Energy changes

• What is an exothermic reaction?

  Releases energy to surroundings as heat
• What is an example of an exothermic reaction?

  Burning wood
• How does an exothermic reaction affect its surroundings?


  Makes the surroundings warmer
• What is the enthalpy change for an exothermic reaction?

  ΔH < 0
• What is an endothermic reaction?

  Absorbs energy from surroundings
• What is an example of an endothermic reaction?

  Melting ice
• How does an endothermic reaction affect its surroundings?


  Makes the surroundings cooler
• What is the enthalpy change for an endothermic reaction?

  ΔH > 0
• How do the energy levels of reactants and products compare in an exothermic reaction?


  Reactants have higher energy than products
• What must reactants overcome in an exothermic reaction?

  Activation energy
• What does the energy release in an exothermic reaction do to the system's energy?

  It lowers the energy of the system
• How do the energy levels of reactants and products compare in an endothermic reaction?


  Products have higher energy than reactants
• What must reactants overcome in an endothermic reaction?

  Activation energy
• What does the energy absorption in an endothermic reaction do to the system's energy?


  It raises the energy of the system
• What are the key differences between exothermic and endothermic reactions?

  • Exothermic: Releases energy, surroundings warmer, ΔH < 0
  • Endothermic: Absorbs energy, surroundings cooler, ΔH > 0
• What is the definition of enthalpy change (ΔH)?


  Energy absorbed or released during a reaction
• What does a negative enthalpy change (ΔH < 0) indicate?

  Energy is released, exothermic reaction
• What does a positive enthalpy change (ΔH > 0) indicate?

  Energy is absorbed, endothermic reaction
• What does a zero enthalpy change (ΔH = 0) indicate?

  No net energy change, equilibrium process
• What is the enthalpy change for burning methane?

  ΔH = -804 kJ/mol
• What is the enthalpy change for melting ice?

  ΔH = +6.01 kJ/mol
• What do energy profile diagrams illustrate for exothermic and endothermic reactions?

  • Exothermic: Products lower energy than reactants
  • Endothermic: Products higher energy than reactants
  • Both show activation energy needed to start the reaction
• What does the peak in an energy profile diagram represent?

  Activation energy needed to start the reaction
• In an energy profile diagram for an exothermic reaction, what is true about the products?

  Products have lower energy than reactants
• In an energy profile diagram for an endothermic reaction, what is true about the products?

  Products have higher energy than reactants
• What is the significance of the activation energy in a reaction?

  It is the minimum energy needed to start the reaction
• What type of reactions absorb energy from the surroundings?

  Endothermic reactions
• What is the enthalpy change for endothermic reactions?

  ΔH > 0
• What does an energy profile diagram illustrate?

  • Energy changes during a reaction
  • Peak represents activation energy
  • Shows energy levels of reactants and products
• In an endothermic reaction, how do the energy levels of products compare to reactants?


  Products have higher energy than reactants
• How does Hess's Law help in calculating enthalpy changes?


  It allows calculation through multiple steps
• What type of reaction occurs when grilling meat on a barbecue?

  Exothermic reaction
• Which of the following is an endothermic process?


  Melting ice
• What happens during an endothermic reaction?

  Energy is absorbed from the surroundings
• What happens during an exothermic reaction?

  Energy is released to the surroundings
• Which process is an example of an exothermic reaction?

  Burning candles
• What are some real-life examples of exothermic and endothermic reactions?

  Exothermic:

  • Burning candles
  • Cooking food
  • Hand warmers

  
  

  Endothermic:

  • Melting ice
  • Photosynthesis
  • Cooking eggs
• If A → B is -100 kJ and A → C is +50 kJ, what is ΔH for C → B?

  • 150 kJ
• If A → B is exothermic with ΔH = -100 kJ, what is the ΔH for A → C → B?

  ΔH = ΔH(A→C) + ΔH(C→B)
• What does Hess's Law state about enthalpy changes?

  Overall ΔH is the same regardless of steps