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12.512.5Do Chemical Reactions Always Release Do Chemical Reactions Always Release

Energy? Energy?

Heolddu Comprehensive School

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Rates of Reaction – A reminderRates of Reaction – A reminder

Chemical reactions occur when different atoms or molecules collide:

For the reaction to happen the particles must have a certain amount of energy – this is called the ACTIVATION ENERGY.

The rate at which the reaction happens depends on four things:

1) The temperature of the reactants,

2) Their concentration

3) Their surface area

4) Whether or not a catalyst is used

18/04/23Endothermic and exothermic Endothermic and exothermic reactionsreactions

Step 1: Energy must be SUPPLIED to break bonds:

Step 2: Energy is RELEASED when new bonds are made:

A reaction is EXOTHERMIC if more energy is RELEASED then SUPPLIED. If more energy is SUPPLIED then is

RELEASED then the reaction is ENDOTHERMIC

Energy

Energy

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Energy level diagramsEnergy level diagrams

Energy level

Reaction progress

Activation energy

Energy given out

by reactionUsing a catalyst might lower the

activation energy

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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)

18/04/23Examples of Energy Profile Examples of Energy Profile DiagramsDiagrams

Very endothermic reaction with a big activation energy.

Very exothermic reaction with a small activation energy.

Moderately endothermic reaction with moderately high activation energy.

Moderately exothermic reaction with a moderately high activation energy.

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A small activation energy reaction with no net energy change. (Possible if the total energy absorbed by the reactants in bond breaking equals the energy released by bonds forming in the products)

Energy level diagram for an exothermic chemical reaction without showing the activation energy.

Energy level diagram for an endothermic chemical reaction without showing the activation energy.

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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 + H2OCuSO4.5H2OThe reverse reaction can be used as a test for

water

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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

H Tier only

H Tier only

18/04/23Reversible reactions and effect Reversible reactions and effect

of temperatureof temperature• If the temperature is raised, the yield from the

__________ reaction increases and the yield from the ________ reaction decreases.

• If the temperature is lowered, the yield from the endothermic reaction _______ and the yield from the exothermic reaction ________.

A + B C + D

Exothermic

Endothermic

Equilibrium in reactions involving Equilibrium in reactions involving gasesgases

• In gaseous reactions, an increase in pressure will favour the reaction that produces the least number of molecules as shown by the symbol equation for that reaction.

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N2 + 3H2 2NH3

There are 4 molecules on the left and 2 on the right

Therefore an increase in pressure would shift this reaction to the right – more ammonia is made!

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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:

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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.

18/04/23Higher Tier OnlyHigher Tier Only-Haber 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