C3.2 Energetics

• What are the two types of reactions discussed in C3.2.1?

  Exothermic and endothermic reactions
• What is the definition of an exothermic reaction?

  An exothermic reaction releases energy to the surroundings
• What is the definition of an endothermic reaction?

  An endothermic reaction absorbs energy from the surroundings
• What is a study tip for remembering endothermic reactions?


  • Think of a cold drink (absorbs heat)
  • Relate it to the word "endothermic" (energy in)
• What is thermal decomposition?

  Thermal decomposition is a type of reaction where a substance breaks down into simpler substances when heated
• What is a reaction profile?

  A reaction profile is a graphical representation of the energy changes during a chemical reaction
• What does the activation energy represent in a reaction profile?

  Activation energy is the minimum energy required for a reaction to occur
• How do you draw a reaction profile for an exothermic reaction?

  Start with the reactants at a higher energy level than the products and draw a curve downwards
• What happens to energy during an exothermic reaction?

  Energy is released to the surroundings
• What happens to energy during an endothermic reaction?


  Energy is absorbed from the surroundings
• What is the formula for calculating energy changes in reactions?

  Energy change = energy required to break bonds - energy released when bonds form
• What are bond energies?

  Bond energies are the amount of energy required to break a bond between two atoms
• How do you calculate the energy change for the combustion of ethene?

  By calculating the total energy required to break the bonds in the reactants and subtracting the energy released when the bonds in the products are formed
• What does a negative energy change indicate about a reaction?

  A negative energy change indicates that the reaction is exothermic
• What does a positive energy change indicate about a reaction?

  A positive energy change indicates that the reaction is endothermic
• How would you determine if a reaction is exothermic or endothermic based on energy changes?

  By calculating the energy change and checking if it is negative (exothermic) or positive (endothermic)
• What is the energy change when nitrogen reacts with hydrogen to produce ammonia?

  The energy change can be calculated using bond energies
• What is an exothermic reaction?

  It gives out energy, usually heat and light
• How is an exothermic reaction indicated?

  By a fall in temperature
• What is an endothermic reaction?

  It takes in energy from surroundings
• How is an endothermic reaction indicated?

  By a rise in temperature
• What do reaction profiles show?

  • Energy levels of reactants and products
  • Indicate if energy is given out or taken in
• What does a reaction profile indicate for an exothermic reaction?

  Products are lower in energy than reactants
• What does the height difference in a reaction profile represent for exothermic reactions?

  The energy given out in the reaction
• What does a reaction profile indicate for an endothermic reaction?

  Products are higher in energy than reactants
• What does the height difference in a reaction profile represent for endothermic reactions?

  The energy taken in during the reaction
• What is activation energy?

  The minimum energy needed to start a reaction
• How is activation energy represented on a reaction profile?

  As the energy difference between reactants and peak
• What analogy is used to explain activation energy?

  Climbing a hill before skiing down
• What happens if energy input is less than activation energy?

  No reaction will occur
• What does "exo-" mean in exothermic?

  Exit
• What does "thermic" mean in exothermic?

  Heat
• What is the relationship between bond energies and reactions?

  • Energy is required to break bonds (endothermic)
  • Energy is released by forming bonds (exothermic)
• What is bond energy?

  The energy associated with a chemical bond
• Why is it important to learn bond energies?

  To calculate overall energy change for reactions
• What does a positive energy change indicate?

  An endothermic reaction
• What does a negative energy change indicate?

  An exothermic reaction
• How do you calculate overall energy change for a reaction?

  1. Calculate energy to break original bonds
  2. Calculate energy released by forming new bonds
  3. Determine overall energy change
• What is the bond energy for H–H?

  436 kJ/mol
• What is the bond energy for Cl–Cl?

  242 kJ/mol