This is part of the HSC Chemistry course under Module 8 Section 3: Chemical Synthesis and Design.

HSC Chemistry Syllabus

Evaluate the factors that need to be considered when designing a chemical synthesis process, including but not limited to:

• availability of reagents

• reaction conditions (ACSCH133)

• yield and purity (ACSCH134)

• industrial uses (eg pharmaceutical, cosmetics, cleaning products, fuels) (ACSCH131)

• environmental, social and economic issues

 

What Factors Do We Consider When Designing a Chemical Synthesis Process?

This video explores the factors that are considered when designing a chemical synthesis process. This includes environmental, social and economical issues, as well as factors that affect the efficacy of a chemical reaction.

 

Issues to Consider When Designing a Chemical Synthesis Process

Environmental Issues

• Chemical syntheses are designed to minimise environmental consequences
  • New processes prioritise the synthesis of biodegradable products
  • Renewable methods of producing energy are increasingly utilised to meet the energy requirement of chemical synthesis while limiting the emission of greenhouse gases
• In cases where production of waste products cannot be avoided, they must be appropriately disposed to minimise effect on both the environment and society.
• In many cases, raw materials, that are derived from non-renewable sources, must be used efficiently.

Examples

• Plastic shopping bags have been replaced by more biodegradable alternatives in major supermarkets due to increasing concern of plastic misusage.
• Battery production has become more prevalent in the chemical industry as electric vehicles are seen as the future of sustainable transportation.

 

Social Issues

• Chemical industries design processes to synthesise products in order to satisfy consumer demand
• Consumer demand is shaped by multiple factors, including social trends, attitudes and needs, as well as cultural values.
• In the last decade, consumers have favoured products that are more environmentally friendly. This has prompted the chemical industry to innovate and synthesise products that are biodegradable while minimising consumption of non-renewable materials and energy.
• Mass production of goods are usually carried out in developing countries due to relatively low labour and operating costs. This, along with child labour in some cases, raise ethical concerns and debate over the need for additional regulation over not only the chemical industry but also global conglomerates from other industries.

Examples

• Development of new antibiotics in the pharmaceutical industry to overcome increasing bacterial resistance and satisfy increasing healthcare burden in ageing populations.
• Newer detergents are synthesised to meet consumer needs and desires. Recently synthesised detergents are more biodegradable and have superior cleaning action than previous detergents. In addition, they are designed to suit specific uses e.g. cationic detergents are commonly used in hair conditioners.
• Lithium and cobalt are common raw materials for battery production. Lithium and cobalt mining sites in South America are associated with little concern for safety, child labour and low financial remuneration for local workers. Furthermore, the heavy use of water in mining sites has caused supply issues in the local agricultural sector. 

 

Economical Issues

• Monetary incentives and profitability drive the chemical industry to constantly innovate and produce products that satisfy both social and environmental considerations.
• The extend of social demand and need for a particular product is an important consideration as it is a strong indicator for consumers’ willingness to become customers and pay for the goods.
• The economical viability of chemical synthesis is determined by the availability, access and cost of resources that are required in the process. Resources are generally more affordable when they are abundant and easily accessible.
• Efficiency of chemical synthesis (i.e. reaction rate, yield, purity) determines the operating cost of chemical synthesis. Processes that are fast (high reaction rate), produce a large quantity (high yield) of pure (high purity) products are preferred as they together reduce the time taken to synthesise the product as well as the energy requirement of synthesis and purification.
• Locations of most chemical factories are chosen to optimise social, environmental and economical considerations.
  • Companies are obligated to minimise negative consequences of chemical synthesis on the environment and people by shifting factories away from cities and biological ecosystems (e.g. oceans). Factories, where possible, should not be built near moving waterways as potential pollution of these waterbodies can lead to spreading contamination and cause great difficulty in confining it.
  • Factories should also be built away from cities, towns and villages as chemicals and waste products released from them can have detrimental health consequences. For example, asbestos and silica (both are derivatives of chemical processes) are known causative agents of lung disease. Ground water contaminated with lead ions can cause irreversible neurological damage.
  • Economically, companies prefer factories to be built in sites that allow for easy access to required resources and convenient transportation to and from market where produced goods are sold to consumers. When resources and market are easily accessible, cost is reduced and the overall process is made more economical.

Example – lithium-ion battery production in Australia

• Availability of lithium reserves in Western Australia is catalyst for the emerging battery industry in Australia.
• Mining sites and lithium processing plants are in Western Australia, close to mining sites to allow for easy transportation
• Battery production facilities are located closer to market e.g. Newcastle to minimise cost associated with transportation of goods.

 

Improving Efficacy of Chemical Synthesis

Increasing Reaction Rate

• Higher temperature increases reaction rate:
  • Increases kinetic energy of particles  increases collision rate  increases reaction rate
  • Increases kinetic energy of particles  increase likelihood of reaction upon collision  increases reaction rate
  • However, more energy is required to facilitate higher temperature conditions. In other words, increasing temperature allows for a faster reaction rate at expense of increasing energy requirement.

• Catalyst: reduces the activation energy by providing an alternate reaction pathway
  • Reduces the energy required for particles to result in a reaction upon collision
  • Allows for a faster reaction rate at a lower temperature (without needing more energy)

• Higher pressure: increases collision rate between gaseous molecules
  • Allows for a faster reaction rate
  • For substances to become gaseous, temperature must be raised to reach their boiling points. This again requires additional energy.
  • Pressure may affect yield of reaction

 

Factors affecting yield

• Loss during synthesis e.g. transfer between beakers, leakage of reaction chambers
• Product is impure, due to:
  • Multiple reactions are happening simultaneously
  • Reaction produces by-products, resulting in a mixture that requires separation
• Reversibility of reaction (dynamic equilibrium); equilibrium constant (K_eq)

 

Factors That Affect Yield in Reversible Reactions

• Concentration
  • ↑ [reactant] shifts equilibrium to product side. Reactants of reversible reactions are constantly supplied to the reaction chamber.
  • ↓ [product] shifts equilibrium to product side. Products of reversible reactions are constantly removed from the reaction chamber. 

• Pressure/volume
  • Effect of pressure and volume changes on yield depends on the relative number of gaseous substances on the reactant and product side of a reaction
  • ↑ pressure shifts equilibrium to side with less gaseous substances
  • ↓ pressure shifts equilibrium to side with more gaseous substances 

• Temperature
  • Effect of temperature on yield depends on reaction’s change in enthalpy (∆H)
  • ↑ temperature increases rate of forward and reverse reaction (improves reaction rate)
  • Increases collision rate of endothermic reaction more than exothermic reaction
  • ↑ temperature favours and increases yield and reaction rate for an endothermic reaction
  • ↓ temperature favours and increases yield for an exothermic reaction. However, this is at the expense of lowering the reaction rate. Therefore, a moderate temperature is usually used to optimise yield and reaction rate for exothermic reactions.