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