HSC Chemistry Module 9.4 Summary

HSC Chemistry Module 9.4 Summary

Much of the work of chemists involves monitoring the reactants and products of reactions and managing reaction conditions

Outline the role of a chemist employed in a named industry or enterprise, identifying the branch of chemistry undertaken by the chemist and explaining a chemical principle that the chemist uses

NAME

Robert Evans

EMPLOYER

Government agency – Department of Mines and Energy

BRANCH OF CHEMISTRY

Analytical chemistry => inorganic chemistry in environmental monitoring

DUTIES

Fieldwork collection of water and soil samples from mining sites and mine rehabilitation areas
Coordinate the duties of technicians within the laboratory
Maintain analytical standards of the laboratory to Australian standards specifications so that results can be compared between laboratories
Preparing reports on the samples for both private groups and other government agencies (e.g. EPA, Department of Land and Water Conservation) and making recommendations
Liaise with community groups, local councils and mine managers on managing contamination of soil by runoff and groundwater near mine sites

ANALYTICAL METHODS USED

Flame Atomic Absorption Spectrometry (AAS) for heavy metal analysis

CHEMICAL PRINCIPLE

Many ores contain metal sulfides, which are very insoluble and in this form do not pose a problem
When exposed to air and water the metal sulfides oxidise to sulfates. The solubility of sulfates is much greater than for sulfides, so heavy metals can enter ground water as metal sulfate salts.

Maintaining acidity of mullock or slag heaps helps to prevent the mobilisation of heavy metals into ground water, so the acidity of the ores must be monitored

Gather, process and present information from secondary sources about the work of practising scientists identifying:

the variety of chemical occupations
a specific chemical occupation for a more detailed study

VARIETY OF CHEMICAL OCCUPATIONS

Analytical chemistry
Bio-molecular chemistry
Colloid and surface science chemistry
Environmental chemistry
Industrial chemistry
Inorganic chemistry
Electrochemistry
Organic chemistry
Physical chemistry (the study of physical aspects of compounds and reactions, such as rates of reactions, energy aspects, and the structures of compounds)
Polymer chemistry

ENVIRONMENTAL CHEMISTRY

An environmental chemists deals with the following jobs:
Reviewing the operation of effluent water treatments systems, and ensuring compliance with government environmental regulations
Reviewing industry’s compliance with government environmental noise standards
Assessing levels of potential contamination in wastes (e.g. soil) intended for landfill disposal and classifying them in accordance with government guidelines
Managing disposal of contaminated wastes
Investigating reports of contamination in soil or groundwater to determine source and then arranging to correct it
Determining whether gas stack emissions contain unacceptable levels of regulated materials
Advising engineers and managers of corrective actions needed if any of the above parameters show faults in systems
Answering public or professional enquiries or complaints regarding environmental performance

Identify the need for collaboration between chemists as they collect and analyse data

As can be seen above, chemistry is a broad field of knowledge with many specialist areas
In real life however, many chemical problems require expertise and in-depth knowledge from a wide range of chemical branches
Hence collaboration between chemists is essential for solving chemical issues, or when dealing with large amounts of collected data, as the chemists provide input and expertise from their own particular field to solve problems
As chemists often work in teams, collaboration and communication is required to collectively benefit the team as they collect and analyse data

Describe an example of a chemical reaction such as combustion, where reactants form different products under different conditions and thus would need monitoring

COMBUSTION

Combustion is an example of a reaction that form different products under different conditions
Consider the combustion of a simple hydrocarbon, octane (C8H18)
In an environment where the oxygen to fuel ratio is high, octane combusts completely, producing only carbon dioxide and water

In an environment which has an insufficient oxygen to fuel ratio (i.e. oxygen is the limiting reagent), octane combusts incompletely, which liberates less energy and can form a range of different products, such as carbon, carbon monoxide, carbon dioxide and water

MONITORING

As can be seen, combustion reactions can produce different products under different conditions, such as carbon monoxide and carbon
Carbon monoxide is a toxic gas, as it can cause respiratory and circulatory problems if inhaled
Carbon is carcinogenic to humans, and can be irritating to the lungs
Incomplete combustion also decreases fuel efficiency, as it results in a decreased energy yield from the fuel
Thus in most situations (such the combustion of octane in car engines), the oxygen-to-fuel ratio needs to be monitored to reduce production of toxic gases and to increase the efficiency of the fuel

Chemical processes in industry require monitoring and management to maximise production

Identify and describe the industrial uses of ammonia

Ammonia (NH3) is a gas that has many industrial uses, including:
Solid and liquid fertilisers (sulfates of ammonia, ammonium nitrate, urea)
Nitric acid (through the Ostwald Process)
Industrial production of detergents and cleaners
Fibres and plastics (rayon, acrylics, nylon)
Production of explosives
Pharmaceuticals

Identify that ammonia can be synthesised from its component gases, nitrogen and hydrogen

Under pressure and heat, nitrogen gas (N2) and hydrogen gas (H2) can be synthesised to produce ammonia (NH3) according to the reaction below

Describe that synthesis of ammonia occurs as a reversible reaction that will reach equilibrium

The industrial production of ammonia is called the Haber Process
As can be seen above, the synthesis of ammonia from nitrogen and hydrogen gas is a reversible reaction, thus the reaction will reach a point of equilibrium rather than going to completion

Identify the reaction of hydrogen with nitrogen as exothermic

The synthesis of ammonia in the Haber Process is shown below

As can be seen from the chemical equation, ΔH is less than zero, hence the reaction is an exothermic reaction

Explain that the use of a catalyst will lower the reaction temperature required and identify the catalyst(s) used in the Haber process

The activation energy of a reaction is the minimum input energy necessary for a reaction to proceed
The addition of a catalyst reduces the activation energy required for a reaction to proceed, which reduces the reaction temperature required
For the Haber process, the catalyst used is Fe3O4 (iron oxide)
Iron oxide is finely ground to produce a larger surface area, and has its surface partly reduced to elemental iron
The addition of a catalyst to the reaction vessel does not alter the position of the final equilibrium, but allows the equilibrium to be reached faster
The energy profile diagram for the Haber process is shown below. As can be seen, the activation energy for the catalysed reaction is less than that of the non-catalysed reaction

Explain why the rate of reaction is increased by higher temperatures

The rate of reaction refers to how fast a reaction proceeds
The rate of reaction increases at higher temperatures for two reasons:
As temperature increases, the kinetic energy of the particles increases, so the particles move faster. This increases the probability of collisions between particles, hence the reaction rate increases
At higher temperatures, more particles have the required activation energy, thus the probability of colliding particles have the necessary activation energy for the reaction to take place
For reversible reactions such as the Haber process, the rates of reaction of both the forward and reverse reactions increase at higher temperatures. Thus the equilibrium is reached faster, but the yield of product is not increased.

Explain why the yield of product in the Haber process is reduced at higher temperatures using Le Chatelier’s principle

For industrial reversible reactions, the yield of product describes how much product is formed at the point of equilibrium (it has nothing to do with the rate of reaction)
As the Haber process is an exothermic reaction, the effect of increased temperatures can be predicted using Le Chatelier’s principle
If the temperature of the reaction vessel is increased, the equilibrium will favour the reverse reaction since it is endothermic, which will minimise the change to the system
As the reverse reaction is favoured at higher temperatures, the yield of product (i.e. ammonia) is decreased

Analyse the impact of increased pressure on the system involved in the Haber process

In the synthesis of ammonia, there are four moles of gas of reactants (one mole N2, three moles H2) for every two moles of NH3
According to Le Chatelier’s principle, increasing the pressure of the reaction vessel in the Haber process will favour the forward reaction, as this reduces pressure by decreasing the number of moles of gas in the system.
Thus increasing the pressure on the system in the Haber process will increase the yield of ammonia
In addition, higher pressures also increase the rate of reaction, as the gas molecules are closer together and at higher concentrations

Explain why the Haber process is based on a delicate balancing act involving reaction energy, reaction rate and equilibrium

The Haber process is the most economical when yield and reaction rate are maximised, but costs are minimised
These factors conflict with each other, as shown in the table below

COST / REACTION RATE / YIELD
PRESSURE / Low pressure / High pressure / High pressure
TEMPERATURE / Low temperature / High temperature / Low temperature

Thus the Haber process is a delicate balancing act involving reaction energy, reaction rate and equilibrium, and requires a set of compromise conditions

COMPROMISE CONDITIONS

Temperature: Higher temperatures (reaction energy) will produce ammonia faster (higher reaction rate), but lower temperatures will produce a higher yield of ammonia (equilibrium consideration). Hence a compromise temperature of around 500°C is used, together with the iron/iron-oxide catalyst
Pressure: Increased pressure will produce more ammonia and increase the reaction rate, but is expensive to build and maintain high-pressure equipment. Thus a compromised pressure of 250-350atm is used
Products: Ammonia is continuously liquefied and removed as it is produced. This reduces the concentration of ammonia, and hence encourages the forward reaction according to Le Chatelier’s principle

Explain why the monitoring of the reaction vessel used in the Haber process is crucial and discuss the monitoring required

The reaction vessel used in the Haber process is monitored to ensure its safe, economical operation. Both the reaction conditions and reactants are continuously monitored.

MONITORING OF REACTION CONDITIONS

Chemical engineers or technicians monitor the reaction vessel to ensure that the appropriate temperature and pressure conditions are maintained within an acceptable range, so that around 30% yield is achieved
Temperature: The temperature of the vessel must be monitored so that it is kept within an acceptable range (around 500°C) due to the yield and rate considerations. In addition, the iron catalyst will melt and be permanently damaged at excessively high temperatures
Pressure: The pressure of the vessel must be monitored for yield and rate considerations. Also, the operation of the Haber process will become extremely dangerous at excessively high pressures, as the reaction vessel may explode

MONITORING OF REACTANTS

Ratio of reactants: The ratio of the incoming gases has to be monitored so that the ratio of N2 to H2 is maintained at a constant 1:3, as a build-up of any gas may dangerously increase the pressure within the reaction vessel
Contaminants: The concentration of contaminants must be monitored for various reasons
O2 must be completely removed, as it introduces the risk of an explosive reaction with H2
CO, CO2 and sulfur compounds must be at very low levels, as they may damage the surface of the catalyst
Argon and methane must be kept at low concentrations, as they lower the efficiency of the reaction
Catalyst: The quality of the surface of the iron catalyst must be monitored to ensure the efficient absorption of N2 and H2 molecules

Gather and process information from secondary sources to describe the conditions under which Haber developed the industrial synthesis of ammonia and evaluate its significance at that time in world history

Before the development of the Haber process in 1913, the primary source of nitrates for fertilisers and explosives came from the mineral saltpetre from Chile
Available supplies of nitrate minerals were dwindling in the early 20th century, whilst the global population boomed => this placed a greater need on developing a synthetic method of producing fertilisers and explosives on a commercial and industrial scale
Haber developed the process for ammonia production to relieve Germany’s reliance on natural fertilisers, and to provide a cheap, synthetic alternative
The Allied blockade restricted Germany’s supply of natural nitrates, which led to the Haber process to be adopted on a widespread scale to produce ammonia for fertiliser and explosives
The Haber process was very significant at that time in world history, as it allowed cheaper method of manufacturing fertilisers and weapons. This allowed for greater agricultural output, which helped mitigate potential food shortages in Germany due to its booming population, and blockades on trade during WWI

Manufactured products, including food, drugs and household chemicals, are analysed to determine or ensure their chemical composition

Deduce the ions present in a sample from the results of tests

The ions to be tested are listed below:
Cations: Ba2+, Ca2+, Pb2+, Cu2+, Fe2+, Fe3+
Anions: PO43-, SO42-, CO32-, Cl-
The table below shows the solubility of the compounds of the above ions => MEMORISE

IONS / PO43- / SO42- / CO32- / Cl- / OH-
Ba2+ / White ppt / White ppt / White ppt / NO REACTION / White ppt
Ca2+ / White ppt / White ppt / White ppt / NO REACTION / White ppt
Pb2+ / White ppt / White ppt / White ppt / White ppt* / White ppt
Cu2+ / Blue ppt / NO REACTION / Green ppt / NO REACTION / Blue ppt
Fe2+ / Green ppt / NO REACTION / Grey ppt / NO REACTION / Green ppt
Fe3+ / Pink ppt / NO REACTION / NO REACTION / NO REACTION / Brown ppt

* - Soluble when heated
Precipitation reactions are used to identify ions in solution. Gravimetric analysis, volumetric analysis, and AAS can be used for quantitative analysis (see practical below)
In ion detection tests, there may be only one ion in the sample, or a mixture of ions in the sample. The tests below explain the individual detection tests, whilst the flowcharts highlight the order of tests when more than one ion is present in the sample
All tests should use 1-2mL samples of concentration of at least 0.1M
For samples containing a mixture of ions, each test needs to be conducted in a specific sequence (i.e. the ion is removed via complete precipitation and filtration, then the next test is conducted)

CATION TESTS

To determine the cations present, HCl, H2SO4 and NaOH is added in that order, and the formation of a precipitate indicates which ion is present
The flowchart below shows describes the relevant cation testing

The table below describes the individual cation tests. The tests must be conducted in this order in a sample containing multiple cations, so that the cation can completely precipitate out of the solution (i.e. until no more precipitate forms)

CATION / TEST / VERIFICATION
Pb2+ / Add HCl solution (Cl- ions) => a white precipitate indicates the presence of Pb2+ ions / Add a few drops of potassium iodide (KI) to a fresh sample => a vibrant yellow precipitate should form
Ca2+ and Ba2+ / Add H2SO4 solution (SO4 ions) => a white precipitate indicates the presence of Ca2+ or Ba2+ ions / Add a few drops of sodium fluoride (NaF) to a fresh sample => Ca2+ forms a precipitate with F, whilst Ba2+ doesn’t.
Cu2+ / Add NaOH solution (OH- ions) => a blue precipitate indicates the presence of Cu2+ ions, an initially green precipitate indicates Fe2+ ions (though it may rapidly turn brown), and a brown precipitate indicates Fe3+ ions / Dissolve the precipitate in ammonia (NH3) => Cu(OH)2 will form a deep-blue solution (only form single ion tests)
Fe2+ / Add a few drops of purple potassium permanganate solution (KMnO4) => the decolourisation of the solution indicates the presence of Fe2+
Fe3+ / Add a few drops of potassium thiocyanate (KSCN) => a blood red precipitate indicates the presence of Fe3+ ions

The presence of Pb2+, Ca2+ and Ba2+ can also be identified by flame tests => this does not work for samples containing multiple ions due to contamination of the solution

ANION TESTS

The table below describes the individual tests and verification of anions. The tests must be conducted in this order if multiple anions are present in the sample

ANION / TEST / VERIFICATION
CO32- / Add dilute nitric acid (HNO3) => the evolution of a colourless gas indicates the presence of CO32- ions / Test the pH of the original solution => a pH of 8-11 indicates the presence of CO32- ions
SO42- / Add barium nitrate (Ba(NO3)2) to an acidified sample => the formation of a thick white precipitate indicates the presence of SO42- ions / Add lead (II) nitrate (Pb(NO3)2) to an acidified sample => the formation of a white precipitate indicates the presence of SO42- ions
PO43- / Add barium nitrate (Ba(NO3)2) to an alkaline sample => the formation of a white precipitate indicates the presence of PO43- ions / Add ammonium molybate solution ((NH4)MoO4) to an acidified sample => the formation of a yellow precipitate indicates the presence of PO43- ions
Cl- / Add silver nitrate (AgNO3) to an acidified sample => the formation of a white precipitate that decomposes in sunlight indicates the presence of Cl- ions / Add 1-2mL of 10% ammonia solution to the precipitate suspension => decolourisation indicates the confirms the formation of the AgCl precipitate

When acidifying the solution, HNO3 must be added, as other acids may contaminate the tests (e.g. H2SO4, HCl). Use ammonia (NH3) to make a sample basic

Gather,process and present information to describe and explain evidence for the need to monitor levels of one of the above ions in substances used in society

LEAD (Pb2+)

Sources:

Lead was widely used as an additive in petrol, which led to high levels of lead in the atmosphere via vehicle exhaust
Paints manufactured before the 1950s contained a high percentage of lead. Lead in paints is released into the atmosphere by deterioration with age, sanding, burning, or demolition.
Mining and refining of lead produces fumes, dust, tailings, and slag wastes contaminated with lead
Plumbing fixtures that contain trace amounts of lead can cause lead to be dissolved in drinking water supplies

Effects: