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CCJ 320: Exam 3
| Question | Answer |
|---|---|
| Where does oxidation always occur? | Anode |
| Where does reduction always occur? | Cathode |
| Why does oxidation happen? | The action of a reducing agent |
| Why does reduction occur? | The action of the oxidizing agent |
| Cell Potential | Ecell=E+ - E- |
| Reduction potentials | E+ and E- |
| Which side is the anode on? | Left |
| Which side is the cathode on? | Right |
| Oxidation | Loss of electrons (increase oxidation state) |
| Reduction | Gain of electrons (decrease oxidation state) |
| Oxidizing agent | Substance that takes electrons (standard reduction potential is more positive) |
| Reducing agent | Substance that gives up electrons (standard reduction potential is more negative) |
| Anode | Electrode that oxidation takes place (positive polarity) |
| Cathode | Electrode that reduction takes place (negative polarity) |
| Coulombs | Unit of charge |
| Volt | Unit of potential |
| Ampere | Unit of current (coulomb/sec) |
| Joule | Unit of work |
| Watt | Unit of power (work/sec) |
| Parts of electrochemical cells | -Anode -Cathode -Completed circuit (for electrons to flow) -A salt bridge (usually!) -An electrolyte solution -Chemical species that undergo reaction |
| Galvanic cell | Uses spontaneous chemical reactions to generate electricity |
| Electrolytic cell | requires an electrical potential to be applied to the cell to drive some reaction |
| Single vertical line | Marks the phase difference |
| Double vertical line | Marks the salt bridge |
| Standard potentials | Used in predicting the action in either a galvanic cell or how much energy would be needed to force aa specific reaction in a non-spontaneous cell |
| Nernst equation | Accounts for potentials of cells where the reagents are not an activity of 1, number of electrons transferred, the temp, used to calc E+ and E- under non-standard conditions |
| What are standard potentials at? | Standard potential=1 |
| E^degree | Used in biochemistry to express the standard reduction potentials for conditions at pH 7 |
| What are the two types of indicator electrodes? | Metal and membrane |
| Indicator electrode | One that develops a potential whose magnitude depends on the activity of one or more species in contact with the electrode |
| Metal indicator electrodes | Develop an electric potential in response to a redox reaction at the metal surface, measurements made against a reference electrode, "first kind" respond directly to changing activity of electrode ion |
| Membrane indicator electrodes | Low solubility, some electrical conductivity, selectivity (part of membrane binds/reacts with analyte) |
| Ion selective electrodes | Physical phenomena which do not involve explicit redox reactions, but whose initial conditions have a non-zero free energy, also will generate a potential |
| Reference electrodes | Used for half of the cell to determine the potential of the analyte of interest, maintains a fixed potential -contrast , the indicator electrode responds to the analyte activity, follows Nernst equation |
| Potentiometry | An analytical method in which an electric potential difference ( a voltage) of a cell is measured |
| Overpotential | The voltage needed to overcome the activation energy for a redox reaction to occur at the electrode. If you want the reaction to go fast, then you apply high voltages -ELECTRODE ISSUE |
| Ohmic potential | The voltage needed to overcome the resistance of the solution (high resistance solutions do not provide easy migration of the ions) -SOLUTION ISSUE |
| Concentration polarization | The concentration of ions at the surface of the electrode are less than they are in bulk solution -SURFACE CHEMISTRY ISSUE |
| How do you overcome or compensate for issues with overpotential? | Using an inert metal for the working electrode material |
| How do you overcome or compensate for issues with ohmic potential? | Add an electrolyte to increase the ionic strength of the solution which improves conductivity of the solution (decreases resistance). Higher conductivity means the ions are more mobile |
| How do you overcome or compensate for issues with concentration polarization? | Stirring the solution |
| Three-electrode system | -When current is flowing, the potential is changing -We want to affect the current/potential at the working electrode -we want the reference electrode to stay stable -With 2 electrodes, there will be current flowing making the reference unstable |
| What is the purpose of the 3rd electrode? | To allow current to flow between working and auxiliary electrodes but measure potential between working and reference electrodes, to overcome instability in reference electrode under electrolysis by adding a 3rd electrode |
| Auxiliary electrode | Current-carrying partner of the working electrode in an electrolysis |
| Coulometry | Total number of electrons used for a reaction tells us how much analyte is present |
| Aperometry | Electric current is proportional to the concentration of the analyte |
| Voltammetry | An analytical method in which the relation between current and voltage is observed during an electrochemical reaction |
| Diffraction | Refers to the constructive and destructive interference pattern that is formed when light passes through an opening of soze d which is about the same order as wavelength |
| Refraction | Is the change of direction which occurs when light passes from one medium to another |
| Dispersion | Refers to the apparent "spreading out" in distance or angle when light is diffracted or refracted |
| Reflection | Is obvious, but importantly exploited in optical fibers which work based upon "total internal reflection" |
| Scattering | An "elastic" Rayleigh and an inelastic "Raman" variety |
| Polarization | Is used in observing the properties of optically active compounds |
| Transmission | Refers to when light passes through a medium without a net change |
| Absorption | Refers to when the energy of the EMR is transferred to atoms or molecules of the absorbing medium, which subsequently wind up in higher energy states |
| Emission | Is the opposite of absorption: an atom or molecule in an excited state gives off a photon and returns to a lower-lying energy state |
| Light as a wave | light is described as a periodically oscillating electric and magnetic field |
| Light as a particle | Light energy is transmitted as discrete "quanta" or packets called "photons" |
| Absorption of matter | Atom or molecule "absorbs" a photon of specific energy, goes to a higher energy state |
| Emission of matter | Atom or molecule releases a photon or specific energy, goes to a lower energy state; nonradiative excitation |
| Fluorescence | Absorption followed by re-emission |
| Ground state | The state of least possible energy in a physical system, as of elementary particles |
| Excited state | Being at an energy level higher than the ground state |
| Chromophore | Is a part of a molecule that is responsible for the absorption of light |
| Beer's law | A=ebc |
| Relationship between absorbance and transmittance | A=-log10T |
| Why is it generally preferable to use absorbance as a measure of absorption rather than % Transmittance? | Because absorbance is proportional to the concentration of the analyte, whereas %T is not |
| Does a compound with high molar absorptivity have a higher or lower limit of detection than a compound with low molar absorptivity? | Lower limit of detection -more sensitivity due to more efficient absorbance |
| Solvent extraction | The relative solubility of an analyte in teo immiscible liquids, is used to remove interferences, concentrate analytes, produce measurable forms of specific analytes |
| Partition coefficient | The distribution of the analyte between the two phases |
| What is the most efficient way to get the analyte extracted? | -One extraction with lots of volume -Lots of extractions with smaller volumes |
| Chromatography | Seperation of components of a mixture by exploiting differences in partitioning between a stationary phase and a mobile phase |
| Analytical chromatography | Determine chemical composition of a sample |
| Preparative chromatography | Purify and collect one or more components of a sample |
| Adsorption | For polar non-ionic compounds |
| What is used for gas mobile phase? | Temperature |
| What is used for liquid mobile phase? | Solvents |
| Ion exchange -anion | Analyte is anion: bonded phase has positive charge |
| Ion exchange -cation | Analyte is cation; bonded phase has positive charge |
| Partition | Based on the relative solubility of analyte in mobile and stationary phases |
| Partition -normal | Analyte is nonpolar organic; stationary phase MORE polar than the mobile phase |
| Partition -reverse | Analyte is polar organic; stationary phase LESS polar than the mobile phase |
| Size exclusion | Stationary phase is a porous matrix; sieving |
| Mobile phase | Phase that moves analyte along the solid phase and through the chromatograph |
| Concept of theoretical plates | -Theoretical plate corresponds to the length of stationary phase for one “equilibration” or “extraction step” of the solute between the 2 phases - There is no “plate” but relates the width of a band of solute to the distance it travels in the column |
| Plate height | The smaller the plate height, the more theoretical plates present, the narrower the band width of the peak, the more extraction steps, the better the separation |
| How do you optimize separation? | In order to optimize separation, you don’t have to calculate H, but understand how different physical phenomenon contribute to H so you can optimize flowrate, type of column, etc. |
| Consequences of Van Deemter Equation -Flow too high | Separation is encumbered by the C term - mass transport rate limits establishment of equilibrium |
| Consequences of Van Deemter Equation -Flow too low | Separation is encumbered by the B term - diffusion in longitudinal and axial directions causes excessive “spreading” (dispersion) of solute band |
| Consequences of Van Deemter Equation -Minimize the A term: | Need good packing or open tubular column |
| Retention time | The time from injection for an individual solute to reach the detector of a chromatography column |
| Purpose of GC | -Separate mixtures - analytical and preparative -Allow identification of components -“Fingerprinting” of mixtures - e.g. accelerants used in arson |
| Requirements for GC | -Thermal stability -Volatile (a few millitorr at column temp.) -Ability to detect the substance |
| Purposes and applicability of HPLC | -Separate mixtures where GC won’t work -Application to small, large molecules -Ionic, highly polar, labile compounds -Polymers |
| Requirements for HPLC | -Solute is soluble in mobile phase, does not react adversely with stationary phase -Elution occurs in a timely fashion (retention times are generally longer in HPLC compared to GC) -Detection must be compatible with “liquid” |
| Why is GC usually preferred to LC? | -Faster -More theoretical plates ===> better resolution available -Lower operating costs =Better detection schemes available |
| Limitations of HPLC | -Low number of theoretical plates -No “universal” separation mode -Operating costs and waste disposal - use of solvents |
Created by:
corty.19
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