Energy change

• different chemicals store different amounts of energy in their bonds
• Exothermic reaction profile

  A) reactants

  B) products

  C) overall energy change

  D) activation energy

  
• Endothermic reaction profile

  A) reactants

  B) products

  C) activation energy

  D) overall energy change

  
• exothermic reactions 


  energy is released to the surroundings

  most common = combustion (fuels are burnt in the presence of oxygen)

  also in neutralisation and oxidation

  energy being released to surrounding - most common way = heat
• endothermic reaction


  take in energy from surroundings

  products have have a higher amount of energy (so they are higher on the graph)
• Activation Energy


  Eact

  the minimum amount of energy the reactant particles need in order to start colliding with each other and reacting

  Higher arc, bigger amount of energy needed to start
• Bond energies 


  the amount of energy needed to break 1 mole of a particular covalent bond
• 1 mole 


  $6.02\times10^{23}$

  Avogadro's constant
• Breaking bonds = endothermic (requires energy from the surroundings)
• Bond formation = exothermic
• Total energy required to break a bond - total energy required to make bonds = overall reaction (KJ/mol)


  negative value = exothermic

  positive value = endothermic
• something is exothermic because more energy is released when the bonds are made than is needed to break the bonds
• something is endothermic because more energy is needed to break the bonds then the energy being released when bonds are formed
• cells 


  electrochemical cells, use chemicals to produce electricity

  cells can be made by connecting 2 different electrodes with a wire and placing them in an electrolyte

  a cell only produces electricity for a certain amount of time - chemicals will eventually run out - reaction stops

  cells only produce electricity if you use metals with different reactivities (the greater the reactivity difference, the greater the potential difference produced by cell) - electrolyte also effects potential difference
• electrodes 


  2 electrically conductive metals
• electrolyte


  a solution that can conduct electricity

  solution of ionic compounds = most common is potassium hydroxide (KOH)
• Cells diagram

  A) metal 1 - electrode

  B) metal 2 - electrode

  C) voltmeter

  D) electrolyte

  
• batteries -


  two or more cells connected in a series to produce a greater voltage

  A) cell 1

  B) cell 2

  C) cell 3

  D) voltmeter

  E) battery

  
• non - rechargeable cells/batteries 


  alkaline

  some point chemicals run out - no more electricity is produced - no way to reverse the reaction
• rechargeable cells/batteries


  can be recharged - we can reverse the chemical reactions when we apply an electrical current
• factors that effect voltage on cells/batteries

  • metals used for electrodes (the greater the difference in reactivity, the greater the voltage)
  • the type and concentration of electrolyte used
  • the conditions - eg temperature
• Fuel cells


  fuel cells get electrical energy from the movement of electrons through a wire

  as fuel (hydrogen) enters the cell, it becomes oxidised (loss of electrons) this sets up potential differences across the cell (difference in charge between electrodes - drives electrons around the circuit to generate electricity)
• fuel cells - structure

  A) anode

  B) cathode

  C) hydrogen inlet

  D) oxygen inlet

  E) water outlet

  F) electrolyte

  G) voltmeter

  
• How do fuel cells work


  hydrogen goes in - oxidised by the anode (each hydrogen looses an electron to become a hydrogen ion)

  electrons go through the wire to the cathode and hydrogen goes through electrolyte to cathode -- these react with oxygen that has come in

  Oxygen reacts with the hydrogen and electrons to make water (need 2 hydrogen for one oxygen)

  water leaves the outlet
• Fuel cell - equations


  negative electrode - $2H_2\ \rightarrow\ 4H^+$$\ +$$4e^-$

  positive electrode - $O_2+$$4H^+$$+$$4e^-\rightarrow\ 2H_2O$

  overall equation - $2H_2+$$O_2\ \rightarrow\ 2H_2O$
• Pro's of fuel cells

  • hydrogen fuel cells will produce electricity for as long as you provide Hydrogen -> renewable
  • they don't get less effective the longer they run
  • can be a source of drinking water, eg on a space craft
  • only require hydrogen and oxygen
  • no Carbon Dioxide or other pollutants given off
  • simple device
  • last longer then batteries
  • less polluting to dispose
• main idea of fuel cells - replace fossil fuels engine and batteries we use in vehicles - pollution
• cons of fuel cells

  • hydrogen fuel cells run on hydrogen which is a gas - more space to store then fossil fuels and batteries
  • hydrogen is explosive when mixed with air - dangerous source
  • requires energy to obtain Hydrogen - fossil fuels
  • hydrogen fuel cells produce a relatively low potential difference or voltage so several are needed