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C7 ChemistryC7 Chemistry

Reversible ReactionsReversible Reactions

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Exothermic vs endothermic:Exothermic vs endothermic:

EXOTHERMIC – more energy is given out than is taken in (e.g. burning, respiration)

ENDOTHERMIC – energy is taken in but not necessarily given out (e.g. photosynthesis)

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Reversible ReactionsReversible ReactionsSome chemical reactions are reversible. In other words, they can go in either direction:

A + B C + D

NH4Cl NH3 + HCl

e.g. Ammonium chloride

Ammonia + hydrogen chloride

If a reaction is EXOTHERMIC in one direction what must it be in the opposite direction?

For example, consider copper sulphate:

Hydrated copper sulphate (blue)

Anhydrous copper sulphate (white)

+ Heat

+ Water

CuSO4 + H2OCuSO4.5H2O

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Reversible ReactionsReversible ReactionsWhen a reversible reaction occurs in a CLOSED SYSTEM (i.e. no reactants are added or taken away) an EQUILIBRIUM is achieved – in other words, the reaction goes at the same rate in both directions:

A + B C + D

Endothermic reactions

Increased temperature:

Decreased temperature:

A + B C + D

A + B C + D

More products

Less products

Exothermic reactions

Increased temperature:

Decreased temperature:

A + B C + D

Less products

More products

A + B C + D

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Making AmmoniaMaking Ammonia

Nitrogen + hydrogen Ammonia N2 + 3H2 2NH3

•High pressure

•450O C

•Iron catalystRecycled H2 and N2

Nitrogen

Hydrogen

Mixture of NH3, H2 and N2. This is cooled causing NH3 to liquefy.

Fritz Haber, 1868-1934

Guten Tag. My name is Fritz Haber and I won the Nobel Prize for chemistry. I am going to tell you

how to use a reversible reaction to produce ammonia, a very important chemical. This is called

the Haber Process.

To produce ammonia from nitrogen and hydrogen you have to use three conditions:

A NOTEA NOTE

• A NOTE:

• The HABER process is not part of the curriculum however it provides a very good example of how equilibrium can impact an industrial process

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Le ChatelierLe Chatelier

Which way will the balance lie?

Le Chatelier did cry.

“It will move to cancel out

Whichever change you bring about!”

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Uses of AmmoniaUses of Ammonia

Nitrogen monoxide

Hot platinum catalyst

Ammonia gas

Oxygen

Cooled

Water and oxygen

Nitrogen monoxide

Nitric acid

Ammonia + nitric acid Ammonium nitrate NH3 + HNO3 NH4NO3

Ammonia is a very important chemical as it can be used to make plant fertilisers and

nitric acid:

More ammonia can then be used to neutralise the nitric acid to produce AMMONIUM NITRATE (a fertiliser rich in

nitrogen).

The trouble with nitrogen based fertilisers is that they can also create problems – they could contaminate our drinking

water.

12/04/23Haber Process: The Haber Process: The economicseconomicsA while ago we looked at reversible reactions:

A + B C + D

Endothermic, increased temperature

A + B C + D

Exothermic, increase temperature

ExothermicEndothermic

1) If temperature was DECREASED the amount of ammonia formed would __________...

2) However, if temperature was INCREASED the rate of reaction in both directions would ________ causing the ammonia to form faster

3) If pressure was INCREASED the amount of ammonia formed would INCREASE because there are less molecules on the right hand side of the equation

Nitrogen + hydrogen Ammonia

N2 + 3H2 2NH3

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Haber Process SummaryHaber Process Summary

•200 atm pressure

•450O C

•Iron catalyst

Recycled H2 and N2

Nitrogen

Hydrogen

Mixture of NH3, H2 and N2. This is cooled causing NH3 to liquefy.

To compromise all of these factors, these conditions are used:

A low temperature increases the yield of ammonia but is too slow

A high temperature improves the rate of reaction but decreases the yield too much

A high pressure increases the yield of ammonia but costs a lot of money