CHM 11600 Exam 2 Study Guide
• Difference between “completion” for irreversible chemical reactions and for reversible chemical reactions
o Irreversible reactions cannot go back to reactants after products have formed (Completion = complete consumption of reactants)
o Reversible reactions are constantly going back and forth between reactants and products (Completion = when there is a large amount of product, we say the reaction “almost goes to completion” until limiting reactant gets used up)
• A system at chemical equilibrium: when the amount of reactants and products remain constant over time, and the rates of the forward and reverse reactions are equal.
• The “position” of equilibrium: specific concentrations (or pressures) at a certain time. o Shifts in position occur, called net reactions, and a new position is obtained that reduces the effect of the disturbance.
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• Law of chemical equilibrium (law of mass action): At a given temp, a chemical system reaches a state where a particular ratio of reactant and product concentration is constant.
• Equilibrium constant expression for a reversible reaction in terms of reactant and product concentrations or partial pressures (gases)
For the reaction aA + bB ???? cC + dD, the equilibrium constant Kc = [!]![!]!
For a reaction involving only gases, K is written using partial pressures: �! = !!!∗!!!
• Calculating the value of the equilibrium constant for a reversible chemical reaction given the equilibrium concentrations of all reactants and products: plug in equilibrium concentrations into the Kc equation; Kc = [!"#$%&']!"#$$%!%#&'[!"#$%&']!"#$$%!%#&'
• Why the concentrations of the reactants and products do not change when a chemical reaction has reached equilibrium: concentrations of reactants and products do not change when a reaction has reached equilibrium because a change in one direction is balanced by a change in the other direction since the forward and reverse rates are equal.
• Chemical equilibrium in terms of the rates of the forward and reverse reactions
o In chemical equilibrium, the rates of forward and reverse reactions are equal, so the equilibrium constant is a ratio of the two rates. Don't forget about the age old question of ua 280
• Information provided by the value of the equilibrium constant for a chemical reaction o Small K = mostly reactants present at equilibrium
o Large K = mostly products present at equilibrium
• Calculating the value of Kp given the value of Kc (and vice versa)
o Kp = ��(��)!!!"#$ !" !"#$% (!)
• Calculating the value of the reaction quotient, Q, given the concentrations or partial pressures (gases) of reactants and products at any moment in time
o Q, the reaction quotient, changes value as concentrations of products and reactants change, until equilibrium is reached.
o At equilibrium, Q = K
o Follows the same formula as K
• Using Q to tell whether a reaction is in equilibrium, and determine direction it must go in to achieve equilibrium
o If Q = K, reaction is in equilibrium
o If Q > K, reaction must proceed to the left toward reactants to achieve equilibrium o If Q < K, reaction must proceed to the right toward products to achieve equilibrium • LeChatelier’s principle: a change in one or more variables describing a system at equilibrium produces a shift in the system at equilibrium position that counteracts the effect of the change.
• Describe the effect(s) on a system at chemical equilibrium when each of the following is changed: − o Concentration (either reactants or products)
▪ No matter the disturbance, the system will react to consume the added substance or produce removed substance Don't forget about the age old question of What is free variation, complementary distribution?
▪ When reactant concentration increases, the other reactant concentration will decrease because the reaction moves to the right to consume the reactants and make products, therefore increasing product concentration.
o Pressure (amounts of either reactants or products)
▪ An increase in pressure will cause the reaction to shift towards the side with fewer moles of gas
▪ A decrease in pressure will cause the reaction to shift towards the side with more moles of gas
o Addition of an inert gas − volume of the container
▪ Adding an inert gas will cause an increase in the total pressure of the system, but will have no effect on equilibrium condition (no effect on concentrations or partial pressures of reactants and products)
o Temperature (reaction enthalpies)
▪ An increase in temperature will lead to an increase in K (shift to the right) for a system with a positive enthalpy (an endothermic reaction)
▪ An increase in temperature will lead to a decrease in K (shift to the left) for a system with a negative enthalpy (an exothermic reaction)
• van’t Hoff equation ln !!
!! = ! !!!"#$ !" !"#!!"#$
!!.!"#!$ ( !!! − !!!)
o For endothermic reaction, K2 > K1
o For exothermic reaction K1 > K2
• Interplay between the rate of reaction and the magnitude of the equilibrium constant for reversible chemical reactions
o If K >> 1, the forward rate is faster, products are favored
o If K << 1, the reverse rate is faster, reactants are favored Don't forget about the age old question of define cephalocaudal development
• “Haber-Bosch Process” and how the experimental conditions were chosen maximize the yield of ammonia
o �! � + 3�! � → 2��! � ΔH = -91.8 kJ
o Conditions to yield max ammonia:
▪ Decrease ��! If you want to learn more check out drosophila development notes
▪ Decrease volume (increase pressure)
▪ Decrease temperature
• Effect of a catalyst on the equilibrium position for a reversible chemical reaction o A catalyst shortens the time it takes for a reaction to reach equilibrium, but has no effect on equilibrium position
• Arrhenius model for acids and bases
o Acid: substance with H in its formula that dissociates in water to yield H3O+
▪ Eg. HCl, HNO3, HCN
o Base: substance with OH in its formula that dissociates in water to yield OH-
▪ Eg. NaOH, KOH, Ba(OH)2
o Exceptions: NH3 and K2CO3 also yield OH- in water, but don’t have OH in their formula. • Strong Acids: 1) HCl, 2) HBr, 3) HI, 4) HNO3, 5) HClO4, 6) H2SO4
• Strong Bases: 1) LiOH, 2) NaOH, 3) KOH, 4) RbOH, 5) CsOH, 6) Ca(OH)2, 7) Sr(OH)2, 8) Ba(OH)2 • Difference(s) between strong acids/bases and weak acids/bases
o Strong acids and bases are those that dissociate completely into ions in water, and have K >>1. They can sometimes be expressed with ???? because their reaction is essentially complete. o Weak acids and bases are those that only dissociate slightly into ions, and have K << 1, and low concentrations of H3O+ and OH-.
• Molecular, total ionic and net ionic equations for an acid-base reaction
o Molecular: shows reactants and products as if they were intact, undissociated compounds o Total ionic: shows all soluble substances dissociated into ions (liquid and aqueous) o Net ionic: eliminates spectator ions and shows the actual chemical change only o Spectator ions: present only as part of reactants, appear unchanged on both sides of the equation • Bronsted-Lowry model for acids and bases
o Acid: proton donor, any species that donates H+ ions, and has H in its formula (ALL ARRHENIUS ACIDS ARE BRONSTED LOWRY ACIDS)
o Base: proton acceptor, any species that accepts H+ ion, has lone e- pair to bind to H+ (ALL ARRHENIUS BASES CONTAIN THE B-L BASE OH-)
o Idea of conjugate acid-base pairs
o Reaction occurs when an acid and base react to form a conjugate base and acid ▪ Conjugate base has 1 less H and 1 more – charge than acid
▪ Conjugate acid has 1 more H and 1 less – charge than the base
▪ Acid loses H, base gains H
• Describe an “amphoteric” substance
o Substance that can either accept or donate a proton, eg. H2O
• Write the equilibrium reaction for the dissociation of pure water to produce H3O + and OH ions o 2�!� � → �!� � + �� � �� = �!� �� = 1 x 10^-14 @ 25 C. (water doesn’t appear in the equilibrium reaction because it is a liquid)
• Molar concentrations of hydronium and hydroxide
o [H3O], [OH] = 1 x 10^-7 M
• Describe pH and pOH
o pH = -log[H3O]
o pOH = -log[OH]
o pKw = pH + pOH = 14.00
• Describe the “strength” of an acid or base in terms of the extent to which its molecules donate (acids) or accept (bases) protons
o If an acid donates (gives up) its protons easily, it has a high Ka, low pKa, so it is a Strong Acid o If a base accepts protons easily, it has a high Kb, low pKb, so it is a Strong Base • Write the equilibrium constant expression for the reaction of an acid (Ka ) or a base (Kb ) with water o Acid: �� + �!� → �!� + � �� = !!! [!]
o Base: � + �!� → �� + �� �� = !" [!"]
• Describe how the strength of either an acid or a base is indicated by the magnitude of its equilibrium constant (i.e., Ka or Kb)
o High Ka/Kb = strong acid/base, because dissociation occurs completely, therefore products are favored, which is why equilibrium constant is large.
• “Ionic strength”
o Ionic strength (u) = !! (�!�!! + �!�!!…)
M = molar concentration of ions, Z = charges of ions
• Relationship between both equilibrium constant expressions and pH, and activities/activity coefficients o Kw = Ka * Kb = 1 x 10^14
o pKw = pKa + pKb = 14.00
o Activity = [A] * fa (assume fa = 1)
• Describe the main assumption that is used when an equilibrium constant, or pH, expression is written using molar concentrations.
• Describe the condition(s) where molar concentration and activity are not equal to one another. • Determining if a substance will behave as an acid, a base, or neither
▪ Group 7 with H or OH
▪ Carboxylics or alcohols
▪ Groups 1 or 2 with H or OH
▪ Groups 1 or 2 with NH2 (amine)
• Trends in acid-base properties for compounds having the structure H-X
o When neither H or X are more polar, ???? nonpolar hydrides
o When H is more electronegative, (X is anything under H) ???? basic
o When X is more electronegative, ???? acidic (strength trend is opposite of EN)
• Trends in acid-base properties for compounds having the structure: H-O-X, and how these properties are affected by bonding of electronegative atoms to atom X
o If X is a metal ???? basic
o If X is a nonmetal ???? acidic
o Acidity can be increased by the addition of electronegative atoms to atom X
• Zwitterions: neutral molecules with no net charge but different charges on different sides • Conditions under which the contribution of H3O + (OH- ) from the dissociation of water will be important in a solution containing a strong, monoprotic acid (base)
o If the concentration of the acid (base) is less than 4.5 x 10^-7, then water contributes MORE than 5% H3O+ (OH-), and must be taken into account, and quadratic equation must be solved. o If concentration of the acid (base) is more than or equal to 4.5 x 10^-7, then water dissociation contributes less than 5%, and concentration of acid (base) = [H3O+]total
• Steps to calculate the pH, pOH, [H3O ]tot, [OH ]tot, [H3O ]water, and [OH ]water in a solution containing a strong acid (base) given the initial concentration of the acid (base)
o Check rules.
o Write equation and expression and Q.
o Evaluate the magnitude of K, find out which is favored.
o ICE table, fill in with x for changes
o Write out K with x’s
o Check assumptions
• The “leveling effect”
o Water exerts a leveling effect on any strong acid or base by reacting with it to form the product of water’s autoionization, and equalizes their strengths.
o The strongest acid that can exist in water is H3O+, strongest base is OH-