Energetics

• What is enthalpy (H) a measure of?

  Total energy of a thermodynamic system
• What does enthalpy change (ΔH) help us understand in chemistry?

  Heat exchange during chemical reactions
• How is enthalpy change defined in a chemical reaction?

  Difference in enthalpy between products and reactants
• What characterizes an endothermic reaction in terms of ΔH?

  ΔH > 0 (absorbs heat)
• What characterizes an exothermic reaction in terms of ΔH?

  ΔH < 0 (releases heat)
• What happens to energy during an exothermic reaction?

  Energy is lost to the environment
• What is the standard enthalpy change for the combustion of methane?

  ΔH = −890 kJ/mol
• What occurs during an endothermic reaction regarding energy?

  Energy is taken in from the environment
• What is the standard enthalpy change for the decomposition of calcium carbonate?

  ΔH = +178 kJ/mol
• What are standard conditions for measuring enthalpy changes?

  298 K, 1 atm, 1 M concentration
• How is bond enthalpy defined?

  Energy required to break one mole of a bond
• What is the bond enthalpy of a hydrogen-hydrogen bond (H-H)?

  436 kJ/mol
• What type of process is breaking bonds?

  Endothermic process
• What type of process is forming bonds?

  Exothermic process
• What are mean bond enthalpies?

  Average energy needed to break a bond type
• How do bond enthalpies vary?

  They can vary depending on the molecule
• What is the overall enthalpy change for the formation of ammonia from hydrogen and nitrogen?

  -97 kJ/mol
• What is the bond energy required to break the bonds of the reactants in the formation of ammonia?

  2249 kJ/mol
• What is the bond energy released when the bonds of the products in the formation of ammonia are made?

  2346 kJ/mol
• What is the bond enthalpy of a C-H bond in methane?

  Approximately 413 kJ/mol
• What is the bond enthalpy of a C-H bond in ethane?

  Approximately 410 kJ/mol
• What are the different types of enthalpy change?

  • Enthalpy change of formation (ΔH∘f)
  • Enthalpy change of combustion (ΔH∘c)
  • Enthalpy change of reaction (ΔH∘r)
• What is the enthalpy change of formation for water?

  ΔH∘f = -286 kJ/mol
• What is the enthalpy change of combustion for methane?

  ΔH∘c = -890 kJ/mol
• What is the enthalpy change of reaction for hydrogen with chlorine?

  ΔH∘r = -184 kJ/mol
• How can the enthalpy change of a reaction be measured in the lab?

  • Use a calorimeter
  • Record initial temperature
  • Measure temperature changes during reaction
• What is the formula to calculate heat energy transferred (Q)?

  Q = m × c × ΔT
• How do you calculate the temperature change (ΔT) of water in the example given?

  ΔT = 363 - 90 = 273 K
• What is the heat energy change calculated for the water in the example?

  68,250 J
• How do you calculate the heat energy change per gram of fuel?

  68,250 ÷ 24.8 = 2752 J/g
• What is the molar enthalpy change for butane based on the example?

  -158.7 kJ/mol
• What is Hess's Law?

  • Total enthalpy change is constant
  • Independent of reaction pathway
  • Useful for calculating unknown enthalpy changes
• Who introduced Hess's Law?

  Germain Hess
• How can Hess's Law be applied to calculate unknown enthalpy changes?

  Using known values from related reactions
• How can you find the enthalpy change for the formation of carbon dioxide using Hess's Law?

  ΔH1 = ΔH2 + ΔH3
• What is the calculated enthalpy change for the formation of carbon dioxide?

  -393.5 kJ/mol
• How do you calculate enthalpy change using formation values?

  • Use enthalpy of formation values for reactants and products
  • Total enthalpy change based on formation and decomposition
  • For elements in standard states, enthalpy of formation is zero
• What are the enthalpy of formation values for SO₂, H₂S, and H₂O?

  -296.8, -20.6, -285.8 kJ/mol
• How do you calculate ΔHr for the reaction involving SO₂ and H₂S?

  ΔHr = -296.8 + (2 × -20.6) + ΔHr
• What is the final calculated ΔHr for the reaction involving SO₂ and H₂S?

  -233.6 kJ/mol