Friday, September 22, 2017

CSEC Chemistry Organic Chemistry Notes - Syllabus Section B1



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B 1.1 - Identify natural gas and petroleum as natural sources of hydrocarbons.

Petroleum and Natural Gas


Although how they were produced is not completely understood, petroleum and natural gas were most likely formed from the remains of marine organisms that lived approximately 500 million years ago. Petroleum is a thick, dark liquid composed mostly of compounds called hydrocarbons that contain carbon and hydrogen only. (Carbon is unique among elements in the extent to which it can bond to itself to form chains of various lengths.)

The table which follows gives the formulae and names for several common hydrocarbons. Natural gas, usually associated with petroleum deposits, consists mostly of methane, but it also contains significant amounts of ethane, propane, and butane. The composition of petroleum varies somewhat, but it consists mostly of hydrocarbons having chains that contain from 5 to more than 25 carbons. To be used efficiently, the petroleum must be separated into fractions by boiling. The lighter molecules (having the lowest boiling points) can be boiled off, leaving the heavier ones behind.


The commercial uses of various petroleum fractions are shown in the next table.


The petroleum era began when the demand for lamp oil during the Industrial Revolution outstripped the traditional sources: animal fats and whale oil. In response to this increased demand, Edwin Drake drilled the first oil well in 1859 at Titusville, Pennsylvania. The petroleum from this well was refined to produce kerosene (fraction C10–C18), which served as an excellent lamp oil. Gasoline (fraction C5–C10) had limited use and was often discarded.

However, this situation soon changed. The development of the electric light decreased the need for kerosene, and the advent of the “horseless carriage” with its gasoline-powered engine signaled the birth of the gasoline age. As gasoline became more important, new ways were sought to increase the yield of gasoline obtained from each barrel of petroleum. William Burton invented a process at Standard Oil of Indiana called pyrolytic (high-temperature) cracking. In this process, the heavier molecules of the kerosene fraction are heated to about 700 °C, causing them to break (crack) into the smaller molecules of hydrocarbons in the gasoline fraction.

As cars became larger, more efficient internal combustion engines were designed. Because of the uneven burning of the gasoline then available, these engines “knocked,” producing unwanted noise and even engine damage. Intensive research to find additives that would promote smoother burning produced tetraethyl lead, (C2H5)4Pb, a very effective “antiknock” agent.

The addition of tetraethyl lead to gasoline became a common practice, and by 1960, gasoline contained as much as 3 grams of lead per gallon. As we have discovered so often in recent years, technological advances can produce environmental problems. To prevent air pollution from automobile exhaust, catalytic converters have been added to car exhaust systems. The effectiveness of these converters, however, is destroyed by lead.

The use of leaded gasoline also greatly increased the amount of lead in the environment, where it can be ingested by animals and humans. For these reasons, the use of lead in gasoline has been phased out, requiring extensive (and expensive) modifications of engines and of the gasoline refining process.

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B 1.2 - List the main uses of at least three fractions obtained from the fractional distillation of petroleum.

Uses should include fuels, petrochemicals, lubricants. Refer to SO A 2.5.

Fractional Distillation of Crude Oil

The fundamental separation process in refining petroleum is fractional distillation (next diagram). Practically all crude petroleum that enters a refinery goes to distillation units, where it is heated to temperatures as high as 370 to 425°C and separated into fractions. Each fraction contains a mixture of hydrocarbons that boils within a particular range.

Fractional distillation of petroleum. The lighter, more volatile fractions are removed from higher up the column; the heavier, less volatile fractions are removed from lower down.

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B 1.3 - Describe cracking of petroleum fractions.

Thermal and catalytic cracking of alkanes.

Cracking


Excessive amounts of solid residue and other long chain hydrocarbons are usually produced during fractional distillation of crude oil. Cracking breaks them into more useful molecules with shorter chains and also produces ethene.

Thermal cracking uses high temperature and pressure to split long chain alkanes into short chain alkanes and alkenes. Hydrogen is a useful byproduct.

Catalytic cracking uses low pressure, high temperature and zeolite catalysts to split long chain alkanes in to fractions used to make petrol, together with arenes (aromatic hydrocarbons containing the benzene ring).



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Some Recent CSEC Past Paper Questions And Answers

Links to the past papers covered below were working as of September 2017 and are:

January 2016 P2: No online source known at this time.
June 2015 P2: No online source known at this time.
January 2015 P2: https://goo.gl/RuQdgm
June 2013 P2: No online source known at this time.


Questions which follow are based on theory required for SS B 1.1 -1.3.

January 2016 P2 Q5 (a) - (c)

5(a) Petroleum.

5(b) Fraction: Kerosene.
       Use: Jet fuel.

       Fraction: Lubricating oil.
       Use: Lubricants (for machines).

5(c)(i) The breakdown of long chain hydrocarbon molecules into shorter chain molecules.

5(c)(ii) Catalytic cracking uses a catalyst and relatively lower temperatures. Thermal cracking uses no catalyst and relatively higher temperatures.

CXC's comments on this question are now shown below.





June 2015 P2 Q1 (a)(i)

1(a)(i) Natural gas and petroleum.

CXC's comments on this question are now shown below.



January 2015 P2 Q3 (a), (b)

3(a) Natural gas and petroleum.

3(b)(i) Fraction 1: refinery gas.

3(b)(ii) Paving of roads.

CXC's comments on this question are now shown below.




June 2013 P2 Q3 (a)

3(a) The breakdown of long chain hydrocarbon molecules into shorter chain molecules.

Wednesday, March 1, 2017

CSEC Chemistry January 2017 P2 - Full Solution

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Question 1

1(a)  A standard solution is one whose concentration is accurately known.

1(b)    Mass of sample = 3.16 g

1(c)(iFinal volumes
  
           Titration 1: 25.60 cm3
           Titration 2: 26.30 cm3
           Titration 3: 24.50 cm3

1(c)(ii)    Volume used:
  
         Titration 1: 24.60 cm3
         Titration 2: 24.25 cm3
         Titration 3: 24.35 cm3

1(c)(iii)  According to the wording in this question, the student is required to use values from all three titrations to calculate the average volume used. This   however, is not what should actually be done in the lab. Correct treatment of this data says that you should ignore Titration 1 and find the average of   volumes used in Titrations 2 and 3. Both versions are shown here. The one   in which all three values were used, is in red and the other in green. Please   ask your teacher at school whether they subscribe to the literal interpretation of this question (in red) or not.
             
             Average volume used = 24.40 cm3
             Average volume used = 24.30 cm3

1(c)(iv)  0.000244 mol
              0.000243 mol

1(d)(i)  0.001220 mol
             0.001215 mol

1(d)(ii)  0.01220 mol
              0.01215 mol

1(e)  1.85 g
        1.85 g

1(f)  1.31 g
        1.31 g

1(g)  0.07278 mol
         0.07278 mol

1(h)  5.97 which is 6 to the nearest whole number.
         5.97 which is 6 to the nearest whole number.

1(i)  The solution goes from pale green to very pale pink/purple.

1(j)  The unreacted potassium permanganate present show that the end point has been reached because its colour does not disappear.

1(k)(i)  Dirty green precipitate is formed.
            Precipitate is in insoluble in excess aqueous NaOH.

1(k)(ii)  Precipitate slowly changes colour from dirty green to dirty brown/rust   brown.

1(k)(iii)  White precipitate formed.
               Precipitate is insoluble in dilute nitric acid.

Question 2

2(a)(i)  Electrolysis is defined as the chemical decomposition of an electrolyte by the passage of an electric current.

2(a)(ii)  Electroplating is the deposition of a metal on the cathode of an electrolytic cell, via the passage of an electric current, if the right combination of   electrolyte and electrodes (or electrode system) is used.

2(a)(iii)  The extraction of reactive metals e.g. Na and Al from their ores/compounds.
 
               Extraction of active non-metals e.g. the halogens, like Cl2 from NaCl(aqusing the diaphragm cell.

              Anodizing of aluminium.

2(b)(i       


              The electrodes should be made of graphite.

2(b)(ii)  Equation at anode: 2Br-(l) Br2(g) + 2e- 
              Equation at cathode: Pb2+(l) + 2e- Pb(l)  


Question 3

3(a)(i)  Compound A is an alkane.

3(a)(ii)  Compound A can be used as a fuel, i.e. cooking gas/LPG.

3(a)(iii)  One necessary condition is UV light.

3(a)(iv) 

3(a)(v)  A glass rod when dipped in (concentrated) aqueous ammonia and placed in the gaseous by-product, produces dense white fumes.

3(b)(i)  A polymer is a substance with (macro) molecules built up from many smaller  repeating units, which are connected by covalent bonds: -P-P-P-P-P- or – (P)n -.

3(b)(ii)  Type of polymerization: Addition polymerization.
  
              Name of polymer: Polypropene
  
             Use of polymer: In the manufacture of bottles and containers.

3(c)(i)  The functional group present is the ester linkage (group).

3(c)(ii)  Type of polymerization: Condensation polymerization.
  
             Use of polymer: Manufacture of automobile bodies.

3(c)(iii)  Water is the by-product formed in this polymerization reaction.

Question 4


4(a)  An isotope is a species of an element having a different mass number, but  the same atomic number as another species of the    same element, due to a difference in the number of neutrons in the nuclei.

For carbon-12, number of protons = 6, and mass number, i.e. number of   (protons + neutrons) = 12 therefore number of neutrons = (12-6) = 6.

For carbon-13, number of protons = 6, and mass number, i.e. number of (protons + neutrons) = 13 therefore number of neutrons = (13-6) = 7.

So carbon-12 and   carbon-13 have the same number of protons (6) and different numbers of   neutrons (6 and 7 respectively). They are therefore isotopes of carbon.

4(b)  Radiocarbon dating uses carbon-14.
         Treatment of cancer uses cobalt-60
         Tracing of blood flow and locating of obstructions in the circulatory system   uses sodium-24.

4(c)(i)  Period number = number of occupied electron shells. Group number =   number of outer electrons. W is in period 2, group 7 and X is in period 3,   group 2.  X is a metal and W is a non-metal. The bonding between them is   ionic bonding. W forms W- and X forms X2+, so the formula of the   compound formed is XW2.

4(c)(ii)  The compound will dissolve in water. This is because it consists of positive   and negative ions whose charges result in ion-dipole attractions with water,   a polar solvent. In reality, this compound, MgF2 is one of a small number of   insoluble fluorides so the correct answer is that it is insoluble in water, but a   CSEC student normally does not know how to explain why a compound is   insoluble. I can only guess what would be done in a case where this   question was answered correctly in terms of lattice and hydration   enthalpies.

Question 5

5(a)  Ammonia is a colourless gas with a pungent odour.

5(b)(i

5(b)(ii)  2NH4Cl(s) + Ca(OH)2(s) 2NH3(g) + CaCl2(aq) + 2H2O(l)

5(b)(iii)  Ammonia gas is a weak base and will react, in a neutralization reaction ,  with sulfuric acid. Hence calcium oxide (CaO(s)) can be used as the drying   agent since CaO is basic and will not react with/neutralize the ammonia gas.

5(c)  Moist red litmus when placed in contact with ammonia gas turns blue.

5(d)  Three harmful effects of excessive nitrates in the environment are:

  1. Eutrophication of water bodies results from high nitrate content.
   
    2. Nitrates in drinking water is poisonous to humans, especially babies.

  3. Nitrosamines, which have been found in foods are carcinogenic and are thought to orignate from nitrates.


Question 6


6(a)  1. Water is a very good solvent as it dissolves both ionic and covalent compounds. Since chemical reactions occur much more readily when in solution, water is important for metabolic processes such as digestion and respiration. The ability of water to dissolve oxygen is necessary for the survival of aquatic life.

  2. Water has a high specific heat capacity and can absorb a large amount of heat without a large change in temperature. This helps living organisms, which contain approximately 70% water, to maintain a relatively constant body temperature.

6(b)(i)  Ca2+(aq) + Na2CO3(aq) CaCO3(s) + 2Na+(aq)

6(b)(ii)  Boiling can remove temporary hardness. The soluble hydrogencarbonates decompose to form the insoluble carbonates. Boiling therefore softens the water by removing the Ca2+ and Mg2+ ions that were in solution:

  M(HCO3)2(aq) MCO3(s) + H2O(l) + CO2(g)

Where M is Ca and/or Mg.

6(c)  Hard water is water that does not readily form a lather with soap.
  Soft water is water that readily forms a lather with soap.

  To distinguish between hard and soft water samples, the following method can be used. Equal volumes of both samples are added to identical test tubes and the test tubes labelled. 1 cm3 of soap solution is added to both water samples and shaken for 30 seconds. The height of lather produced by each sample is measured. The water sample giving the greater height of lather is the soft water. The hard water may either give a lower height of lather, or no lather at all depending on the degree of hardness.