Chemistry – May 7, 2018 P3 Challenge – Balance _____C2H2 + _____O2  _____CO2 + _____H2O Today’s Objective – Equilibrium Constants LeChatelier’s Principle Agenda Reversible reactions Equilibrium Expressions Calculations with Keq LeChatelier’s Principle Assignment: Equilibrium Worksheet

Equilibrium Defined Equilibrium is any two opposing processes occurring at the same rate such that there is no net change. All reactions are equilibrium reactions. All reactions are theoretically reversible. When rate of forward reaction = rate of reverse reaction, there is equilibrium.

The Equilibrium Constant For the general equation, aA + bB  cC + dD at equilibrium, we can write an expression that demonstrates where the equilibrium is located. Keq is called the equilibrium constant and is equal to a ratio of concentrations for products to reactants raised to powers.

Size of Keq, Describing Keq If K is very large, e.g. 1025 or larger, then at equilibrium the concentration of products vastly outnumber the concentration of reactants and a reaction is said to “go to completion.” If K is very small, e.g. 10-25 or smaller, then at equilibrium the concentrations of reactants vastly outnumber the concentration of products and there is said to be “no reaction.” If K is moderate, e.g relatively close to 1, then at equilibrium there will be significant concentrations of both reactants and products. “Lies to the right” if greater than 1. “Lies to the left” if less than 1.

Writing Equilibrium Expressions Equilibrium expressions must be written based on the balanced reaction. In general for a reaction: aA + bB  cC + dD the equilibrium expression is STATES MATTER!!!! Included are any reactants or products in the (aq) or (g) state in Molarity. Solids and Liquids are defined with a molarity of 1, so they have no effect on K.

Practice Writing K Expressions N2O (g) + 4 H2 (g)  2 NH3 (g) + H2O (g) 2 POCl3 (g)  2 PCl3 (g) + O2 (g) Cu2+(aq) + 4 NH3 (aq)  Cu(NH3)42+(aq)

Calculations with Keq If you know the equilibrium concentrations of all reactants and products you can calculate the Keq Keq values vary with temperature, so a temperature should be stated for any equilibrium. If temperature is missing, the default is 25C. Ex: 2 POCl3 (g)  2 PCl3 (g) + O2 (g) At 60C, a 2.00 L flask was found to contain 3.50 mol of POCl3, 2.15 mol of PCl3 and 1.08 mol of O2. Find the equilibrium constant.

Calculations with Keq If you know the Keq value at a given temperature, and know all but one of the equilibrium concentrations, you can find the last concentration. Ex: 2 POCl3 (g)  2 PCl3 (g) + O2 (g) At 30C, the equilibrium constant is 5.2 x 10-3 . A flask at equilibrium was found to contain 0.875 M POCl3 and 0.049 M O2.. What is the concentration of PCl3 in the flask?

LeChatelier’s Principle A system at equilibrium will oppose any change to the system in a way that helps eliminate the change. The types of changes that a chemical equilibrium may experience include changes in…. Concentrations of reactants or products Pressure Volume Temperature An equilibrium responds by shifting left (more reactants) or shifting right (more products).

Concentration changes To determine how an equilibrium responds imagine a set of marbles balanced on a seesaw. If you increase a reactant concentration, that’s like pushing up on the left side. The marbles and the equilibrium move right. If you decrease a reactant concentration, that’s like pulling down on the left side. The marbles and the equilibrium move left. If you increase a product concentration, that’s like pushing up on the right side. The marbles and the equilibrium move left. If you decrease a product concentration, that’s like pulling down on the right side. The marbles and the equilibrium move right.

Practice: Consider: 2 C2H2 + 5 O2  4 CO2 + 2 H2O Predict if the equilibrium will shift right or left when the equilibrium is disturbed in the following ways: Increase in [O2] Decrease in [CO2] Decrease in [H2O] Increase in [CO2]

Pressure and volume changes Pressure and volume changes are all about the gasses that are present in a reaction. Count up the number of moles of gas on each side of the reaction. If they are equal, changing pressure and/or volume will have no effect. If they are different, both pressure and volume will effect an equilibrium in a similar way. An increase in pressure (or decrease in volume) will shift the equilibrium to the side with fewer moles of gas. A decrease in pressure (or increase in volume) will shift the equilibrium to the side with more moles of gas.

Practice: Consider: 2 C2H2 (g)+ 5 O2 (g) 4 CO2 (g)+ 2 H2O (g) Predict if the equilibrium will shift right or left when the equilibrium is disturbed in the following ways: Increase pressure Decrease pressure Decrease volume Increase volume

Temperature changes Temperature changes effect an equilibrium in a different way. All other changes keep the same equilibrium constant value, which in our model is the location of the fulcrum. Changing temperature changes the value of the equilibrium constant How a reaction responds depends on whether it is endothermic, requiring a heat source, or exothermic, producing heat. Treat heat as a reactant (endothermic, positive H) or as a product (exothermic, negative H) and then use the same reasoning as concentration changes.

Practice: Consider: 2 C2H2 + 5 O2  4 CO2 + 2 H2O (exothermic) Predict if the equilibrium will shift right or left when the equilibrium is disturbed in the following ways: Increase temperature Decrease temperature

Exit Slip - Homework Exit Slip: What is the equilibrium constant expression for ____H2O (l)  _____ H2 (g) + _____ O2 (g) What’s Due? (Pending assignments to complete.) Equilibrium worksheet. What’s Next? (How to prepare for the next day) Read p496 - 510