Ch. 15: Chemical EquilibriumCh. 15: Chemical Equilibrium

Dr. Namphol Sinkaset

Chem 152: Introduction to General Chemistry

I. Chapter OutlineI. Chapter Outline

I. IntroductionII. The Equilibrium Constant (K)III. Le Châtelier’s Principle

I. IntroductionI. Introduction• Most reactions are reversible, meaning

they can proceed in both forward and reverse directions.

• This means that as products build up, they will react and reform reactants.

• At equilibrium, the forward and backward reaction rates are equal.

I. Example EquilibriumI. Example Equilibrium

II. Equilibrium ConcentrationsII. Equilibrium Concentrations

• Equilibrium does not mean that concentrations are all equal!!

• However, we can quantify concentrations at equilibrium.

• Every equilibrium has its own equilibrium constant.

II. The Equilibrium ConstantII. The Equilibrium Constant

• equilibrium constant: the ratio at equilibrium of the [ ]’s of products raised to their stoichiometric coefficients divided by the [ ]’s of reactants raised to their stoichiometric coefficients.

• The relationship between a balanced equation and equilibrium constant expression is the law of mass action.

II. The Equilibrium ConstantII. The Equilibrium Constant

• For a general equilibrium aA + bB cC + dD, the equilibrium expression is:

II. Example Equilibrium II. Example Equilibrium ConstantConstant

II. Sample ProblemII. Sample Problem

• Write the equilibrium constant expression for the reaction:

2H2(g) + O2(g) 2H2O(g).

II. Physical Meaning of KII. Physical Meaning of K

• Large values of K mean that the equilibrium favors products, i.e. there are high [ ]’s of products and low [ ]’s of reactants at equilibrium.

• Small values of K mean that the equilibrium favors reactants, i.e. there are low [ ]’s of products and high [ ]’s of reactants at equilibrium.

II. Values of KII. Values of K

• Values of K are most easily calculated by allowing a system to come to equilibrium and measuring [ ]’s of the components.

• For the equilibrium H2(g) + I2(g) 2HI(g), let’s say equilibrium [ ]’s at 445 °C were found to be 0.11 M, 0.11 M, and 0.78 M for molecular hydrogen, molecular iodine, and hydrogen iodide, respectively.

II. KII. Kcc for a H for a H22/I/I22 Mixture Mixture

• Note that units are not included when calculating K’s.

• Thus, equilibrium constants are unitless.

II. Equilibrium [ ]’s Vs. KII. Equilibrium [ ]’s Vs. K

• For any reaction, the equilibrium [ ]’s will depend on the initial [ ]’s of reactants or products.

• However, no matter how you set up the reaction, the value of the equilibrium constant will be the same if the temperature is the same.

II. Equilibrium [ ]’s Vs. KII. Equilibrium [ ]’s Vs. K

III. Predicting Qualitative III. Predicting Qualitative Changes to EquilibriumChanges to Equilibrium

• If a system is at equilibrium, what happens when it is disturbed?

• Le Châtelier’s Principle allow us to make qualitative predictions about changes in chemical equilibria: When a chemical system at equilibrium is

disturbed, the system shifts in the direction that minimizes the disturbance.

III. Three Ways to Disturb an III. Three Ways to Disturb an EquilibriumEquilibrium

• We look at three ways that disturb a system at equilibrium:

1) Adding/removing a reactant or product.2) Changing the volume/pressure in

gaseous reactions.3) Changing the temperature.

III. Adding Reactant/ProductIII. Adding Reactant/Product

• If we add or remove a reactant or product, we’re changing the [ ]’s.

• The equilibrium will shift in a direction that will partially consume a reactant or product that is added, or partially replace a reactant or product that has been removed.

III. Adding ReactantIII. Adding Reactant

III. Adding ProductIII. Adding Product

III. Changing VolumeIII. Changing Volume• Via PV = nRT, changing volume is

related to changing pressure and vice versa. For example, decreasing volume is

equivalent to increasing pressure.• Reducing volume of a gaseous reaction

mixture shifts the equilibrium in the direction that will, if possible, decrease the # of moles of gas.

III. Changing PressureIII. Changing Pressure

III. Changing TemperatureIII. Changing Temperature• To determine the effect of changing the

temperature, we need to know the heat of reaction, ΔHrxn.

• Once we know if a reaction is exo or endo, we can write “heat” as a product or reactant.

• We then apply Le Châtelier’s Principle in the same manner as when we considered the add product/reactant case.

III. Changing TemperatureIII. Changing Temperature

III. Sample ProblemIII. Sample Problem• For the reaction N2(g) + 3H2(g) 2NH3(g),

ΔHrxn = -46.19 kJ/mole. Which way will the equilibrium shift when each of the following occurs?

a) NH3 is removed.

b) The reaction vessel is opened.c) The reaction vessel is cooled by 25 °C.