4. energetics

• What does the symbol Delta H represent?

  Enthalpy change
• What are the standard conditions for measuring enthalpy change?

  100 kPa and 298 K (25°C)
• What are endothermic reactions?

  Reactions that absorb energy from surroundings
• How does an endothermic reaction feel when touched?

  Colder
• What are exothermic reactions?

  Reactions that release heat energy
• How do exothermic reactions feel when touched?

  Warmer
• What happens during bond breaking?

  Energy is absorbed
• What is the sign of Delta H when breaking bonds?

  Always positive
• What happens during bond making?

  Energy is released
• What is the sign of Delta H when forming bonds?

  Always negative
• What is mean bond enthalpy?

  Average energy required to break bonds
• Why do bonds of the same type have different bond enthalpies?

  Due to their position in the molecule
• How do you calculate the energy change for a reaction?

  Total energy to break bonds minus forming energy
• What is calorimetry used for?

  To determine enthalpy change of combustion
• What is the formula for calculating energy in calorimetry?

  Q = mcΔT
• What does Q represent in the calorimetry equation?

  Energy gained or lost
• What is the specific heat capacity value used in calculations?

  4.18
• How do you calculate the energy transferred for 100 g of water heated from 23°C to 57°C?

  Q = 100 g × 4.18 × 34
• How do you calculate the number of moles of ethanol used?

  Mass of ethanol / Molar mass of ethanol
• How do you calculate the enthalpy change of the fuel?

  Energy transferred / moles of fuel
• What is the enthalpy change of the fuel if energy is -14212 J and moles are 0.039?

  -364.4 kJ per mole
• What is the purpose of a polystyrene cup in calorimetry?

  To prevent heat loss to surroundings
• What type of reactions are typically measured in calorimetry experiments?

  Acid-base reactions
• What is measured before and after adding reactants in calorimetry?

  Temperature change
• Why is it important to measure temperature before mixing reactants?

  To establish a baseline for comparison
• What is the purpose of the polystyrene cup in calorimetry experiments?

  • Prevents heat loss to surroundings
  • Contains the solution being tested
• What do we add first in the calorimetry experiment?

  Acid
• Why do we use acid-alkali reactions in calorimetry?

  They show a reasonable temperature change
• If 25 cm³ of 1 M HCl is at 20°C, what is the next step?

  Add sodium hydroxide and stir
• How do we calculate the enthalpy change of neutralization for hydrochloric acid?

  Using Q = MCΔT
• What mass do we consider when calculating enthalpy change?

  Mass of both liquids mixed
• What is the assumed density of the liquids in the experiment?

  1 g/cm³
• How do we convert volume from cm³ to dm³?

  Divide by 1000
• What is Hess's Law about?

  • Total energy change is independent of the route taken
  • Useful for calculating enthalpy changes not easily measured
• Who formulated Hess's Law?

  Jérôme Hess
• What are the two types of Hess cycles?

  • Formation cycle
  • Combustion cycle
• What is the enthalpy of formation symbol?

  ΔH_f
• When should you use a formation cycle?

  When given formation data
• How do you check the accuracy of your Hess cycle calculations?

  They should add up to zero
• What is the combustion cycle structure?

  • Reactants and products
  • Combustion products at the bottom
  • Ensure everything is balanced