Module 8: Applying Chemical Ideas – Environment & Analytical Techniques (Vocabulary Flashcards)

A set of vocabulary flashcards covering environmental monitoring concepts, qualitative/quantitative analytical techniques, and common instrumental methods from the lecture notes.

Flame test

Qualitative test where metal ions produce characteristic flame colors.

Precipitation reactions

Tests where a cation forms an insoluble precipitate with reagents, guided by solubility rules.

NAG SAG

Mnemonic: Nitrates, Ammoniums, Group 1 soluble; Sulphates soluble except PMS/Castro Bear; Acetates soluble; Group 17 soluble except PMS.

PMS

Lead(II), Mercury(II), Silver(I) — common insoluble halide/sulfate exceptions.

Castro Bear

Mnemonic for Ca^{2+}, Sr^{2+}, Ba^{2+} sulfates being insoluble; used in cation identification tests.

Ba^{2+}

Apple green flame color in a flame test. Forms a white precipitate (BaSO4) with sulfate ions (SO42−). Forms a white precipitate with hydroxide (OH−) that is slightly soluble, but not amphoteric.

Ca^{2+}

Brick red flame color in a flame test. Forms a white precipitate (CaSO4) with sulfate ions (SO42−). Forms a white precipitate with hydroxide (OH−) that is sparingly soluble, but not amphoteric.

Cu^{2+}

Blue-green flame color in a flame test. Forms a pale blue precipitate (Cu(OH)2) with dilute hydroxide (OH−) or dilute ammonia (NH3). This precipitate dissolves in excess dilute ammonia to form a deep blue complex solution ([Cu(NH3)4(H2O)2]2+).

Fe^{2+}

Gold flame color in a flame test. Forms a green precipitate (Fe(OH)2) with dilute hydroxide (OH−) or dilute ammonia (NH3), which quickly turns reddish-brown on exposure to air due to oxidation.

Fe^{3+}

Orange-brown flame color in a flame test. Forms a reddish-brown precipitate (Fe(OH)3) with dilute hydroxide (OH−) or dilute ammonia (NH3).

Pb^{2+}

Blue-white flame color in a flame test. Forms a white precipitate (Pb(OH)2) with hydroxide (OH−) which dissolves to form a complex ([Pb(OH)4]2−) with excess hydroxide (amphoteric). Forms a white precipitate (PbCl2) with chloride (Cl−) (soluble in hot water) and a yellow precipitate (PbI2) with iodide (I−). Forms a white precipitate with sulfate (SO42−).

Na^{+}

Orange flame color in a flame test. Generally does not form precipitates with common reagents as all Group 1 salts are soluble (NAG SAG).

K^{+}

Purple/lilac flame color in a flame test (typically observed through cobalt blue glass to obscure any yellow Na^+ contamination). Generally does not form precipitates with common reagents as all Group 1 salts are soluble (NAG SAG).

Li^{+}

Mauve/purple flame color in a flame test. Generally does not form precipitates with common reagents as all Group 1 salts are soluble (NAG SAG).

Chloride (Cl^-)

Forms a white precipitate $$(AgCl)$Forms a white precipitate (AgCl) with acidified Ag+ solution; AgCl dissolves in dilute NH3 to form a complex. Forms a white precipitate (PbCl2) with Pb2+ (soluble in hot water).

Bromide (Br^-)

Forms a cream precipitate (AgBr) with acidified Ag^+ solution; AgBr dissolves in concentrated NH_3 to form a complex. Forms a cream precipitate with Pb^{2+}.

Iodide (I^-)

Forms a yellow precipitate (AgI) with acidified Ag^+ solution that does not dissolve in ammonia. Forms a yellow precipitate (PbI_2) with Pb^{2+}.

Hydroxide (OH^-)

Alkaline solution (pH>7). Forms precipitates with many metal cations (e.g., Cu(OH)2 blue, Fe(OH)2 green, Fe(OH)3 reddish-brown, Pb(OH)2 white amphoteric).

Acetate (CH_3COO^-)

Generally soluble (NAG SAG). Can be identified via reaction with concentrated H2SO4 to produce the smell of ethanoic acid. Also, can form a fruity-smelling ester when reacted with an alcohol in the presence of an acid catalyst (esterification).

Carbonate (CO_3)^{2-}

Reacts with dilute acids (e.g., HCl) to produce carbon dioxide gas (CO_2), which turns limewater milky. Forms a white precipitate with Ba^{2+}.

Sulfate (SO_4)^{2-}

Forms a white precipitate (BaSO4) with acidified barium chloride solution (BaCl2) that is insoluble in excess dilute acid. Forms precipitates with Pb2+(PbSO4).

Phosphate (PO_4)^{3-}

Forms a yellow precipitate with ammoniacal ammonium molybdate solution upon warming. Forms a white precipitate (Ba3(PO4)2) with Ba2+ in alkaline conditions, but not acidic conditions.

Why (PO_4)^{3-} will precipitate in alkali conditions but not acidic conditions

In acidic conditions, phosphate ions (PO43−) are protonated to H3PO4, H2PO4− or HPO42−, which are generally soluble. In alkaline conditions, the equilibrium shifts towards the unprotonated (PO43−) ion, allowing it to react with metal cations (e.g., Ba2+) to form insoluble phosphate salts.

Gravimetric analysis

Gravimetric analysis is a quantitative determination method by measuring masses of precipitates to find composition. Write the equation, find moles, mass and then percentage composition.

Assumptions and sources of error in gravimetric analysis

Assumptions in gravimetric analysis include that the precipitate is pure, completely insoluble, and fully recovered. Sources of error can include: relying on the assumption that all sulphate reacts and precipitates out, contamination from other precipitates (e.g. Ba3(PO4)2), sample could be hydrated or wet when measuring, particles can pass through the filter

Precipitation titrations

Precipitation titration uses a precipitation reaction to determine the amount of a species. It works by adding a titrant that forms an insoluble precipitate with the analyte. An indicator is used to detect the endpoint when the precipitation of the analyte is complete, typically by reacting with the titrant to form a colored precipitate or complex.

Mohr’s method

Mohr’s method is a direct precipitation titration for halides (Cl−, Br−) using Ag+ as the titrant and chromate ions (CrO42−) as an indicator. The endpoint is indicated by the formation of a reddish-brown precipitate of silver chromate (Ag2CrO4) after all the halide ions have precipitated as silver halide. It operates optimally within a pHpH range of 6.5–9.

Volhard’s method

Volhard’s method is a back titration for halides (Cl^-, Br^-, I^-). An excess, known amount of Ag^+ is added to the halide sample, precipitating AgX. The unreacted Ag^+ is then back-titrated with a standard thiocyanate (SCN^-) solution using Fe^{3+} as an indicator. The endpoint is indicated by the formation of a red FeSCN^{2+} complex. This method is highly accurate and is suitable for acidic solutions.

Fajan’s method

Fajan’s method is a direct precipitation titration for halides using Ag^+ as the titrant and an adsorption indicator (e.g., dichlorofluorescein). The indicator molecules are adsorbed onto the surface of the silver halide precipitate at the equivalence point, causing a sharp color change of the precipitate, often from white to pink or red. This method relies on the indicator's ability to be adsorbed onto the surface of the precipitate.

The method for AAS

Atomic Absorption Spectroscopy (AAS) measures concentrations by atomizing a sample (usually in a flame or graphite furnace), then passing light from a hollow cathode lamp (specific to the element of interest) through the atomized sample. Atoms in the sample absorb light at their characteristic wavelengths. The decrease in light intensity (absorbance) is measured and correlated to the concentration using a calibration curve.

Bromine water test

The bromine water test is used to detect carbon-carbon double bonds (alkenes). When bromine water (reddish-brown) is added to an alkene, an electrophilic addition reaction occurs, breaking the C=C bond and adding bromine atoms. This causes the bromine water to be decolorized (turn colorless). Alcohols (with an -OH group) will not react with bromine water.

Acidified KMnO_4 test for carbon-carbon double bond

The acidified potassium permanganate (KMnO4) test is used to detect carbon-carbon double bonds (alkenes). When acidified purple KMnO4 solution is added to an alkene, the alkene is oxidized, and the MnO4− ion is reduced, causing the purple coloration to disappear. This indicates the presence of an oxidizable group, such as a C=C bond.

Alkoxide test for -OH group

The alkoxide test for -OH groups (alcohols) involves reacting an alcohol with a highly reactive metal, typically sodium metal. This reaction produces an alkoxide and hydrogen gas: 2ROH + 2Na \rightarrow 2RONa + H_2. The evolution of hydrogen gas is an indication of the presence of an acidic hydrogen, such as that in an alcohol's -OH group.

Esterification test (for -OH group and carboxylic acid)

The esterification test detects the presence of both -OH groups (alcohols) and carboxylic acid groups. When a carboxylic acid and an alcohol are heated together in the presence of a strong acid catalyst (e.g., concentrated H2SO4), they react to form an ester (known for its characteristic sweet or fruity smell) and water.

Reaction with PCl_5 for -OH group

The reaction with PCl5PCl5 is used to detect -OH (hydroxyl) groups in alcohols. Alcohols react vigorously with phosphorus pentachloride (PCl5) at room temperature to produce chloroalkanes, phosphorus oxychloride, and hydrogen chloride gas: ROH+PCl5→RCl+POCl3+HCl. The presence of white fumes of HCl (which turn blue litmus red) indicates an -OH group.

pH test for carboxylic acid

The pH test for carboxylic acids involves measuring the pH of an aqueous solution. Carboxylic acids are weak acids and their aqueous solutions will have a pH below 7 (typically in the range of 2-5, depending on concentration and acid strength), which can be detected with pH paper or a pH meter.

Reaction with Na2CO3 for carboxylic acid

The reaction with sodium carbonate (Na2CO3) is used to detect carboxylic acids. Carboxylic acids are acidic enough to react with carbonates to produce carbon dioxide gas, water, and a salt: 2RCOOH+Na2CO3→2RCOONa+H2O+CO2. The effervescence (bubbling) of CO2 gas is a positive indication.

Components of an IR spectrometer

Source of IR radiation: a simple electrically heated tungsten filament
A sample cell and reference cell: compared to ensure the IR spectra is only for the molecule in the sample and not the solvent or container
Monochromator: select the frequencies that will be observed. They will generally scan through a wide range of frequencies

Components of the NMR instrument and what each is used for

Sample: placed in the centre of a chamber and is normally mixed with some solvent
Electromagnets: provide the magnetic fields that cause alignment, strength can be varied easily
RF Generator: produces pulses of radio waves at different frequencies, this is transmitted to an antenna that exposes them to the sample, causing nuclei to resonate
RF Detector: detects radio pulses produced by the relaxing nuclei

Features of the ^1H NMR spectra

The features of ^1H NMR spectra include:

• Number of Signals: Corresponds to the number of chemically distinct hydrogen environments.
• Chemical Shift: The position of a signal (in ppm relative to TMS) indicates the electronic environment of the protons (shielding/deshielding).
• Integration (Area Under Peak): The relative area under each signal is proportional to the number of protons in that specific chemical environment.
• Splitting (Multiplicity): Signals are split into multiple peaks by spin-spin coupling with neighboring non-equivalent protons (following the n+1 rule), indicating the number of neighboring protons.

Features of the ^{13}C NMR spectra

The features of ^{13}C NMR spectra include:

• Number of Signals: Corresponds to the number of chemically distinct carbon environments in the molecule.
• Chemical Shift: The position of a signal (in ppm relative to TMS) indicates the electronic environment of the carbon atom, with a much wider range than ^1H NMR.
• Decoupling: Commonly, ^{13}C NMR spectra are proton-decoupled, meaning signals appear as single peaks (singlets) without splitting from neighboring protons.
• No Integration: Peak areas are generally not directly proportional to the number of carbons, so integration is typically not used for quantitative analysis in routine ^{13}C NMR.