Unit 2 - Getting The Most From Reactants Flashcards
Feedstocks
Feedstocks are the reactants for a chemical reaction. Feedstocks are usually extracted from raw materials and then purified before use.
Raw Material
A raw material is a substance available naturally in the Earth’s crust i.e in the ground, sea, atmosphere, or living material
Factors influencing industrial process design
- availability, sustainability and cost of feedstock(s)
- opportunities for recycling
- energy requirements
- marketability of by-products
- product yield
Environmental Considerations
Modern chemical plants are designed to minimise waste and to avoid the use and production of toxic substances. The air and water quantity in the vicinity of chemical plants is monitored regularly.
Products are also designed to be biodegradable where possible.
Molar Volumes of Gases
The molar volume is the volume occupied by one mole of a gas at a certain temperature and pressure. The molar volume is the same for all gases at the same temperature and pressure. The units are litres per mol (l mol-1).
At room temperature and pressure (abount 25°C adn 1 atmosphere pressure) the molar volume of any gas is about 24 litres per mole.
Volumes of Gases in Reactions
One mole of any gas has the same volume under the same conditions of temperatue and pressure. This measn that eqaul volumes of gases (at the same temperatue and pressure) will contain the same number of moles.
Any reactant and/or product substances that are solids or liquids have a negligible volume compared with any gases present. In these calculations, the volumes of solids and liquids are assumed to be zero.
Limiting and Excess Reactants
The reactant that is left over at the end of a reaction is said to be in excess. The reactant which is not in excess will be used up. This is known as a limiting reactant as it limits how much product will be formed. The chemical reaction stops when all the limiting reactant has been used up. The limiting reactant can be used to calculate the mass of product formed.
Percentage Yield
The yield of a chemical reaction is the quantity of product obtained. There are two types of yield:
* The theoretical yield is the quantity of desired product obtained, asssuming full conversion of the limiting reagent, as calculated from the balanced equation.
* The actual yield is the quantity of the desired product formed under the prevailing reaction conditions.
The actual yield is usually less than the theoretical yield. There could be several reasons for this:
* Impure reactants
* Reversible reaction
* Side-reactions may occur in adddition to the main reaction
* Loss of desired product during seperation or purification steps
For a particular set of reaction conditions, the percentage yield provides a measure of the degree to which the limiting reagent is converted into the desired products.
The percentage yield can be used to calculate the cost of reactant(s) required to produce a given mass of product.
Atom Economy
The atom economy measures the proportion of the total mass of all starting materials successfully converted into the desire prodcut.
Reactions which have a high percentage yield may have a low atom economy value if large quantities of unwanted by-products are formed.
Reaction Efficiency In Industry
In order to ensure that costly reactant(s) are converted into product, an excess of less expensive reactant(s) can be used.
Whilst the use of excess reactants may help to increase the percentage yield, this will lower the atom economy. Recycling unused reactants improves the efficieny of the process. A balance between economic and environmental considerations must be achieved.
