Energy changes

• exothermic reaction

  Is one that transfers energy to the surroundings so the temperature of the surroundings increases

  -includes combustion

  -neutralization reactions

  -oxidation reactions
• everyday uses of exothermic reactions:

  -Hand warmers

  -exothermic oxidation of iron in the air to release energy

  -Self-heating cans
• endothermic reaction

  Is one that takes in energy from the surroundings, so the temperature of the surroundings decreases.

  -includes thermal decompositions

  -the combustion of gasoline

  -the reaction between baking soda and citric acid
• everyday uses of endothermic reactions:

  -some sports injury packs chemical reaction allows the pack to become instantly cooler without having to put it in the freezer.
• bond breaking

  endothermic- energy must be supplied to break existing bonds
• bond forming

  exothermic-energy is released when new bonds are formed
• what happens during a chemical reaction?

  during a chemical reaction, old bonds are broken and new bonds are formed
• what happens in exothermic reactions?

  on exothermic reactions, the energy released by forming bonds is greater than the energy used to break them
• what happens in endothermic reactions?

  in endothermic reactions, the energy used to break bonds is greater than the energy released by forming them
• how to calculate bond energies?

  1)find energy required to break original bonds

  2)find energy released by forming new bonds

  3)find overall change for reaction
• activation energy

  minimum amount of energy the reactants need to collide with each other and react to start a reaction
• Exothermic(reaction profile)

  -products are at a lower energy level than reactants

  -when reactants form products, energy is transferred to the surroundings

  
• Endothermic(reaction profile)

  -products are at a higher energy level than reactants

  -reactants form products, energy is transferred from the surroundings to the reaction mixture

  -temperature of surroundings decreases

  
• the greater the activation energy


  the more energy needed to start the reaction
• what is a fuel cell?

  a fuel cell is an electrical cell that's supplied with fuel and oxygen and uses energy from the reaction between them to produce electrical energy effieciently
• what does the hydrogen-oxygen fuel cell combine?

  the hydrogen-oxygen fuel cell combines hydrogen and oxygen to produce clean water and release energy
• positives of hydrogen-oxygen fuel cells


  -fuel cell vehicles don't produce as many pollutants

  -no greenhouse gases

  -By-products are only water and heat
• negatives of hydrogen-oxygen fuel cells

  -Expensive to produce and requires energy for the production process.

  -Difficult and dangerous to store and move around (usually stored as liquid hydrogen in highly pressurized containers) The production of hydrogen process releases carbon dioxide.