Chemical Evolution

1
Chemical Evolution

• Reactions and Energy Changes
• Energy Requirements for Reactions
• Energy Sources on Primitive Earth

2
Energy Changes (Real world)

• All chemical reactions are accompanied by energy
  changes usually as heat
• Exothermic reaction - reaction, which when once
  initiated, produces heat and sustains itself
• Endothermic reaction- reaction, which when once
  initiated, requires heat to sustain the reaction

3
Thermochemical Equations (Symbolic world)

• Modification of a chemical equation that includes
  the heat change which accompanies the reaction
• States of reactants and products given by (s)
  solid, (g) gas, (l) liquid
• Indicates heat change for moles specified by
  equation as DH lt 0 for exothermic and DH gt 0 for
  endothermic

4
Continued...

• 2 H2(g) O2(g) gt 2 H2O(g) DH -136 kcal
• 2 moles of H2(g) react with 1 mole of O2(g) to
  produce 2 moles of H2O(g) while producing 136
  kcal of heat (Exo)
• 2 HgO(s) gt 2 Hg(l) O2 (g) DH 43 kcal
• 2 moles of HgO(s) produces 2 moles of Hg(l) and 1
  mole of O2(g) while consuming 43 kcal of heat
  (Endo)

5
Energy Change and Energy Requirement for a
Reaction (Imaginary world)

• In a chemical reaction, energy must be supplied
  (endothermic) to break bonds in reactants and
  energy is produced (exothermic) when bonds are
  formed in the products
• The difference between energy supplied to break
  bonds (endo) and energy produced when bonds form
  (exo) is DH

6
Continued...

• The minimum energy required to initiate a
  reaction is called the energy of activation, Ea
• The energy profile of a reaction is a graph of
  energy vs. progress of reaction

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Energy Profile of a Reaction
Ea
The energy change for a reaction is DH, while
the energy requirement for a reaction is Ea
http//gcsechemistry.com/rc1.htm
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Sources of Energy on Primitive Earth

• Nuclear radiation
• Solar radiation
• Cosmic radiation
• Lightning
• Thermal energy from volcanoes

9
Nuclear Radiation

• Many of the atoms produced during a supernova are
  unstable and decay by alpha or beta radiation to
  produce more stable nuclei and heat
• Ionizing radiation can break bonds, produce free
  radicals and heat and thus can cause chemical
  reactions

10
Solar Radiation

• Surface of the sun is about 6000 K
• All matter having T gt 0 K, radiate
  electromagnetic radiation
• Electromagnetic radiation is energy moving
  through space in the form of a wave having the
  following properties

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Electromagnetic Radiation

• Wavelength, l, is distance between consecutive
  troughs or crests of a wave
• Frequency, n, is the number of wavelengths
  passing a point in one second

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Relations 1. c ln 3 x 108 m/s
speed of light (or any form of electromagnetic
radiation) 2. Energy, E å n or E å 1/l
n
n
n
Silberberg"Chemistry", Mosby, NY, 1996, 257
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Electromagnetic Spectrum
Visible (VIS)
Tro, 162
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Prism Bending Rays of White Light
Short wavelengths bend more than long wavelengths
giving rainbow ROYGBIV
Brescia et al, General Chemistry, 5th Saunders,
1988, 140f
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Visible Spectrum (VIS)
Tro, 162
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Energy Distribution in Solar Radiation
UV radiation can break chemical bonds, produce
free radicals, and thus can cause chemical
reactions
Schwartz et al, Chemistry in Context,Brown, IA,
1994, 37
17
Cosmic Radiation

• Particles in or near a supernova are accelerated
  to tremendous velocities and make up cosmic
  radiation
• When they collide with molecules, cosmic rays can
  break chemical bonds and create free radicals,
  and thus can cause chemical reactions

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Other Energy Sources

• Electrical energy associated with lightning can
  easily break chemical bonds and produce free
  radicals and thus can cause chemical reactions
• Volcanoes are sources of thermal energy which can
  increase the rates of reactions

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Chemical Evolution

• Reactions Occurring on Primitve Earth

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Summary of Chemical Evolution
Fox/Dose, Molecular Evolution and the Origin of
Life, rev ed, Dekker, NY,1977, 67
21
Fox/Dose, Molecular Evolution and the Origin of
Life, rev ed, Dekker, NY,1977, 67
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Time Frame for Chemical Evolution on Earth

• Earth is estimated by ratios of Pb to U to be
  about 4.5 to 4.8 billion years old
• Life is estimated from fossil records to have
  begun about 3.3 billion years ago
• Chemical evolution of precursors to life thus
  occurred over 1.2-1.5 billion years

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Conditions for Precursor Reactions

• Earliest atmosphere contained H2 and was
  reducing, so major components were H2O, CH4 , NH3
  and N2
• Assume average temperature was about same as
  today, 288 K
• Sources of energy were cosmic rays, nuclear
  radiation, UV light, lightning, and heat
  (volcanoes and nuclear decay)

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First Reactions

• Formation of HCN
  2 CH4 N2 gt 2 HCN 3 H2
• Formation of formaldehyde CH4
  H2O gt CH2O 2 H2
• These and the other reactions present-ed on later
  slides have all been demon-strated to have been
  feasible on prim-itive planet earth

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Apparatus for producing organ-ic compounds from a
reducing atmosphere by spark discharges as used
by S.L. Miller in 1955.
Miller, Livingin Environment, 12th, Thompson, CA,
2002, 104
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Formation of Molecules Associated with Life

• Formation of amino acids (components of proteins)
  
  CH4 HCN CH2O NH3 gt amino acids
• Formation of carbohydrates (sugars)
  n(CH2O) gt carbohydrates
• Formation of DNA bases n(HCN) gt
  bases for DNA

Details of these reactions are given on following
slides
27
Important Side Reactions

• 2 CH4 N2 gt C2N2 4 H2
• C2N2 2 OH- gt CN- NCO- H2O
• 6 CH4 N2 gt 2 HCCCN 11 H2

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Formation of Amino Acids
Mason, Chemical Evolution,Oxford, UK, 1991, 237
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Formation of Carbohydrates
Mason, Chemical Evolution,Oxford, UK, 1991, 241
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Formation of Purine Bases
Mason, Chemical Evolution,Oxford, UK, 1991, 240
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Synthesis of the Pyrimidine Bases
Mason, Chemical Evolution,Oxford, UK, 1991, 240