C5.1 - Monitoring Reactions Flashcards
how do you calculate the percentage yield?
(real yield / theoretical yield)x 100
what does calculating the percentage yield show?
the efficiency of a reaction
define the term ‘actual yield’
mass of product made in a chemical reaction
define the term ‘theoretical yield’
maximum mass it is possible to make from a given mass of reactants (in perfect conditions)
define the term ‘percentage yield’
proportion of the theoretical yield actually obtained during a reaction
state 3 factors that prevent a percentage yield from reaching 100%
- reactants may burn in different ways than expected
- some reactants present may not react
- loss of some reactants
5.75g of sodium is reacted with chlorine to produce sodium chloride. If the actual yield is 10.2g, calculate the percentage yield. (4 marks)
2Na + Cl2 -> 2NaCl
10.2 / 14.625
= 70%
define atom economy
a measure of the amount of reactants that become useful products
what is the equation to calculate atom economy?
(sum of Mr of desired product / sum of Mr of all products) x 100
how much is one cm3 in dm3?
1 cm3 = 0.001 dm3
how much is 1 dm3 in cm3?
1 dm3 = 1000cm3
what is the equation to calculate concentration in g/dm3?
mass of solute (g) / volume of solution (dm3)
what is the equation to calculate concentration in mol/dm3?
amount of solute (mol) / volume of solution (dm3)
what are the factors when choosing g a reaction pathway?
5
- yield of the product
- atom economy of the product
- usefulness or otherwise of by-products
- rate of reaction
- equilibrium position, if it is a reversible reaction
if the atom economy of a process is high, how does it affect the efficiency?
it has a higher efficiency (less waste)
how does the by-product of a reaction affect the atom economy?
- if the by-product is useful the atom economy is higher (can sell it for another use)
- if not, the atom economy is lower
what makes the reaction pathway of a substance less desirable?
by products which are toxic, or of little use
what are titrations for?
to calculate the volume of acid/alkali needed to neutralise the acid/alkali
describe colour changes of methyl orange
yellow in alkali → red in acid
describe the colour changes of phenolphthalein
pink in alkali → colourless in acid
describe the process of how titrations are done
1) add acid to a burette using a funnel, record the volume in the burette to start
2) add 25cm3 of alkali to a conical flask with volumetric pipette and pipette filler
3) and add some indicator
4) place conical flask on a white tile (see colour change clearly)
5) add acid to alkali slowly until you see a colour change completely
6) calculate how much acid has been added (TITRE)
7) repeat until you get concordant titres
what do indicators show?
the end point of titrations (changes colour)
what is a burette?
a long graduated glass tube with a gap at the bottom
what is used to measure a alkali/acid when adding it to the conical flask?
volumetric pipette/ measuring cylinder
what must be used to fill a pipette safely?
pipette filler (cone)
what is a titre?
the difference between the final burette reading and the inital burette reading / the volume of acid added to the alkali
describe two precautions needed to obtain a repeatable titre?
- burette must be clamped vertically
- swirl the flask to mix its contents
Define the term ‘limiting reactant’
- the reactant that is completely used up
- therefore, it limits the amount of product
how does the concentration of a solution relate to the mass of the solute and the volume of the solution?
smaller volume/ larger number of moles = higher concentration
larger volume/smaller number of moles = lower concentration
describe the relationship between molar amounts of gases and their volumes
- equal amounts of moles of gases occupy the same volume under the same conditions and pressure (RTP)
how do you calculate the volume a gas takes up in room temperature (RTP - room temperature + pressure)
- if you have the number of moles
volume of gas at RTP (dm^3) = mol x 24
how do you calculate the volume of gas at RTP in cm^3
volume (cm^3) = mol x 24 000
1cm3 = (in ml)
1 ml
1dm3 = (in l)
1 litre
Pb - why don’t you get 100% yield
- side products form
- reaction not going to completion.
- losing products when separating it making the substance
Pb - definition of atom economy
A measure of how many atoms in the products, make up the desired product
Atom economy is a measure of how many of the atoms in the reactants end up in the desired product.
How do you read a Burette
Water forms a crescent-shape called a meniscus.
The volume of a burette is designed be read from the bottom of the meniscus. If you are not at eye-level you will not get the correct reading.
What type of error is caused by not reading off the bottom of a meniscus
Systematic error
Explain why it is important to rinse the burette and volumetric pipettes with their solutions before using them in the experiment.
This removes any chemicals left behind that might react with the substances in this experiment. The solution used to rinse is considered contaminated and is discarded.
Why is a “rough titration” performed before taking other results?
In order to know the approximate volume needed to reach the end point. It would be extremely time-consuming to add drop-wise from the very beginning. The rough result usually overshoots the end-point and so can’t be used.
Why is it important a single indicator is used, rather than a mixture of indicators such as universal indicator?
A mixture of indicators would make it very difficult to see a clear colour change at the end point.
Why is it important to only add a few drops of indicator to the conical flask?
You could alter the volume of solution, but more importantly: indicators may form precipitates that make the solution cloudy.
What is the molar gas volume in words
1 mole of a gas takes up 24dm3 of volume under standard conditions.
What does standard conditions mean
room temperature and pressure (25°C and 1 atmosphere of pressure).
