AP Chemistry Unit 3

Big Idea:

Any bond or intermolecular attraction that can be formed can be broken. These two processes are in a dynamic competition, sensitive to initial conditions and external perturbations.

Student Expectations:

Focus Standards

Enduring understanding 6.A: Chemical equilibrium is a dynamic, reversible state in which rates of opposing processes are equal.

Essential knowledge

• 6.A.1: In many classes of reactions, it is important to consider both the forward and reverse reaction.
• 6.A.2: The current state of a system undergoing a reversible reaction can be characterized by the extent to which reactants have been converted to products. The relative quantities of reaction components are quantitatively described by the reaction quotient, Q.
• 6.A.3: When a system is at equilibrium, all macroscopic variables, such as concentrations, partial pressures, and temperature, do not change over time. Equilibrium results from an equality between the rates of the forward and reverse reactions, at which point Q = K.
• 6.A.4: The magnitude of the equilibrium constant, K, can be used to determine whether the equilibrium lies toward the reactant side or product side.

Enduring understanding 6.B: Systems at equilibrium are responsive to external perturbations, with the response leading to a change in the composition of the system.

Essential knowledge

• 6.B.1: Systems at equilibrium respond to disturbances by partially countering the effect of the disturbance (Le Chatelier’s principle).
• 6.B.2: A disturbance to a system at equilibrium causes Q to differ from K, thereby taking the system out of the original equilibrium state. The system responds by bringing Q back into agreement with K, thereby establishing a new equilibrium state.

Enduring understanding 6.C: Chemical equilibrium plays an important role in acid-base chemistry and in solubility.

Essential knowledge

• 6.C.1: Chemical equilibrium reasoning can be used to describe the proton-transfer reactions of acid-base chemistry.
• 6.C.2: The pH is an important characteristic of aqueous solutions that can be controlled with buffers. Comparing pH to pK_a allows one to determine the protonation state of a molecule with a labile proton.
• 6.C.3: The solubility of a substance can be understood in terms of chemical equilibrium.

Enduring understanding 6.D: The equilibrium constant is related to temperature and the difference in Gibbs free energy between reactants and products.

Essential knowledge

• 6.D.1: When the difference in Gibbs free energy between reactants and products (ΔG°) is much larger than the thermal energy (RT), the equilibrium constant is either very small (for ΔG° > 0) or very large (for ΔG° < 0). When ΔG° is comparable to the thermal energy (RT), the equilibrium constant is near 1.

Ongoing Standards

Science Practice 1 The student can use representations and models to communicate scientific phenomena and solve scientific problems.

Science Practice 2 The student can use mathematics appropriately.

Science Practice 3 The student can engage in scientific questioning to extend thinking or to guide investigations within the context of the AP course.

Science Practice 4 The student can plan and implement data collection strategies in relation to a particular scientific question. (Note: Data can be collected from many different sources, e.g., investigations, scientific observations, the findings of others, historic reconstruction and/or archived data.)

Science Practice 5 The student can perform data analysis and evaluation of evidence.

Science Practice 6 The student can work with scientific explanations and theories.

Science Practice 7 The student is able to connect and relate knowledge across various scales, concepts and representations in and across domains.

Student Learning Targets:

• Use LeChatelier’s principle to design a set of conditions that will optimize a desired outcome, such as product yield. [LO 6.9, SP 4.2]
• Given data (tabular, graphical, etc.) from which the state of a system at equilibrium can be obtained, calculate the equilibrium constant, K. [LO 6.5, SP 2.2]
• Use LeChatelier’s principle to predict the direction of the shift resulting from various possible stresses on a system at chemical equilibrium. [LO 6.8, SP 1.4, SP 6.4]
• Connect LeChatelier’s principle to the comparison of Q to K by explaining the effects of the stress on Q and K. [LO 6.10, SP 1.4, SP 7.2]
• For a reversible reaction that has a large or small K, determine which chemical species will have very large versus very small concentrations at equilibrium. [LO 6.7, SP 2.2, SP 2.3]
• Given a set of experimental observations regarding physical, chemical, biological, or environmental processes that are reversible, construct an explanation that connects the observations to the reversibility of the underlying chemical reactions or processes. [LO 6.1, SP 6.2]
• Given a manipulation of a chemical reaction or set of reactions (e.g., reversal of reaction or addition of two reactions), determine the effects of that manipulation on Q or K. [LO 6.2, SP 2.2]
• Connect kinetics to equilibrium by using reasoning about equilibrium, such as LeChatelier’s principle, to infer the relative rates of the forward and reverse reactions. [LO 6.3, SP 7.2]
• Given a set of initial conditions (concentrations or partial pressures) and the equilibrium constant, K, use the tendency of Q to approach K to predict and justify the prediction as to whether the reaction will proceed toward products or reactants as equilibrium is approached. [LO 6.4, SP 2.2, 6.4]
• Given data (tabular, graphical, etc.) from which the state of a system at equilibrium can be obtained, calculate the equilibrium constant, K. [LO 6.5, SP 2.2]
• Use LeChatelier’s principle to predict the direction of the shift resulting from various possible stresses on a system at chemical equilibrium. [LO 6.8, SP 1.4, SP 6.4]
• Given a set of initial conditions (concentrations or partial pressures) and the equilibrium constant, K, use stoichiometric relationships and the law of mass action (Q equals K at equilibrium) to determine qualitatively and/or quantitatively the conditions at equilibrium for a system involving a single reversible reaction. [LO 6.6, SP 2.2, SP 6.4]
• Use LeChatelier’s principle to make qualitative predictions for systems in which coupled reactions that share a common intermediate drive formation of a product. [LO 5.16, SP 6.4, connects to 6.B.1]
• Design and/or interpret the results of an experiment regarding the absorption of light to determine the concentration of an absorbing species in a solution. [LO 1.16, SP 4.2, SP 5.1]
• Make quantitative predictions for systems involving coupled reactions that share a common intermediate, based on the equilibrium constant for the combined reaction. [LO 5.17, SP 6.4, connects to 6.A.2]
• Generate or use a particulate representation of an acid (strong or weak or polyprotic) and a strong base to explain the species that will have large versus small concentrations at equilibrium. [LO 6.11, SP 1.1, SP 1.4, SP 2.3]
• Reason about the distinction between strong and weak acid solutions with similar values of pH, including the percent ionization of the acids, the concentrations needed to achieve the same pH, and the amount of base needed to reach the equivalence point in a titration. [LO 6.12, SP 1.4, SP 6.4, connects to 1.E.2]
• Identify a given solution as containing a mixture of strong acids and/or bases and calculate or estimate the pH (and concentrations of all chemical species) in the resulting solution. [LO 6.15, SP 2.2, SP 2.3, SP 6.4]
• Identify a given solution as being the solution of a monoprotic weak acid or base (including salts in which one ion is a weak acid or base), calculate the pH and concentration of all species in the solution, and/or infer the relative strengths of the weak acids or bases from given equilibrium concentrations. [LO 6.16, SP 2.2, SP 6.4]
• Design, and/or interpret data from, an experiment that uses titration to determine the concentration of an analyte in a solution. [LO 1.20, SP 4.2, SP 5.1, SP 6.4]
• Interpret titration data for monoprotic or polyprotic acids involving titration of a weak or strong acid by a strong base (or a weak or strong base by a strong acid) to determine the concentration of the titrant and the pKa for a weak acid, or the pKb for a weak base. [LO 6.13, SP 5.1, SP 6.4, connects to 1.E.2]
• Draw and/or interpret representations of solutions that show the interactions between the solute and solvent. [LO 2.8, SP 1.1, SP 1.2, SP 6.4]
• Create or interpret representations that link the concept of molarity with particle views of solutions. [LO 2.9, SP 1.1, SP 1.4]
• Interpret titration data for monoprotic or polyprotic acids involving titration of a weak or strong acid by a strong base (or a weak or strong base by a strong acid) to determine the concentration of the titrant and the pKa for a weak acid, or the pKb for a weak base. [LO 6.13, SP 5.1, connects to 1.E.2]
• Reason about the distinction between strong and weak acid solutions with similar values of pH, including the percent ionization of the acids, the concentrations needed to achieve the same pH, and the amount of base needed to reach the equivalence point in a titration. [LO 6.12, SP 1.4, SP 6.4, connects to 1.E.2]
• Identify a given solution as being the solution of a monoprotic weak acid or base (including salts in which one ion is a weak acid or base), calculate the pH and concentration of all species in the solution, and/or infer the relative strengths of the weak acids or bases from given equilibrium concentrations. [LO 6.16, SP 2.2, SP 6.4]
• Given an arbitrary mixture of weak and strong acids and bases (including polyprotic systems), determine which species will react strongly with one another (i.e., with K >1) and what species will be present in large concentrations at equilibrium. [LO 6.17, SP 6.4]
• Relate the predominant form of a chemical species involving a labile proton (i.e., protonated/deprotonated form of a weak acid) to the pH of a solution and the pKa associated with the labile proton. [LO 6.19, SP 2.3, SP 5.1, SP 6.4]
• Explain the relative strengths of acids and bases based on molecular structure, interparticle forces, and solution equilibrium. [LO 2.2, SP 7.2, connects to Big Idea 5, Big Idea 6]
• Identify a solution as being a buffer solution, and explain the buffer mechanism in terms of the reactions that would occur on addition of acid or base. [LO 6.20, SP 6.4]
• Design a buffer solution with a target pH and buffer capacity by selecting an appropriate conjugate acid-base pair and estimating the concentrations needed to achieve the desired capacity. [LO 6.18, SP 2.3, SP 4.2, SP 6.4]
• Identify a solution as being a buffer solution, and explain the buffer mechanism in terms of the reactions that would occur on addition of acid or base. [LO 6.20, SP 6.4]
• Predict the solubility of a salt, or rank the solubility of salts, given the relevant Ksp values. [LO 6.21, SP 2.2, SP 2.3, SP 6.4]
• Interpret data regarding solubility of salts to determine, or rank, the relevant Ksp values. [LO 6.22, SP 2.2, SP 2.3, SP 6.4]
• Interpret data regarding the relative solubility of salts in terms of factors (common ions, pH) that influence the solubility. [LO 6.23, SP 5.1, SP 6.4]
• Explain observations regarding the solubility of ionic solids and molecules in water and other solvents on the basis of particle views that include intermolecular interactions and entropic effects. [LO 2.15, SP 1.4, SP 6.2, connects to 5.E.1]

Essential Questions:

• Hydrofluoric acid (HF) is banned from high school chemistry storerooms. If it is so dangerous, why is it classified as a weak acid?
• A sealed bottle of a saturated solution of calcium hydroxide (Ca(OH)2 ) sits on a shelf, unchanging for one month. Why, when the seal is broken, does a thin white layer appear on the surface?
• The clear liquid above a saturated solution of copper sulfate (CuSO4 ) is removed and placed in a test tube. Is the solution in the test tube at equilibrium?
• At the equivalence point of a weak acid–strong base titration, what three amounts are equal?

Extra Information:

Adopted Textbook:

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