Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110

General Chemistry

Summer 2006

© R. Paselk 2002
 
 

Exam I Study Guide

 

Summer 2006, Final Version

Equilibrium Review

Le Chatelier's Principle-be able to use it to guess outcomes with equilibria. What do we mean when we say that chemical equilibria are dynamic? Think about how systems, starting from different initial conditions approach equilibria. What is the "mass action expression"? Define Q. How is Q different from K, the equilibrium constant? When are they equal? Is an equilibrium constant "constant" at different concentrations? at different temperatures? What is the equilibrium condition? Do we use units with K's? What is a homogeneous equilibrium? Heterogeneous equilibrium? Be able to solve problems involving both. What is the relationship between values of Keq for forward and reverse reactions? Be able to solve equilibrium problems like the examples we have seen in class. When are approximations legitimate? What assumptions are involved? Be sure you note assumptions. Be able to use quadratic equation if necessary.

Acid-base Equilibria: What is meant by "strong" and "weak" in reference to electrolytes, acids and bases? Degree of dissociation. Aqueous Solution Reactions: What do we mean by conjugate acid and/or conjugate base in the "Brönsted" definition. What are the meanings of "strong" and "weak" for acids and bases? What are precipitation and complexation reactions? Be able to write net ionic equations! What is Kdiss? Ka? Kb? Ki? Be able to write K's for acid/base. Be able to solve all manner of acid/base equilibrium problems. % dissociation problems. What is an ion product? Know the ion product (Kw) for water. What is pH? Be able to solve pH problems. What is the relation of pH to pOH? What is the pH of a neutral solution? How do we find the pH of salt solutions? What does the pH of a salt solution depend on (why is it generally NOT 7)? What is Kh? What is its relationship to Ka and/or Kb? Correlate the Kh expression with chemical equation. Why are the various terms where they are? Titrations: Be able to draw and interpret titration curves as we have seen in class and lab. Be able to find the equivalence point, buffer range, Ka or Kb and to identify the acid and base species in solution at any point along the curve. What is an indicator? How does it work? Buffers: What is a buffer? Define precisely. Be able to do buffer problems. How can you make a buffer? Common ion effect: What is the common ion effect? Buffers and the suppression of dissociation.  Henderson-Hasselbalch equation: pH = pKa + log([A-] / [HA]) Be able to solve problems using this equation. In what way does the legitamacy of this equation depend on the common ion effect? Be able to describe how you would make up a buffer, and make the appropriate calculations given a set of acid salts and/or acids and bases.

Aqueous Ion Solubility: Remember the activity of any pure substance in its own phase is one. Thus pure solids disappear from mass-action and equilibrium expressions. We can thus write a simpler expression for the equilibrium, Ksp = the solubility product. Be able to determine: solubility from Ksp values; Ksp values from solubility data; relative solubilities using Ksp values. Be able to solve Ksp problems involving common ion effects on ionic solubility and pH effects on ionic solubility.

Complex Ion Equilibrium: What is a complex ion? ligand? How do ligands and cations interact in complex ions? What is the coordination number of a complex ion? Are hydrated metal ions complex ions? What is a formation constant? How are formation constants related to dissociation constants? How are step-wise formation constants combined to give an overall formation constant? Be able to solve equilibrium problems involving complex ions.

Note that our qualitative analysis lab consists largely of applications of the materials above, with great examples of acid-base equilibria, buffers, common ion effects, Ksp, Ka, step wise equilibria, complex ions, net-ionic equations, etc. Thinking your way through the lab provides great examples and practice questions! For example, we have used complex ion formation to dissolve ionic solids in the presence of common ions in the Ksp expressions. How does this occur? Could you set up equations to calculate solubility under these circumstances?

Thermodynamics, Spontaneity, Entropy, and Free Energy

Thermodynamics: What are the First and Second Laws of Thermodynamics (know)? Be able to explain these laws and their implications. What are the three forms of energy we have discussed? How are they related? What is heat? What are state functions, (pathway independent, depend only on the initial and final states).

Spontaneity and Entropy: What is meant by the term spontaneous? How is it related to entropy? What is entropy? Why is it important? What determines whether a process is spontaneous? If all processes result in an increase in the entropy of the Universe, how is it possible for reactions to have a decease in entropy for the products? Why is entropy different from other thermodynamic functions (absolute value).

Free Energy: What is "free energy" and why is it important to us? deltaG = deltaH - TdeltaS. For a spontaneous process deltaG = (-). Be able to solve problems involving the free energy equation. Be able to determine if a reaction is spontaneous given tables of free energy of formation, enthalpies of formation, S° and temperature. What are the units of each of these parameters? Under what circumstances do you know the free energy without tables? the entropy? Why do you need to write a chemical equation to solve thermo problems? (functions are /mole). You should be able to calculate free energy by four different ways we discussed:

  1. By the formula for free energy: deltaG° = deltaH° -Tdelta
    1. Calculate deltaH° and deltaS° from tabulated values.
    2. Use these values and the specified temperature to calculate deltaG°.
  2. By the Hess's Law approach - deltaG is a State Function, so it is pathway independent. Look at any combination of reactions which will add up to the desired reaction, regardless of how impractical, and find the value of deltaG for the overall reaction.
  3. Look up and add together values of free energies of formation (if they are available): deltaG° = Sum(ndeltaf(prod)) - Sum(ndeltaf(rct)).
  4. Calculate from the equilibrium constant.

How (and why) is deltaG dependent on pressure? What is the relationship between free energy and the equilibrium constant? deltaG° = - RTlnKeq. How is the free energy of a reaction affected by concentration? deltaG = deltaG° + RTlnQ. Under what concentration conditions is the free energy of a reaction equal to zero? Why would you expect the lowest free energy of a reaction mixture to lie somewhere between pure products and pure reactants even when both enthalpy and entropy favor one or the other?

How are Keq and Temperature related? ln K = (-deltaH°/R)(1/T) + deltaS°/R Be able to use data to find the enthalpy and entropy of a reaction. What is the van't Hoff Equation (ln (K1/K2) = (deltaH°/R)[(T1-T2)/(T1-T2)]) and how can it be used to find thermodynamic values given T and Keq values?

How is free energy related to work? What does it tell us about the work that can be done? the energy available to do work? Is all of the free energy ever realized as work in a real process?

Electrochemistry

Review Redox chemistry! Terms: Oxidation, oxidizing agent, oxidized, reduction, reducing agent, reduced, etc. Remember that oxidation and reduction are always coupled in chemical reactions. Be able to balance half-reactions. Be able to write and balance half reactions for the two half cells of an electrochemical system. Be able to identify anode (oxidation) and cathode (reduction) half reactions.

Know electrical units and what they measure. What is the Faraday constant? Why is it important?

Galvanic Cells: What is a Galvanic cell? Be able to draw a Galvanic cell and label it (anodic and cathodic half cells, direction of electron flow, direction of ionic flows in salt bridge (or junction), salt bridge (or junction), half reactions, and electrodes).

SHE and Standard Reduction Potentials: What is the SHE (Standard Hydrogen Electrode) and why is it important? What is its voltage (E°)? Why is it not widely used? Why do we need reference electrodes for making electrochemical measurements? What is a standard reduction potential? If we have two half reactions with their standard reduction potentials, how do we determine electron flow between them? For a spontaneous reaction which becomes the anode reaction and which the cathode reaction? Which is reversed? Be able to use a table of Standard Reduction potentials to solve electrochemical problems. Is the potential of a redox reaction affected by stoichiometry? the current?

Cell Diagrams: Be able to diagram a cell using the cell diagram convention - "Line Notation." Why do we sometimes see two parallel lines? What do the lines symbolize? Be able to explain or draw a cell given a cell diagram.

Cell Potential and Free Energy: What is the relationship between free energy and potential? (deltaG° = -nFE°) What is the Nernst Equation? (E = E° - (RT/nF) ln Q) Does this look similar to another equation we know? (deltaG = deltaG° + RT ln Q). Why is n needed in the Nernst equation? Be able to use the Nernst equation to calculate cell voltages or use cell voltages to determine concentrations (as we did in lab).

Cell Potential and Concentration: What is a concentration cell? Be able to calculate voltages, or given voltages, concentrations with such cells.

Potentiometry: What is potentiometry? What is a Standard Calomel Electrode (SCE)? Why is it important? Why is the SCE a preferred reference electrode? Be able to diagram this electrode. Why have other reference electrodes become more popular then the SCE? What is a glass electrode? What is is used for (measuring pH). Why is it so important?

You may bring a single sheet of 8.5 x 11 paper with formulae etc. on one side only

as a "cheat sheet" for the exam.

Make sure you have all relevant formulae and constants on your sheet!

You should be careful and thorough in designing and creating this sheet.

However, remember it is for reference and insurance, the less you rely on it the better off you will be!

You will be provided with a Periodic Chart and tables of required data (i.e. values of E°, Ksp, etc.)


Syllabus/Schedule
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© R A Paselk

Last modified 9 June 2006