Energy changes

• What are chemical reactions characterized by?


  Breaking and forming of chemical bonds
• What accompanies chemical reactions?


  Energy changes
• What is required to break chemical bonds?


  Energy
• What happens when new bonds form?


  Energy is released
• What determines if a reaction is exothermic or endothermic?


  The net energy change
• How can chemical reactions be compared to building with LEGO?


  • Breaking bonds is like taking apart LEGO pieces (requires energy)
  • Forming new bonds is like putting LEGO pieces together (releases energy)
  • Overall energy change determines the reaction type
• What type of reaction occurs if more energy is released when forming new bonds than was used to break the original bonds?


  Exothermic
• What happens to the kinetic energy in exothermic reactions?


  It increases
• What happens to the potential energy in exothermic reactions?


  It decreases
• What is a common example of an exothermic reaction?


  The combustion of natural gas (methane)
• What do endothermic reactions do?


  Absorb energy from their surroundings
• What happens to the potential energy in endothermic reactions?


  It increases
• What happens to the kinetic energy in endothermic reactions?


  It decreases
• What is a common example of an endothermic reaction?


  The process of photosynthesis
• Compare exothermic and endothermic reactions based on energy flow.


  • Exothermic: Releases energy
  • Endothermic: Absorbs energy
• Compare exothermic and endothermic reactions based on potential energy.


  • Exothermic: Decreases
  • Endothermic: Increases
• Compare exothermic and endothermic reactions based on kinetic energy.


  • Exothermic: Increases
  • Endothermic: Decreases
• What is the enthalpy change (ΔH) for exothermic reactions?


  Negative ΔH
• What is the enthalpy change (ΔH) for endothermic reactions?


  Positive ΔH
• What is the activation energy in exothermic reactions?


  Typically lower than in endothermic reactions
• What is the chemical equation for the neutralization reaction between hydrochloric acid and sodium hydroxide?


  HCl + NaOH → NaCl + H2O + heat
• What happens to the temperature during the neutralization reaction?


  It initially increases and then decreases
• Why does the temperature decrease after reaching a peak in the neutralization experiment?


  The excess alkali dilutes the solution without further reaction