5.2 Quantitative analysis Flashcards
Concentration
Concentration (moles / dm^3) = number of moles of solute (mol) / volume of solution (dm^3)
Titration calculations
Moles (mol) = volume (dm^3) x concentration (moles / dm^3)
Yield
Yield is the term used to describe the amount of product you get from a reaction.
In practice, you never get 100% yield in a chemical process for several reasons.
These include:
Some reactants may be left behind in the equipment.
The reaction may be reversible and in these reactions a high yield is never possible as the products are continually turning back into the reactants.
Some products may also be lost during separation and purification stages such as filtration or distillation.
There may be side reactions occurring where a substance reacts with a gas in the air or an impurity in one of the reactants.
Products can also be lost during transfer from one container to another.
Actual and theoretical yield
The actual yield is the recorded amount of product obtained
The theoretical yield is the amount of product that would be obtained under perfect practical and chemical conditions.
It is calculated from the balanced equation and the reacting masses
The percentage yield compares the actual yield to the theoretical yield
For economic reasons, the objective of every chemical producing company is to have as high a percentage yield as possible to increase profits and reduce costs and waste.
Percentage yield
Percentage yield = (actual yield / theoretical yield) x 100
Atom economy
Along with the percentage yield, atom economy is used to analyse the efficiency of reactions.
Most reactions produce more than one product and very often some of them are not useful.
Atom economy studies the amount of reactants that get turned into useful products.
It illustrates what percentage of the mass of reactants become useful products.
It is used extensively in the analysis of systems and procedures in industries, in an effort to obtain sustainable development.
It is also a very important analysis for economic reasons as companies prefer to use processes with higher atom economies.
The higher the atom economy of a process then the more sustainable that process is.
Atom economy = (total Mr of desired product / total Mr of all product) x 100
Choosing a reaction pathway
Reactions that have low atom economies use up a lot of resources and produce a lot of waste material which then needs to be disposed of, a very expensive procedure.
These reactions are thus unsustainable as they use up too much raw material to manufacture only a small amount of product.
They are not economically attractive as raw materials tend to be expensive, as does waste disposal which requires chemicals, equipment, space and transport.
Companies continually analyse reactions and processes and evaluate several factors in an effort to improve efficiency.
Atom economy, percentage yield, rates of reaction and equilibrium position are important factors which need to be considered when choosing a reaction pathway.
High percentage yields and fast reaction rates are desirable attributes in industrial chemical processes.
In reversible reactions, the position of the equilibrium may need to be changed in favour of the products by altering reaction conditions.
If the waste products can be sold or reused in some way that would improve the atom economy.
Alternative methods of production could also be considered that may produce a more useful by-product.
Molar volume
At room temperature and pressure, the volume occupied by one mole of any gas was found to be 24 dm3 or 24,000 cm3.
This is known as the molar gas volume at RTP.
RTP conditions are 20 ºC and 1 atmosphere (atm).
Volume (dm^3) = amount of gas (mol) x 24 (dm^3 / mol)
Avogadro’s law
In 1811 the Italian scientist Amedeo Avogadro developed a theory about the volume of gases.
Avogadro’s law (also called Avogadro’s hypothesis) enables the mole ratio of reacting gases to be determined from volumes of the gases.
Avogadro deduced that equal volumes of gases must contain the same number of molecules.
At room temperature and pressure(RTP) one mole of any gas has a volume of 24 dm3.
Practical 5 (acid alkali titration)
Aim: analyse the concentration of a solution
Procedure - Use the pipette and pipette filler and place exactly 25 cm3 sodium hydroxide solution into the conical flask.
Place the conical flask on a white tile so the tip of the burette is inside the flask.
Add a few drops of a suitable indicator to the solution in the conical flask.
Perform a rough titration by taking the burette reading and running in the solution in 1 – 3 cm3 portions, while swirling the flask vigorously.
Quickly close the tap when the end-point is reached (sharp colour change) and record the volume, placing your eye level with the meniscus.
Now repeat the titration with a fresh batch of sodium hydroxide.
As the rough end-point volume is approached, add the solution from the burette one drop at a time until the indicator just changes colour
Record the volume to the nearest 0.05 cm3.
Repeat until you achieve two concordant results (two results that are within 0.1 cm3 of each other) to increase accuracy.
