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4d) Equilibria & Haber Process (effect of heat on Ammonium Chloride…
4d) Equilibria & Haber Process
reversible reactions
definition = a reversible reaction is one which occurs simultaneously in both directions (i.e. forwards & backwards)
symbol ⇋
Dehydration of hydrated copper (II)
method
Place a spatula measure of hydrated copper (II) sulphate in a test tube
Heat gently over a Bunsen burner
Allow to cool
Add drops of distilled water
changes include:
when heated it turns white
when water was added back it turns blue again
The water of crystallisation is lost when heated, but is restored when cold water is added
Dynamic Equilibrium
forward reaction = the reaction from left to right (i.e. the normal way of reading a reaction) reactants to products
back reaction = the reaction from right to left - products to reactants
at equilibrium
rate of forward reaction = rate of backward reaction
concentration of reactants & products don't change anymore - become CONSTANT
but both reactions are still happening = dynamic
must be in a sealed container
manufacture of ammonia
equation for production of ammonia in Haber process
N2 (g) +3H2 (g) ⇋ 2HN3 (g)
△H = -92 KJ/mol
P↑ goes to side with fewer mols therefore exothermic forward reaction
P↓ to right hand side more NH3 made therefore endothermic back reaction
450 degrees C is a compromise between high yield/slow rate & low yield /fast rate.
low temp required to give a high yield of NH3; one forward reaction is exothermic. However at low temperatures one rate of reaction is too slow.
450 degrees C - (compromise ideally should be at low temp for high yield)
atm = unit you use to measure pressure
200atm used for this process because high P = high yield
pressure: compromise - as there are fewer molecules of gas on the right hand side, a high pressure is favoured for ammonia production. Increasing pressure also increases the rate of reaction. (brings mols close together). However, achieving very high pressure is uneconomic (high costs including safety)
(comprimise Lower P = lower yield but it's cheaper)
effect of heat on Ammonium Chloride
place a spatula measure of solid ammonium chloride in a test tube
heat gently over a Bunsen burner
white solid at the bottom of the tube disappears. A white gas can be observed in the test tube, a white solid forms near the mouth of the test tube
as the NH4Cl is heated it decomposes to form HCl (the white gas) and NH3 (a colourless gas). When it's cool the HCl and NH3 react to reform the NH4Cl
NH4Cl (s) ⇋ NH3 (g) + HCl (g)
NH3 (g) = alkali
HCl (g) = acid
NH4Cl = neutralised
cooling of the ammonia production reaction
the cooling of the reaction mixture liquefies the ammonia produced and allows the unused hydrogen and nitrogen to be recirculated
only about 15% of the nitrogen and hydrogen are converted at a time which is not a problem because any unreacted N2 and H2 can be recycled & used again
the ammonia is collected as NH3 has a higher boiling point than N2 or H2. So by cooling the mixture, NH3 can be collected as a liquid
the use of ammonia in the manufacture of nitric acid & fertilisers
ammonia reacts with acids to form ammonium salts, which make good fertilisers
e.g. 2NH3 (g) + H2SO4 (aq) → (NH4)2SO4 (aq)
Haber Process
this is a chemical process used industrially to form ammonia (NH3), a chemical which is commercially important.
as an industrial process conditions are chosen not to maximise yield, but to maximise profit
what is pressure a measure of? - number of gas molecules present
↑Pressure if the pressure is increased equilibrium will move to the side with fewest gas molecules (or moles) in order to reduce the pressure i.e. reduce the number of molecules
P↑ equilibrium moves to the right
P↓ equilibrium moves to the left
