AQA GCSE FOUNDATION Chemistry: Percentage Yield and Atom Economy

AQA GCSE Foundation Chemistry: Percentage Yield and Atom Economy

Introduction

This tutorial focuses on two important concepts in chemistry: percentage yield and atom economy. These concepts are crucial for understanding the efficiency of chemical reactions and their impact on sustainability.

Efficiency of Reactions

When we carry out a chemical reaction, we want to maximise the amount of desired product formed. This means we need to consider how much of the reactants actually get converted into the product. Two key factors that determine the efficiency of a reaction are:

• Percentage yield: How much product we actually get compared to the theoretical maximum amount.
• Atom economy: How much of the reactants ends up in the desired product, minimizing waste.

Sustainable Chemistry

Sustainable chemistry aims to minimize the environmental impact of chemical processes. This includes:

• Reducing waste: Using reactions with high atom economy to minimize the production of unwanted byproducts.
• Using renewable resources: Finding alternative sources of raw materials that can be replenished.
• Minimizing energy consumption: Designing reactions that require less energy to proceed.

Percentage Yield

Definition: The percentage yield is the actual amount of product obtained in a reaction divided by the theoretical maximum amount of product that could be obtained, multiplied by 100%.

Formula:

Percentage Yield = (Actual Yield / Theoretical Yield) x 100%

Example:

Suppose you want to make 10 g of sodium chloride (NaCl) by reacting sodium (Na) with chlorine (Cl2). However, you only manage to produce 8 g of NaCl.

• Theoretical yield: 10 g
• Actual yield: 8 g

Percentage Yield = (8 g / 10 g) x 100% = 80%

Factors affecting percentage yield:

• Incomplete reaction: Some reactants may not react completely.
• Side reactions: Unwanted reactions may occur, forming byproducts.
• Loss of product during purification: Some product may be lost during separation and purification steps.

Improving Percentage Yield:

• Optimizing reaction conditions: Adjusting temperature, pressure, or catalyst use can improve the yield.
• Preventing side reactions: Carefully selecting reagents and reaction conditions can minimize side reactions.
• Minimizing product loss: Improving purification techniques can reduce product loss.

Atom Economy

Definition: Atom economy is a measure of how much of the reactants end up in the desired product. It is expressed as a percentage.

Formula:

Atom Economy = (Mass of desired product / Total mass of reactants) x 100%

Example:

Consider the following reaction:

CH4 + 2O2 ? CO2 + 2H2O

• Desired product: CO2
• Total mass of reactants: 16 g (CH4) + 64 g (2O2) = 80 g

Atom Economy = (44 g (CO2) / 80 g (total reactants)) x 100% = 55%

Factors affecting atom economy:

• Number of byproducts: Reactions with fewer byproducts have higher atom economy.
• Type of reaction: Some reactions inherently have lower atom economy.

Improving Atom Economy:

• Choosing reactions with fewer byproducts: Look for reactions that produce minimal waste.
• Using catalytic reactions: Catalysts can speed up reactions without being consumed, leading to less waste.
• Developing alternative synthetic pathways: Finding new routes to produce the desired product with higher atom economy.

Summary

• Percentage yield measures the efficiency of a reaction by comparing the actual yield to the theoretical yield.
• Atom economy measures how much of the reactants end up in the desired product, minimizing waste.
• Both concepts are essential for understanding the efficiency and sustainability of chemical processes.

By focusing on improving percentage yield and atom economy, we can create more efficient and environmentally friendly chemical processes.