Lecture 5: Concentration Cells, The Periodic Table, and Properties of Light

Concentration Cell

When the concentrations are different electrons flow from the side with the less concentration solution (Anode) to the side with the more concentration solution.

The cell stops working when the concentration is equal

ZnZn2+Cu2+CuZn|Zn^{2+}||Cu^{2+}|Cu Cr2+Cr3Cr^{2+}|Cr^{3-}

\uparrow \uparrow \uparrow \uparrow

electrode salt electrode Platinum

bridge


  • In a cell with different concentrations (same metals) the EMF is higher than when the concentrations are equal.
  • In the normal cell - one metal gets oxidized, the other reduced (Driving force) Difference in Potential energy
  • The difference in energy is due to the entropic difference in the solution - the more concentrated the solution \rightarrow the lower entropy than the less concentrated solution.
  • Electrons will flow from the electron with less concentrated solution to the electrode in the more concentrated solution
  • Oxidation of the electrode in the less concentrated solution will increase the ion concentration in the solution - less concentrated solution has the anode. (And opposite)
  • The cell has the same half reaction in both cell compartment but with different concentrations of electrolytes.as long as the concentration of the solutions are different the cell potential is >0 and the cell can do work.


Ag+(aq:0.01mol/L)[HalfcellA]Ag+(aq:4.0104mol/L)[HalfcellB]Ag^{+} (aq:0.01 mol/L\left) [ Half cell A \right ]\rightarrow Ag^{+}(aq:4.0\cdot 10^{-4}mol/L)[Half cell B]

.Ecell=RTFE^{\circ }_{cell}= \frac{RT}{F} ln[Ag+]dil[Ag+]concln \frac{\left [ Ag^{+} \right ]dil}{\left [ Ag^{+} \right ]conc} Ecell=Ecell0.0592V1log[Ag+]dil[Ag+]conc\Rightarrow E_{cell}=E^{\circ }_{cell}- \frac{0.0592V}{1}log\frac{\left [ Ag^{+} \right ]dil}{\left [ Ag^{+} \right ]conc}

=0.0V(0.0592log4.01040.01)=0.0827V=0.0V - (0.0592 log \frac{4.0\cdot 10^{-4}}{0.01})=0.0827V


The Periodic Table - Atomic Radius, Ionization Energy and Electronegativity

All have the same number of Valence electron (outer shell)

Periodic Trends

  1. Atomic Radius as we go down the table, the atomic size increases. As we move to the right, atomic ratios decreases.
  2. Ionic Radius - Electrons repel each other, so adding an electron makes the atom larger. Removing an electron makes it smaller.
  3. Ionization Energy - Energy required to remove an electron from the atom (always the one in the outer most shell) The farther the electron from the nucleus, the easier it is to pull it away. On the Periodic table, opposite from the atomic radius.
  4. Successive Ionization energies (KJ/mol) the more electrons removed, the less stable the atom becomes.
  5. Electron Affinity - (Opposite of ionization of energy) How much an atom wants to gain an electron.
  6. Electronegativity - Ability of an atom to hold electrons tightly


Ionization Energy

  • The energy required to remove an electron from an atom
  • The less likely an atom gives up an electron, the more energy is required to take that electron away.
  • Increases across a period

F=Kq1q2r2F=\frac{Kq_{1}\cdot q_{2}}{r^{2}}

q1q_{1} = Charge due to the nucleus

q2q_{2} = Charge due to the electrons


Electronegativity

  • As we go across a period from left to right, electronegativity increases. As we have more protons in the nucleus, higher nucleus charge/force. The force which the nucleus pulls the electrons is higher. (Ratio of protons in nucleus and electrons in outer shell)
  • Measured on a Pauling scale
  • As we proceed from top to bottom of a group, the electronegativity decreases
  • For noble gases, electronegativity is undefined


Electron Affinity

Increases from left to right across a period and decreases from top to bottom


Basic Parts of an Atom

  1. Protons
  2. Neutrons
  3. Electrons
  4. Nucleus

Atom - The smallest particle of an element - Retains the characteristics of the element

Nucleus - Contains the protons (+) and Neutrons (neutral)

Electron Cloud - ee^{-} move outside the nucleus in orbitals

Proton - (+) Very small mass 1 a.mu

Neutron - Neutral 1a.m.u

Electron - (e)(e^{-}) Λl2000^{\Lambda }l_{2000} a.m.u , In a high speed


Atomic Structure Meets the Periodic Table

Atomic number - number of protons in the nucleus

Electron Shell - Row in the Periodic Table

Shell 1 - 2 electrons

Shell 2 - 8 electrons

Shell 3 - 8 electrons

Shell 4 - 18 electrons

Shell 5 - 18 electrons

Shell 6 - 32 electrons...


Frequency, Wavelength and Speed of Light

Frequency - (V) How often a wave cycle passes a given point per sec.

Wave Length - (λ\lambda ) The distance from one wave cycle to the next

As the wave length is smaller the Frequency increases.

Measuring Frequency - Frequency = cyclessecond\frac{cycles}{second} (unit - ls) / (Hz)

Speed of Light - 300,000,000 m/s \Rightarrow 3.00 \cdot 108^{8} meters/sec (c)

Wavelength of 1 m = Frequency of 3 \cdot 108^{8} ls

(each wave length has a corresponding Frequency)

Wavelength (λ\lambda ) \cdot Frequency (V) = Speed of Light (Constant)


What is Light?

All electromagnetic radiation moves at the speed of light.

All waves move at a speed equal to wavelength \cdot Frequency

How is Electromagnetic Radiation Produced?

\cdot E = h \cdot V Quantum Revolution

h - Planks Constant

V - Frequency of Photon

6.62 \cdot 1034^{-34} m2Kgs\frac{m^{2}\cdot Kg}{s}


Photo electric Effect, Work Function, Threshold Frequency, Wavelength, Speed and Kinetic Energy


Photo electric Effect

The energy carried by a photon can be transferred to an electron. When hitting a metal, there is enough Kinetic energy to escape and bounce of the metal. The frequency needs to be at a certain wave length to eject an electron from the surface of a certain metal. This doesn't change if the amount of light is added, only wave length matters, to get an electron to eject, If the wave length is high enough to eject an electron, then adding more light will increase the amount of electrons ejected. You just need to pass the threshold)


Threshold Frequency

E=hVE_{\circ }= h \cdot V_{\circ } E=E_{\circ }= Work

h - Planks Constant

VV_{\circ } - Threshold Frequency


Kinetic Energy

KE=EphotonEKE = E_{photon}-E_{\circ }

EphotonE_{photon} - hV

λ\lambda - wave length

EE_{\circ } - Required to eject the e^{-}

C=λVEphoton=hCλC=\lambda \cdot V \Rightarrow E_{photon}=\frac{h\cdot C}{\lambda}

KE=hCλEKE = \frac{hC}{\lambda}-E_{\circ}


Speed of the e^{-}

KE=12mV2KE = \frac{1}{2}mV^{2}

m - (kg)

V - Speed


Maximum Wave Lenth

λ=hCE\lambda _{\circ }= \frac{h\cdot C}{E_{\circ }}

EE_{\circ} - Work function (in J)

To see if the light can eject e^{-}





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