Chemical Energetics

Energetics of a Reaction

  • Exothermic reaction: one that releases heat energy into the surrounding

  • Endothermic reaction: one which absorbs heat energy from the surroundings

  • Bond breaking is endothermic

  • Bond making is exothermic

Exothermic reactionEndothermic reaction
Energy given out to the surroundingsEnergy is taken in from the surroundings
Surroundings become hotSurroundings become cold
Bond making -- exothermicBond breaking -- endothermic

Energy Level Diagrams

Bond Energy

  • This is the amount of energy consumed or liberated when a bond is broken or formed in kJ/mol

ΔH=Bond Breaking+Bond FormingΔH=BondBreaking+BondForming

  • If overall heat energy is negative, reaction is exothermic

  • If overall heat energy is positive, reaction is endothermic

Production of Energy

  • A fuel is a substance which can be conveniently used as a source of energy.

  • Burning fuels (like oil) 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.

  • In order for any material to combust three things must be present:

    • Fuel

    • Heat

    • Oxygen

  • A good fuel would:

    • Be cheap

    • Be available in large quantities

    • Ba a liquid at room temperature

    • Produce a large amount of energy when combusted

    • Not produce polluting gases

Hydrogen

  • Burns explosively with oxygen, so it is used in rockets.

  • In a fuel cell, it combines with oxygen without burning.

  • Produced by reacting methane gas with steam

AdvantagesDisadvantages
Produces a lot of energyDifficult to transport as it is a gas at room temperature
Abundant on earth (sea)Forms explosive mixture with air -- very dangerous
Less pollutant
Renewable fuel

Simple Cells

  • A cell is a device which converts chemical energy into electrical energy and is composed of two metals of different reactivity connected by an external circuit and an electrolyte

  • The process works due to the different reactivity of metals

  • Consists of a negative pole (the more reactive metal) and a positive pole (less reactive metal) and an electrolyte.

  • The greater the difference in reactivity of the two metals, the greater the voltage will be.

  • The electrons flow because one metal is more reactive, so it has a stronger drive to give up its electrons.

  • The atoms give up electrons and enter the solution as ions.

Radioactive Isotopes

  • Uranium-235 can be used in nuclear power stations to produce electricity

  • The radioactive isotope is bombarded by neutrons resulting in a lot of heat being produced

  • Small amount of radioactive fuel produces large amount of heat

  • Advantages: lots of energy is from a small amount and no CO~2~

  • Disadvantage: radioactive waste produced and non-renewable