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
Product Design
When designing the practical route required to produce a marketable product, there are several key considerations.
availability and cost of
feedstocks
sustainability of feedstocks
size of yield
energy requirements
formation of side products that can be recycled, used in the process or sold to increase profit
Ways to Minimise Energy Cost
- Conserving energy: Heat loss is minimised by insulating hot pipes throughout the plant
- Balancing heat energy: Heat energy from exothermic stages can be used to provide the energy needed for endothermic stages.
- Using catalysts: They allow reactions to take place at lower temperatures thus reducing energy costs.
- Recycling energy: Many processes require feedstocks to be heated and the products to then be cooled. With gasses or liquids, the heating and cooling can be achieved using heat exchangers.
- Site locations: Some industries save energy by siting their operating plant close to cheap power sources.
- On-site power: Most oil refineries save on energy costs by producing their own steam and electrical power on site by burning their own hydrocarbon gas.
- Selling energy: ‘Waste heat’ can be piped out as steam or hot water to provide heat for local hosuing or industry
- Alternative routes: When energy costs are large, chemical companies invest in research to find alternative manufacturing routes which use less energy.
In many chemical processess, a by-product is formed in addition to the desired product. If this by-product can be sold then this can make a process profitable.
It is important that the yield of the porudct is as high as possible. The reaction temperature, pressure and reactant concentrations can be altered to find the combination that gives the highest yield of product at the lowest cost.
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.
Solutions Calculations
For solutions, the mass of solute (g), the no. of moles of solute (mol), the volume of solution (l) or the concentration of the solution (mol l-1) can be calculated.
Molar Volumes of Gases
The mass of one mole of different substances usually differs. Although the mass of one mole is different, the volume of one mole of each gas is the same under the same conditions of temperature and pressure.
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 compare =d 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.
Reaction Efficiency
The efficiency with which reactants are converted into the desired product is measured in terms of the percentage yield and atom economy.
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.
