1. AQA C2 4.4 Reversible Reactions & The Haber Process

2. Most chemical reactions only ‘go’ one way. A few reactions go both ways. These are called ‘REVERSIBLE’ Reactants Products Reactants Products

4. In a reversible reaction, the REACTANTS turn into PRODUCTS AND the PRODUCTS can be turned back into the REACTANTS FORWARD REACTION REVERSE REACTION Heat IN Heat OUT ENDO-THERMIC EXO-THERMIC BLUE COPPER WHITE COPPER + WATER SULPHATE SULPHATE (STEAM) REACTANT PRODUCTS

5. 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 White copper sulphate (PRODUCT) Blue copper sulphate (REACTANT) Water (PRODUCT)

6. 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

8. THE REACTION Production of ammonia from nitrogen and hydrogen N 2 + 3H 2 2NH 3 Nitrogen (g) + hydrogen (g) ammonia (g) H N N H H H H H H N N H H H H H N H H H H H H H N N H H H H H H H H N H H H

9. FORWARD REACTION: Hydrogen & Nitrogen make Ammonia REVERSE REACTION: Ammonia makes Hydrogen & Nitrogen BOTH REACTIONS ARE HAPPENING AT ONCE N H H H N H H H H N N H H H H H EXOTHERMIC ENDOTHERMIC

10. HYDROGEN & NITROGEN IN UNUSED HYDROGEN & NITROGEN RECYCLED TEMPERATURE and PRESSURE of the reaction vessel can be controlled Mixture cooled here. AMMONIA condenses LIQUID AMMONIA REMOVED HYDROGEN NITROGEN AMMONIA hydrogen + nitrogen ammonia % YIELD = % AMMONIA in main reaction vessel

11. 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 ££

12. EFFECT OF TEMPERATURE 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. COOL HOT

13. EFFECT OF PRESSURE 4 molecules 2 molecules 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. H H H H N N H H H H H H H H N H H H N H H H HYDROGEN NITROGEN AMMONIA

14. 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 Gives about 30% yield 100 200 300 400 500 0 20% 40% 60% 80% 100% 0% 200 °C 300 °C 400 °C 500 °C Pressure (atmospheres) Yield of ammonia IRON CATALYST used to speed reaction up further