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AQA Module C2 Reversible Reactions & The Haber Process. Reactants Products. Reactants Products. Most chemical reactions only ‘go’ one way. A few reactions go both ways. These are called ‘REVERSIBLE’. HAND WARMERS USE A REVERSIBLE REACTION. Click disc.
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AQA Module C2 Reversible Reactions & The Haber Process
Reactants Products Reactants Products Most chemical reactions only ‘go’ one way. A few reactions go both ways. These are called ‘REVERSIBLE’
HAND WARMERS USE A REVERSIBLE REACTION Click disc • These are plastic bags containing sodium acetate crystals in solution. • Put bag in hot water for a few minutes and the cloudy liquid goes clear as the crystals dissolve • Let it cool down • Click the metal disc inside and the clear liquid goes cloudy again and gets HOT. HEAT IN ENDOTHERMIC HEAT OUT EXOTHERMIC FORWARD REACTION REVERSE REACTION
In a reversible reaction, the REACTANTS turn into PRODUCTS AND the PRODUCTS can be turned back into the REACTANTS BLUE COPPER WHITE COPPER + WATER SULPHATE SULPHATE (STEAM) REACTANT PRODUCTS FORWARD REACTION REVERSE REACTION ENDO-THERMIC EXO-THERMIC Heat IN Heat OUT
White copper sulphate (PRODUCT) Water (PRODUCT) Blue copper sulphate (REACTANT) Equilibrium If 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
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 Heat IN
H H H H H N N N The HABER PROCESS for producing AMMONIA • The production of ammonia is a very important process as it is used to make fertilizers to grow more food crops. • Ammoniais made from: • nitrogen, which is removed from the air • hydrogen, made from methane (natural gas) The Haber Process is a reversible reaction where both reactants and the product are GASES.
THE REACTION Production of ammonia from nitrogen and hydrogen H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H N N N N N N N N N2 + 3H2 2NH3 Nitrogen (g) + hydrogen (g) ammonia (g)
FORWARD REACTION: Hydrogen & Nitrogen make Ammonia EXOTHERMIC H H H H H H H H H H H H N N N N ENDOTHERMIC BOTH REACTIONS ARE HAPPENING AT ONCE REVERSE REACTION: Ammonia makes Hydrogen & Nitrogen
hydrogen + nitrogen ammonia HYDROGEN NITROGEN AMMONIA % YIELD = % AMMONIA in main reaction vessel HYDROGEN & NITROGEN IN TEMPERATURE and PRESSURE of the reaction vessel can be controlled UNUSED HYDROGEN & NITROGEN RECYCLED Mixture cooled here. AMMONIAcondenses LIQUID AMMONIA REMOVED
How to make the most ammonia quickly & cheaply ? We can change 2 conditions inside the reaction vessel: PRESSURE TEMPERATURE 500 atmospheres 500°C ££ EXPENSIVE ££ ££ CHEAP ££ 20°C (normal temp) 1 atmosphere (normal pressure)
EFFECT OF TEMPERATURE COOL HOT HYDROGEN 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) 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.
HYDROGEN NITROGEN AMMONIA H H H H H H H H H H H H H H H H H H N N N N EFFECT OF PRESSURE 2 molecules 4 molecules Because the FORWARD reaction produces a SMALLER NUMBER OF MOLECULES, a HIGHERPRESSURE 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.
A COMPROMISE solution 100% 200°C 80% 300°C Yield of ammonia 60% 400°C IRON CATALYST used to speed reaction up further 40% 500°C 20% 0% 0 100 200 300 400 500 Pressure (atmospheres) 450°C Gives about 30% yield 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