c2 4.4 haber process optimisation
DESCRIPTION
TRANSCRIPT
AQA C2 4.4
Reversible Reactions & The Haber Process
Most chemical reactions only ‘go’ one way.
A few reactions go both ways. These are called ‘REVERSIBLE’
Reactants Products Reactants Products
HAND WARMERS USE A REVERSIBLE REACTIONThese are plastic bags containing
sodium acetate crystals in solution. 1. Put bag in hot water for a few
minutes and the cloudy liquid goes clear as the crystals dissolve
2. Let it cool down3. Click the metal disc inside and the
clear liquid goes cloudy again and gets HOT.
FORWARD REACTION REVERSE REACTION
Click disc
HEAT INENDOTHERMI
C
HEAT OUTEXOTHERMI
C
In a reversible reaction, the REACTANTS turn into PRODUCTSANDthe PRODUCTS can be turned back into the REACTANTS
BLUE COPPER WHITE COPPER + WATERSULPHATE SULPHATE (STEAM)
REACTANT PRODUCTS
FORWARD REACTION
REVERSE REACTION
Heat IN Heat OUT
ENDO-THERMIC
EXO-THERMIC
White copper sulphate (PRODUCT)
Blue copper sulphate(REACTANT)
Water (PRODUCT)
EquilibriumIf we make a CLOSED SYSTEM by preventing any reactants or products escaping….
The RATE of the FORWARD REACTION balances the RATE of the REVERSE REACTION
So we have both REACTANTS AND PRODUCTS CONSTANTLY TURNING INTO EACH OTHER
And the system is in EQUILIBRIUM
Heat IN
Removal of Product (s)
If we want to make and collect one (or both) of the products in a reversible reaction, it must be steadily removed from the system as we go
WATER REMOVED so REVERSE REACTION can’t happen
so PRODUCT can be collected
The production of ammonia is a very important process as it is used to make fertilizers to grow more food crops.
Ammonia is made from:
• nitrogen, which is removed from the air
• hydrogen, made from methane (natural gas)
The HABER PROCESS for producing AMMONIA
The Haber Process is a reversible reaction where both reactants and the product are GASES.
N N
H H
NH H
H
H
N N
H
H H
H H
H
N N
H
H H
H H
NH H
H
THE REACTION Production of ammonia from nitrogen and hydrogen
N2 + 3H2 2NH3
H HH H
N N H HH H
H HH H
NH H
H
Nitrogen (g) + hydrogen (g) ammonia (g)
NHH
H
NHH
H
H
N N
H
H H
H H
FORWARD REACTION: Hydrogen & Nitrogen make Ammonia
REVERSE REACTION: Ammonia makes Hydrogen & Nitrogen
EXOTHERMIC
ENDOTHERMIC
BOTH REACTIONS ARE HAPPENING AT ONCE
HYDROGEN NITROGEN AMMONIA
HYDROGEN & NITROGEN IN
UNUSED HYDROGEN & NITROGEN RECYCLED
hydrogen + nitrogen
ammonia
TEMPERATURE and PRESSURE of the reaction vessel can be controlled
Mixture cooled here. AMMONIA condenses
LIQUID AMMONIA REMOVED
% YIELD =
% AMMONIA in main reaction
vessel
How to make the most ammonia quickly & cheaply ?
TEMPERATURE PRESSURE
We can change 2 conditions inside the reaction vessel:
500°C
20°C (normal temp)
500 atmospheres
1 atmosphere(normal pressure)
££ CHEAP ££
££ EXPENSIVE ££
EFFECT OF TEMPERATUREHYDROGEN NITROGEN AMMONIA Because the forward reaction is
exothermic (’releases heat’), the % YIELD of ammonia is GREATER at LOWER TEMPERATURES
(The ammonia molecules tend to split up again at high temps)
COOL HOT
So it would seem that the temperature needs to be LOW……
BUT… LOW TEMPERATURES make the rate of reaction SLOW so you would have to wait a long time…
So a higher temperature (450°C) is actually used to make the ammonia FASTER even though the yield is lower.
EFFECT OF PRESSUREH HH H
N N H HH H
H HH H
NH H
H
NH H
H
4 molecules 2 molecules
HYDROGEN NITROGEN AMMONIA
Because the FORWARD reaction produces a SMALLER NUMBER OF MOLECULES, a HIGHER PRESSURE makes a LARGER YIELD of AMMONIA
BUT using a HIGH PRESSURE means much stronger and MORE EXPENSIVE pipes and reaction vessels are needed.
So.. a MEDIUM PRESSURE of 200 atmospheres is used.
100 200 300 400 5000
20%
40%
60%
80%
100%
0%
200°C
300°C
400°C
500°C
Pressure (atmospheres)
Yie
ld o
f a
mm
on
ia
A COMPROMISE solution
450°C and 200 atmos.
TEMP:LOW enough for a reasonable yield but HIGH enough for a fast reaction
PRESSURE: LOW enough to not need expensive reinforced apparatus but HIGH enough to give a reasonable yield
450°C
IRON CATALYST
used to speed reaction up
further
Gives about 30%
yield