Blog Archives

February 28, 2018

2/26/2018

C5: Environmental Impact

Above image courtesy of Pearson.

Evidence exists that increased levels of greenhouse gases in the atmosphere produced by human activities are changing the climate. This upsets the balance between radiation entering and exiting the atmosphere that leads to climate change. Sunlight has a range of wavelengths (see p. 680 in your text). The highest frequencies are absorbed by the upper atmosphere, allowing some UV, visible and longer wavelengths to reach the surface where they are absorbed. The waves re-emitted from the surface are longer wavelength infrared. These waves interact with carbon dioxide, methane and water vapor (the main greenhouse gases) which capture this energy so that it remains trapped in Earth's atmosphere.

The IR radiation interacts with the covalent bonds of greenhouse gas molecules, causing them to bend and stretch. The IR radiation causes the molecules to vibrate, achieving resonance. The C-H, C=O, and O-H bonds in greenhouse gases have resonant frequencies of vibration in the IR region of the electromagnetic spectrum.

Image courtesy of Wikipedia.

Water vapor accounts for 95% of all greenhouse gases. As the Earth warms up, more surface water evaporates and this increases the atmospheric water vapor concentration. The atmosphere then absorbs more IR radiation and causes increased warming. However, much of the water vapor condenses into clouds which block sunlight, causing global dimming and cooling the planet.

Carbon dioxide emissions have increased dramatically since the industrial revolution. Page 682 in your text shows the average CO2 increases for the 20th century and the global land-temperature index.

The main sources of greenhouse gas emissions are: burning fossil fuels (which accounts for 50% of anthropogenic greenhouse gases), industrial gases from factories that produce not only CO2, but also nitrogen oxides (which account for 25% of human greenhouse gas production, and agriculture/deforestation (the remaining 25%).

Burning fossil fuels release the carbon dioxide that comes from hydrocarbons previously stored underground. Industrial gases introduce not only CO2 but also other gases, such as chlorofluorocarbons (which do not occur naturally). Agriculture increases methane concentration from farty animals such as sheep and cows who generate methane in their digestive systems. Deforestation increases CO2 because with fewer trees, less carbon dioxide is absorbed from the atmosphere and used in photosynthesis.

A carbon sink is anything that absorbs more carbon than it releases as carbon dioxide. Of all the CO2 gas that is released into the atmosphere by human activity, approximately half has remained in the atmosphere. The rest is removed to carbon sinks, such as the oceans, resulting in CO2 concentrations rising by about 1% per year. About 30% of anthropogenic CO2 is absorbed by the oceans.

Page 684 breaks down the chemistry that occurs between CO2 gas and aqueous CO2 occurring at the ocean's surface. CO2 itself is not very soluble. The overall process produces a small positive delta H, which increases the temperature and shifts equilibrium to the left and lowers the ability of CO2 to dissolve in water.

Measures to Reduce Greenhouse Gas Emissions. Carbon capture and storage (CCS) is the process of capturing waste CO2 from where it is produced, transporting it to a storage site, and storing it where it will not enter the atmosphere, such as an underground geological formation. See page 685 for a detailed list.

Methane and nitrous oxide are the main greenhouse gases produced in agriculture. Methane is 25X as powerful a greenhouse gas as CO2 while nitrous oxide has over 300X the impact. Changing from nitrogen-based fertilizers to crop rotation methods could increase the levels of CCS and reduce emissions. The use of urban space to grow crops could subsidize local communities and reduce transport costs.

Smoke, dust particles and clouds reflect sunlight back into space, causing global dimming which cools the Earth's surface. Soot and ash can change the properties of clouds, polluting them and causing them to reflect more light than normal clouds. Global dimming has harmful effects: causing acid rain, decreased evaporation rate for water, and health problems such as asthma.

0 Comments

February 28, 2018

2/25/2018

0 Comments

C4: Solar Energy

Photosynthesis is the process by which sunlight is converted to chemical energy. Sunlight is absorbed into chloroplasts by the chemical chlorophyll. Visible light can be absorbed by molecules that have a conjugated structure with an extended system of alternating single and multiple bonds. When light is absorbed, electrons get excited and jump to higher energy levels within the visible wavelength region. When the electrons calm down, they jump back to lower energy levels and emit a photon of light in a particular color or wavelength. During photosynthesis, the return of the electron to the ground state takes place during a complex series of chemical reactions.

Pigments in plants are colored due to conjugated double bond systems. If a certain pigment absorbs red and green and/or yellow light as a result of its conjugation, then blue or purple light will be reflected. Chlorophyll absorbs red and blue light, but reflects green.

Image courtesy of biologyexams4u.com.

The conversion of carbon dioxide to carbohydrates using solar energy by photosynthesis produces our food and fuels. Biofuels such as ethanol are obtained from corn sugar or glucose by fermentation. See the equation on page 676. The ethanol produced in this manner can be blended with gasoline. Biodiesel is another sustainable fuel that can be grown and used as a diesel substitute. It is produced from vegetable oils, which can release similar amounts of energy to diesel when burned. However, it is viscous and therefore, unable to flow easily. This can be overcome by converting the vegetable oils to a less viscous ester with fewer intermolecular forces. See the transesterification process on page 676.

In this process to form biodiesel, the vegetable oil is typically heated with a sodium or potassium hydroxide catalyst along with methanol to produce the methyl ester, or ethanol to produce the ethyl ester of the triglyceride.

Page 677 provides a list of the advantages and disadvantages of biodiesel. My understanding is that you need to know these for the exam.

0 Comments

February 27, 2018

2/25/2018

1 Comment

C3: Nuclear Fusion & Fission

The source of energy for the sun is the fusion of hydrogen nuclei into helium. This process releases much more energy than the fission of U-235 or Pu-239, the fuels that are used in nuclear reactors. This process releases a tremendous amount of heat and almost no nuclear waste. However, it takes a vast amount of energy to initiate the reaction. Hydrogen bombs use a nuclear fission reaction (a small atomic bomb) to provide this energy. The heat released comes from nuclear fusion, but it has associated nuclear fallout from the fission reaction.

There is an abundance of fuel for nuclear fusion and the lack of waste makes it an attractive prospect for energy generation. However, there are technological issues involved. Fusion takes place at high temperatures and there is no material that can contain it. In the sun, hydrogen converts to helium, but there is a difference in the mass of a helium nucleus and the sum of the masses of 2 protons and 2 neutrons. This is known as mass defect. The missing mass is converted directly into energy, which can be predicted using E = mc2. This is energy calculated per atom, and the unit is the electron volt (eV).

Many different chain reaction mechanisms can occur to produce the helium nucleus and most of them occur in the sun and stars. On Earth, the process incorporates the fusion of deuterium and tritium. Deuterium is a hydrogen isotope with 1 proton and 1 neutron. Tritium is a hydrogen isotope with two neutrons. The reaction is shown on page 668 in your text. There is a difference in the binding energy per nucleon between helium and the two hydrogen isotopes. This means there is a mass defect and that mass is converted directly into energy.

Fusion of lighter elements into heavier ones increases the binding energy and the mass defect is converted into energy. The heavier transuranium elements (Z=92+) can undergo fission to form lighter nuclei. The binding energy of the two lighter elements is greater than the binding energy of a uranium isotope and therefore, the mass defect is converted into energy. Controlled nuclear fission is the process that powers nuclear plants nowadays.

Read about critical mass on page 669.

Fission and/or fusion involve the capture or emission of subatomic particles. The conversion of one element into another is called transmutation. Familiarize yourself with the above chart from the Pearson textbook.

Some heavier atoms are radioactive, which means they undergo spontaneous decay to produce daughter products, releasing alpha, beta, and/or gamma radiation in the process. Radioactive decay is a first-order reaction, which means that they have a constant half-life. The half-life refers to the time it takes for one half of the number of atoms in a sample to decay. There is an equation for half-life in your text on page 671.

The following material is for Advanced Higher Level.

Uranium-235 is the only natural occurring uranium that is fissionable. However, there is only 0.72% of naturally occurring U-235. In order to obtain fissile material, U-235 must be enriched so that the percentage of U-235 is large enough. To do this, the U-235 isotope must be separated from the U-238 isotope. Pages 704-705 goes into detail how this is done.

Graham's Law of Effusion allows you to calculate the relative rates of diffusion of the uranium hexafluoride.

Radioactive decay is a first order process. The time it takes for half a sample to decay is called half-life. The decay constant is related to the half-life (see page 706). The level of radioactive decay decreases in proportion to the quantity of material remaining.

1 Comment

February 26, 2018

2/25/2018

0 Comments

C2: Fossil Fuels

The formation of fossil fuels from decaying organisms is an example of reduction. Recall that reduction is the gain of hydrogen with the loss of oxygen. Many fossil fuels contain saturated alkanes and during the formation of fossil fuels, carbon atoms became more and more saturated with hydrogen and have fewer bonds to nitrogen, sulfur and/or oxygen than existed in the living form.

There are three main fossil fuels: coal, gas and crude oil. Coal is the most abundant but crude oil, also known as petroleum, is the most important. However, petroleum is difficult to use in its natural form. Petroleum contains a mixture of hydrocarbons of varying lengths. Long chain hydrocarbons are stronger intermolecular forces than short chain hydrocarbons. As a result, boiling points can be used to separate crude oil into fractions of varying chain lengths. At oil refineries, the fractions are separated by fractional distillation.

First image courtesy of the Pearson text. Second image comes from the Oxford text.

The crude oil is heated and becomes less viscous. Temperatures are lower at the top, so substances with low boiling points leave the column whereas fractions with higher BPs condense at higher temperatures near the bottom. The longer chain hydrocarbons are more viscous, darker in color and less volatile, therefore they are less flammable than the shorter chains hydrocarbons.

The more volatile short chain hydrocarbons make better fuels and burn cleaner. However, there are more long chain hydrocarbons in crude oil than short ones. In order to get more short chains, a process called cracking is used. This process is extremely important in the production of gasoline and diesel fuel.

When fuels are burned in automobile engines, they are compressed and then lit with a spark. Some hydrocarbons have a higher tendency to auto-ignite, which produces an effect called knocking. This can damage an engine. A measure of the fuel's ability to resist auto-ignition is its octane rating. Higher octane fuels can be compressed more and give a better performance than fuels with lower octane ratings. Read pages 660-661 to find out more about the relationship between octane rating and and branched-chain hydrocarbons.

Catalytic reforming is used to convert low-octane numbered alkanes such as heptane or octane into higher-octane numbered isomers such as methylbenzene or 2,2,4-trimethylpentane. I strongly suggest watching Mr. Thornley's YouTube videos on cracking and reforming petroleum.

Coal is the most abundant fossil fuel. It is formed from the remains of plant life that have been subjected to geological heat and pressure. It can be converted to more useful forms that are cheaper than crude oil. One method is coal gasification in which synthesis gas (aka coal gas or syngas) is produced by reacting coal with oxygen and steam in a gasifier to create hydrocarbons. Coal gasification can occur in an underground cavity, giving low plant costs as no gasifier needs to be constructed, no coal needs to be brought to the surface, and the carbon dioxide formed can be stored underground. This process is an example of coal capture and storage and reduces the amount of carbon dioxide entering the atmosphere. Examine Figure 8 and Table 2 on page 663 in your text.

Coal liquefaction takes filtered and cleaned synthesis gas and adds water or carbon dioxide over a catalyst. This is known as indirect coal liquefaction (ICL). In direct coal liquefaction (DCL), hydrogen is added to heated coal in the presence of a catalyst. Both methods adjust the carbon-to-hydrogen ratio and produce synthetic liquid fuels known as the Fischer-Tropsch process. The equation is on page 663 in your text.

The production of energy by burning fuels produces carbon dioxide. The carbon foot print of a reaction is a measure of the net quantity of carbon dioxide produced by the process. Even though biofuels may cost more to produce, their carbon footprint is less because carbon dioxide is absorbed by photosynthesis while the fuel is growing.

Be sure to examine the worked problems in the textbook.

0 Comments

February 5 - 9, 2018

2/1/2018

0 Comments

C1: Energy Sources

A good energy source should contain a large quantity of potential energy and said potential energy should be able to be released or converted, at a reasonable rate, to a useful form with minimal pollution and unwanted products. If the conversion is too fast, a large quantity of the energy is dispersed, and if it's too slow, it isn't useful.

All energy conversions undergo some form of quality degradation as some of the energy is dispersed as heat. The energy and materials in the original source change from a concentrated to dispersed form and the energy available to do useful work diminishes.

The videos below talk about useful energy source requirements and calculating efficiency.

The more quality of energy is degraded, the less efficient the fuel is:

efficiency of energy transfer = (useful output energy/total input energy) * 100. See the worked example on p. 655.

Energy density (ED) is a useful measure of the quality of a fuel that compares the energy released per unit volume:

energy density = energy released from fuel/volume of fuel consumed

Specific energy (SE) is the energy contained per unit mass of a fuel:

specific energy = energy released from fuel/mass of fuel consumed. See the worked example on p. 656.