5.15C - Choosing reaction pathways using data

What is being chosen

A reaction pathway is the route used to make a desired product. It may be a single reaction, or a sequence of reactions, but this lesson focuses on choosing between routes when the question supplies the relevant data.

The specified product is the product the chemist wants to make. Other substances formed may be by-products. Some by-products are useful and can be sold or reused. Others are waste and may need separating, treating or disposing of.

For this specification point, the usual evidence includes:

An Edexcel Explain answer needs reasons. Instead of writing Pathway A is better because it has the highest atom economy, write what that means for the route: Pathway A has the highest atom economy, so a larger proportion of the reactant mass is converted into useful product and less waste is formed.

What each data type shows

Each piece of data answers a different question. That is why two pathways can look good in different ways.

Atom economy is based on the balanced equation for the reaction. A high atom economy means a high proportion of the reactant mass end up in the desired product. It is a measure of waste built into the equation, not a measure of how well the reaction worked in the laboratory or factory.

Percentage yield compares the actual mass of product obtained with the theoretical maximum mass. A high percentage yield means a large fraction of that route’s theoretical product is obtained. Comparing actual masses also requires the theoretical yields: a larger percentage alone does not guarantee a larger product mass. A low yield may happen because the reaction is incomplete, product is lost during separation, or side reactions form unwanted products.

Rate tells you how quickly product is formed. In industry, an acceptable rate matters because slow reactions use equipment and energy for longer. A pathway with a high yield but a very slow rate may be less useful if the product is needed quickly.

Equilibrium position matters for reversible reactions. If the equilibrium mixture contains a large proportion of the desired product, the reaction is more favourable for making that product. If the equilibrium mixture contains mostly reactants, the process may need repeated removal of product or recycling of unreacted reactants.

By-products matter because they affect the overall usefulness of the route. A useful by-product can be sold, reused or used as a fuel. An unwanted by-product may reduce atom economy and may also need extra separation or disposal.

Weighing the evidence

To choose a pathway, start with the purpose of the process. Then decide which data matter most for that purpose.

For a bulk chemical made on a large scale, high yield and fast rate are often important because large masses must be made in an acceptable time. Atom economy still matters because wasted reactants cost money and produce waste. If the process is reversible, the equilibrium position matters because it limits how much product is present at equilibrium.

For a product where waste is a major concern, atom economy and by-products may be more important. A pathway with a lower yield might still be chosen if it makes far less waste, forms a useful by-product, or avoids an unwanted side reaction.

The key skill is to make a balanced judgement. A good answer usually follows this pattern:

1. Identify the pathway chosen.
2. Use at least two pieces of data to support the choice.
3. Acknowledge an important disadvantage or trade-off.
4. Finish with a judgement linked to the specified product.

Avoid vague phrases such as it is better for the environment unless the data show why. Say what the data show: less waste, higher atom economy, useful by-product, lower amount of unwanted product, faster production, or more product in the equilibrium mixture.