How to solve for K when you are not yet at equilibrium! ICE Box Problems How to solve for K when you are not yet at equilibrium!

Quick Review of What We’ve Done! We have been using equilibrium expressions to solve the equilibrium constant for situations that are already AT EQUILIBRIUM. I think you know what coming….. Lets say we know the initial concentrations of the compounds, but want to know what the concentrations will be once they REACH EQUILIBRIUM.

I – Initial Concentration C – Change in Concentration So How do we solve Equilibrium Expressions when we are NOT already at Equilibrium? We are going to do something called an ICE Box problem. ICE stands for…. I – Initial Concentration C – Change in Concentration E – Equilibrium Concentration

Change in Concentration ICE CHART TEMPLATE [A] [B] [C] Initial Concentration Change in Concentration Concentration at Equilibrium

PCl3(g) + Cl2(g)  PCl5(g) Here is the type of question we can see. Initially the reactions starts with 0.50 M of PCl3 and 0.50 M of Cl2. When equilibrium is reached, 0.30 M of is PCl5 present. Determine the concentration of each compound when equilibrium is reached, and then determine the Keq.

Where to start? At the Initial! We can start by filling in the initial concentrations. [PCl3] and [Cl2] were given in the problem but what about [PCl5]? Initially, how much of the product will we have? [PCl3] [Cl2] [PCl5] Initial 0.50 Change Equilibrium

PCl3(g) + Cl2(g)  PCl5(g) Next we need to think about how the concentrations will change over time. We don’t know exactly much these compound will change yet, but we do know how they will change. What will be true of the conc. of our reactants? What will be true of the conc. of our products? Will they all change at the same rate? YES! We know this because all of these compounds are in a 1:1 molar ratio.

PCl3(g) + Cl2(g)  PCl5(g) Initial Change Equilibrium 0.50 - X X So the reactants are decreasing, but we don’t know by how much, so we put an…. The product is increasing, but we don’t know by how much, so we put an…. [PCl3] [Cl2] [PCl5] Initial 0.50 Change - X X Equilibrium

PCl3(g) + Cl2(g)  PCl5(g) Initial Change Equilibrium 0.50 - X X 0.30 For the equilibrium, we have one value given to us initially in the problem. “When equilibrium is reached, 0.30 M of is PCl5 present”, where should it go? [PCl3] [Cl2] [PCl5] Initial 0.50 Change - X X Equilibrium 0.30

What about the rest of the Equilibrium? To fill in the rest of the boxes, it is always: INITIAL + CHANGE = EQUILIBRIUM Go down the columns, and write in the expression. [PCl3] [Cl2] [PCl5] Initial 0.50 Change - X X Equilibrium 0.50 - X 0.30

Final Part! We need to know the concentrations at equilibrium to solve for K. What are their values? We need to know X. Is there a way to figure that out? INITIAL + CHANGE = EQUILIBRIUM Focusing on PCl5,we can determine that x = 0.30 M. And if we know X somewhere, then we know X everywhere! [PCl5] X 0.30

Solving Problems….It’s What We Do! Watch and prepare to be amazed! If X = 0.30 then… And then!!! And BAM! I’M READY TO CALCULATE K! Why??? [PCl3] [Cl2] [PCl5] Initial 0.50 Change Equilibrium - 0.30 - X - 0.30 - X 0.30 X 0.50 - X 0.20 0.50 - X 0.20 0.30

PCl3(g) + Cl2(g) <==> PCl5(g) Determine the concentration of each compound when equilibrium is reached, and then determine the Keq. Part one is done, now we need to write the equilibrium expression and solve for Keq. [PCl5] [0.30 M] K = [PCl3][Cl2]  [0.20 M][0.20 M] Keq = 7.5 – K > 1 means products favored. [PCl3] [Cl2] [PCl5] Equilibrium 0.20 0.20 0.30

A(g) + B(g)  C(g) In a closed container, 0.550 M of A, 0.750 M of B, and 0.100 M of C are present. When equilibrium is reached 0.488 M of C is present. Determine the concentrations of A and B at equilibrium and then solve for the equilibrium constant. How will I know when to use an ICE box type of problem? The reaction is not already at equilibrium.

A(g) + B(g)  C(g) Initial Change Equilibrium Start with your Initial Concentrations. Then determine how they will change! Do we know concentrations at equilibrium? [A] [B] [C] Initial 0.550 0.750 0.100 Change - X X Equilibrium 0.550 - X 0.750 - X 0.488

Do we have enough to solve for X??? Plug in X values and solve for the rest of the concentrations at equilibrium. [A] [B] [C] Initial 0.550 0.750 0.100 Change - X X Equilibrium 0.550 - X 0.750 - X 0.488 - 0.388 - 0.388 0.388 0.162 0.362

A(g) + B(g)  C(g) [C] [0.488 M] K = [A][B] [0.162 M][0.362 M] [A] Use the concentrations at equilibrium to calculate Keq [C] [0.488 M] K = [A][B] [0.162 M][0.362 M] Keq = 8.32 [A] [B] [C] Equilibrium 0.162 0.362 0.488