D) Percentage Yield and AtomEconomy
Learning intention
Learn how the efficiency of achemical reaction can bemeasured in terms of percentageyield and atom economy.
Making only as much as we need
PERCENTAGE YIELD
and ATOM ECONOMY
Reactants
(rawmaterials)
Products
Most of the substances we use every day are madefrom RAW MATERIALS, often through complexchemical reactions.
Chemical reactions
Pottington
Braunton
The chemical industry is a multi billion pound international industryproducing millions of products vital to our civilisation and well being.
Chemical Engineers play a crucial role and are much in demand –there are many opportunities and high levels of pay!
Chemical Engineers are much concerned with:
% YIELD and ATOM ECONOMY..
What isgreen chemistry?
•The sustainable design of chemical products andchemical processes.
•It minimises the use and generation of chemicalsubstances that are hazardous to human health or theenvironment.
Green chemistryprinciples
•Better to prevent waste than to treat it or clean it up.
•Chemical processes should aim to incorporate allreactants in the final product.
•Chemical processes should aim to use and generatesubstances with minimal toxicity to human health andthe environment.
The greenchemical industry
•Modern chemists design reactions with the highestpossible atom economy in order to minimise environmentalimpact.
•Chemists achieve this by reducing raw material andenergy consumption.
Percentage yield
•Historical method for evaluating reactionefficiency.
•Measures the proportion of the desired productobtained compared to the theoretical maximum.
•Gives no indication of the quantity of wasteproduced.
% YIELD is the amount of product you actually makeas a % of the amount you should theoretically make
+
SHOULDmake thismuch
ACTUALLYmake thismuch
REACTANTS
PRODUCT
%YIELDABOUT75%
Old fashioned example: Cement from limestone
Limekiln
CaCO3 CaO + CO2
LIMESTONE (calcium carbonate) is used to makeQUICKLIME (calcium oxide) for cement making
RFM: 100 56 44
So, THEORETICALLY, 100 tonnes of limestone shouldproduce 56 tonnes of quicklime.
BUT the ACTUAL YIELD is only 48 tonnes
So..the PERCENTAGE YIELD is only 48 x 100 = 87.5%
56
Why? –nextslide
PERCENTAGE YIELD 1
RAM
Ca 40
O 16
C 12
Very few chemical reactions have a yield of100% because:
• The raw materials (eg limestone) may not be pure
• Some of the products may be left behind in theapparatus
• The reaction may not have completely finished
• Some reactants may give some unexpected products
•Some reactions involve an equilibrium
Careful planning anddesign of the equipmentand reaction conditionscan help keep % yield high
PERCENTAGE YIELD 2
Atom economy
•In an ideal reaction, all reactant atoms end upwithin the useful product molecule. No waste isproduced!
•Inefficient, wasteful reactions have low atomeconomy.
•Efficient processes have high atom economy andare important for sustainable development. Theyconserve natural resources and create less waste.
Atom economy
•A measure of the proportion of reactant included inthe final useful product.
•A reaction may have a high percentage yield but alow percentage atom economy, or vice versa.
High atom economy
All reactant atoms included in the desiredproduct.
Low atom economy
Some reactant atoms not included in thedesired product.
CaCO3 → CaO + CO2
2Mg + O2 → 2MgO
USEFUL PRODUCT(antacids, fire resistantcoatings, electricalinsulators)
USEFUL PRODUCT(cement, glass,agriculture etc)
Atom Economy
Compare these two industrial reactions
What do you notice abouteach one?
Think raw materials, usefulproducts, waste products
REACTANTS
+
PRODUCTS
ATOM ECONOMY is the mass of the product youwant as a % of the mass of all the products you make
Stuff youwant
Stuff youalso getbut don’twant
ATOMECONOMYabout 50%
CALCULATING ATOM ECONOMY
Often, chemical reactions produce unwanted products alongwith the product you want.
CaCO3 → CaO + CO2
RFM: 100 56 44
Useful product
Waste product
ATOMECONOMY
mass useful product
mass of all reactants
=
X 100%
A
t
o
m
E
c
o
n
o
m
y
=
5
6
/
(
5
6
+
4
4
)
=
5
6
/
1
0
0
=
5
6
%
ATOM ECONOMY is the mass of product you want as a% of the mass of all the reactants
Fe2O3 + 3CO → 2Fe + 3CO2
160 84 112 132
Atom Economy = 80 / 80 x 100% = 100 % (obviously)
ATOM ECONOMY is the mass of product you want as a% of the mass of all the reactants
2Mg + O2 → 2MgO
RFM: 48 32 80
RAM
Mg 24Fe 56C 12
O 16
Atom Economy = 112 / 244 x 100% = 45.9 %
Find the atom economy for these 2 methods of extractingcopper:
1. Heat copperoxide with carbon
2. Heat coppersulphide withoxygen
2CuO + C → 2Cu + CO2
CuS + O2 → Cu + SO2
RAM Cu 64, O 16, C 12, S 32
2(80) 12 128 44
96 32 64 64
Mass of allreactants
160 + 12 = 172
Mass of allreactants
= 96 + 32 = 128
ATOM
ECONOMY
128
172
X 100
=
ATOM
ECONOMY
64
128
X 100
=
= 74.4 %
= 50 %
Mass ofcopper =128
Mass ofcopper = 64
Real example: Paracetamol
The non-prescription analgesicmarket (paracetamol, aspirin,ibuprofen) is worth about £21billion annually.
Maximising % yield and atomeconomy in the reactions at left isvital to save money and conserveenergy and resources
ATOM ECONOMY is themass of the product youwant as a % of the mass ofall the products you make
Stuff youwant
Stuff youalso get butdon’t want
ATOMECONOMYabout 50%
% YIELD is theamount of product youactually make as a % ofthe amount you shouldtheoretically make
SHOULDmake thismuch
ACTUALLYmake thismuch
%YIELDABOUT75%
SUMMARY
Choosing the mostEFFICIENT REACTIONto make the product
to…
Using the most EFFICIENTREACTION CONDITIONS& APPARATUS
to…
Raw materials are scarce and expensive and so mustbe carefully conserved. Also, chemical processesneed to produce as little waste as possible, minimisecosts, energy use and pollution.
CHEMICAL ENGINEERS must plan to maximise:
PERCENTAGE YIELD
by…
ATOM ECONOMYby…
Reduce energy use, costsand conserve raw materials
Reduce waste and pollution
Example 1
What is the percentage atom economy for the following reaction formaking hydrogen by reacting coal with steam?
C(s) + 2H2O(g) → CO2(g) + 2H2(g)
12 g 2(2 + 16) g [12 + (2 × 16)] g 2(2 × 1) g
12 g 36 g 44 g 4 g
Total mass of reactants Mass of desired product
= 12 + 36 = 48 g = 4 g
Example 1 (contd)
% atom economy = mass of desired product × 100
total mass of reactants
= 4 × 100
48
= 8.3%
This reaction route has a very low atom economy andis an inefficient method of producing hydrogen.
Example 2
Calculate the percentage atom economy for the reaction below.
C6H12 C6H12
Total mass of reactants Mass of desired product
= [(6 × 12) + (12 × 1)] = [(6 × 12) + (12 × 1)]
= 84 g = 84 g
Example 2 (contd)
% atom economy = mass of desired product × 100
total mass of reactants
= 84 × 100
84
= 100%
This reaction route has a very high atom economy asall reactant atoms are incorporated into the desiredproduct.
Example 3
Hydrazine (N2H4) is used for rocket fuel. Calculate the atom economy forhydrazine production.
Total mass of reactants Mass of desired product
= 34 + 74.5 = 108.5 g = 32 g
NH3
2 mol
34 g
NaOCl
1 mol
74.5 g
N2H4
1 mol
32 g
NaCl
1 mol
58 g
H2O
1 mol
18 g
Example 3 (contd)
% atom economy = mass of desired product × 100
total mass of reactants
= 32 × 100
108.5
= 30%
This reaction route has an atom economy of 30%. Theremaining 70% is waste product (NaCl and H2O).
Catalysts
•Have a crucial role in improving atom economy.
•Allow the development of new reactions requiring fewer startingmaterials and producing fewer waste products.
•Can be recovered and re-used.
•Allow reactions to run at lower temperatures, cutting energyrequirements.
Methanol and atom economy
Methanol
•Methanol is a potentialfuel of the future
•It can be made frommethane which in turncould be made byanaerobicfermentation of wasteorganic material
Laptop with methanol fuel cell
green technology
Methanol
•It is added to other fuels, it is the fuel fordrag car racing and it is being widely used infuel cells to power vehicles as it has not beeneasy to set up the infrastructure needed forhydrogen based fuel cells
Carbon neutral?
•If methane is combusted in a limited supply ofoxygen, carbon monoxide forms
• CH4 + ½ O2 → CO + 2H2
•The products are then passed over a catalyst at50-100 atmospheres and 250oC
•CO + 2H2 → CH3 OH
Carbon neutral?
CO + 2H2 → CH3 OH
28 4 32
Atom economy = mass of desired products x 100
Total mass of reactants
= 32 x 100 = 100%
16 + 16
Carbon neutral?
All the carbon in the methane is converted intocarbon in the methanol – it is said to be carbonneutral
The carbon dioxide released when methanol burnscontains the same amount of carbon that wasreclaimed from the CO
The amount of carbon dioxide released when themethanol burns is the same as the amount themethane would have released if it was burnedcompletely. No additional carbon dioxide resultsfrom the process
Carbon neutral?
•What were the conditions needed forstep 1? How were they achieved?
•What were the conditions needed forstep 2? How would these have beenachieved?
•In your opinion, how true is the claimthat the process is carbon neutral?
Limited supply of oxygen
High pressure and temperature – ieenergy costs (where will this energycome from?
Green Chemistry
•Look at the 12 principles of GreenChemistry
•How many of them apply to thisprocess? The catalysts are zincoxide, aluminium oxide or copper.
% yield
Percentage yield = actual yield/theoretical yield x 100%
Preparation of zinc sulphate
•Zinc sulphate is used as a paste,mixed with zinc oxide to treat acne.
•Aim to produce zinc sulphate andcalculate the % yield
Preparation of zinc sulphate
•It is an astringent– it closes thepores of the skinto keep outbacteria
•This productcontains peroxide,zinc sulphate, teatree oil and sage
Preparation of zinc sulphate
•It can also be used to preventsunburn!
What to do
•Add a weighing boat to the balance and zero it
•Add between 2.2 – 2.5 g zinc oxide
•Record the accurate mass
•Measure out 25cm3 1.0 mol l-1 H2SO4
•Heat the acid to about 50 oC
•Add the zinc oxide, stirring constantly
What to do next
•Cool the reaction mixture
•Weigh an evaporating dish and record the mass accurately
•Weigh a filter paper, fold the filter paper, place in a funnel andfilter the zinc sulphate solution into the dish
•Heat to nearly dry, then leave to fully evaporate and cool.
•Reweigh the dish and product
Calculation % Yield
Write the balanced equation
Which reactant is in excess and which onelimits the amount of product formed?
How much product is expected (using thelimiting reactant in the calculation)
Calculate the yield = actual mass x 100
expected mass
Preparation of sodiumcitrate % yield
Sodium citrate is a food additive(E331) e.g. in pepsi
What to do
•Weigh a beaker accurately
•Add approx 2g citric acid to the beakerand reweigh
•Add 25cm3 1.0 mol l-1 NaOH and stir untilall the crystals have dissolved
•Evaporate the beaker almost to drynessthen leave to dry completely
•Reweigh the beaker
For the calculation
3 mol NaOH reacts with 1 mol citric acid
Na
Na
Na
+ 3NaOH ↔
+ 3H2O
The calculation
1. 3NaOH + C6H8O6 → CH5O7Na3 + 3H2O
2. Work out the theoretical yield.
3. Use the actual yield from your results andthe theoretical yield to calculate thepercentage yield.