This is a summary of the topic "Energy changes" in the GCE O levels subject: Chemistry. Students taking either the combined science (chemistry/physics) or pure chemistry will find this useful. These slides are prepared according to the learning outcomes required by the examinations board.
2. ENERGY CHANGES
Energy cannot be created or destroyed, it can only be changed from one
form to another. All chemical reactions involve energy changes.
Energy change has taken place when there is a rise or drop in temperature
to the surroundings.
Two types of energy change
Exothermic
Endothermic
3. EXOTHERMIC CHANGES
An exothermic change is a change in which heat is given out to the
surroundings.
When an exothermic change occurs,
Heat is released from the reactants and transferred to the surroundings
The temperature of the reaction mixtures rises
The container feels warmer
4. EXOTHERMIC CHANGES
Physical process Chemical reaction
Condensation Combustion of fuels
Freezing Respiration
Dissolving of anhydrous salts (sodium
carbonate) in water
Neutralisation (reaction between acid and
alkali)
Dissolving of acids (e.g. concentrated
sulfuric acid) in water
Corrosion of metal (e.g. rusting of iron)
6. EXOTHERMIC CHANGES
During an exothermic change, the temperature of the reaction mixtures
rises until it reaches the highest temperature. When the reaction is
completed, no more heat is given off and the temperature of the reaction
mixtures falls back to room temperature.
7. ENDOTHERMIC CHANGES
An endothermic change is a change in which heat is absorbed from the
surroundings.
When an endothermic change occurs,
Heat is absorbed and transferred from the surroundings to the reactants
The temperature of the reaction mixture falls
The container feels cooler
8. ENDOTHERMIC CHANGES
Physical process Chemical reaction
Evaporation Photosynthesis
Melting Action of light on silver bromide in
photographic film
Dissolving of some ionic compounds (e.g.
ammonium chloride, potassium chloride
and hydrated copper (II) sulfate) in water
Thermal decomposition (e.g. heating of
calcium carbonate to produce calcium
oxide and carbon dioxide)
10. ENDOTHERMIC CHANGES
During an endothermic change, the temperature of the reaction mixtures
falls until it reaches the lowest temperature. When the reaction is
completed, no more heat is absorbed and the temperature of the reaction
mixtures rises back to room temperature.
11. ENTHALPY CHANGE IN A REACTION
The enthalpy change refers to the amount of energy involved in the
reaction.
Measured in kilojoules (kJ) and represented by the symbol △H. △H simply
means “change in energy content”.
12. ENTHALPY CHANGE IN A REACTION
The enthalpy change, △H, represents the difference in energy content of
the reactants and products.
△H = total energy of products – total energy of reactants.
When △H is negative, it is an exothermic reaction.
When △H is positive, it is an endothermic reaction.
13. ENERGY LEVEL DIAGRAMS
Energy level
Reactants
Products
heat of reaction
(△H = negative)
Exothermic reaction: △H < 0
Exothermic reaction
14. ENERGY LEVEL DIAGRAMS
Energy level
Reactants
Products
heat of reaction
(△H = positive)
Endothermic reaction: △H > 0
Endothermic reaction
15. BOND BREAKING AND BOND MAKING
All reactions involve the breaking of bonds or the forming of new bonds.
Energy changes in reactions are caused by the making and breaking of
chemical bonds.
Heat energy is absorbed when breaking bonds while heat energy is
released when forming bonds.
Bond breaking is an endothermic process.
Bond forming is an exothermic process.
16. EXOTHERMIC OR ENDOTHERMIC
CHANGE?
The overall enthalpy change will indicate whether a reaction is exothermic
or endothermic.
Overall enthalpy change (△H) = total energy absorbed for bonding
breaking – total energy released for bond making
17. EXOTHERMIC OR ENDOTHERMIC
CHANGE?
Exothermic:
total energy absorbed for bond breaking < total energy released for bond
making
Endothermic
total energy absorbed for bond breaking > total energy released for bond
making
18. EXOTHERMIC OR ENDOTHERMIC
CHANGE?
Let’s consider the reaction between hydrogen and oxygen to produce
water:
hydrogen + oxygen water
2H2(g) + O2(g) 2H2O(l)
Is this reaction exothermic or endothermic? We will need to do some
calculations to find out.
19. EXOTHERMIC OR ENDOTHERMIC
CHANGE?
Given data:
Energy required to break 1 mol of O=O bonds = 496 kJ
Energy required to break 1 mol of H=H bonds = 436 kJ
Energy required to break 2 mol of H=H bonds = 2 X 436 kJ = 872 kJ
Total energy absorbed for bond breaking = 872 + 496 = 1368 kJ
20. EXOTHERMIC OR ENDOTHERMIC
CHANGE?
Given data
Energy released on forming 1 mol of O-H bonds = 463 kJ
Energy released on forming 2 mol of O-H bonds in 1 mol of H2O molecules = 2
X 463 kJ = 926 kJ
Energy released on forming 2 mol of H2O molecules = 2 X 926 kJ = 1852 kJ
21. EXOTHERMIC OR ENDOTHERMIC
CHANGE?
Overall heat change:
△H = total energy of products – total energy of reactants = 1368 – 1852 = -484
kJ
The negative △H indicates that the reaction is exothermic.
22. ACTIVATION ENERGY
Activation energy, Ea, is the minimum amount of energy that reactant
particles must possess in order for a chemical reaction to occur.
For a reaction to occur, reactant particles must collide to form products.
They must possess energy equal to or more than the activation energy in
order for bond breaking, followed by bond forming.
23. ENERGY PROFILE DIAGRAMS
Energy profile diagrams show the enthalpy change and activation energy
of a reaction.
24. FUELS
Fuels are a source of energy and we burn these substances to give off
energy, an exothermic process.
Examples include wood, coal, petrol and natural gas.
25. COMBUSTION OF FUELS
Coal is made up of carbon and it burns in excess air to give carbon dioxide
and produce heat energy.
carbon + oxygen carbon dioxide + heat energy
C(s) + O2(g) CO2(g) + heat energy
26. COMBUSTION OF FUELS
Most fuels are hydrocarbons (compounds containing hydrogen and
carbon only) and when burnt, produces carbon dioxide, water and heat.
We have an example of the combustion of natural gas in excess air.
methane + oxygen carbon dioxide + water vapour + heat energy
CH4(g) + 2O2(g) CO2(g) + 2H2O(g) + heat energy
27. INCOMPLETE COMBUSTION
Incomplete combustion occurs when there is limited supply of oxygen.
Carbon particles (soot) and a poisonous gas, carbon monoxide are
produced. We have an example of the incomplete combustion of natural
gas in limited air.
methane + oxygen carbon monoxide + water vapour + carbon + heat
energy
4CH4(g) + 5O2(g) 2CO(g) + 8H2O(g) + 2C(s) + heat energy
28. FUEL CELLS
A fuel cell uses a fuel and oxygen to produce electricity directly.
Example of a fuel cell is the hydrogen-oxygen fuel cell.
29. FUEL CELLS
A hydrogen-oxygen fuel cell is a chemical cell which uses hydrogen as the
fuel. The combustion process produces water and heat energy.
hydrogen + oxygen water
2H2(g) + O2(g) 2H2O(g) △H = -484 kJ (exothermic reaction)
30. ADVANTAGES OF USING HYDROGEN
AS A FUEL
Hydrogen does not cause pollution because only water is produced during
combustion.
Hydrogen is a renewable source of energy.
Hydrogen is an efficient source of energy. It gives off more than twice as
much energy compared to other fuels.
31. ADVANTAGES OF USING HYDROGEN
AS A FUEL
There are no cheap sources of hydrogen. Hydrogen is obtained by
electrolysis, which is an expensive process. Hydrogen is currently via
cracking of alkanes.
Hydrogen is highly flammable and explosive.
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