Module 8: Chemistry Applying Chemical Ideas
Why do environmental considerations need to be considered when designing a chemical synthesis process? What might be done to take this into consideration? Why?
(C) Considerations must be made waste management as many waste products are harmful to environment (YP) Waste may be toxic, explosive, radioactive or harmless. Where possible, production of harmful waste should be minimised (I) If they are not useable elsewhere, waste may be treated to convert harmful substances that can then be released into the environment (ESE) Measures must also be in place to prevent and minimise impact of chemical spills and accidents that could cause the release of harmful substances into the environment
Why do the reaction conditions need to be considered when designing a chemical synthesis process? What might be done to take this into consideration?
(I) Investigations into the optimal conditions of conducting a synthesis reaction are done to maximise yield, rate, and minimise energy expenditure and cost Conditions such as: Temperature, pressure, concentration, surface area, catalysts
Why does the yield need to be considered when designing a chemical synthesis process? What might be done to take this into consideration?
(R) Many synthesis processes involve equilibrium reactions (C) To maximise yield, factors that affect equilibrium position should be considered and adjusted to shift equilibrium to favour products eg continual removal of product
Why does the purity need to be considered when designing a chemical synthesis process? What might be done to take this into consideration?
(YP) Reactions often produce by-products alongside the desired compound and reactants may remain in the mixture, may be toxic (R) Purification processes are therefore required to isolate product (ESE) Quality assurance tests are performed on the final product to check its purity. The minimum purity level required depends on the particular product and its final use
What are the limitations of gravimetric analysis?
- Calculations assume 100% of analyte is precipitated out of solution and none is lost in filtration and transfer steps - Calculations also assume filtered residue is completely dry and no solvent remains - Risk of contaminants also precipitating out of solution so sample needs to be pure (or at least not contain any other species that will also form a precipitate with the added reagent) Assumes: Only the target ion precipitates out Improve: Selective reagent, Remove or prevent the precipitation of other salts All the target ion precipitates out Improve: Selective reagent (Highly insoluble precipitate, really small Ksp), Added in excess All of the precipitate is collected and in a pure form Improve: Filtration (quantitative filter paper), Wash precipitate with warm distilled water multiple times, Ensure dry when measuring its mass
What is Volhard's method?
- Can be used to measure a range of anions (Cl-, Br-, I-, CN-, PO43-, Cr2O72-, S2-, CO32-) via a back titration: - Anion initially precipitated out with excess silver nitrate. Depending on analyte, the mixture may be titrated as is, or precipitate is removed via filtration and then the filtrate is titrated with potassium thiocyanate giving a silver thiocyanate precipitate. - Fe3+ used as indicator, forms blood red complex with excess thiocyanate once all silver ions have been precipitated out. - Can also be used as a direct titration to measure Ag+ content of a sample. Consideration: pH must be acidic → avoid precipitation of Fe(OH)3, interferes with formation of coloured complex [FeSCN]2+ If analyte precipitate is more soluble than AgSCN (its Ksp is higher than Ksp of AgSCN), added SCN- will displace analyte from silver, and Ag+ will appear more abundant, resulting in underestimation of anion analyte. In such cases, analyte precipitate must be filtered out before titration with thiocyanate
What is the equation for beers law?
A = εlc → on data sheet A = absorbance, ε = molar absorptivity in L mol-1 cm-1 (constant for a given substance at a given wavelength of light) l = path length in cm, c = concentration in mol/L
How does infrared spectroscopy work?
A beam of IR radiation is passed through a sample of a chemical, and is absorbed by covalent bonds in the molecule (increasing vibrational energy). Vibrating bonds in molecules stretch and bend as they absorb infrared radiation The amount of energy absorbed varies according to bond length, strength and size of atoms. Can be used to identify functional groups present in a substance (qualitative analytical technique) Infrared spec is a relatively cheap and simple analysis technique
How do you use AAS to measure a substance? What are the limitations of AAS?
A calibration graph is constructed to plot absorbance against concentration using standard solutions of known concentration Concentration of an unknown sample can be determined by measuring its absorbance and using line of best fit on graph Only interpolate, not extrapolate since at higher temps, the curve does not follow a linear trend
Covalent bond
A chemical bond that involves sharing a pair of electrons between atoms in a molecule
When do complexation reactions occur? What are they?
A complexation reaction occurs when a coordination compound or complex forms. Consist of a central metal ion (typically a transition metal) coordinated to molecules or ions (polar & lone pair of electron molecules). The coordinating species are called ligands. They are typically neutrally or negatively charged. (E.g. 𝐻2𝑂, 𝑁𝐻3, 𝐶𝑙 −, 𝐶𝑁 −)
Pros and cons of gravimetric analysis
Adv Very accurate, if gravimetric analysis is carefully designed (assumptions) and conducted. - Does not require speciality equipment - Does not require construction of a calibration curve, which can be time consuming Cons Target ion of interest must be able to form a sufficiently insoluble precipitate for accurate results (A limitation of the technique) - The precipitate must be highly pure (Impurities can be absorbed or trapped in precipitate, increased mass and inaccurate results) - It is extremely time consuming compared to modern analytical techniques - Concentration of the target ion must be high enough to produce a precipitate that can be accurately weighed.
What are the advantages and disadvantages of AAS?
Advantages - Simple and fast procedure - Very sensitive (detects down to ppb amounts) - Very accurate - Highly specific (result is not affected by other components in mixture) - Only a small amount of samples needed (but technique is destructive) - Relatively inexpensive when compared with more advanced techniques Disadvantages - Can only be used to analyse metals and a select few non-metals - High initial cost for instrument set up - Many instruments can only test for one element at a time (though some can analyse up to 4 at a time)
What are the advantages and disadvantages of Mass spectrometry
Advantages Highly sensitive technique, Can detect compounds in low concentrations (ppm or ppt), Small sample size needed (microlitre), High resolution MS can distinguish between compounds of similar molecular weight (accurate to m/z +- 0.001) masses are different. Disadvantages Cost (expensive to buy, to operate (cleaning is expensive) and maintenance is costly), Sample reaching the mass analyser and detector must be pure, Contamination will mask the specific fragmentation pattern due to the analyte. Therefore a separation instrument (liquid/gas chromatography, maps compound with time, so the graph can be used to map analysis with time the compound passed through the MS) is usually coupled to the mass spectrometer, adding cost.
How can alkenes/alkynes be identified?
Alkenes and alkynes undergo addition reactions with halogens Bromine water with the yellow to brown colour can be used to identify the presence of carbon-carbon double or triple bonds, as a colour change occurs only if the alkene is in excess
How can alkenes be distinguished from alkanes?
Alkenes will undergo oxidation Addition of acidified potassium permanganate can distinguish alkenes from alkanes Potassium permanganate has a deep purple colour which will turn colourless or form a brown precipitate as the permanganate is converted to manganese ions
Why is AAS still used?
Although more advanced instruments now exist, AAS is still widely used due to its lower cost and robustness
Describe the acceleration stage of mass spectrometry
An electric field accelerates the particle along the path to the mass analyser Constant vacuum must be maintained inside the mass spectrometer as any air particles will interfere with the path of the ions
What happens in UV Vis?
As with colourimetry, a sample is placed in the path of the light beam and a detector on the other side measures the amount of transmitted light and the instrument calculates absorbance
How do flame tests work?
Atom can be excited through heating. The main coloured light emitted is the average of the emitted protons Spectra produced
What are factors to be considered when designing a chemical synthesis process?
Availability of reagents (R) Reaction conditions (C) Yield and purity (YP) Industrial uses (I) Environmental, social and economic issues (ESE)
What are errors caused by colourimetry?
Beer's law does not hold true at very high concentrations, so a calibration curve may not be linear if the upper concentration limit is too high. If not coloured, not as coloured The more dilute a solution the less coloured.
What is signal intensity in Proton NMR? What info does it give?
Can be used to determine number of protons in a specific environment The height of the peaks can give some indication of area under the peak, but this can be complicated by differing splitting patterns. NMR analysis software quantitatively measures the area under the curve and ratios between areas of each signal are used to determine the number of protons in each environment. The height of the integral curves are proportional to the number of protons in that environment The example shown gives a ratio of 1:2:3 which corresponds to the number of protons in those distinct environments of propanoic acid
Outline the parts of an Atomic Absorption Spectrometer
Capillary tube - Draws up aqueous solution of sample into the nebuliser. Flame - Atomises the sample Hollow cathode lamp - Shines specific wavelengths of light through the gaseous sample. Monochromator - Filters light and selects the particular wavelength of interest to pass through to the detector. Detector - Measures the intensity of the incoming light and relays data to analysis software.
How do you test for carboxylic acids?
Carboxylic acids are weak acids blue litmus test for acidity, if red it is acidic Acids and carbonates react to form a salt, water and carbon dioxide gas. Sodium carbonate solution can be used to test for carboxylic acids, a positive result is recorded if effervescence is observed. Additionally the gas can be confirmed to be carbon dioxide by bubbling through limewater
Give 2 case studies on the factors that need to be considered when designing a chemical synthesis process
Case study: Solvay process A major waste product in the production of sodium carbonate (via the Solvay Process) is calcium chloride. If not recovered for other industries, can be safely disposed of into the ocean (salty already), but not into freshwater since it could increase salinity of water and soil and kill organisms living in area. Case Study: Haber process Cooling towers use flowing cold water to cool reaction vessels Resulting heated water shouldn't be released directly into bodies of water, as thermal pollution may affect wildlife. Hot water must be first directed into holding reservoirs to cool before being released into natural water bodies.
How can you determine transition metal cations?
Complexation reactions are commonly used to identify transition metal cations (due to their d orbitals). - This includes 𝐴𝑔 +, 𝐶𝑢 2+, 𝐹𝑒 2+, 𝐹𝑒 3+ and 𝑃𝑏 2+
What process could be used to determine anion and cation present unknown aqueous solutions of single ionic compounds?
Construct a flow chart based on expected precipattes after the addition of certain ions
How do complexation identify ions?
Coordination complexes are usually coloured and are indicated by the formula being enclosed in square brackets. E.g. [𝐶𝑢(𝑂𝐻2 )6 ] 2+ and [𝐶𝑢𝐶𝑙4 ] 2−.
Describe the detection stage of mass spectrometry
Detector used to measure the intensity of the ion beam signal it receives. This is converted to a measure of abundance for each m/z detected
Give a gravimetric analysis experiment done in class
Determine the concentration of lead ions in a water sample using gravimetric analysis Materials: ~50 mL of sample of water to be analysed, Potassium iodide solution, 25 mL pipette and filler, 2 x 100 mL beakers, Filter paper, Filter funnel, Wash bottle, Electronic balance Method: 1. Rinse pipette with water sample and transfer 25ml to a clean beaker, 2. Add potassium iodide to the sample until no more precipitate forms 3. Accurately weigh a piece of filter paper and fold into a filter funnel. 4. Pour the solution including the lead iodide precipitate through the funnel. 5. Rinse precipitate with a small amount of solution 6. Allow precipitation to dry 7. Weigh filter paper with precipitate Results: Weight of filter paper: 0.86g Weight of filter paper with precipitate: 1.47 Calculations: 1.47 - 0.86 = 0.61g of precipitate, n = c/v, n = m/MM = .61/(207.2 +2(126.9)) = .00132 mol. n(Pb2+) = 0.00132 mol, c(Pb2+) = n/v = .00132/0.02500L = 0.0528M Discussion: Discuss the errors associated with this method and how these may have affected your result Measurement, parallax, calibration
What is Atomic Absorption Spectroscopy
Developed by Australian scientist Sir Alan Walsh and his team at CSIRO in 1950s quantitative determination of chemical elements using the absorption of optical radiation (light) by free atoms in the gaseous state. Atomic absorption spectroscopy is based on absorption of light by free metallic ions. Very sensitive and specific technique I.e it can detect trace amounts (ppm levels) of a particular element (most commonly metal)
How do you calculate absorbance?
Digital colorimeters convert the incident and transmitted light intensity data into an absorbance value Log10 (I₀/I) = A → in HSC data sheet Where I₀ is incident light intensity, I is the transmitted light intensity and A is absorbance (unitless)
What is Mohr's method?
Direct titration of a solution containing Br-, or CN- with silver nitrate and chromate(CrO4 2-) Silver preferentially precipitates with halide ions When excess silver is added, it will react with chromate ions to produce a reddish-brown precipitate 𝐴𝑔2𝐶𝑟𝑂4(𝑠), the endpoint of the titration is the point where a permanent colour change is observed (middle flask) Considerations: Adding Ag+ to CrO4 2- doesn't give an immediate colour change. Excess of Ag+ is required for the precipitate to from, excess dependent on the concentration of CrO4 2- in solution A "blank" titration must be conducted to determine excess volume of Ag+ required and the same amount of CrO4 2- must be used in each solution pH must also be controlled (pH 6-9) Too alkaline → AgOH precipitate forms instead Too acidic → Chromate ion is converted to chromic acid (H2CrO4) so insufficient chromate is available to react with the excess silver ions
What does each signal in a 13C NMR graph represent?
Each signal in at 13C NMR spectrum represents either a single carbon or a number of carbons in equivalent chemical environments Multiple carbon nuclei in a molecule can be in the same chemical environment due to molecular symmetry
What is chemical shift in 13C NMR?
Electrons around each nucleus make the nuclei partially 'shielded' from the external magnetic field. Causes variation in the resonance frequency of the nuclei and results in chemical shift (δ) in the spectrum.
Coloured complex example (cobalt)
Equilibrium between hydrated and dehydrated cobalt(II) chloride
What are conditions needed for the Haber Process?
Exothermic → cold gas needs to be pumped through, but not too cold so that according to collision theory, it forms in a reasonable time Pressure, but not too pressured because expensive Iron Catalyst, speed up getting to equilibrium Monitoring products - Increasing the amount of reactants or reducing the amount of product will favour an equilibrium to the right, The extraction of liquid ammonia is used to prevent this. Monitoring reactants - Using exact molar ratio in the equation means that catalyst is more efficient as the right proportion of molecules and therefore space in the reactor is not wasted. Monitoring the volume of nitrogen and hydrogen gas to ensure that the mixture is a ratio of 1:3 Monitoring products
Coloured complex example (iron)
Fe2+ and Fe3+ also form coloured complexes with water molecules as ligands - hexaaquairon(II) is green, hexaaquairon(III) is yellow Thus, complexation reactions with qualitative colour change can be used to identify some metal cations
Why does the availability of reagents need to be considered when designing a chemical synthesis process? What might be done to take this into consideration?
For large scale industrial processes, the raw materials required must be readily available (C) Includes considerations of purity ease of transport and cost of reagents (YP) Chemical plants may be close to a source of a particular raw material to reduce transport costs and other issues (eg environmental and safety concerns of transporting the material)
What is on the x axis of a 13C NMR graph? What is it dependent on?
Frequency Dependent on the strength of the external magnetic field (which can be changed). Thus, frequency is converted to ppm which is independent of the strength of the magnetic field and spectra can be compared across measurements made with differing magnetic field strengths.
How does a colourimeter function?
Function by passing a beam of light at a specified wavelength through a sample of the solution and measuring the amount of light that reaches the detector on the other side Light filter chosen for analysis should correspond to wavelength that the sample absorbs most strongly (colour complementary to colour of solution on colour wheel) Eg. for the analysis of copper sulfate solutions, 635 nm is appropriate (orange is complementary to blue). Digital colorimeters convert the incident and transmitted light intensity data into an absorbance value
What does IR Spectra show?
IR spectra shows % transmittance. Troughs, upside down peaks, are observed wherever the molecule absorbs energy By convention, the unit for energy is the wavenumber cm-1. The usual range on the x axis is 4000-400cm-1
Qualitative analysis
Identification of the constituents of components in a sample.
What is peak splitting?
In proton NMR, additional information is given by peak splitting patterns and signal intensity. Signals in PNMR spectra can exhibit 'splitting patterns depending on the number of hydrogen atoms that are present on neighbouring carbon atoms The number of peaks can be predicted using n+1 Eg. One neighbouring proton means the peak will be a doublet
What is Fajan's method?
In the analysis of chloride, an anionic dye is added to the sample. - A standard solution of AgNO3 is added and the precipitation occurs: 𝐴𝑔+ + 𝐶𝑙− →𝐴𝑔𝐶𝑙 - Before equivalence point, a small excess of chloride ions is incorporated into the precipitate, so it carries a negative charge. Dye is repelled and remains in solution. - After EP, small excess of silver ions. So precipitate becomes slightly positively charged, and the dye adsorbs onto it, resulting in a precipitate colour change. The most suitable indicator depends on the analyte.
Why do social and economic issues need to be considered when designing a chemical synthesis process? What might be done to take this into consideration?
Industrial processes are usually commercial, so reasonable profit needed, otherwise it wouldn't be done. In some cases, such as in the pharmaceutical or defence industries, governments may subsidise certain processes using tax proceeds. Many chemical syntheses are performed to produce chemicals that can replace others that are no longer naturally available or economically viable.
Precipitate
Insoluble ionic salt formed when 2 ionic solutions are mixed together.
What is mass spectrometry?
Involves ionisation (and usually the fragmentation) of analyte molecules, which can then be separated using a mass analyser based on their mass/charge (m/z) rations Relative abundance of each ion fragment detected ad the software output gives a spectrum of (m/z) ratios This spectrum can be used to identify the molecular weight of the analyte, and any molecule fragments which can help with determining the structure of the analyte molecule
Briefly outline the steps in mass spectrometry
Ionisation, acceleration, deflection, detection, interpreting spectra
Describe the deflection stage of mass spectrometry
Magnet in the mass analyser focuses one beam of particles at a time, based on their mass-to-charge (m/z) ratio. We assume that most particles will have a charge of +1, so their m/z is equivalent to their mass. Particles that are too light will deflect too far, and particles that are too heavy will not deflect far enough. These will collide with the sides of the chamber, pick up an electron and become neutralised. These neutralised particles will be sucked out by vacuum
AAS vs UV Vis
Main point of difference for UV-vis is that it measures the transmittance of light as a function of wavelength (ie. it produces a spectrum showing the absorbance of the sample across the UV-vis range)
Describe the interpreting spectra stage of mass spectrometry
Mass spectrum shows relative abundance % against m/z Two key features of a mass spectrum are the molecular ion peak and the base peak
Quantitative analysis
Measurement/determination of the amount (expressed in a concentration) of the constituent.
How do isotopes impact mass spectrometry
Much shorter peak is often seen at 1m/z higher than the molecular ion peak → due to a small number of molecules containing a carbon-13 isotope Other atoms have high abundances of more than one isotope namely Br and Cl. Causes a pair of peaks two m/z apart in a fragment containing the Br or Cl atom Approximate 50% abundance of each of Br-79 and Br-81 so peak pairs with Br will be almost equal in height. Cl-35 has an approx. abundance of 75% and 25% is Cl-37, so the smaller m/z peak will be roughly three times the height of the other. Fragmentation patterns become more complex with larger molecules
What are the limitations of Volhard's method?
Must be performed in low pH to prevent precipitation of Fe3+ indicator as Fe(OH)3. Less valid if the first precipitate formed is more soluble than AgSCN. For example, AgCl (𝐾𝑠𝑝 = 1.77 × 10−10) is more soluble than the AgSCN (𝐾𝑠𝑝 = 1.03 × 10−12). This means that AgCl can dissolve during the titration leading to a greater titration volume required. To avoid this, procedure can be modified. - First precipitate can be removed via filtration before titration. Can lead to inaccuracies - Dense organic liquid like nitrobenzene or chloroform can be added to conical flask to act as a barrier between precipitate and aqueous layer.
Why is it necessary to monitor the environment?
Needed to ensure the health and wellbeing of both plants and animals. Allows for more effective management. Eg. destruction of UV-protective ozone, caused by chlorofluorocarbons (CFCs) Hazardous ions eg. mercury have also been identified and monitored using chemical techniques. Nutrients required by humans and other living organisms have also been monitored
Explain spin flip
Nuclei parallel to the external MF→ lower energy state Nuclei antiparallel → higher energy state Nuclei at the lower energy state can absorb energy (between 25-1000 MHz in the radio wave region of the EM spectra) and flip to the higher energy state The frequency at which this flip occurs is the resonance frequency of the nuclei. There is effectively an absorption of energy that occurs when the flip occurs which is the basis for nuclear magnetic resonance spectroscopy
What does the position of peaks on the 13C NMR spectrum mean?
Peaks at lower shift (closer to the TMS standard) → upfield and are most shielded. Peaks of higher shift (away from TMS) → deshielded and appear further downfield.
How can 1º and 2º alcohols be distinguished?
Primary alcohols readily oxidise to aldehydes (and subsequently carboxylic acids) secondary alcohols readily oxidise to ketones Acidified potassium permanganate will decolourise in the presence of 1° and 2° alcohols. Acidified dichromate solution can also be used - colour will change from orange to greenish blue as Cr2O72- is reduced to Cr3+
What is spin in an atom?
Protons and neutrons have angular momentum (spin), Nuclei are charged (contain protons), Nuclei containing an odd number of protons or neutrons or both have an overall spin state Results in magnetic behaviour and when placed in an external magnetic field, the nuclei align parallel or antiparallel to the field
How does UV Vis qualitatively and quantitatively analyse?
Qualitatively - confirms identity of substance by comparing its spectrum to spectrum of pure sample of compound Quantitative - analysis of a known substance: wavelength is set to the λmax of the species (the wavelength it absorbs most strongly) and standard solutions of varying concentrations are run in order to construct a calibration graph. Sample of unknown concentration is also run and concentration determined using its measured absorbance at λmax and the calibration graph.
What are the pros and cons of Fajans method?
Relies on a large precipitate surface area, to allow sufficient dye to adsorb so colour change is easily visible. - Dextrin (starch) can be added to prevent silver halide precipitates from aggregating - High concentration of spectator ions can cause aggregation, so method cannot be used with all samples. - The analyte must be concentrated enough, colour will not be seen if there is too little precipitate. pH of titration reaction must be controlled - anionic dye indicators are conjugate bases of weak acids. So will react with free H+ and will not adsorb onto the surface of the precipitate.
What ion is commonly used in precipitation reactions?
Silver cation Equivalence point is reached when all of the anion has been precipitated out of a solution. Difficult to observe with precipitation reactions Indicator used, one that contains an anion that also precipitates with silver cation, producing distinct colour Ag+ must preferentially precipitate with the ion of interest, so it does not form a coloured precipitate with the indicator before all analyte ion is precipitated out of solution.
What is proton NMR?
Similar principles of distinct chemical environments and degree of shielding
What is used to identify organic compounds?
Simple chemical tests can be used to help identify organic compounds by exploiting the reactivity of the functional groups they contain. In course: distinguish between simple alkanes, alkenes, alcohols and carboxylic acids.
How can you test for hydroxyl groups?
Sodium metal can also be used to test for hydroxyl groups Adding sodium to an alcohol results in a sodium alkoxide (RO-Na+) and H2 gas (which can be tested for with the pop test) This test identifies the presence of hydroxyl groups, it also gives a positive result with carboxylic acids and water (so samples must be dry) Sodium does not react with alkanes or alkenes Phenolphthalein added to the dissolved sodium solution, to check alkalinity. As alkoxides are strong bases, so phenolphthalein checks the resulting solution.
What are the solubility rules?
Soluble: All Group 1 (Li+ , Na+ , K+ , Rb+ , Cs+ ) and ammonium (NH4 + ) are soluble All nitrate (NO3 - ) and acetate (CH3COO- ) salts are soluble All chloride (Cl- ) and bromide (Br- ) salts are soluble (EXCEPT Ag+ , Pb+ Hg+) All iodide (I- ) salts are soluble (EXCEPT Ag+ , Pb+ , Hg+) All sulfate (SO4 2- ) salts are soluble (EXCEPT Ag+ , Pb2+, Ba2+, Ca2+, Sr2+, Hg+) Insoluble All hydroxide (OH- ) salts are insoluble (EXCEPT Group 1 and NH4 + salts are soluble. Ba2+, Ca2+ are slightly All carbonate (CO3 2- ) and phosphate (PO4 3- ) are insoluble (EXCEPT Group 1 and NH4 + salts are soluble)
What are the problems of mass spectrometry
Some compounds are more easily fragmented than others. A molecule that is not readily fragmented may only show a molecular ion peak, or the molecular ion peak is also the base peak and only a few small fragment peaks are seen in the spectrum. Some compounds are too easily fragmented Molecular ion is not observed (can adjust instrument settings to try avoid this)
Why do the industrial uses need to be considered when designing a chemical synthesis process? What might be done to take this into consideration?
Sometimes by-products and waste of one synthesis reaction are the reactants in another (R) Plants may be located in close proximity to each other in order to utilise useful by-products and minimise transport costs Many compounds have uses in a variety of industries eg. sulfuric acid in fertilisers, metal processing, manufacturing rayon, paper and batteries
What are the steps of a gravimetric analysis?
Steps 1. Accurately weigh substance and dissolve/accurately measure volume of solution 2. Add excess reactant that will form a precipitate with only the ion of interest 3. Filter the precipitate and wash with small amount of solvent 4. Dry precipitate to remove all solvent and weigh 5. Conduct calculations to determine analyte content
When is TMS suitable for use? Why is it suitable?
Suitable as a standard for both 13C and 1H NMR since it has only one carbon and hydrogen environment but 4 carbons and 12 hydrogens, giving a strong, single peak in either spectrum. Low electronegativity of silicon→ means the nuclei are highly shielded and so the carbon and hydrogen nuclei in most organic compounds will have shifts appearing to the left of the TMS peak. The TMS signal is often subtracted from the spectrum during post-processing and so it is not always present in a given spectra.
Destructive testing
Test that irreversibly alter the composition of a sample.
What is TMS standard?
Tetramethylsilane (Si(CH3)4) (TMS) - often used as a reference standard Standard is added to the sample compound prior to analysis and shows up as a single peak in the spectrum. The x axis is aligned so the TMS is at a shift of 0ppm
What are the key features of a mass spectrometry graph?
The (parent) molecular ion peak corresponds to the heaviest ion and represents the ionised analyte molecule (M+•). Furthest line to the right that isn't tiny Useful as gives the molecular weight of the compound Base peak→ Tallest peak and represent the most abundant fragment It is given a relative abundance of 100% and all other peak abundances are relative to the base peak When a clear fragmentation pattern occurs, abundant fragments and the difference in m/z between the peaks can be used to get information about the molecules
What does electron density do to the nucleus?
The greater the electron density around a nucleus, the stronger the shielding effects of the electrons, and the more energy is required to flip spin states.
What is deshielding? What does this mean 13C NMR?
The presence of electron withdrawing groups, such as oxygen, have deshielding effects as the electrons are pulled away from the carbon nucleus. Therefore peaks due to these carbons will appear further downfield as less energy is required to achieve resonance.
What are the limitations of Mohr's method?
The presence of multiple anions in a sample can lead to invalid results. Carbonate and phosphate ions will precipitate with silver. The conical flask must be thoroughly stirred during the titration to ensure the Ag+ ions are evenly distributed, so AgCrO4 will not precipitate early. The titration must be conducted at a pH 6.5-9.0 Not suitable for iodide or thiocyanate ions. Excess titrant required to cause a colour change is relatively large, as the concentration of the indicator used is quite low Excess silver required to reach the end point leads to a systematic error, which can be corrected using a blank titration.
Give an experiment investigating colourimetry?
To determine the concentration of CuSO4 in a solution using colourimetry. Method: Make 0.50 M stock standard solution of CuSO4. Use stock and distilled water to make up 10mL of further standard solutions at 0.10, 0.20, 0.30, and 0.40 M using a measuring cylinder 'Zero' the colourimeter using a cuvette filled with distilled water. Use 0.50 M standard solution to determine which coloured light gives the highest absorbance reading for the solution. Once filter is chosen, take absorbance readings for each standard solution and unknown sol. (Rinse cuvette with solution to be tested before filling to take a reading). Construct a calibration curve using data from standard solutions and use to determine the concentration of the unknown solution.
Coordinate covalent bond
Type of covalent bond where both electrons are donated by the same atom
Precipitation titration
Type of titration where a precipitation reaction is used to determine amount of analyte present
What is UV-vis?
Ultraviolet-visible (UV-Vis) spectrophotometry measures the transmittance of light as a function of wavelength (ie. it produces a spectrum showing the absorbance of the sample across the UV-vis range) essentially the atoms will
How can you determine an unknown ion?
Use solubility rules and precipitation rules
Describe the parts of a mass spectrometer
Vary in their mode of ionisation, also the type of mass analyser used to focus the fragments.
Describe the ionisation stage of mass spectrometry
When a vaporised sample is injected into the ionisation chamber, the particles are bombarded with high energy electrons. These electrons knock an electron (or more than one electron) off the molecules, resulting in positively charged particles If only one electron is lost from the molecule, it now has an unpaired electron and the molecule is called a radical denoted by a "•"
What do you do when molar absorptivity of the species is unknown, in terms of a colourimeter?
When molar absorptivity of the species is unknown, standard solutions of known concentration can be used to construct a calibration curve. Calibration curve can then be used to determine concentration of an unknown sample of the same substance based on its absorbance. Beer's law does not hold true at very high concentrations, so a calibration curve may not be linear if the upper concentration limit is too high.
Bromine water
a dilute solution of bromine with a yellow to brown colour
Colourimetry
an instrumental analysis technique used to determine concentration of coloured compounds in solution, using a colorimeter
What are the precipitates and solutions made from: barium (Ba2+), calcium (Ca2+), magnesium (Mg2+), lead(II) (Pb2+), silver ion (Ag+), copper(II) (Cu2+), iron(II) (Fe2+), iron(III) (Fe3+), chloride (Cl-), bromide (Br-), iodide (I-), hydroxide (OH-), acetate (CH3COO-), carbonate (CO32-), sulfate (SO42-), phosphate (PO43-)
and ammonia forms white precipitate with silver
What is the axis range of Proton NMR
around 0-12 ppm.
Give the flame test colours for barium (Ba2+), calcium (Ca2+), magnesium (Mg2+), lead(II) (Pb2+), silver ion (Ag+), copper(II) (Cu2+), iron(II) (Fe2+), iron(III) (Fe3+), chloride (Cl-), bromide (Br-), iodide (I-), hydroxide (OH-), acetate (CH3COO-), carbonate (CO32-), sulfate (SO42-), phosphate (PO43-)
barium (Ba2+) = apple green, calcium (Ca2+)=orange, magnesium (Mg2+)=none (intense white light), lead(II) (Pb2+)=light blue, silver ion (Ag+) = none, copper(II) (Cu2+) = green, iron(II) (Fe2+) = gold, iron(III) (Fe3+)=orange-brown [chloride (Cl-), bromide (Br-), iodide (I-), hydroxide (OH-), acetate (CH3COO-), carbonate (CO32-), sulfate (SO42-), phosphate (PO43-)] = none
Complex ions
consist of a central cation -usually a transition metal- surrounded by anions or polar molecules called ligands.
Beers law
the absorbance of a solution is directly proportional to it concentration
What is the axis range of 13C NMR
typically 0-220 ppm,
