Choosing a reaction pathway (Edexcel GCSE Chemistry): Revision Notes
📚Revision Notes
3.3.4 Choosing a Reaction Pathway
What is a Reaction Pathway?
Note
A reaction pathway is the series of steps or reactions needed to produce a desired chemical product. Often, there is more than one way to make the same substance, so choosing the best reaction pathway is important. The decision on which pathway to use depends on several factors.
Note
Key Factors in Choosing a Reaction Pathway:
- Percentage Yield: This tells us how much of the reactants are successfully turned into the desired product. A higher percentage yield means more product is made from the same amount of reactants.
- Atom Economy: Atom economy measures how much of the reactants end up as the desired product and how much ends up as waste. A higher atom economy is better because it means less waste is produced.
- Rate of Reaction: This is how fast the reaction happens. Faster reactions are usually preferred because they produce the product more quickly.
- Equilibrium Position: In reversible reactions, the reaction doesn't go to completion, and the final mixture contains both reactants and products. The position of equilibrium tells us how much of the reactants are converted into products.
- The usefulness of By-products: Sometimes, the reaction produces other substances besides the desired product. If these by-products are useful, they can be sold or used, improving the overall efficiency of the process.
Example
Example: Making Ethanol Ethanol, which is used as a fuel and in alcoholic beverages, can be produced in two main ways:
- Fermentation of Plant Sugars:
- Equation:
- Chemical Formula:
- Summary: In this method, sugars from plants are converted into ethanol and carbon dioxide by fermentation, a process that's been used for thousands of years.
- Hydration of Ethene:
- Equation:
- Chemical Formula:
- Summary: Ethene, which comes from crude oil, is reacted with steam to produce ethanol. This is a more modern and industrial method.
Comparing the Two Methods:
| Process | Yield | Atom Economy | Reaction Rate |
|---|---|---|---|
| Fermentation of Sugars | 15% | 51.1% | Low |
| Hydration of Ethene | 95% | 100% | High |
Note
Worked Example:
- Evaluating the Processes:
- Fermentation has a lower yield (15%) and a lower atom economy (51.1%) compared to the hydration of ethene, which has a yield of 95% and an atom economy of 100%.
- The rate of reaction is also much lower in fermentation. This means that, overall, the hydration of ethene is a more efficient way to produce ethanol.
Equilibrium Position:
- The hydration of ethene is a reversible reaction, meaning not all the ethene is turned into ethanol in one go. The equilibrium lies to the left, so only about 5% of the ethene is converted to ethanol at any one time. However, by recycling the unreacted ethene, the overall yield can reach 95%.
Using By-products:
- During fermentation, carbon dioxide is produced as a by-product. This gas can be captured and sold to companies that make fizzy drinks, increasing the process's atom economy. If all the carbon dioxide is used in this way, the atom economy could technically reach 100%.