C5 Flashcards
What is the ‘yield of a product’?
the mass of that product made in a chemical process
What is theoretical yield?
Maximum mass it is possible to make from a given mass of reactants
What law is used to calculate theoretical yield?
Law of Conservation of Mass
What do you need to calculate theoretical yield?
- mass of limiting reactant
- relative formula masses of reactants and products
- the balanced equation for the reaction
Formula for theoretical yield?
theoretical yield = (mass of limiting reactant / sum of Mr for limiting reactant) x Sum of Mr for all products
What is sustainable development?
The idea that you should meet your needs without damaging the ability of future generations to meet their needs
What does having high percentage yields and atom economies do?
Reduces the use of raw materials and production of waste
What is ‘actual yield’?
The mass of the product you actually make in a chemical reaction
Formula for percentage yield?
Percentage yield = (actual yield / theoretical yield) x 100
What
is the first thing that affects percentage yield?
Reactants may react in a different way than expected e.g. If you burn lithium in air to make lithium oxide, you might make lithium nitride instead
What is the second thing that affects percentage yield?
The reaction may not go to completion e.g. Some of the reactants present do not react
When might a reaction not go to completion?
In reversible reactions
What is the third thing that affects percentage yield?
You may lose some of the product when you separate it from the reaction mixture / purify it e.g. During filtration or when transferring between containers
What is ‘atom economy’?
A measure of how many atoms in the reactants form a desired product
Formula for atom economy
Atom economy % = (sum of Mr of the desired product / sum of Mr of all products) x 100
What is a ‘reaction pathway’?
A reaction or a series of reactions for making a particular substance
What factors do chemical engineers consider when choosing a reaction pathway?
- Yield of the product
- Atom economy of the reaction
- Usefulness or otherwise of by-products
- Rate of the reaction
- Equilibrium position, if it is a reversible reaction
Chemical formula for epoxyethane?
C2H4O
What is epoxyethane made from?
Ethane, a compound obtained from crude oil
How many steps was the original pathway for making epoxyethane?
2
Why was the original pathway for epoxyethane changed?
- It was a complex pathway
* It had high percentage yield (80%) but low atom economy (24.5%)
Overall reaction pathway that was original for epoxyethane
ethene + chlorine + calcium hydroxide -> calcium chloride + water + epoxyethane
What is the modern process for epoxyethane?
Ethene + oxygen -> epoxyethane
What does the modern process for epoxyethane use?
Silver as a catalyst
What is the difference between the original pathway and the modern pathway?
- Both yields are 80%
- Modern process has 100% atom economy
- Both processes waste some reactants
- The higher atom economy of the modern process means it is more efficient
What is a ‘by-product’?
A substance formed in reaction in addition to the desired product
How do you improve the atom economy of a reaction?
By selling the by-products because they then become a desirable product
What is a reason for low atom economy?
Toxic and useless by-products
Convert from dm3 to cm3
x 1000
Formula for concentration in g/dm3
concentration in g/dm3 = mass of solute in g / volume of solution in dm3
Formula for concentration in mol/dm3
concentration in mol/dm3 = amount of solute in mol / volume of solution in dm3
Formula for moles
moles = mass / molar mass
What is a titration?
A technique that uses a neutralisation reaction to find the concentration of an acid or an alkali
How do you calculate the concentration in a titration?
- Find the amount of solute in mol of the standard solution using the concentration formula
- Mol of standard solution / volume (titre value) of unknown solution
What is a standard solution?
A solution with a known accurate concentration and volume, used in titrations
Steps for titration:
- Use a volumetric pipette and pipette filler to measure alkali into a conical flask.
- Add a few drops of phenolphthalein indicator
- Fill a burette to the top with acid until the bottom of the meniscus touches the last line.
- Take an initial burette reading.
- Add the acid.
- Swirl the flask to mix its contents.
- Stop at the end-point.
- Take a final burette reading.
- Calculate the difference between the two readings (titre value).
- Repeat steps 1-8 two more times until you obtain around 2 concordant titres (within 0.10cm3 of each other).
To get a repeatable titre, you need to…
- Swirl the flask to mix its contents
- Do a rough run to get an idea of what the titre is, so you can quickly add the acid within a few cm3 of the rough titre, then add drop by drop
You get accurate readings by…
- Clamping the burette vertically
* Using a volumetric pipette