Chemical Energetics

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  • %%Energetics of a Reaction:%% Chemical reactions involve a transfer of energy between the system (the chemical reaction) and its surroundings.
Exothermic reactionEndothermic reaction
Heat energy is released into the surroundingsHeat energy is absorbed from the surroundings
Bond making reactionsBond breaking reactions
Surrounding temperature increasesSurrounding temperature decreases

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Energy Level Diagrams

  • Energy level diagrams show the relative energies of the reactant and product.

  • The energy change of a reaction is represented through the difference in height between the reactant and its product.

  • Activation energy: the minimum energy required for the reaction to take place

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Endothermic energy level diagram:

  • Energy is gained by the system; higher activation energy required

    Exothermic energy level diagram:

  • Energy is lost by the system; lower activation energy required

Bond Energy

  • This is the amount of energy required or released when a bond is formed or broken respectively. The unit measure of this energy is kJ/mol.

  • Formula for energy change:

  • %%If overall heat energy value is negative, reaction is exothermic%%

  • %%If overall heat energy value is positive, reaction is endothermic%%

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Production of Energy

  • %%Fuel%%: substance that can be used as a source of energy.
  • Burning fuels to form oxides is an exothermic reaction.
  • The heat from burning fuels is used in power plants to create steam from water and turn turbines.
  • A combustion process requires the presence of a fuel, oxygen and heat.

%%A good fuel is:%%

  1. Cheap
  2. Available in large quantities
  3. Liquid at room temperature
  4. Have high efficiency (produce a large amount of energy)
  5. Does not produce polluting gases

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Hydrogen

  • Produced by reacting methane gas with steam
  • Used in fuel cells and rockets
AdvantagesDisadvantages
Releases a lot of energyDifficult to transport as it is a gas at room temperature
Does not produce pollutantsForms explosive mixture with air when stored under pressure
Renewable and abundantIs expensive to produce (requires a lot of energy)

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%%Fuel Cell%%

  • In this electrochemical cell, fuel loses electrons at one porous electrode while oxygen gains electrons at the alternate porous electrode.
  • The product is water: 2H2 + O2 → 2H2 O

Reaction at anode:

2H2 → 4H+ + 4e-

Reaction at cathode:

4H+ + O2 + 4e- → 2H2 O

  • The flow of the electrons, through the electric circuit, from the cathode to the anode generates a current.

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