Mr. Kinton Honors Chemistrry Enloe High School Chemical Equilibrium Mr. Kinton Honors Chemistrry Enloe High School

Chemical Equilibrium Condition which the concentrations of all reactants and products in a closed system cease to change with time Occurs when opposing reactions are proceeding at equal rates Equal rates DOES NOT mean equal amounts With gaseous reactions, we will use partial pressures in aqueous reactions we will examine concentration When a reaction reaches equilibrium reactants do not stop reacting

Chemical Equilibrium Vocabulary Law of mass action: rules by which the equilibrium constant is expressed in terms of the concentrations of reactants and products, based on a balanced chemical reaction Equilibrium-Constant Expression: expression that describes the relationship among the concentrations (partial pressures) of the substances present in a system at equilibrium Equilibrium Constant: The numerical value of the equilibrium- constant expression for a system at equilibrium, denoted by Keq

Equilibrium Constant Expression Let’s examine 2 different general expressions aA + bB cC + dD If the reactants and products are gases the equilibrium expression is written as follows: Keq= 𝑃𝐶 ^𝑐 𝑃𝐷 ^𝑑 𝑃𝐴 ^𝑎 𝑃𝐵 ^𝑏 If the reactants and products are aqueous solutions: Keq= [𝐶]^𝑐[𝐷]^𝑑 [𝐴]^𝑎[𝐵]^𝑏

Equilibrium-Constant Expressions Given the following reactions write an equilibrium expression: N2O4(g) 2NO2(g) Ag+(aq) + 2NH3(aq) Ag(NH3)2+(aq)

Magnitude of Equilibrium Constants Equilibrium constants can have very large or very small values The magnitude of the equilibrium constant will tell us which side will be most prevalent Keq » 1: equilibrium lies to the right; products predominate Keq « 1: equilibrium lies to the left; reactants predominate

Other ways to Manipulate an Equilibrium Expression The equilibrium expression for a reaction written in one direction is the reciprocal of the one written in reverse The direction of the reaction is what is most important The equilibrium constant of a reaction can be multiplied by a number An equilibrium of a net reaction made up of multiple steps is the product of the equilibrium constant for the individual steps Go over examples that are on page 584 and 585 in the Brown and LeMay textbook

Homogeneous and Heterogeneous Equlibrium All of the examples that we have examined have had the reactants and products all in the same phase (Homogenous) In other cases, the substances in equilibrium will occur in different phases (Heterogeneous) Partial pressures of gases are substituted into the equilibrium expression Molar concentrations of dissolved substances are placed in the equilibrium expression Pure solids, liquids, and solvents are not included in the equilibrium expression

Heterogeneous Equilibrium Given the following chemical reactions, give the correct equilibrium expression: CO2 (g) + H2 (g) CO (g) + H2O (l) SnO2 (s) + 2CO (g) Sn (s) + 2CO2 (g) Sn (s) + 2H+ (aq) Sn2+ (aq) + H2 (g)

Calculating Equilibrium Constants For the scope of our class if you are asked to calculate the value of Keq all of the concentrations/pressures will be given to you All you will need to do is write the equilibrium expression and then perform the calculation Example: A mixture of H2 and N2 in a reaction vessel is allowed to attain equilibrium at 472oC. At equilibrium the partial pressure of each gas was measured and found to be 7.38 atm H2, 2.46 atm N2 and 0.166 atm NH3. Calculate Keq.

Le ChÂtlier’s Principle If a system at equilibrium is disturbed by a change in temperature, pressure, or concentration, the system will shift its equilibrium to counteract the disturbance Le Châtlier’s Principle can be used to make qualitative predictions about a reaction Adding or removing a reactant or product Changing the pressure Changing the temperature

Change in Reactant/Product Concentration At equilibrium a reaction is in a state of balance, if a change is made that balance is disrupted In order to re-establish equilibrium the reaction will shift to counteract that change Adding reactant/product will cause the reaction to shift to the opposite side of where substance was added Removing reactant/product will cause the reaction to shift toward the side that had a substance removed Adding or removing a pure liquid or solid will not alter the equilibrium

Changing Volume and Pressure This change will only influence our reactions that contain gases in their equilibrium Reducing the volume (increasing pressure) will cause the equilibrium to shift to the side with the fewest moles of gas Increasing the volume (decreasing pressure) will cause the equilibrium to shift to the side with the most moles of gas Additionally, these changes are assumed to be made at a constant temperature Changing the concentrations or pressure DOES NOT change the value of Keq

Changing Temperature Changing the temperature on a reaction at equilibrium will change the value of the equilibrium constant For endothermic reactions, increasing T will increase Keq For exothermic reactions, increasing T will decrease Keq When temperature is being changed, we will treat heat like a reactant or product Endothermic: Reactants + Heat Products Exothermic: Reactants Products + Heat Do example problems on page 600 to practice predicting shifts in equilibrium

Entropy and Equilibrium Entropy is a measure of the amount of disorder in a system When entropy increases it has a positive value, when entropy decreases it has a negative value Entropy increases when: Phase changes occur resulting in a less organized state (solid to liquid) The number of particles increases (solid ground to a powder) The number of product molecules is greater than the number of reactant molecules When the temperature increases