Improving Processes and Products
Atom Economy and Percentage Yield
Extension for separate Chemistry: the calculations in this section are not examined on Combined Science, but the ideas help you explain why some processes are more sustainable.
In industrial chemistry, it is not enough to simply make a product — the process must be efficient and produce as little waste as possible. Two key measures of efficiency are atom economy and percentage yield.
Atom economy measures how much of the mass of the reactants ends up as the useful product. A high atom economy means less waste is produced, which is better for both the environment and the company’s costs. It is calculated using:
$$\text{atom economy} = \frac{M_r \text{ of desired product}}{\text{sum of } M_r \text{ of all products}} \times 100$$
Percentage yield compares the actual amount of product obtained to the maximum theoretical amount (from the balanced equation):
$$\text{percentage yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100$$
Percentage yield is often less than 100% because of incomplete reactions, side reactions, product lost during purification, or reversible reactions that do not go to completion. The limiting reactant is the one that runs out first and determines the maximum amount of product that can be formed. The other reactant is said to be in excess — some of it will be left over when the reaction finishes. For example, if you react 2 g of magnesium with 100 cm³ of excess hydrochloric acid, the magnesium is the limiting reactant and determines how much hydrogen is produced.
Atom economy measures the proportion of reactant atoms that become the desired product. Percentage yield measures how much product was actually made compared to the theoretical maximum. Both should be as high as possible for economic and environmental reasons. Industry aims for reactions with 100% atom economy (where all atoms end up in the desired product), but this is not always achievable, especially in reactions that produce by-products.
Life Cycle Assessments
A life cycle assessment (LCA) evaluates the environmental impact of a product at every stage of its life. The four main stages are: (1) extracting and processing raw materials, (2) manufacturing the product, (3) using the product (including transport), and (4) disposing of the product at the end of its life.
LCAs consider: energy consumption at each stage, raw material use, water use, emissions to air (including greenhouse gases), emissions to water, waste produced, and the potential for recycling or reuse. However, some aspects are difficult to quantify (e.g. the impact of noise pollution or visual impact), which makes LCAs partly subjective. Different analysts may reach different conclusions depending on the weighting they give to different factors. Companies may also use LCAs selectively to support marketing claims, so it is important to evaluate LCA data critically.
Recycling
Recycling materials reduces the need to extract new raw materials, saves energy, and reduces waste going to landfill. Factors affecting recycling decisions include: the cost of collection and processing, the value of the recycled material, the energy savings compared to making new material, and public willingness to recycle. Metals such as aluminium are particularly worth recycling because extracting aluminium from bauxite by electrolysis is very energy-intensive — recycling aluminium uses only about 5% of the energy needed to extract it from ore. Glass, paper and many plastics can also be recycled, though the economics depend on collection costs, sorting complexity and the market value of recycled material.
Crude Oil and Hydrocarbons
Crude oil is a finite resource and a vital feedstock for the petrochemical industry. It is a complex mixture of hundreds of different hydrocarbons — mostly alkanes, which have the general formula \(C_nH_{2n+2}\). Crude oil is separated into useful fractions by fractional distillation. The oil is heated and vapours rise up a fractionating column; fractions with lower boiling points (shorter molecules with weaker intermolecular forces) are collected near the top, and those with higher boiling points (longer molecules with stronger intermolecular forces) near the bottom. The main fractions include refinery gases, petrol (gasoline), kerosene, diesel, fuel oil and bitumen.
Cracking breaks long-chain hydrocarbons (which are less useful) into shorter, more useful molecules. There is high demand for short-chain hydrocarbons (for fuels) and for alkenes (used to make polymers). Cracking can be done by passing hydrocarbon vapour over a hot catalyst (catalytic cracking) or by mixing with steam at very high temperatures (steam cracking). The products include shorter alkanes (used as fuels like petrol and diesel) and alkenes (unsaturated hydrocarbons used as the starting materials for making polymers such as polyethylene and polypropylene). The general equation for cracking might look like: \(C_{10}H_{22} \rightarrow C_8H_{18} + C_2H_4\), where a long-chain alkane is split into a shorter alkane and an alkene.
Crude oil is a finite resource and a vital feedstock for the petrochemical industry. It is separated by fractional distillation based on boiling point. Long-chain hydrocarbons are cracked to produce shorter, more useful alkanes (fuels) and alkenes (for making polymers). Modern life is crucially dependent on hydrocarbons for energy, transport and materials.