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Effect of Concentration Changes on Equilibrium Yields

Part 1: Effect of concentration changes on equilibrium yields THEORY The solution of Fe(SCN)2 with which you have been supplied contains the ions Fe3 , SCN- and Fe(SCN)2 at equilibrium according to the equation :
Fe3 (aq) SCN-(aq) Fe(SCN)2 (aq) (pale yellow) (colourless) (blood red)
The intense, blood-red colour of the solution is due to the presence of the Fe(SCN)2 (aq) ion. The colour of the solution in each test-tube, when viewed down the tube, is a measure of the concentration of Fe(SCN)2 (aq) in the tube. If the volumes are the same in each tube, then the colour can also be used as a measure of the amount, in mol, of Fe(SCN)2 (aq) in the tubes. By noting how the intensity of this colour changes, it is possible to deduce the effect of each of the tests on the equilibrium. For instance, if the colour deepens, the amount of Fe(SCN)2 (aq) ions has increased and the amount of the Fe3 (aq) and SCN-(aq) ions must have simultaneously decreased since they are used up to form more Fe(SCN)2 (aq). The equilibrium would be described as having a net forward reaction (the position of equilibrium would have ‘shifted to the right’).
In tests A to E the amount, in mol, of Fe3 (aq) or SCN-(aq) ions present in the solution is initially changed as follows:
In Test A addition of Fe(NO3)3 increases the amount of Fe3 .
In Test B addition of KSCN increases the amount of SCN-.
In Test C addition of NaF decreases the amount of Fe3 because F- ions react with Fe3 ions to form FeF63-
In Test D addition of AgNO3 decreases the amount of SCN- because Ag ions react with SCN-ions to form a white precipitate of AgSCN.
In Test E addition of water has no effect on the initial amounts of the two ions, but affects the concentration of ALL components in the mixture.
Refer to Chemistry 2, Chapter 9, for a discussion of the effects of changes in conditions on equilibria.
PROCEDURE and RESULTS 1 Fill each of six semi-micro test-tubes to 1/3 of its volume with Fe(SCN)2 (aq) solution. Check that the liquid in each tube has the same intensity of colour when you look down the tube using a white tile or sheet of paper as a background. If necessary, add more solution so that the liquid in each tube is the same colour. Label the tubes A to F.
2 Using test-tube F for the purposes of comparison, perform each of the tests described in the table and record the change that occurs in the colour of the solution when viewed down the tube.
Test-tube Test Colour change
A 1 drop of Fe(NO3)3 added
B 1 drop of KSCN added
C 1 drop of NaF added
D 1 drop of AgNO3 added
E Equal volume of water added
F None No change
Fe3 (aq) SCN-(aq) Fe(SCN)2 (aq) (pale yellow) (colourless) (blood red)
Test Tube A
B
C
D
E
Test
Add
Fe(NO3)3
Add
KSCN
Add
NaF
Add
AgNO3
Add
H2O
Initial effect on …
[ Fe3 ]
INCREASES [ SCN- ]
[ Fe3 ]
[ SCN- ]
[ Fe3 ]
Change in colour
Change in
[Fe(SCN)2 ]
Consequent change in [Fe3 ] and [SCN-]
Comparison of final concentration of the ion with the initial concn.
[ Fe3 ]
GREATER [ SCN- ]
[ Fe3 ]
[ SCN- ]
[ Fe3 ]
Comparison of final amount (mol) of the ion with the initial amount
n (Fe3 )
GREATER n (SCN-)
n (Fe3 )
n (SCN-)
n (Fe3 )
Direction of the shift in equilibrium
ï‚® RIGHT Draw graphs showing the changes in concentration due to the change in the position of equilibrium in each of the tests A to E (the graph for Test A has been drawn as an example).
Note : Parts of graphs showing system at equilibrium should be horizontal lines
Changes in concentration should be approximately in proportion
Concentrations at new position of equilibrium must not go above/below the original values
TEST A SCN- Fe(SCN)2 Fe3 Concentration
Time
Initial
Equilibrium
Final
Equilibrium
TEST B Concentration
Time
Initial
Equilibrium
Final
Equilibrium
SCN- Fe(SCN)2 Fe3 TEST C Concentration
Time
Initial
Equilibrium
Final
Equilibrium
SCN- Fe(SCN)2 Fe3 TEST D Concentration
Time
Initial
Equilibrium
Final
Equilibrium
SCN- Fe3 Fe(SCN)2 TEST E Concentration
Time
Initial
Equilibrium
Final
Equilibrium
Fe(SCN)2 SCN- Fe3 Part 2 : Effect of changes in volume on a gaseous equilibrium THEORY The syringe contains the gases nitrogen dioxide (NO2) and dinitrogen tetroxide (N2O4) in equilibrium
: N2O4(g) 2 NO2(g) (colourless) (dark brown)
N2O4 is a colourless gas, whilst NO2 is a dark brown gas. Consequently, changes in the concentration of NO2 can be monitored by observing changes in the intensity of the colour of the gas mixture. In this way, shifts in the position of equilibrium can be identified. In this experiment the temperature of the mixture is constant so the value of the equilibrium constant, K, is unchanged. The effect of a change in volume on gaseous equilibria is described in Chemistry 2, Chapter 9.
PROCEDURE Test 1 Hold a syringe containing a mixture of NO2 gas and N2O4 gas in equilibrium and rapidly withdraw the plunger, holding it in position once you have done so. Note and record the change which occurs in the intensity of the brown colour the instant the volume is increased and the change in colour which occurs a moment later.
(You may need to do the test a few times in order to identify both of these changes.)
Observations : ……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
Test 2 Hold the syringe securely, this time firmly holding the sealed end of the syringe. Rapidly push in the plunger to decrease the volume of gas. Hold the plunger in the new position. Again, note and record the change which occurs in the intensity of the brown colour the instant the volume is decreased and the change that occurs a moment later.
(You may need to do the test a few times in order to identify both of these changes.)
Observations : ……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
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QUESTIONS 1. Write an expression for the equilibrium constant, K, for the reaction being investigated.
……………………………………………………………………………………………………………………
2. For Test 1 : a) Account for the initial colour change in terms of the instantaneous change in concentration of NO2.
…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………
b) What does the subsequent change in colour tell us about the concentration of NO2 ?
……………………………………………………………………………………………………………………
…………………………………………………………………………………………………………..
c) Complete the following explanation of the effect of the increase in volume on the equilibrium.
The increase in volume initially causes the concentration of NO2 to …………………………….. and the concentration of N2O4 to ……………………………….. The concentration fraction is ……………………… than the equilibrium constant, K, and the system is no longer at equilibrium. To regain equilibrium the concentration fraction must …………………………………
The concentration of NO2 must ………………………………. while the concentration of N2O4 must ………………………………………..
There is a shift in equilibrium to the ………………………… causing the amount of NO2 to …………………….…..and the amount of N2O4 to ……………….…………………..
3. For Test 2: Use Le Chatelier’s Principle to explain the change in equilibrium observed when the plunger was pushed in and the volume of the gas mixture decreased.
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………………………….…………………………………………………………………………………………………
……………………………………………………………………………………………………………………
……………………………………………………………………………………………………………………
Part 3: Effect of temperature on equilibrium mixtures THEORY: According to Le Chatelier’s principle, a temperature change of an equilibrium mixture will elicit a response by the equilibrium mixture, so as to partially oppose the change. The effect on the concentration of the equilibrium components, and hence on the equilibrium constant, depends on whether the reaction is exothermic or endothermic, and on the direction of the temperature change.
Refer to Heinemann Chemistry Two, Chapter 9, for further discussion of the effect of temperature change on chemical equilibrium.
PROCEDURE AND RESULTS A. N2O4 (g) 2NO2 (g) ; endothermic. (Note : N2O4 is colourless and NO2 is brown)
Carefully place one of the stoppered test-tubes containing an N2O4 / NO2 gas mixture in a beaker of hot water for about one minute, holding the stopper firmly in place. Place the other tube in a beaker of ice water.
Remove the tubes from the hot water and ice-water and immediately compare the intensities of the brown colour in the tubes. Record your observations.
Hot water ……………………………………………………………………………………
Cold water……………………………………………………………………………………
B. Fe3 (aq) SCN- (aq) Fe(SCN)2 (aq) ; exothermic. (Red colour is due to Fe(SCN)2 )
Half-fill two semi-micro test-tubes with Fe(SCN)2 solution.
Place one test-tube in a beaker of ice water. Place the other test-tube into a beaker of very hot water.
Compare the colour of the mixtures in the two test-tubes. Record your observations.
Hot water ……………………………………………………………………………………
Cold water……………………………………………………………………………………
C. H3PO4 (aq) H2PO4- (aq) H (aq) ; exothermic (Methyl violet indicator)
(Green – Blue – Violet as pH increases)
Pour 1M orthophosphoric acid (H3PO4), into each of two semi-micro test-tubes, to a depth of about 3 cm. Add one drop of methyl violet indicator to each test-tube.
Place one test-tube in a beaker of ice water. Place the other test-tube into a beaker of very hot water. Record the colour of the indicator in each of the two test-tubes.
Hot water ……………………………………………………………………………………
Cold water……………………………………………………………………………………
Note the acidity of each solution. Methyl violet indicator is yellow in solutions with a high concentration of H (aq) (low pH) and its colour changes through green to blue and to violet as the H (aq) concentration reduces (ie with increasing pH).
QUESTIONS In Part A, the brown colour in the test-tubes supplied is due to the presence of nitrogen dioxide (NO2) gas.
How does the concentration of nitrogen dioxide in the equilibrium mixture change as the temperature is increased?
What does your experiment indicate happens to the value of the equilibrium constant, K, for this reaction as the temperature increases? Explain.
Describe (using Le Chatelier’s principle) how the equilibrium position has shifted.
In Part B, the red colour is caused by the ion Fe(SCN)2 .
How does the concentration of Fe(SCN)2 ions in the equilibrium mixture change as the temperature is increased?
What does this indicate about the value of the equilibrium constant, K, for this reaction as the temperature increases? Explain.
Describe (using Le Chatelier’s principle) how the equilibrium position has shifted..
Account for your observations in Part C of this experiment (H3PO4 (aq))

Benefits of DNA Technology on Forensic Science

Discuss how DNA technologies have been applied to these cases including reference to the challenges facing Forensic Scientists in these cases. Illustrate your answer with appropriate case studies.
Introduction
Deoxyribonucleic acid (DNA) profiling has been used as an investigative tool since the discovery of the polymorphic nature of short tandem repeats by Jefferys in 1985 (Aronson 2005). The benefits of using DNA profiling in the identification and/or exclusion of perpetrators in unsolved cases was initially shown when it was used to solve the Colin Pitchfork case in 1986 just a year after it was reported.
Since this success, DNA technology has been employed in the conviction of perpetrators in a multitude of cases, spanning a variety of crimes from volume crimes such as burglary to serious crimes of murder. In addition to this DNA testing has been successfully used in post-conviction exonerations that are enabled by organisations such as the Innocence Project. The effectiveness of this application is shown in the statistics – since 1989 there have been 312 individuals exonerated in the US post conviction, of these, 18 served time on death row (Innocence Project 2014). In times that pre-date the advent of DNA technology this type of investigation was not possible, and the scientific methods available for forensic investigation (such as serology, hair comparison and blood grouping) lacked statistical accuracy and discriminatory power which often meant that the evidence collected in relation to a crime in conjunction with the traditional policing techniques did not yield useful lead, or a suspect and as a result, many crimes went unsolved, and these unsolved cases are also referred to as cold cases.
According to research by the BBC’s freedom of information team, in the UK alone there are 1,143 unsolved killings on police record (Casciani 2010). As a result of recent and previous progress in DNA technologies many of these cold case murder investigations have been reopened worldwide, with the hope that the reinvestigation of evidentiary material that was collected but not examined (Goodwin et al. 2007). Or those that were examined for a specific reason e.g. latent prints for fingerprint identification with new technologies and methodologies may provide increased support to cases that have weak suspects or the lack of a suspect.
The advent of PCR-based techniques has made it possible to obtain usable DNA from smaller quantities and poor quality samples that are indicative of old evidentiary material by multiplying the available genetic material (Taberlet et al. 1996). The breadth of methods that have been developed and subsequently been influential in assisting in closing cold cases will be discussed here.
Short Tandem Repeat
Current DNA testing is based on the primary use of STR technology. This technology involves the evaluation of specific loci in the nuclear genome and it is the individual variations at these particular STR loci that allow human identification and the ability to distinguish between different individuals profiles. As previously mentioned the initial success of this technique was shown in the Colin Pitchfork example. In 1983, the body of 15-year old, Lydia Mann was found in Narborough, Leicestershire having been raped and murdered (Aronson 2005).
It was not until three years later, when a second similar crime occurred which did have a suspect that investigators asked Jefferys to compare a DNA sample from the suspect in the new case, to DNA extracted from sperm collected during the Manns case. They did not match, it did however match samples taken from the second case (Figure 1). This testing in conjunction with a dragnet investigation involving 4582 men and a phone call from an informant led to the identification of Colin Pitchfork as the perpetrator (Aronson 2005).
This technology has since been improved and simplified for high-throughput analysis through the development of commercial STR typing kits, which allow amplification of up to 16 loci simultaneously (Butler 2007). In the 1990s, early 2000s the first commercial kit for multiplex STR typing became available for use (Roland et al. 2010). Since then, through the application of PCR and laser fluorescence detection, detection limits have been reduced to less than 100 pg therefore improving the chances of producing a useful DNA profile from poor quality and or quantity DNA (Senge et al. 2011).
Mitochondrial DNA analysis
Nuclear DNA is not the only genomic information that has been identified as being applicable in forensic investigation. Mitochondrial DNA (mtDNA) analysis can be applied as an alternative method to develop DNA profiles from evidence in incidences were STR profiling is unsuitable. For example in cases where the amount of material is very small and nuclear DNA cannot provide a definitive result (Goodwin et al. 2007; Bender et al. 2000), although this does not offer the same discriminatory power.
The value of mtDNA in criminal investigations was shown in its use to solve the 33-year old murder case of Rosa Cinnamon in 2007. Re-examination of fingernail scrapings that were taken in the original autopsy and stored for more than 30 years produced a mtDNA profile match to Edward Delon Warren, who died in 2003 whilst incarcerated for robbery and murder convictions (Bernstein 2009).
MtDNA has a high copy number per cell, which makes it more readily available in very low quantity and highly degraded DNA samples, therefore, it is the preferential method for old remains and evidence samples that lack nucleated cells such as hair shafts, bones and teeth. This is the reason it is such a valuable technique to use when re-investigating evidentiary material from old unsolved cases (NIJ special report 2002). Hairs are one of the most common sample types to undergo mtDNA testing, and it is very likely that hair found at past crime scenes or on clothing taken from a victim or suspects clothing, would have been collected for hair comparison. They are then available for mtDNA analysis during re-investigation (Melton et al. 2005).
This was the case in the conviction of William Gregory, who was arrested, charged and subsequently sentenced to 70 years for the attempted rape of a woman who lived in his apartment building (Scheck and Neufeld 2010). The only evidence available for investigation were six hairs, that were found in a stocking cap worn and discarded by the perpetrator (Scheck and Neufeld 2010). The hairs underwent analysis by microscopy, and it was stated during the trial in 1993 that they could have come from Gregory, this evidence aided in his prosecution (Melton 2009). Post-conviction analysis of the hairs using mtDNA technologies, assisted by the Innocence Project produced results that excluded Gregory as being a contributor, and he was subsequently released in 2000 after serving 7 years of his sentence (Innocence project 2014).
MtDNA is also useful in cold cases when the victim or suspect is missing, unavailable or has deceased. The maternal inheritance nature of mtDNA can be used to provide a reference sample, which can then be compared to crime scene samples. This could provide valuable insight for cold case investigators (Melton 2009).
Mini STR and LCN
The ability to utilise DNA in solving criminal investigations has been even further increased by the development of low copy number (LCN) methodologies (Goodwin et al. 2007). Low copy number or LCN is an approach that is sensitive enough to analyse a few cells to produce a DNA profile, the ability to do this was declared in 1997 (Peter Gill 2001; Aditya et al. 2011). Obtaining profiles from trace DNA has, opened up the opportunity to obtain DNA from a wider range of exhibits that previously were thought to hold no evidentiary material (Roland et al. 2010).
Further advances have been made to techniques to help enhance the results in LCN samples through mini-STRs. Mini-STRs assists in typing degraded and trace DNA. It was a method that emerged out of the 9/11 attack on the World Trade Center to assist in the study of heavily degraded bone material that were collected (Lederman 2005). The usefulness of LCN in solving cold cases can be shown in the Jacqueline Poole case. In February 1983 Jacqueline Poole’s body was found in her northwest London, Ruislip home, she had been strangled and sexually assaulted (Summers 2001). A man called Anthony Ruark was a prime suspect at the time, but there was not enough evidence for him to be charged, so the investigation was slowed down but not closed. Fourteen years later the case was reopened, and the original forensic items and samples from the prime suspect were re-examined. “Miniscule amounts of semen were found on Mrs. Poole’s clothes”, this was put forward for LCN DNA typing (Summers 2001). Twelve months later this technique successfully identified the semen to belong to Ruark, and he was subsequently charged with the murder (Summers 2001).
LCN analysis has had an equally effective application in exoneration cases; in particular it was applied in the UKs first exoneration case of Michael Shirley in 2003. Linda Cook was raped and killed in Portsmouth in 1986 (Johnson and Williams 2004). The blood group match of semen extracted from Linda’s body to Shirley was a key component in the prosecution, along with other evidentiary material. Despite DNA profiling being available at the time of the trial, there was insufficient sample to use the techniques of the time. However, in 1999 developments in DNA technologies allowed LCN analysis to take place on intimate swabs stored since 1986 (Johnson and Williams 2004). Comparison of the profile from the intimate swabs to reference samples from Cook and Shirley in 2001, found that Shirley was not a contributor to the DNA profiles of the mixed sample (Johnson and Williams 2004). His conviction was subsequently quashed in 2003 after serving 16 years in prison (Johnson and Williams 2004).
Y-STRs
Y-STR is a technique that focuses on DNA analysis of regions on the male specific Y-chromosome. This method has seen its most useful application forensically in sexual assault cases in particular it is used for evidence types including fingernail scrapings, ligatures, rape kit swabs and microscope slides from cold cases (Clay 2007).
Y-STR profiling does not allow for unique identification; this is because the same DNA profile will appear for all male individuals that belong to the same paternal lineage. As a result, its use today is often in conjunction with familial searching using National DNA Databases (NDNAD). The value of Y-STRs has been shown in a wide variety of cases including its use in solving the historic 1983 Colette Aram case. Sixteen-year-old Colette was found murdered in a field having been strangled and sexually assaulted. A cold case review in 1997 and a further one in 2005 allowed the development of a Y-chromosome profile in August 2007, which was compared to over 300 samples on the NDNAD. This search produced a match to John-Paul Hutchinson, who was too young to have committed the original crime; however, his father Paul Hutchison was later confirmed to be the perpetrator (Clayton and Flint 2014).
This case is also a good example of the issues that come with examining old evidentiary material. The poor quality and quantity as with the microscopic slides in this case only allowed for the identification of three alleles. This meant that using this incomplete profile to search the NDNAD still produced an overwhelming 600 matches, even with a restriction to Nottingham (Clayton and Flint 2014). Another problem that occurred with this case was a lack of discrimination with Y-STR analysis. Hutchinson attempted to implicate his deceased brother as the perpetrator and had the prosecution used Y-STR as the main source of evidence for conviction, this would have presented an issue in court.
Practical issues
Although DNA technologies have clearly been shown to aid in the solving of cold cases and have been successfully used in the exoneration of innocent individuals, there are many practical issues that arise with re-examination of cold case evidentiary samples. Some of these issues have been highlighted in previously mentioned cases, but there are many other incidences where forensic scientists have had difficulties. The 2008 mistake of re-examining the evidence from the Tapp murders is a perfect example of how the lack of stringency in procedures to prevent contamination in times prior to DNA technologies makes cold case investigation difficult.
In August 1984, 35 year old Margaret and her daughter Seana, 9 were strangled whilst they lay in their beds; Seana was also sexually assaulted (Moor 2013). In 2008, a DNA sample that had previously been taken from Seana Tapp’s nightclothes was compared to profiles on the NDNAD, it was found to match Russell Gesah. He was subsequently charged (Moor 2013). After his implication in the murder it was found that the biological material was contaminated in the 1999 analysis, by clothing from an unrelated offence, which was found to contain DNA matching Russell John Gesah. This contamination occurred as a result of clothing from both the Tapp murder case and the unrelated case being examined in the same laboratory area on the same day (Moor 2013).
Previous contamination is a very possible risk in the examination of any cold case samples, so very strict procedures must be followed to ensure steps are taken to confirm any results that are produced, and to take measures to evaluate the size of the sample, to determine if it is, in fact, contamination or a genuine deposit.
A similar problem arose in the more recent, and extensively publicised Stephen Lawrence case. The defence in this case recognised the high possibility that the new DNA evidence that resulted from the 2007 cold case review, and subsequently helped to cement the convictions of Gary Dobson and David Norris was a product of contamination. It is thought that dried blood flakes had been transferred from Lawrence’s blood-stained jacket and cardigan to each of the suspect’s jacket (Dobson) and jeans (Norris) when they were all packaged unsealed in the same ‘over bag’ (Anon 2012). It is thought that, during the use of new DNA technologies in the cold case review re-examination, a flake from the bag containing Dobson’s jacket and Lawrence’s cardigan could have dissolved during subsequent saliva tests as well as being transferred onto the suspects clothing. This made it very difficult for forensic analysts to support the reliability of their findings, which were presented to support prosecution (Hughes 2011).
Conclusion
When taking into account the examples that have been presented here, be it the Cinnamon case that utilised mtDNA to identify the perpetrator responsible after 30 years, or the historic 1983 Colette Aram case which used Y-STR profiling in conjunction with familial searching. It becomes evident that advances in DNA technology have provided an invaluable premise for solving previously unsolved cold cases. This is not only in instances were perpetrators have previously been unidentified, but also in cases where they have been wrongly recognised. DNA testing also has a clear value in exonerating individuals, despite this not necessarily being the application that initially comes to mind. It has successfully changed the lives of 312 individuals so far in the US alone, and its value is being recognised more recently in the UK since the 2003 Shirley case (Innocence Project 2014; Johnson and Williams 2004). However despite the array of successful cases, as explained complications can arise when re-examining cold case evidentiary material. As a result of this, laboratory practices must be implemented to prevent problems such as the contamination of minute samples, or to allow incidences of previous contamination to be recognised. In some cases, contamination must be ruled out as the source of the implicating evidence, as this is a common defense used in court as shown in the Stephen Lawrence case.
Continued use of current technology as well as continued advances in methodologies, including identifying more ways to utilise DNA databases and technologies such as using forensic DNA phenotyping will allow further exonerations, possibly the identification of the true perpetrator and the continued closure of cold cases.

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