Energetics

• What does a positive value of ΔH indicate?


  It indicates an endothermic process
• What does a negative value of ΔH indicate?

  It indicates an exothermic process
• What factors affect the enthalpy changes during reactions?

  Temperature, pressure, and physical state of substances
• What are the standard conditions for measuring standard enthalpy changes (ΔH°)?

  • Pressure = 100 kPa (10⁵ Pa)
  • Temperature = 298 K (25 °C)
  • All substances in their normal physical states
• What is the sign of ΔH for combustion reactions?

  All ΔH values are negative
• What is the difference between exothermic and endothermic reactions?


  Exothermic:

  • Heat released to surroundings
  • Products more stable than reactants

  
  

  Endothermic:

  • Heat absorbed from surroundings
  • Products less stable than reactants
• How is the enthalpy change (ΔH) measured in experiments?

  At constant pressure during reactions
• What are the two methods for determining ΔH in experiments?

  1. Reactions in aqueous mixtures
  2. Enthalpy of combustion using a spirit burner
• What is the equation used to calculate heat change (q)?

  q = mcΔT
• What does 'm' represent in the heat change equation?

  Mass of water in grams
• What is the specific heat capacity of water?

  4.18 J g⁻¹ K⁻¹
• How do you calculate the enthalpy change (ΔH) in kJ mol⁻¹?

  ΔH = - q / (moles x 1000)
• What does a positive temperature change indicate for ΔH?

  ΔH will be negative
• What does a negative temperature change indicate for ΔH?

  ΔH will be positive
• What are the limitations of using bond enthalpies in calculations?


  • Mean bond enthalpies are approximations
  • Assumes all reactants/products are gaseous
  • Requires adjustments for solids and liquids
• Why are bond enthalpy values only approximations?

  Actual bond enthalpies may differ slightly
• What is the bond enthalpy for a C-H bond in CH4?

  Varies compared to CH3Cl
• What must be considered if solids and liquids are present in a reaction?

  Enthalpy of fusion or vaporization must be allowed for
• What is the standard state of water?

  Liquid at standard conditions
• What are the key concepts in thermochemistry?

  • Enthalpy change (ΔH)
  • Standard enthalpy changes (ΔH°)
  • First Law of Thermodynamics
  • Hess's Law
  • Exothermic and endothermic reactions
• Why might a certain x-x bond be a mean average 


  Because it exists in many different compounds , whereas soemthing like cl-cl doesnt have a mean average value due to only existing as that
• Why might an enthalpy of combustion answer calculated by students differ to data book 


  Because the data book does the condition under standard conditions and so includes additional exothermic process
• Reasons why an enthalpy of combustion is determined experimentally less exothermic than what is calculated

  Heat loss to environment OR incomplete combustion
• If the temperature change is negative, what sign is heat change (q)

  Positive
• If temp change is positive, what sign is heat change q

  negative
• Exam q :How do you measure mass of water without a balance

  the density is known
• EQ review: suggest why it’s difficult to measure the standard enthalpy of formation of CO

  easily oxidises to CO2
• How to improve percentage uncertainty of temperature change ( delta T)

  increase the CONCENTRATION of the solutions