energy changes

• an exothermic reaction is one that causes a rise in temperature to its surroundings. it releases energy
• an endothermic reaction is one that causes a decrease in temperature in the surroundings. takes in energy
• reacting particles must collide with sufficient energy in order for bonds to be broken
• in a chemical reaction:

  • atoms are rearranged
  • old bonds broken
  • new bonds formed
• activation energy is the minimum amount of energy that particles must have for a reaction to take place
• catalysts increases the rate of reaction by reducing the activation energy and providing an alternate pathway
• bond breaking = endothermic

  bond making = exothermic
• cells release energy by the chemicals inside reacting and releasing electricity
• simple cells are made by placing 2 different metals into a beaker containing an electrolyte
• a battery is 2 or more cells connected together
• in a hydration fuel cell, hydrogen and oxygen are supplied to the cell. the fuel is oxidised to produce a potential difference, the hydrogen is oxidised to produce water
• fuel cells are becoming more popular as traditional methods involve many stages, are less efficient and more polluting
• advantages of hydrogen fuel cells:

  • produce water (not a pollutant)
  • lightweight and small
  • no moving parts(unlikely to break down)
• disadvantages of hydrogen fuel cells:

  • explosive
  • stored in pressurised containers