Briony Brundidge AP Chemistry Chemical Equilibrium Briony Brundidge AP Chemistry

Key Terms Chemical equilibrium Reversible reaction Law of mass action Equilibrium constant Equilibrium position Equilibrium expression Homogeneous equilibria Heterogeneous equilibria Reaction quotient Le Chatelier’s principle

What is it? The state where the concentrations of all reactants and products remain constant with time chemical reaction and its reverse proceed at equal rates

Types of Equilibrium Homogeneous: one phase Normally gas Heterogeneous: at least 2 phases solid-liquid, solid-gas, liquid-gas Gaseous: involving molar concentrations or partial pressures Aqueous: Acid/base Precipitate Buffers

Chemical Equilibrium Chemical: bonds breaking and bonds forming (change in chemical composition) Ex: dissociation of acetic acid HC2H3O2+ + H2O  H3O+ + C2H3O2-

Physical Equilibrium Physical: change in state (not involving change to chemical properties) Ex: sealed bottle of water- partly filled Rate of water evaporating= rate of water condensing

Dynamic Equilibrium Rate of forward rxn= rate of reverse rxn No net change in concentration of products or reactants Same total number of particles Both some of products and reactants left over after reaction occurs

Law of Mass Action *General description of the equilibrium condition Law of Mass Action: states that the molecularity of a balanced chemical equation can be used to find a constant Principle that the rate of a chemical reaction is proportional to the masses of the reacting substances *General description of the equilibrium condition

Equilibrium Constants aA + bB  cC + dD Concentrations can be found in Appendix Kc = [C]c[D]d [A]a[B]b Kp = (PC)c (PD)d (PA)a (PB)b  Represents coefficient in chemical equation Concentration of products Concentration reactants Partial pressure of products Partial pressure of reactants

Practice Write the equilibrium constant expression for the following reaction: 4NH3 (g) + 7O2 (g)  4NO2 (g) + 6H2O (g)

Calculate the value of the equilibrium constant: Practice Calculate the value of the equilibrium constant: 2NH3 (g)  N2 (g) + 3H2 (g) [NH3]= 3.1 X 10-2 , [N2]= 8.5 X 10-1, [H2]= 3.1 X 10-3

More on Keq… NO Units!!! The K value of the reverse reaction is the reciprocal of the K value of the original reaction If rxn is multiplied x2, Keq is squared, etc. Dependent on temperature Defined for only 1 temperature

Gaseous Equilibrium So… Kp = Kc (RT)Δn(gas) Molarity is directly related to the pressure for gaseous equilibrium PV = nRT P = (n/V)RT So… Kp = Kc (RT)Δn(gas) Δng = change in moles of gas Products - Reactants

Kp = 4.34 x 10-3 calculate Kc at 300°C Practice N2 (g) + 3H2 (g)  2NH3 (g) Kp = 4.34 x 10-3 calculate Kc at 300°C

Determining Concentrations at Equilibrium… Method: use ICE table I initial C change E equilibrium Additional Method: Neglect x When k is very small, less than 5%

Practice H2(g) + I2(g)  2HI(g) At a particular temperature, K = 1.00 x 102 for the reaction: H2(g) + I2(g)  2HI(g) In an experiment, 1.00mol H2, 1.00mol I2, and 1.00mol HI are introduced into a 1.00-L container. Calculate the concentrations of all species when equilibrium is reached.

Practice At 25°C, Kc, for the following equilibrium is 1.00 x 10-13 if the initial concentrations of IO3- and H+ are, 0.00200M and 0.065M respectively what is the equilibrium concentration of IO3-? 7IO3(aq)- + 9H2O(l) + 7H-(aq)  I2(g) + 5H5IO6(aq)

Le Chatelier‛s Principle Equilibrium shifts in the opposite direction of the substance whose concentration is increased and towards the substance of decreasing concentration. Adding a reactant or product Increase reactant [ ] = equil shifts right Increase product [ ] = equil shifts left Removing a reactant or product Decrease reactant [ ]= equil shift left Decrease product [ ] = equil shifts right

Common Ion Effect States that if the concentration of any one of the ions in solution is increased, then according to Le Chat’s principle, the excess number of ions should combine with those of opposite charges

Le Chatelier’s Principle Changing pressure Adding an inert gas (that isn't involved in the reaction) No effect on equilibrium position, total pressure, or partial pressures of reactants and products because inert gas already does not participate in reaction Changing the volume of the container Decrease in volume = equil shifts right because System decreases its own volume as a result and therefore its total number of gaseous molecules Can be further proved by ideal gas law V = (RT/P)n

Le Chatelier’s Principle Adding a catalyst Decreases energy needed for reaction to occur (activation energy) but does not affect equilibrium Only speeds up the rate for which a reaction reaches dynamic equilibrium

Le Chat’s Principle Changing temperature Only stress that changes equilibrium constant, K Equilibrium shifts towards direction that consumes energy Ex: N2 (g) + 3H2 (g)  2NH3 (g) + 92kj Exothermic so equilibrium shifts left Value of K decreases as result of increase in reactant [ ]’s

NH4NO2 (s) + O2 (g)  2NO2 (g) + 2H2O(g) Practice NH4NO2 (s) + O2 (g)  2NO2 (g) + 2H2O(g) What is the effect of each of the following on the equilibrium position? Increase Volume Decrease [O2] Increase [NO] Decrease Temperature Add inert gas Increase NH4NO2

Reaction Quotient When the concentration of one of the substances in a reaction is zero, equilibrium shifts towards the direction of the missing component Hard to determine shift when all of the initial concentrations are zero, so the reaction quotient is used to determine the shift

Reactant Quotient If K = Q, then no shift in equilibrium Ex: N2 (g) + 3H2 (g)  2NH3 (g) Q = [NH3]02 [N2]0[H2]0 If K = Q, then no shift in equilibrium If K > Q, then equil shifts right because ratio of [ ] of products to reactants is too large, products favored If K < Q, then equil shifts left because ratio of [ ] of products to reactants is too small, reactants favored

Labs Determination of Equilibrium Constant, K Lab Fun with Cobalt Lab Finding value of Keq for the reaction after determining the molar [ ]’s of ions present and testing solutions of unknown molar [ ]’s Fun with Cobalt Lab Determining effects on the reactants and products, observe shifts in equilibrium

Works Cited… http://www.examstutor.com/chemistry/resources/studyroom/chemical_equilibria/reversible_reactions/pictures/figure_7_2.gif