Monthly Archives: March 2017

Application Exercises (Group 18)

Week 10 – Application Exercise Answers

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Question 1

Acetone dissolves the polymer, allowing the gas of the foaming agent to escape. The polymer collapses on itself and is dense because the gas has been removed.

Question 2

(a) LDPE cannot be used in this application because it does not have the required strength.

(b) The molecules of HDPE must be arranged in a way that produces the required strength. Using a thin layer of HDPE allows sufficient flexibility.

Question 3

When the piece of plastic is stretched, the strip narrows and “necks down”. The molecules become aligned parallel to each other and in the direction of the pull. This alteration of the three-dimensional structure is irreversible. If the pulling continues, the plastic breaks. When the same pulling force is applied to a piece of paper, the paper tears rather than stretch to any significant extent. The cellulose molecules in the paper are held far more rigidly in place and are not free to become aligned.

Question 4

(a) The benefits for polymers intended for use in the body should far outweigh any risks. The two main properties are (i) stable over time of intended use and (ii) non-toxic. Other factors to consider are low cost, lack of solubility in body fluids, lack of reactivity in body fluids, and the ease of implantation.

(b) Several different types of contact lenses are on the market and each uses a different type of polymer. Polymethyl methacrylate (PMMA), one of the earliest polymers used for rigid non-gas permeable lenses, is structurally similar to Lucite and Plexiglas. Silicone-acrylate materials are now more commonly used under trade names such as Kolfocon. Newer rigid gas permeable (RGP) polymers contain fluorine: fluro-silicone-acrylate polymers and fluoro-silicones. Polymacon (38% water) is typical of the polymers used for soft lenses and is a polymer of 2-hydroxyethyl methacrylate (HEMA). Other methacrylates include hioxifilcon (48% water) and methafilcon (55% water) or even lidofilcon (70% water). Manufacturers’ websites are good sources of information.

Desirable properties include being non-toxic, permeable to oxygen, comfortable to wear, and inexpensive. Additionally, it is preferred if it has the ability to conform to the shape of the eye and easily cleaned (if it is not disposable).

(c) As mentioned in the previous part, hard contact lenses are typically made of PMMA, a rigid non-gas permeable plastic. The soft contact lenses that replaced them are made of silicone, which is flexible and allows oxygen to reach the eye. Because of these properties, the soft lenses tend to be more comfortable.

Application Exercises (Group 18)

Week 9 – Application Exercise Answers

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Question 1

(a) The burning of coal (1-6% sulfur) produces varying amounts of SO2. Once SO2 is in the air,  it reacts with oxygen molecules in the atmosphere to form SO3, which acts as a formation of aerosols.

S (s) + O2 (g) → SO2 (g) [Burning]

2 SO2 (g) + O2 (g) → 2 SO3 (g) [Reaction with Air]

Aerosols do not absorb sunlight but instead reflects it, thereby reducing visibility.

Moreover, combustion from coal plants also releases huge amounts of water vapor.

CH4 (g) + O2 (g) → CO2 (g) + 2 H2O (g)

Water vapor when cooled in the hilly forest becomes visible as fog.

(b) Natural rain is slightly acidic due to atmospheric carbon dioxide dissolving in and reacting with water to produce weak carbonic acid. Hence the approximated pH for natural rain is around pH 5.3.

Given that the pH scale is a log scale, if the acid level is 10 times stronger, it is equivalent to 1 pH more acidic. Hence, the NPCA reporting that the rain in Mammoth Cave National Park is 10 times more acidic than natural would mean that the rainfall in the park is approximately pH 4.3.

Question 2

(a) Instead of using a dryer, hang your laundry to dry it. A large portion of acid rain emissions can be attributed to power (that comes from gas or coal) generated by appliances. The idea is to use less power (reduce SO2 emission).

(b) Automobile transportation emits high levels of NOx, which contributes to the extra acidity in rain. Walk, bike, or take public transportation to work helps reduce NOx emission rate (less cars).

(c) Avoid running dishwashers and washing machines with small loads helps to reduce energy and power consumption (reduce SO2 emission).

(d) Adding additional insulation on hot water heaters and pipes helps to reduce energy use by preventing heat and cold from escaping (reduce SO2 emission).

(e) Non-locally produced food requires transportation (e.g. trucks, planes, cars, trains, boats) that are powered by fossil fuels. By purchasing locally produced and grown foods, it helps to reduce transportation-related emissions (NOx) being released to the atmosphere.

Question 3

(a) Names and chemical formulas for 5 Acids:

  1. Sulfurous acid (H2SO3)
  2. Sulfuric acid (H2SO4)
  3. Nitric acid (HNO3)
  4. Hydrochloric acid (HCl)
  5. Carbonic acid (H2CO3)

       Names and chemical formulas for 5 Bases:

  1. Sodium hydroxide (NaOH)
  2. Magnesium hydroxide (Mg(OH)2)
  3. Ammonia (NH3)
  4. Barium hydroxide (Ba(OH)2)
  5. Potassium hydroxide (KOH)

(b) 3 observable properties for Acids:

  1. Sour taste
  2. Turn blue litmus paper red
  3. pH less than 7

       3 observable properties for Bases:

  1. Bitter taste
  2. Turn red litmus paper blue
  3. pH greater than 7

Question 4

The main concern of acid rain in Singapore is the fear of wiping out native species. A study showed that 20 species of animals plentiful in the Singapore’s Bukit Timah Reserve in the 1980s, including frogs, crabs and fish are slowly being wiped out [1]. Researchers have found that the water in the stream on the nature reserves to have a pH value of 4.4 to 4.7, which is believed to be more acidic now than 20 years ago [1]. Singapore is therefore, dealing with acid rain (e.g. National Environment Agency monitoring rainfall acidity) in an aim to preserve the native species and wildlife reserves in Singapore.

Acid deposition can originate both outside and inside of the Singapore’s border. An example of acid deposition from outside is the Haze (which originates from the deforestation of Singapore’s neighboring country, Indonesia) An example of acid deposition from inside is attributed to Singapore being a tropical stream, which are naturally acidic.

[1] http://wildsingaporenews.blogspot.sg/2009/09/native-species-in-singapore-may-be.html#.WMlB3G-GOM8