change in Concentration 2CO (g) + O2 (g)  2CO2 (g) What direction does the equilibrium equation shift when more O2 is added? More CO? More CO2? To The Right! To The Right! To The Left!

Change in temperature 1.) Exothermic reaction vs. Endothermic Think of Heat (ΔH) as a reactant or as a product! 1.) Exothermic reaction vs. Endothermic 2.) Temperature increase or decrease

Temperature, continued In an exothermic rxn, is heat added or released? Released Which side of the equation should it go on? Products, because it is given off. What about endothermic… heat is? Added Heat goes on the ______ side of the eqn. Reactants Now that we have the heat added to the equation, we can simply treat it like we treated concentration. Increasing the temperature increases the heat.

Temperature, Continued Practice Problem: 2CO (g) + O2 (g)  2CO2 (g) Rewrite the eqn if it’s exothermic. Which direction does equilibrium shift if… Temperature Increases? Temperature decreases? Rewrite the eqn if it’s endothermic.

Change in pressure I hope everyone remembered that in equilibrium equations, solids and liquids do not affect equilibrium values. This especially important, b/c in a equilibrium of gases, only pressure counts! Remember that pressure is directly proportional to the # moles of gas Le Chatelier's says that when pressure increases, the equilibrium shifts to make fewer moles of gas

Pressure, continued 2CO (g) + O2 (g)  2CO2 (g) There are three moles of gas on the left and only two on the right, so if the pressure was increased, the reaction would go towards the right. Conversely, if the pressure was decreased, the reaction would go towards the left.

Reactions not affected by pressure changes Consider the following reactions: CO(g) + H2O(g) ⇄ CO2(g) + H2(g); H2(g) + Cl2(g) ⇄ 2HCl(g); Reactions have same number of gas molecules in reactants and products. Reducing or increasing the volume will cause equal effect on both sides – no net reaction will occur. Equilibrium is not affected by change in pressure.

Summary If you want to make more product → Add a reactant Remove product Change in temperature Change in pressure If you want to make more reactant ← Remove a reactant Add a product

Try this! AgCl (s) + Heat ↔ Ag+1 + Cl-1 List three things that you can do that will result in a increase in AgCl (s). __________________