HSC Chemistry Syllabus Notes -
Module 8 / Inquiry Question 3
Overview of Week 16 Inquiry Question – What are the implications for society of chemical synthesis and design?
Learning Objective #1 – Evaluate the factors that need to be considered when designing a chemical synthesis process, including but not limited to:
– Availability of reagents
– Reaction conditions
– Yield and purity
– Industrial uses (eg pharmaceutical, cosmetics, cleaning products, fuels)
– Environmental, social and economic issues
NEW HSC Chemistry Syllabus Video – Chemical Synthesis and Design
Week 16 Homework Questions
Week 16 Curveball Questions
Week 16 Extension Questions
Solutions to Week 16 Questions
Overview of Week 16 Inquiry Question
Welcome to Week 16 of your HSC Chemistry Syllabus Notes!
We will be exploring the importance of having available reagents (reactants) for a chemical reaction involved in producing chemicals used in industry or everyday life. We will also be examining other relevant factors such as operating and maintaining manufacturing plants at the optimal reaction conditions, the yield and purity of the products.
Subsequently, we will examine why such factors are important in industries as pharmacetical, cosmetic, cleaning products and fuel.
Lastly, we will examine the environmental, social and economic implications involving the chemical synthesis and design.
Learning Objective #1 - Evaluate the factors that need to be considered when designing a chemical synthesis process including but not limited to:
- Availability of reagents
- Reaction conditions
- Yield and Purity
- Industrial Uses
- Environmental, Social and Economic Issues
Availability of reagents
The availability of reagents (i.e. reactants) is critical in designing a synthesis process. This is because the quantity (moles) of product formed is limited by the moles of reactants available by mole reaction. Therefore, when all the reagents are consumed, no more products can be produced.
In the case of equilibrium reactions, a high availability of reagents is particularly important in driving the equilibrium position and, thus reaction, in favour to produce more moles of the product (which can be explained using Le Chatelier’s Principle and/or Collision Theory).
This means that it is preferable for manufacturing plants to have easy access to the reagents used to produce the desired product. For such reasons, oil refineries are located very close to shore whereby the transportation distance of ships to transport oil extracted from machines in the middle of the ocean can be reduced. This saves both time and money for the oil supplying companies.
Another example is that coal-fired power stations are built near coal deposit sites so that coal that is obtained from the site can be quickly transported to the power stations. Again, this saves both time and money.
Reaction conditions
It is important to operate and maintain operations at optimal reaction conditions such that the most cost-effective chemicals can be produced.
This would allow to reduce production costs, which imparts lower cost products to the consumers, and greater net profit margins for the manufacturing company to remain competitive.
We have introduced the Haber Process as the chemical reaction to produce ammonia in Module 5.
Let’s explore how ammonia manufacturing plants operate and maintain operations at optimal reactions conditions.
To remind ourselves with the Haber Process, let’s see the following chemical reaction:
3H2(g) + N2(g) <-> 2NH3(g) ; Forward reaction is exothermic.
As we know that the forward reaction is exothermic, it would mean that if the system’s temperature is increased, it will cause the equilibrium position to shift to the left and reduce the yield of ammonia.
Vice versa, if we increase the temperature of the system, we will be able to obtain more ammonia by shifting the equilibrium position to the right and drive the reaction forward.
However, notice that all the species involved in the Haber process equilibrium are in the gaseous state. This would mean a change in the system’s pressure or volume will affect the equilibrium concentration and, thus, yield of ammonia.
More specifically, if we increase the pressure of the system, the equilibrium position will shift to the right in favour of the forward reaction to minimise the increase in gas concentration.
Vice versa, if we decrease the the pressure of the system, the equilibrium will shift to the left, in favour of the reverse reaction, to minimise the decrease in gas concentration.
So, you would think that the manufacturing plant should operate at the lowest temperature and highest pressure possible in order to drive the equilibrium position as far to the right as possible right?
Well, that’s not really the reaction conditions that you will find occurring in ammonia manufacturing plants in reality. Let’s explore why.
Reason for the need to comprise conditions for the Haber Process
So, if we operate at very low temperatures then the rate of reaction will be affected because the collision energy of the reacting species may be lower than the activation energy required to break the necessary chemical bonds to form ammonia. Thus, the rate of the forward reaction will decrease (or be zero) depending on the temperature.
Next, the frequency of collision will also decrease because the average kinetic energy possessed by the hydrogen and nitrogen molecules will be lower than at high temperatures. This means that the rate of reaction will decrease.
However, we cannot operate a high temperatures because the forward reaction of the Haber Process is exothermic. As explained before, increasing the system’s temperature will shift the equilibrium position to the left (which we can explain using Le Chatelier’s Principle, Collision Theory or the nature of the size of the activation energy barrier of endothermic vs exothermic reactions).
So, there must be a comprise in temperature so that the highest amount of ammonia is produced.
A temperature of approximately 450 degrees celsius is used to allow an optimised amount of ammonia being produced at an industrial scale at a given time period.
Moving on to explore the pressure factor. There must be a comprise on pressure too. This is because as the total gas pressure applied onto the reaction vessel walls increases, the danger for an explosion increase.
This also means that the cost to build and maintain such walls would be higher if a higher pressure reaction condition is to be used.
Also the incremental percentage increase of nitrogen and hydrogen gas being converted into ammonia is drops after 200atm.
Therefore, there must be a comprise in the pressure used to manufacture ammonia for cost and safety reasons. This comprised pressure is at 200atm for the Haber Process.
Returning to reaction conditions, an iron oxide catalyst can be used to provide an alternative reaction pathway of lower activation energy for the reaction between nitrogen and hydrogen to produce ammonia.
Yield and Purity
There are two different terminologies for yield – these being the theoretical yield as well as the actual yield.
As the name implies, theoretical yield is basically the moles of product (e.g. chemical) that is expected to form using mole ratio whereby all reactants supplied are expected to be converted into products.
Comparatively, the actual yield refers to the actual moles of product formed for the amount of reactants that is supplied as raw material to manufacture the product.
We are concerned about the theoretical yield prior to the reaction and we measure and compare the actual yield with the theoretical yield after the product has been obtained.
When we compare the actual yield with the theoretical yield, we can use what is called a percentage yield.
Percentage yield = (actual yield / theoretical yield) x 100 = ? %
Now, you may be wondering what are some factors that result in a difference between the theoretical yield and actual yield. There could be many factors but one factor is purity.
We have already touched on the concept of purity in Module 5 when we were examining the purity of the substance when used as primary standard. This is because if the reactant is not pure, the actual moles of the reactant that could produce the desired product or chemical is reduced.
For example, this may be because the total mass of the reactant may consists of other atoms (impurity) that are not used to produce the product but some other by-products. Therefore, our actual yield will be lower than the theoretical yield.
Another factor may be the reaction condition being different in reality than on a theoretical level. We have already explored reaction conditions in the previous section.
However, as an example, if the temperature of the reaction is occur at a lower temperature in reality than expected, the rate of reaction will be lowered and the actual yield of the product per hour may be lower than the theoretical yield.
Returning the concept pertaining to the purity of the chemical, it is important in many industries to measure the purity of their product. The importance of this is very broad, some reasons for the measurement of the chemical composition & purity of a chemical can be:
Government regulations requires accurate reporting of ingredients present inside a product (Nutrition & Ingredients Label). For example, people with an allergy to a specific ingredient can rely on such nutritional labelling before purchase.
NOTE: The purity of a drug is particularly important in the pharmaceutical industry as it involves interaction with living cells and can result in development or worsening of a disease.
Marketing purposes (e.g. 100% pure honey) to deliver true and appealing information to capture consumers’ attention
Sydney Water monitors and test the purity of water to ensure that it meets the regulatory requirements for its water to be qualified as potable (drinkable) water when it reaches the consumers’ pipelines.
Petrol composition & purity are tested to ensure that petrol quality meets standard requirements so that car engines are not damaged.
The petroleum and natural gas obtained from petroleum extraction plants will be test for its chemical composition and purity as it will dictate the wholesale price. Therefore, as the scarcity of petroleum and other non-renewable fuel sources, price will also increase as we have discussed in Module 7’s Notes.
Industrial Uses pertaining to chemical synthesis & Design
Moving onto the next factor when designing a chemical synthesis process being industrial uses.
It is critical to plan and design the physical reaction pathway in reactant and products are inserted and obtained in the manufacturing plant.
This is because, by doing so, any waste or by-products formed as a result of the reaction can be used to as reactant or drivers for the same or a different reaction. This will improve the cost-efficiency and thus lower the cost of production of the chemical.
Let’s take the steel industry which manufactures steel as a chemical as an example.
The blast furnace gas (composed of mainly nitrogen and carbon dioxide) are produced as a result of smelting of iron ores. Iron is a component of steel. The heat derived from such the hot gas is used as source of energy to power up turbines and generate electricity used in the industry.
Furthermore, coke ovens used to heat coal to produce carbon that is as an ingredient in steel production. This process releases ammonia gas as a by-product which can be collected and be used to react with sulfuric acid to form ammonium sulfate.
Ammonium sulfate can be used as a fertiliser in the agricultural industry which is another source of revenue for the steel manufacturers.
In the ammonia manufacturing plant to produce ammonia cleaning product, any unreacted hydrogen and nitrogen gas is designed to be recycled and fed back into the high temperature and pressure compression chamber for a reaction to occur. This therefore reduces the amount of hydrogen and nitrogen lost and lowers the cost of ammonia.
The process of recycling unreacted hydrogen and nitrogen gases will also drive or shift the equilibrium position to the right to form more ammonia when the recycled nitrogen and nitrogen gas is re-added into the reaction vessel.
NOTE: Ammonia that is produced can also be removed from the reaction vessel to help drive the reaction forward (i.e to the right) to form more ammonia.
If you are interested, feel free to research other different industries’ manufacturing plants. Two other good industries to research on are textiles and oil refinery (chemicals derived from petroleum, natural gas, coal, etc).
NOTE: In the oil refineries (oil industry) to produce petrochemicals (e.g. ethylene), heat energy are obtained by recycling from hot wastewater to power equipment and cooling towers. This saves cost as over 2 million gallons of water can be recycled per day.
Environmental Issues pertaining to Chemical Synthesis & Design
There waste product(s) produced as a result of chemical synthesis reaction are often either toxic or harmful to the environment.
There are many government regulations that has been imposed to require that the waste products (e.g. waste effluents) to be of certain acceptable condition (e.g. pH value, ppm, etc) prior to being allowed to be released into the environment.
It is not currently possible for the government to have real time information on the surrounding environment (e.g. soil and waterways) in which the waste product makes contact with. Also, the release of heavy metal by manufacturing plants may have uncertain long-term effects on the health of the environment and living organisms that come into contact with such environments.
The issue of chemical synthesis reactions imposing a negative effect on the environment would be accelerated through accidents. An example is the Bhopol disaster whereby a USA-owned pesticide manufacturing plant in India released over 30 tonnes of methyl isocyanate toxic gas into the atmosphere.
As a result of this accident, thousands of people in Bhopol were killed as a result of the exposure of the toxic gas. Fast forward to today, there are still people dying due to development of cancer as a result of the exposure to such toxic gas. The children of the people that were affected experiences birth defects and surviving victims are suffering from an array of problems such as lost of visual acuity. The water at Bhopol still remains toxic today.
Moving on to explore more common waste secreted by manufacturing plants involved in chemical synthesis, this would include the release of sulfur dixoide from steel manufacturing plants involved in smelting iron ore.
This has the ability to cause acid acid as we have explored in Week 13’s Notes.
SO2(g) + 1/2 O2(g) -> SO3(g)
SO3(g) + H2O(l) -> H2SO4(aq)
The sulfuric acid can then dissolve in waterways when it rains which lowers the pH of the water, resulting the death of living organisms. For example, fish larvae could die if the pH of the water is 5.5 or lower. This would threaten the biodiversity of aquatic organisms in the country which can result in economic implications as we have explored in Week 9’s notes such as declining export revenue, the price of fish increasing and thus reduces the affordability of seafood.
Acidic rain can lower the pH of the soil. Aluminium ions are toxic to plants and their solubility increases exponentially with decreasing soil pH below 5. This could result in the both of plants in areas with high numbers of manufacturing plants secreting sulfur dioxide into the atmosphere.
Also, acid rain can dissolve marble statues in the city via the reaction:
CaCO3(s) + H2SO4(aq) -> CaSO4(s) + H2O(l) + CO2(g)
Another issue involves pesticide and fertiliser run off from agricultural land to ambient waterways when they are dissolved in water (e.g. irrigation or rainwater). This is because such chemicals are toxic to aquatic organisms such asfish as it can reduce the fish’s ability to regulate its internal temperature which can affect enzymes’ ability to catalyse necessary metabolic processes like cellular respiration to sustain life.
Social Issues pertaining to Chemical Synthesis & Design
The social issues here are related to the use of the chemical.
Fertilisers and pesticides are chemicals that are used in the agricultural industry. However, such chemicals are harmful to living organisms including farmers themselves. The exposure to such chemical can result in a range of health issues such as respiratory problems to cancer.
Another problem involves the incorrect labelling of chemicals that are subsequently consumed or used by humans.
For instance, a skin cream (chemical) may contain an ingredient that could cause allergic reactions to the consumer but was not written on the ingredient label of the chemical.
A lawsuit was filed against Mario Badescu by thirty one customers in which one of the company’s skincare cream that was labelled to contain only botanical-only ingredients in fact had incorporated synthetic steroids (chemicals) as ingredients. That is, they incorporated steroids in their chemical design.
It is known that there are serious health risks (e.g. addiction, skin burns) associated with the use of such chemicals.
Thirdly, the misuse of chemicals can also be dangerous to humans.
Plastics are petrochemicals (chemicals derived from petroleum) such as high density polyethyelene (HDPE) which is rigid and hard as explained in Module 7’s Notes under the Polymers topic.
For example, small, hard plastic toys are dangerous when used by babies due to potential choke hazard.
Sharp, hard plastic toys are equally dangerous as they could cause bleedings when mishandled by babies.
Economic Issues to Chemical Synthesis & Design
Economic issues in regards to chemical synthesis and design primarily depends on the profit generated from the sale of the chemical product.
If the price of a chemical product decreases and the quantity sold remains constant, the revenue generated would decline. However, if the quantity supplied remains constant and the cost of production is lowered, the manufacturing company can generate a profit.
This is because its net profit margin would be higher than its competitors. Therefore, it is important to focus on the production and cost efficiency during the manufacturing process. This will come under chemical synthesis and design.
For example, research and development can be performed to discover or develop cheaper and more effective catalysts to increase the rate of reaction and purity of the resulting chemical product. This could lower the cost of production.
Similarly, research can be done to pinpoint the optimal reaction conditions to obtain the greatest yield of the chemical product and/or highest purity at the lowest cost.
However, it is also important to realise that the economy also plays a role on the sale price of the chemical product.
For instance, government regulations may impose greater cost on cost of production by disallowing the use of a certain chemical in the manufacturing process that would otherwise be cheaper to use.
Example: In 2017, the European Union banned the use of Mercury as a catalyst in the production of polyurethane elastomers which is used as doming resins to give artworks a glossy finish. This affects the type of machinery that can be used to produce such doming resins without mercury as a catalyst, increasing the cost to manufacturers.
The government may also impose regulations on the quality of wastewater or products to meet a certain requirement before being allowed to release into the environment.
This would result in the manufacturing having to spend additional cost in purifying the waste products to meet regulatory requirements. This would drive down net profit margins of the chemical product sold.
Example: The wastewater effluent from the textile industry (e.g. manufacturing & dyeing of Nylon) consists of high level of dissolved solids that must be purified before it is allowed to be discharged into the environment. Cationic polyacrylamide can be used as a coagulant and flocculant in the water purification process.