1. 10/11/2018
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Energy Changes
AQA 2016 Chemistry topic 5

2. 5.1 Exothermic and Endothermic Reactions
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5.1 Exothermic and Endothermic Reactions

3. Grouping reactions Can you put these reactions into groups? Burning
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Photosynthesis
Can you put these reactions into groups?
Burning
Self-heating cans
Hand warmer packs
Cooling packs

4. Exothermic and Endothermic Reactions
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Exothermic reactions
Ones that GIVE OUT energy to the surroundings
Endothermic reactions
Ones that TAKE IN energy from the surroundings

5. Energy level diagrams Energy level
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Energy level
Activation energy – the energy needed to start the reaction
Energy given out by reaction
Reaction progress

6. Exothermic vs endothermic:
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EXOTHERMIC – more energy is given out than is taken in (e.g. burning, respiration)
ENDOTHERMIC – less energy is taken in than given out (e.g. photosynthesis)

7. Chemical Reactions recap
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A chemical reaction is when atoms are basically rearranged into something different. For example, consider burning methane:
Methane oxygen
Carbon dioxide + water
Energy is NEVER created or destroyed. Therefore the total amount of energy after the reaction must have been the same as before.

8. Endo and exo reactions in more detail (HT only)
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For a reaction to happen you have to break bonds and then make new ones. How is energy involved?
Step 1: Energy must be SUPPLIED to break bonds (the “activation energy”):
Energy
Energy
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.

9. Measuring energy in methane
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CH4 + 2O H2O + CO2
To burn methane you have to break all of these bonds:
And then you have to make these ones:

10. Bond energies (HT only)
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C-H = 435 Kj
O=O = 497 Kj
Total for breaking bonds = 4x x497 = 2734 KJ/mol
H-O = 464 Kj
C=O = 803 Kj
Total for making bonds = 2x x464 = 3462 KJ/mol
Total energy change = = KJ/mol

11. Drawing this on an energy diagram:
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3462 Kj
2734 Kj
-728 Kj
More energy is given out (3462) than is given in (2734) – the reaction is EXOTHERMIC. The total (“nett”) energy change is –728 Kj/mol. An endothermic reaction would have a positive energy change.

12. Bond energy values C-H = 435 KJ/mol O-H = 464 KJ/mol O=O = 497 KJ/mol
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C-H = 435 KJ/mol
O-H = 464 KJ/mol
O=O = 497 KJ/mol
C=O = 803 KJ/mol
C-O = 360 KJ/mol
C-C = 346 KJ/mol

13. Another example: ethanol
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C2H5OH + 3O CO2 + 3H2O
Total for breaking bonds = 5x435 (C-H) (C-O) (O-H) + 3x497 (O=O) +346 (C-C) = 4836 KJ/mol
Total for making bonds = 4x803 (C=O) + 6x464 (O-H) = 5996 KJ/mol
Energy change = =-1160 KJ/mol

14. Another example: methanol
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2CH3OH + 3O CO2 + 4H2O
Total for breaking bonds = 6x435 (C-H) + 2x360 (C-O) + 2x464 (O-H) + 3x497 (O=O) = 5749 KJ/mol
Total for making bonds = 4x803 (C=O) + 8x464 (O-H) = 6924 KJ/mol
Energy change = (divide this by two as we are dealing with two molecules of methanol) = KJ/mol

15. Example reactions (HT only)
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Reaction
Temp. after mixing/OC
Exothermic or endothermic?
Sodium hydroxide + dilute hydrochloric acid
Sodium hydrogencarbonate + citric acid
Copper sulphate + magnesium powder
Sulphuric acid + magnesium ribbon

16. 5.2 Chemical Cells and Fuel Cells (Chem only)
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5.2 Chemical Cells and Fuel Cells (Chem only)

17. Electric Current
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Here’s a battery (a “cell” really) powering a lightbulb: - + e-
How does the cell work? e-
A “battery” is technically a combination of cells.

18. How cells are made Basically, cells are made out of chemicals:
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Basically, cells are made out of chemicals:
Carbon rod with metal cap cathode
Manganese oxide
Ammonium chloride (electrolyte)
Zinc casing anode
In other words, a battery is basically two DIFFERENT metals in contact with an electrolyte. The voltage produced by the cell depends on the type of metals and electrolyte.

19. Increasing reactivity
Metals in Batteries
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Here are some common metals and their potentials. What do you notice about the potential compared to their reactivity?
Potassium
Sodium
Calcium
Magnesium
Aluminium
Carbon
Zinc
Iron
Lead
Hydrogen
Copper
Silver
Gold
Increasing reactivity
Metal
Potential / V
Lithium
-3.04
Potassium
-2.92
Calcium
-2.76
Sodium
-2.71
Magnesium
-2.37
Zinc
-0.76
Copper
+0.34
Silver
+0.80
Gold
+1.50

20. Alkaline vs Rechargeable Batteries
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In non-rechargeable batteries the chemical reactions stop when one of the reactants has been used up. Alkaline batteries are non-rechargeable.
In rechargeable batteries the chemical reactions are reversed using an external electric current.

21. The whole process is called a “redox” reaction.
Fuel Cells
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Fuel cells work by using the reaction between hydrogen and oxygen to produce an electric current:
The hydrogen atom loses an electron (this is called “oxidation”)
They then pass through the electrolyte
The oxygen atoms gain two electrons each (“reduction”).
The whole process is called a “redox” reaction.

22. Fuel Cell equations (HT only)
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Here are the equations for what happens in a fuel cell:
At the cathode, oxygen gains electrons and reacts with water to make hydroxide ions:
O2 + 4e- + 2H2O 4OH-
Then, at the anode, hydrogen combines with the hydroxide ions to make water and electrons:
2H2 + 4OH H2O + 4e-
Overall – 2H2 + O H2O

23. Words – construct, pollute, energy, efficient
Fuel Cells
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Fuel cells have many advantages over traditional fuels:
They do not _______
They are very _______
They transfer ______ directly
They are simple to ______
Words – construct, pollute, energy, efficient

24. Fuel Cells
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Fuel cells are also useful as a potential energy source for __________ as:
They provide _______
They are very _______
They have no ____ ____
They are _________
They are also being considered for cars but have two main disadvantages: fuel cells often have poisonous ________ and producing the hydrogen and oxygen will use energy elsewhere.
Words – lightweight, spacecraft, water, moving parts, efficient, catalysts