Chemical Equilibrium Unit 11

My Chemistry Presentation Chemical Reactions We usually think of chemical reactions as having a beginning and an end. reactants products The END? Not always! Chemical reactions are reversible when they can go forwards and backwards without any extra energy being used. reactants products This means that as products form, they can react and reform reactants.

My Chemistry Presentation Changes in the Concentration The concentration of the reactants decreases and then remains constant. The concentration of the products increases and then remains constant. Chemical equilibrium is achieved when the concentration of the reactants and the products over time remains constant. A reaction may look "finished" when equilibrium is reached, but actually the forward and reverse reactions continue to happen at the same rate.

My Chemistry Presentation Example of Chemical Equilibrium Consider the following reactions: CaCO 3 (s) + CO 2 (aq) + H 2 O (l)  Ca 2+ (aq) + 2HCO 3 - (aq) (1) and Ca 2+ (aq) + 2HCO 3 - (aq)  CaCO 3 (s) + CO 2 (aq) + H 2 O (l) (2) Reaction (2) is the reverse of reaction (1). At equilibrium the two opposing reactions occur at the same rate. Concentrations of chemical species do not change once equilibrium is established.

My Chemistry Presentation Example of Chemical Equilibrium 2 NO 2 N 2 O 4 See Animation

Reaction Rate Chemical Equilibrium: – The forward reaction rate is equal to the reverse reaction rate. – The concentrations of both the products and the reactants remain constant. What happens to the forward reaction rate (reactants → products) as we approach equilibrium? It decreases, because we are running out of reactants. What happens to the reverse reaction rate (products → reactants) as we approach equilibrium? It increases, because we are making more products

Equilibrium Constant At equilibrium we have a certain concentration of products and reactions. We can compare those and come up with a constant (K) for the reaction that describes this ratio for a specific reaction. Each chemical reaction will have a unique equilibrium constant. These constants have been calculated for most reactions and are easily accessible.

The Equilibrium Constant Consider the following equilibrium system wA + xB ⇄ yC + zD K = The numerical value of K is calculated using the molar concentrations of reactants and products that exist at equilibrium. Pure solids or liquids are not included in the calculation of the equilibrium constant. An example of a pure liquid is water, an example of a pure solid would be silver. Equilibrium Constants are normally calculated at 25 °C unless otherwise noted. Brackets [ ] indicate that values are concentrations (molarity)

Physical Meaning of the Equilibrium Constant What does it mean to have a large equilibrium constant? K [products] [reactants] = It means that the equilibrium favors products, i.e. there are high concentrations of products and low concentrations of reactants at equilibrium. It means that the equilibrium favors reactants, i.e. there are low concentrations of products and high concentrations of reactants at equilibrium. What does it mean to have a small equilibrium constant? K [products] = [reactants]

Expressions for Equilibrium Constants Examples: N 2 (g) + 3H 2 (g) ⇄ 2NH 3 (g) PCl 5 (s) + H 2 O (g) ⇄ 2 HCl (g) + POCl 3 (g) 2 NO 2 (g) ⇄ N 2 O 4 (g) Find K for 2.00 moles of NO 2 and 1.60 moles of N 2 O 4 in a 4.00 L flask.

My Chemistry Presentation What about reactions that are not at equilibrium? Can we quantitatively describe reactions that are not at equilibrium? – Yes! We can use the same calculation as we did to get the equilibrium constant (K) to get a value, which is referred to as the reaction quotient (Q). Consider the following system that is NOT at equilibrium: wA + xB ⇄ yC + zD Q =

My Chemistry Presentation What does the reaction quotient tell us? All chemical reactions strive to reach equilibrium. By comparing the reaction quotient (where the reaction is now) to the equilibrium constant (where it wants to go) we can make a prediction about what will happen. Lets look at an example; Consider the following reaction: SO 2 Cl 2 (g) SO 2 (g) + Cl 2 (g) What is Q before the reaction begins if the concentration of M SO 2 Cl 2, M SO 2, and M Cl 2 ?

My Chemistry Presentation What does the reaction quotient tell us? If K = at 100 o C, which direction will the reaction proceed in order to establish equilibrium? < If the reaction quotient, Q, is going to increase, do we need to increase the concentration of our products or our reactants? Q = [products] [reactants] *Increasing the products simultaneously decreases the reactants. If we need to increase the concentration of our products and decrease the concentration of reactants, the reaction must proceed in the forward direction. reactants products

My Chemistry Presentation What have we been saying all along? A chemical reaction strives to reach equilibrium. Le Chatelier’s Principle: If a stress is put on a reversible reaction at equilibrium, the system will shift in such a way to relieve the stress.

Le Chatelier’s Principle When a system at equilibrium is subjected to a stress, the equilibrium will tend to shift in the direction to relieve the stress. What kind of “stress” could a chemical reaction be subjected to?? – Change in concentration – Change in temperature – Change in pressure Change in pressure affects which state of matter? – GASES ONLY

Change in Concentration To get back to equilibrium, a system will shift to get back to the same ratio of reactants and products. N 2 (g) + 3H 2 (g) ⇄ 2NH 3 (g) What will happened to the reaction if we: – Increase the concentration of N 2 ? Equilibrium shifts to the right (  ). – Increase the concentration of NH 3 ? Equilibrium shifts to the left (  ). – Decrease the concentration of NH 3 ? Equilibrium shifts to the right (  ).

Change in Temperature Consider the following reaction: 2 SO 2 (g) + O 2 ⇄ 2 SO 3 (g) + Heat or Energy What will happened to the reaction if we: – Raise the temperature? Equilibrium shifts to the left (  ) – Lower the temperature? Equilibrium shifts to the right (  ) NOTE: Lowering the temperature will shift the equilibrium to the right (increasing the [products], but as the temperature is lowered, the reaction slows down.

Change in Pressure (Gases ONLY) Consider the following reaction: N 2 (g) + 3H 2 (g) ⇄ 2NH 3 (g) What will happened to the reaction if we: – Raise the pressure Equilibrium shifts to the right (  ) – Lower the pressure Equilibrium shifts to the left (  )