earth metals water - unsw chemistry 4 - energy earth metals water energy prof russell howe school of...

Module 4 - Energy

Earth

Metals

Water

Energy

Prof Russell Howe School of Chemistry, UNSW

Module 4 - ENERGY

Overall content of this module■ Photosynthesis: light energy vs chemical energy

■ Coal, petroleum and natural gas

■ Carbon: allotropes and bonding

■ n-alkanes: properties, dispersion forces and theirconsequences. Distillation.

■ Energy changes in chemical reactions

■ Combustion and pollution

■ Reaction kinetics and catalysis

Energy : General Chemistry Concepts

■ Atomic structure: carbon and carbon allotropes

■ Bonding: bond energies, single, double and triple bonds

■ Intermolecular forces: dispersion forces and theirconsequences

■ Chemical reactions: energetics and energy profiles

■ Chemical reactions: factors affecting reaction rates

■ Carbon chemistry: classification of carbon compounds,alkanes, alkenes, alkynes, carbohydrates

■ Thermodynamics: heats of reaction, combustion

■ Environmental chemistry: fossil fuels, pollution fromcombustion

■ Biological chemistry: photosynthesis

Energy - Relationship to Other Modules

■ Energy uses the following concepts from the Earth,Metals andWater Modules

■ EARTH● Introduction to bonding and Lewis dot structures

● light, heat and electricity as common forms of energy

● bond energies

● physical vs chemical change, including boiling (and presumably other changesof state)

■ METALS● organisation of the periodic table,the mole

■ WATER● intermolecular forces

● distillation

● endothermic and exothermic reactions

Relationship with the Old Syllabus

■ Core 1-atoms and elements● concept of a chemical element

● allotropy

■ Core 2- compounds● systems of formulae and nomenclature

■ Core 5-chemical reactions● equations as useful representations of chemical change

● chemical reactions vary in many ways (endo-or exothermic, reaction rates)

■ Core 7-energy● chemicals as energy, ∆H, chemical energy and people

■ Core 8-structure and bonding● arrangement of electrons in atoms (carbon)

● covalent bonds

● intermolecular forces

Relationship with the Old Syllabus(2)

■ Core 11-carbon chemistry● IUPAC nomenclature (alkanes, alkenes, alkynes, structural formulae)

● Hydrocarbons, structure and physical properties, industrial and domestic use,safely precautions

● suggested experience: distillation

■ Elective 1-chemical energy● chemicals as fuels: heats of combustion, ignition and flashpoint, volatility,

hydrocarbon fuels, safety aspects

● enthalpy, standard enthalpy changes, enthalpy of combustion, bond energies

■ Elective 3-biological chemistry● composition of simple monosaccharides and disaccharides

● photosynthesis

■ Elective 4-chemistry and the environment● the atmosphere (effects of industrial effluents)

General perspective on ENERGY as aGeneral perspective on ENERGY as aGeneral perspective on ENERGY as aGeneral perspective on ENERGY as acontextcontextcontextcontext

■ attempts to link together thermodynamics, carbon chemistry,and reaction kinetics, using fuels and combustion as thecontext

■ many concepts recurr in the 5 different sub-modules (e.g.bond energies)

■ some artificial separation of concepts (e.g. carbon chemistry,carbohydrates encountered before simple alkanes,intermolecular forces, only dispersion forces covered in thismodule)

Living organisms make high energyLiving organisms make high energyLiving organisms make high energyLiving organisms make high energycompounds(1)compounds(1)compounds(1)compounds(1)

■ Recall role of photosynthesis in ecosystems

■ Photosynthesis controls oxygen levels in the atmosphere, andsynthesizes carbohydrates

■ Web resources on photosynthesis■ http://photoscience.la.asu.edu/photosyn/education/photointro.html

■ tutorial on photosynthesis■ http://www.life.uiuc.edu/cheeseman/JC.software.html

■ photosynthesis tutorial program (PC version is currently free,1.36Mb)

■ note that both of these sites give much more detail than isrequired at this point

Photosynthesis: essential featuresPhotosynthesis: essential featuresPhotosynthesis: essential featuresPhotosynthesis: essential features

■ “Carbonated water converted to sugar plus oxygen”

■ Overall: 6 H2O + 6 CO2 + 48 photons → C6H12O6 + 6 O2

■ With the level of chemistry background that students have atthis point, this is probably as far as you should go● One step further is to describe photosynthesis as the oxidation of water by

light:

● 12H2O + light → 6 O2 + 24 H+ + 24 e-

● followed by fixation of carbon (which does not directly require light)

● 6 CO2 + 24 H+ + 24 e- → C6H12O6 + 6 H2O

■ carbon fixation involves ATP and NADPH, regenerates ADPand NADP. These concepts will recur in the year 12Biochemistry Option


■ The role of photosynthesis in transforming light energy tochemical energy

■ relate light energy to wavelength and frequency (concepts ofcolour, wavelength, photons). Energy content of one mole ofphotons of different wavelengths?● Note that the syllabus does not require this, but it may be a useful thing for

better students to consider (see next page)

■ Role of the production of high energy carbohydrates fromcarbon dioxide as the important step in the stabilization of thesuns energy in usable form

■ what is a high energy compound?

■ Combustion as an exothermic process releasing energy

■ C6H12O6 + 6O2 → 6CO2 + 6H2O

Energy content of 1 mole of photonsEnergy content of 1 mole of photonsEnergy content of 1 mole of photonsEnergy content of 1 mole of photons

for 1 photon:E = hν = hc/ λ h = 6.626 x 10-34 Js c = 3 x 108 m s-1

for 1 mole of photons:E = 0.119/ λ ( /m) J mol-1

colour Wavelength/ nm Energy/ kJ mol-1

red 650 183

orange 600 198

yellow 570 209

green 530 225

blue 470 253


■ High energy carbohydrates- which ones?

■ Structural formulae of simple mono, di and polysaccharides(from old syllabus (elective 3, biological chemistry): glucose,fructose, sucrose, starch. Probably only need glucose at thispoint; the others will appear in Year 12.

■ Reinforce concept of bond energies; thus need to knowstructural formulae.

■ Qualitatively, describe energy release on combustion ofglucose as difference between energy required to breakbonds and energy released on forming bonds in● C6H12O6 + 6O2 → 6CO2 + 6H2O

■ Quantitatively, bond energies can be used to estimate this,(next page) but syllabus does not require such calculations


■ Combustion of glucose involves breaking 6 O=O bonds, 7 C-H bonds, 5 C-O bonds, 5 O-H bonds, 5 C-C bonds and 1 C=Obond, and forming 12 C=O bonds and 12 O-H bonds

■ amount of energy released can be estimated from averagebond energies

■ definition: bond energy = enthalpy change on breaking aparticular chemical bond (note sign!)

■ O=O 498 C-H 414 C-O 358 O-H 463 C=O 804C-C 346 kJ mol-1 (SI Chemical data)

■ thus net energy release from combusting one mole of glucoseis difference between bond energies of bonds formed andbond broken: 2679 kJ

■ experimental value = 2800 kJ why the difference?


■ Conversely, forming 1 mole of glucose from CO2 and H2Orequires 2679 kJ, obtained from sunlight.

■ Relate energy content of glucose to typical energyconsumptions (Selinger, Chemistry in the Market Place)

■ e.g. average adult woman requires 9 MJ per day

■ other high energy products of photosynthesis ?

■ Compare energy contents of glucose with other foods● glucose: 15.6 kJ g-1

● sucrose: 16.2

● brown rice: 14.9

● vegetable oil: 37.0

● butter: 30


■ Identify the photosynthetic origins of the chemical energy incoal, petroleum and natural gas

■ describe coal, petroleum and natural gas as naturallysynthesized fuels resulting from geological processes….

■ Chemical structure of coal (typical!!), petroleum, natural gas

■ Coal information:■ http://www.newcastle.edu.au/department/gl/cfkd/undp.htm#physicochemical

■ http://www.isr.gov.au/resources/coal_vl/education.html

■ natural gas site■ http://www.gsenergy.com.au/energy/school/gas/index.htm

■ see also Australian Institute of Petroleum site :http://www.aip.com.au/education/index.html


■ What are the energy contents of typical coal, oil and gas“molecules”?

■ Compare enthalpy of combustion per gram of carbon, octaneand methane

■ How are coal, oil and gas formed?

■ Where are the Australian coal, oil and gas reserves, and whatis their extent?

■ Build into student assignments rather than teach directly

There is a wide variety of carbonThere is a wide variety of carbonThere is a wide variety of carbonThere is a wide variety of carboncompounds(1)compounds(1)compounds(1)compounds(1)

■ Identify the position of carbon in the periodic table anddescribe its electronic configuration

■ describe the allotropes of carbon and relate their physicalproperties to their atomic arrangement

■ identify that carbon can form single, double or triple bondswith other carbon atoms

■ diamond, graphite, fullerenes

■ three dimensional covalent network versus two dimensionallayers versus discrete clusters

■ tetrahedral versus trigonal versus linear bonding

There is a wide variety of carbonThere is a wide variety of carbonThere is a wide variety of carbonThere is a wide variety of carboncompounds(2)compounds(2)compounds(2)compounds(2)

■ Web sites:● http://www.bris.ac.uk/Depts/Chemistry/MOTM/diamond/diamond.htm

● http://www.bris.ac.uk/Depts/Chemistry/MOTM/buckyball/c60a.htm

● http://sbchem.sunysb.edu/msl/fullerene.html

■ Explain the relationship between carbon’s combining powerand ability to form a variety of bonds and the existence of alarge number of carbon compounds● C-C,C=C,C≡C, C-H● C-O, C=O

● C-N, C=N, C≡N● C-X

■ give a few examples of compounds (IUPAC nomenclature);note that other organic compounds will be met in year12

There is a wide variety of carbonThere is a wide variety of carbonThere is a wide variety of carbonThere is a wide variety of carboncompounds(2A)compounds(2A)compounds(2A)compounds(2A)

■ Additional web sites:

■ http://www.science.org.au/nova/● buckyballs-a new sphere of science

● activities:

● is carbon hard or soft

● the geometry of the buckyball

● how hard is diamond

A variety of carbon compounds areA variety of carbon compounds areA variety of carbon compounds areA variety of carbon compounds areextracted from organic sources(1)extracted from organic sources(1)extracted from organic sources(1)extracted from organic sources(1)

■ identify and use the nomenclature for describing straightchained hydrocarbons from C1 to C8

■ compare and contrast their properties : which ones? Meltingpoint, boiling point, volatility, viscosity……….● Need to define these properties (refer back to WATER and CHEMICAL

EARTH)

■ explain the relationship between melting point etc of theabove hydrocarbons and their non-polar nature andintermolecular forces (dispersion forces)● dispersion (London) forces come from instantaneous dipoles, which depend

on polarizability

● polarizability is not mentioned in the syllabus so should probably not be used,but the size of the instantaneous dipoles depends on number of electrons andthe effective volume in which they are confined, thus dispersion forces dependon size of molecule

● recall dipoles and dipole dipole interactions from WATER

Supplementary data: n alkanesSupplementary data: n alkanesSupplementary data: n alkanesSupplementary data: n alkanesCarbonnumber

Meltingpoint/ K

Boilingpoint/ K

∆Hvaporization/kJ mol-1

Viscosity/10-3 kgsec-1m-1

1 90 112 8

2 89 184 15

3 85 231 19

4 135 272.5 22

5 143 309 27 0.289

6 178 342 32 0.401

7 182 371 37 0.524

8 216 399 42 0.706

Water 273 373 50 1.00


■ dispersion forces also depend on shape of molecule

■ e.g. methane BP = -162ºC, n-pentane = 36ºC,

■ 2,2-dimethylpropane = 10ºC

■ compare also dispersion forces with dipole-dipole forces

■ e.g. butane, BP = -0.5ºC, 2-propanone(acetone) = 56ºC

■ compare magnitudes of dispersion forces with dipole:dipoleand hydrogen bonding● this needs to be done with care. Dispersion forces are present in all atoms and

molecules; energy range from 0.05 to 40 kJ mol -1 . Dipole:dipole forcesdepend on size of molecular dipoles; energy range 5-25 kJ mol -1 . Hydrogenbonding spans a wide range of energies from below 10 to 40 kJ mol -1

■ viscosity is another useful illustration of intermolecular forces


■ describe the use of fractional distillation to separate thecomponents of petroleum and identify the relative amountsand uses of each fraction obtained

■ e.g. Selinger, Chemistry of the Car

■ differences between CNG, LPG, petrol, diesel, jet fuel(kerosine), lubricating oil

■ assess the safety issues associated with storage and use ofhydrocarbons in view of their weak intermolecular forces

■ compare volatility, flash point, ignition temperature,flammability limits (this anticipates combustion)

Supplementary data: combustion hazards

Fuel VP at RT / bar Flash point / °C Ignition point/°C

Explos.Limits/ v%

Methane >60 -187.7 5-15

Propane 10 -104 466 2.2-9.5

Butane 2.4 -60 430 1.1-8.4

Octane 0.015 13 220 1.0-4.7

Combustion provides another opportunityCombustion provides another opportunityCombustion provides another opportunityCombustion provides another opportunityto examine the conditions under whichto examine the conditions under whichto examine the conditions under whichto examine the conditions under which

chemical reactions occur(1)chemical reactions occur(1)chemical reactions occur(1)chemical reactions occur(1)

■ Identify combustion as an exothermic chemical reaction● reinforce exothermic versus endothermic reactions

■ outline changes in molecules during chemical reactions interms of bond breaking and bond making

■ explain that energy is required to break bonds……..● reinforce concepts from photosynthesis

■ identify the role of a wick and explain the conditions underwhich it is needed● this appears to be familiar material from the old syllabus

■ this section largely repeats what was covered under Livingorganisms make high energy compounds



■ Describe the energy needed to begin a chemical reaction asactivation energy

■ molecular description: reactant molecules must collide with aminimum amount of energy for reaction to occur

■ Describe the energy profile diagram for both endothermic andexothermic reactions

■ examples:● CO + NO2 → CO2 + NO E = 134 kJ, ∆H = -226 kJ

● 2 HI → H2 + I2 E = 180 kJ, ∆H = 159 kJ



■ explain the relationship between ignition temperature andactivation energy

■ initiation of free radical reactions (C-H bond breaking) requirescollisional activation; note that C-H bonds become weaker inlonger chain alkanes (compare methane, 435 kJ mol -1 andethane, 410 kJ mol -1)

■ identify the sources of pollution which accompany combustionof organic compounds and explain how these can be avoided

■ describe chemical reactions to summarize examples ofcomplete and incomplete combustion

■ pollutants: CO2,CO, NO2 (SO2)



■ CH4 + 1.5 O2 → CO + 2H2O

■ CH4 + 2 O2 → CO2 + 2H2O

■ N2 + O2 → 2 NO

■ NO + 0.5 O2 → NO2

■ (NO produced above 1300C)

■ How to reduce pollution of the atmosphere?

● Ensure complete combustion

● lower combustion temperature (add combustion catalyst)

● catalytic treatment of exhaust gases (this introduces catalysis, which strictlyspeaking shouldn’t happen until the next topic!)



■ Examples of exhaust gas treatment:

● selective catalytic reduction (power stations)

● 6NO2 + 8NH3 → 7N2 + 12H2O (vanadium oxide catalyst)

● selective catalytic reduction (car exhaust)

● 2CO + 2NO → 2CO2 + N2

● 4CO + 2NO2 → 4CO2 + N2

● CH4 + 2NO2 → CO2 + 2H2O + N2 (Pd/Pt/Rh catalysts)

● role of catalyst is to accelerate desired reactions (leads into next topic)

The extent and rate of energy release areThe extent and rate of energy release areThe extent and rate of energy release areThe extent and rate of energy release areaffected by factors…..(1)affected by factors…..(1)affected by factors…..(1)affected by factors…..(1)

■ Describe combustion in terms of slow, spontaneous andexplosive reactions, and explain the conditions under whichthese occur● At the lowest level, this can be described in terms of the rate of an exothermic

reaction. If the reaction is sufficiently slow, combustion will occur in a controlledfashion. If the rate is fast, the heat produced will increase the reaction ratecatastrophically, leading to an explosion.

● Rates depend on :

● nature of reactants

● temperature (refer back to activation energy)

● concentrations

● To go any further, it is necessary to introduce the concept of reactionmechanism: sequence of reaction steps leading to overall reaction (seeexample on following page)

Example: combustion of hydrogen

● Combustion described in terms of chain mechanism:

● initiation

● propagation

● termination

● Relative rates of three steps determine outcome

● explosion occurs if reaction rate increases rapidly with increasing temperature

■ 2H2 + O2 → 2H2O● initiation: H2 + O2 → OH + OH● propagation:

● H2 + OH → H2O + H

● O2 + H → O + OH

● O + H2 → OH + H● termination:

● H + OH → H2O

● OH + W → WHO wall reactions● rate of propagation versus termination determines overall reaction rate


■ Explain the importance of collisions between molecules as acriterion for determining reaction rates

■ Explain the relationship between temperature and the kineticenergy of particles

■ molecules must collide before they can react

■ collision frequency depends on concentration

■ outcome of collision depends on kinetic energy● recall concept of activation energy: minimum kinetic energy needed to

overcome activation barrier

■ concept of distribution of molecular velocities

■ average kinetic energy proportional to T


■ Describe the role of catalysts in chemical reactions, using anamed industrial catalyst as an example

■ Explain a model of the role of catalysts in changing the rate ofchemical reaction

■ Recall reaction profiles: role of catalyst is to provide analternative reaction path with a lower activation energy. Thecatalyst participates in the reaction, but is not consumed.

■ Examples:● ammonia synthesis

● methanol synthesis

● oxidative coupling of methane


■ Ammonia synthesis: iron catalyst● N2 + 3 H2 → 2 NH3

● N2 + 2 Fe → 2 Fe-N

● H2 + 2Fe → 2 Fe-H

● Fe-N + 3 Fe-H → NH3 + 4 Fe● catalyst provides an alternative pathway by dissociating nitrogen and hydrogen

■ Methanol synthesis: copper/zinc oxide catalyst● CO + 2 H2 → CH3OH

● CO + Cu → Cu-(CO)

● H2 + ZnO → Zn(H)O(H)

● Cu-(CO) + 2 Zn(H)O(H) → CH3OH + 2 ZnO● catalyst provides an alternative pathway by activating but not dissociating CO,

and dissociating hydrogen


■ Oxidative coupling of methane: MgO catalyst (doped withalkali metal)● 2 CH4 + O2 → C2H4 + 2 H2O

● CH4 + MgO → CH3 + MgOH

● 2 CH3 → C2H6

● C2H6 + 2 MgO → C2H4 + 2MgOH

● 4 MgOH + O2 → 4 MgO + 2 H2O● catalyst provides alternative pathway by breaking C-H bonds

■ CSIRO (North Ryde) pilot plant ca 1991, but not economic!!

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