3.1: getting the most from reactants

Industrial processes and various calculations

What must be considered when making industrial processes?

Cost and efficiency must be considered when making industrial processes to maximize profit and minimise environmental impact

Feedstock

The reactants in a process

What must be considered when choosing feedstock?

Many factors are considered when choosing feedstock, such as: availability, cost (including the cost of transport), sustainability (including when storing feedstock), and risks to humans/the planet 

The recycle loop

Byproducts of a process can be recycled as feedstock in other processes or sold, and excess reactants can be reused. This reduces cost, maximises profit, and limits waste 

Energy requirements

You must consider if heat produced can be used elsewhere, you should also consider if a catalyst can be used to lower energy requirements

Byproducts

Unwanted or extra products made in an industrial process

How can byproducts be used and what about byproducts must be considered?

Byproducts can be sold or used as feedstock in other processes, if a byproduct is toxic then worker safety and environmental protection laws must be considered 

Product yield

The gain of the desired product, you must consider if this is high enough to make a profit

Health and safety considerations for workers, the public, and the environment 

• You should minimise waste and its toxicity 

• You should avoid the use of toxic substances as feedstock 

• You should design biodegradable products/byproducts wherever appropriate 

Considerations when analysing flowcharts 

• Can alternative, cheaper feedstock be used?

• Can unreacted reactants be separated and reused?

• Can byproducts be used in different processes or sold?

• Can energy inputs be lowered?

Moles

The unit of measurement of a substance based on particles. 1mol = avogrado’s constant = 6.02×10²³ particles

What factors is the volume of a gas dependent on?

Pressure and temperature

As pressure increases, the volume of a gas…

As pressure increases, the volume of a gas decreases 

As temperature increases, the volume of a gas….

As temperature increases, the volume of a gas increases 

TRUE OR FALSE: the volume of 1mol of CO₂ at 150°C and 150Pa is different to the volume of 1mol of CO₂ at 40°C and 150Pa 

True 

TRUE OR FALSE: the volume of 1mol of CO₂ at 40° and 150Pa is different to the volume of 1mol of H₂ at 40° and 150Pa

False 

Molar volume 

The volume occupied by any gas at a specific temperature and pressure 

As temperature increases, molar volume…

As temperature increases, molar volume increases 

As pressure increases, molar volume…

As pressure increases, molar volume decreases 

Formula for molar volume 

l mol^-1 

Consider the equation: 

N₂ + 3H₂ → 2NH₃

If 100cm³ of nitrogen is used, how much hydrogen is used and how much ammonia is produced?

300cm³ of hydrogen is used and 200cm³ of ammonia is produced

Excess reactants 

When more of a reactant is used than necessary so that all of the limiting reactant is used up 

Why is a reactant used in excess?

So that the limiting reactant is used up completely as it may be expensive or toxic 

Steps to calculate limiting and excess reactants 

1. Find the molar ratio of reactants 

2. Find the mass used of each reactant 

3. Find the GFM of each reactant 

4. Calculate the moles of each reactant used and 

Percentage yield 

Percentage yield compares how much of s product is actually made to what the equation predicts 

Why can percentage yield never be 100%?

• Impurities in chemicals 

• Products escaping the system 

• Unfinished reactants 

• Side reactions created by by-products 

Why can percentage yield sometimes exceed 100%?

• Byproducts reacting to form products 

• Impurities 

• Solvents 

Steps to calculate percentage yield

1. Write the balanced equation

2. Find the limiting and excess reactants

3. Calculate the theoretical yield of the product using a balanced equation and the limiting reactant

4. Calculate the percentage yield

Atom economy

Atom economy measures the proportion of reactants that have been turned into useful products (theoretical)

Desirable reactions have a high…

Desirable reactions have a high atom economy

What is the atom economy of reactions with one product?

100%

What are the benefits of efficient processes with high atom economies?

They conserve natural resources and reduce waste 

Steps to calculate atom economy 

1. Write the balanced equation 

2. Find the total mass of reactants and the mass of the desired product 

3. Calculate the atom economy 

Magic triangle for molar volume