energy changes

• What do chemical reactions involve?

  Energy transfers
• What are the two types of energy changes in reactions?

  Endothermic and exothermic
• What happens in exothermic reactions?

  Energy is released to the surroundings
• What is the energy profile of exothermic reactions?

  Reactants have more energy than products
• What happens in endothermic reactions?

  Energy is absorbed from the surroundings
• What is the energy profile of endothermic reactions?

  Products have more energy than reactants
• What are the characteristics of exothermic and endothermic reactions?

  • Exothermic:
  • Releases energy
  • Surroundings get hotter
  • Examples: combustion, respiration
  • Endothermic:
  • Absorbs energy
  • Surroundings get colder
  • Examples: thermal decomposition, photosynthesis
• What is activation energy (Eₐ)?

  Energy needed to start the reaction
• What happens during bond breaking?

  Energy is needed to break bonds
• What happens during bond making?

  Energy is released when new bonds form
• How is overall energy change calculated?

  Energy change = Total energy to break bonds - Total energy released forming bonds
• What is the overall energy change for the reaction H2 + Cl2 → 2HCl?

  -184 kJ
• What does a negative energy change indicate?

  Exothermic reaction
• What are everyday uses of exothermic and endothermic reactions?

  • Exothermic:
  • Hand warmers (oxidation of iron)
  • Self-heating cans (reaction releases heat)
  • Endothermic:
  • Sports injury packs (absorbs heat)
• What is an electrochemical cell?

  A simple cell producing electricity from a reaction
• How does the reactivity of metals affect voltage in electrochemical cells?

  Bigger difference in reactivity means higher voltage
• What is the negative electrode in an electrochemical cell?

  More reactive metal
• What is the positive electrode in an electrochemical cell?

  Less reactive metal
• What is the difference between non-rechargeable and rechargeable batteries?

  Non-rechargeable run out; rechargeable can restore charge
• How do hydrogen fuel cells work?

  Hydrogen reacts with oxygen to produce electricity
• What is produced by hydrogen fuel cells?

  Electricity, water, and heat
• What is an advantage of hydrogen fuel cells?

  Only produces water, no CO₂ pollution
• What is a disadvantage of hydrogen fuel cells?

  Hydrogen is difficult to store
• What is a summary of energy changes in reactions?

  • Exothermic:
  • Releases energy (gets hotter)
  • Examples: combustion, respiration
  • Endothermic:
  • Absorbs energy (gets colder)
  • Examples: thermal decomposition, photosynthesis
  • Bond Breaking:
  • Energy in (endothermic)
  • Bond Making:
  • Energy out (exothermic)