Energetics

• What is the definition of bond energy?

  Energy needed to break a bond
• Why do enthalpy changes occur during a reaction?

  Bonds are being broken and formed
• What type of process is bond breaking?

  Endothermic
• What happens when new bonds are formed?

  Energy is released to the surroundings
• How do you determine if a reaction is exothermic or endothermic?

  By comparing energy in and energy out
• What indicates an endothermic reaction?

  More energy required to break bonds
• What indicates an exothermic reaction?

  More energy released when forming bonds
• What is an energy level diagram?

  A diagram showing energies of reactants and products
• What is the transition state in a reaction?

  A stage where bonds are partially broken and formed
• Why is the transition state unstable?

  It cannot be isolated and is high in energy
• What is activation energy (E)?

  Energy needed to reach the transition state
• How does the energy level of reactants compare in exothermic reactions?

  Higher than the products
• What does a negative ΔH indicate?

  Energy is lost to the surroundings
• How does the energy level of reactants compare in endothermic reactions?

  Lower than the products
• What is the ΔH for the complete combustion of methane?

  -890 kJ mol
• How do you calculate ΔH for the reaction of 2H (g) + O (g) → 2H O (l)?

  ΔH = 2 mol x (-286 kJ mol)
• What is calorimetry?

  Measurement of enthalpy changes in reactions
• What is specific heat capacity (c)?

  Energy needed to raise 1 g by 1 °C
• What is the specific heat capacity of water?

  4.18 J g °C
• How do you calculate energy transferred in a calorimeter?

  Using q = m x c x ΔT
• What are the characteristics of exothermic and endothermic reactions?

  Exothermic Reactions:

  • Products have less energy than reactants
  • Heat is released to surroundings
  • ΔH is negative

  
  

  Endothermic Reactions:

  • Products have more energy than reactants
  • Heat is absorbed from surroundings
  • ΔH is positive
• What are the standard conditions for thermodynamic measurements?

  • Pressure of 100 kPa
  • Temperature of 298 K (25 °C)
  • Each substance in its standard physical state
• What is the difference between the system and surroundings in a chemical reaction?

  • System: Substances that are reacting
  • Surroundings: Everything else (e.g., flask)
• What is the significance of labeling physical states in enthalpy equations?

  • Changes in state can cause large enthalpy changes
  • Important for accurate thermodynamic comparisons
• How do you determine the activation energy from a reaction pathway diagram?

  • Activation energy is the energy difference from reactants to transition state
• What is the relationship between activation energy and reaction rate?

  • Higher activation energy generally means slower reaction rate
• What is the role of the transition state in a chemical reaction?

  • It is a high-energy state where bonds are partially formed and broken
• What is the significance of ΔH being zero for elements in their standard state?

  • It serves as a reference point for enthalpy changes
• What materials can be used to make a simple calorimeter?

  Polystyrene cup, vacuum flask, metal can
• How does a polystyrene cup function in calorimetry?

  It measures enthalpy changes in reactions
• What is specific heat capacity (c)?

  Energy needed to raise 1 g by 1 °C
• What is the specific heat capacity of water?

  4.18 J g^-1 °C^-1
• How is energy transferred as heat calculated in a calorimeter?

  Using the formula q = m x c x ΔT
• What is the process to calculate energy released per gram of methane burnt?

  1. Calculate q using q = m x c x ΔT
  2. Determine total energy from q and percentage absorbed
  3. Divide total energy by mass of methane
• If 2.50 g of methane is burnt and 30% of energy is absorbed by 500 g of water, what is the temperature change?

  43 °C
• How do you find the total energy released by methane if 89,870 J is 30% of it?

  Total energy = 299,567 J
• What is the energy released by 1.00 g of methane if 2.50 g releases 299,567 J?

  120,000 J g^-1
• What happens to energy when new bonds are formed?

  Energy released equals energy absorbed when broken
• What is the relationship between temperature rise and ΔH?

  ΔH is negative for exothermic reactions
• What does a positive ΔH indicate about a reaction?

  It suggests the reaction is endothermic