1. 12.5 Do Chemical Reactions Always Release Energy?
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12.5 Do Chemical Reactions Always Release Energy?
Heolddu Comprehensive School

2. Rates of Reaction – A reminder
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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:
The temperature of the reactants,
Their concentration
Their surface area
Whether or not a catalyst is used

3. Endothermic and exothermic reactions
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Step 1: Energy must be SUPPLIED to break bonds:
Energy
Step 2: Energy is RELEASED when new bonds are made:
Energy
A reaction is EXOTHERMIC if more energy is RELEASED then SUPPLIED. If more energy is SUPPLIED then is RELEASED then the reaction is ENDOTHERMIC

4. Energy level diagrams Energy level Activation energy
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Energy level
Activation energy
Using a catalyst might lower the activation energy
Energy given out by reaction
Reaction progress

5. Exothermic vs endothermic:
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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)

6. Examples of Energy Profile Diagrams
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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.

7. 14/04/2017
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 endothermic chemical reaction without showing the activation energy.
Energy level diagram for an exothermic chemical reaction without showing the activation energy.

8. The reverse reaction can be used as a test for water
Reversible Reactions
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Some chemical reactions are reversible. In other words, they can go in either direction: A + B C D
e.g. Ammonium chloride
Ammonia + hydrogen chloride
NH4Cl
NH3 + HCl
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 + H2O
CuSO4.5H2O
The reverse reaction can be used as a test for water

9. Endothermic reactions
Reversible Reactions
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When 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:
Exothermic reactions
Increased temperature:
Decreased temperature: A + B C D A + B C D
More products
Less products A + B C D A + B C D
H Tier only
H Tier only
Less products
More products

10. Reversible reactions and effect of temperature
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Reversible reactions and effect of 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

11. Equilibrium in reactions involving gases
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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.
N2 + 3H NH3
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!

12. Making Ammonia Nitrogen + hydrogen Ammonia N2 + 3H2 2NH3
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Fritz Haber,
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.
Nitrogen + hydrogen Ammonia
N2 + 3H NH3
To produce ammonia from nitrogen and hydrogen you have to use three conditions:
High pressure
450O C
Iron catalyst
Mixture of NH3, H2 and N2. This is cooled causing NH3 to liquefy.
Nitrogen
Hydrogen
Recycled H2 and N2

13. Ammonia + nitric acid Ammonium nitrate
Uses of Ammonia
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Ammonia is a very important chemical as it can be used to make plant fertilisers and nitric acid:
Water and oxygen
Nitrogen monoxide
Ammonia gas
Oxygen
Hot platinum catalyst
Nitrogen monoxide
Nitric acid
Cooled
More ammonia can then be used to neutralise the nitric acid to produce AMMONIUM NITRATE (a fertiliser rich in nitrogen).
Ammonia + nitric acid Ammonium nitrate
NH3 + HNO NH4NO3
The trouble with nitrogen based fertilisers is that they can also create problems – they could contaminate our drinking water.

14. Higher Tier Only-Haber Process:The economics
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A while ago we looked at reversible reactions: A + B C D
Endothermic, increased temperature A + B C D
Exothermic, increase temperature
Nitrogen + hydrogen Ammonia
N2 + 3H NH3
Endothermic
Exothermic
1) If temperature was DECREASED the amount of ammonia formed would __________...
However, if temperature was INCREASED the rate of reaction in both directions would ________ causing the ammonia to form faster
If pressure was INCREASED the amount of ammonia formed would INCREASE because there are less molecules on the right hand side of the equation

15. Haber Process Summary
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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
To compromise all of these factors, these conditions are used:
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.