Equilibrium I love chemistry!!!

What is Equilibrium? A dynamic condition in which 2 opposing changes occur at equal rates in a closed system Ex. A phase change can be a physical condition that can be described to be in equilibrium Solid + ∆H Liquid

Other examples of Equilibrium Vapor pressure P atm P gas Saturated solutions NaCl (s) Na + (aq) + Cl - (aq)

LeChatlier’s Principle A system will remain at equilibrium until something occurs to change this condition When a system at equilibrium is disturbed by an application of stress, it attains a new equilibrium position that minimizes that stress

What are the possible stressors? Change in temperature Change in pressure Change in volume Addition or removal of reactants Addition or removal of products

Understanding how to minimize the stress Equilibrium is like a teeter totter When stress is placed on it, the system must adjust to rebalance itself

Using color to predict shifting ∆H + Co(H 2 O) 6 +2 (aq) + 4Cl - (aq) CoCl 4 -2 (aq) + 6H 2 O (l) Pale pinkDeep blue At equilibrium, the color of a beaker containing this system would be violet (light purple) What would you see if the following possible stressors were Introduced according to LeChatlier? Addition of Co(H 2 O) 6 +2 Removal of Cl - (aq) Removal of H 2 O (l) Addition of CoCl 4 -2 (aq) Addition of ∆H Removal of ∆H Increase Pressure Decrease Pressure

Sample Problem #1 2NO 2(g) N 2 O 4(g) + ∆H Very dark brown Very light brown *At equilibrium, the system will display a medium brown color as shown in flask #2

2NO 2(g) N 2 O 4(g) +∆H Very dark brownVery light brown Where would the shifting take place when the following possible stressors are added? What would you see experimentally? Removal of heat Increase in pressure Addition of NO 2(g) Decrease in pressure Removal of N 2 O 4(g) Addition of CO 2(g) Addition of heat Increase in volume of container Decrease in volume of container

#1 represents a shift to the right (lighter brown color) #2 represents the original equilibrium situation #3 represents a shift to the left (darker brown color)

Things to remember about equilibrium shifting Pure solids and liquids will not have an effect on equilibrium Only gases will be affected by a change in pressure or volume

How do you know which side of the equilibrium expression is favored? Sometimes at equilibrium, there is a higher concentration of reactants or products To determine this by: 1.Looking at the equilibrium constant 2.Looking at the reversible arrows shown in the equation

HBr (aq) + H 2 O (l) H 3 O + (aq) + Br - (aq) The forward reaction shown above is favored, therefore there is a higher concentration of products than of reactants at equilibrium H 2 CO 3(aq) + H 2 O (l) H 3 O + (aq) + HCO 3 - (aq) The reverse reaction shown above is favored, therefore there is a higher concentration of reactants than of products at equilibrium

In some cases, both the forward and reverse reactions occur nearly at the same rate before chemical equilibrium is established. Neither reaction is favored Do not just assume that if you see an equilibrium arrow ( ) that neither side of the system is favored. Use the equilibrium constant to determine for sure!

The equilibrium expression (K eq ) After equilibrium has been achieved, the concentrations of products and reactants remain constant The ratio of the concentrations should also remain constant ***The only stressor that can affect these concentration ratios is temperature ***K eq is temperature dependant

How do you find K eq ? K eq is the constant that represents the ratio of the mathematical product to the mathematical reactant The general equation for an equilibrium expression is as follows: nA + mB xC + yD *n, m, x, y = coefficients from balanced chemical equation *A, B, C, D = molarities of reactants and products

nA + mB xC + yD K eq = [C] x [D] y [A] n [B] m *K eq will only change if temperature changes *Only gases and aqueous solution concentrations will be included in K eq *Pure solids and liquids are omitted because their concentrations cannot change

Sample Problem Consider the following equilibrium equation at 425°C: H 2(g) + I 2(g) 2HI (g) 1.Write the K eq expression for this reaction 2.Determine K eq mathematically when given the following concentrations: [H 2 ] =.015M, [I 2 ] =.015M, [HI] =.11M 3.Using the known K eq from #2 and given the following concentrations for H 2 and I 2, determine the concentration of HI: [H 2 ] =.018M, [I 2 ] =.018M

How to use K eq to determine if the reactants, products or neither are favored? If K eq > 1, the forward reaction (products) are favored If K eq < 1, the reverse reaction (reactants) are favored If K = 1, neither reaction is favored (the concentrations of both products and reactants should be equal)

For the previous sample problem, which of the reactions is favored? H 2(g) + I 2(g) 2HI (g)