Energetics

• What happens when an enthalpy change occurs?

  Energy is transferred between the system and surroundings.
• What is the definition of the system and surroundings in thermodynamics?

  The system is the chemicals, and the surroundings is everything outside the chemicals.
• In an exothermic change, where is energy transferred?

  Energy is transferred from the system (chemicals) to the surroundings.
• How do the energy levels of products compare to reactants in an exothermic reaction?

  The products have less energy than the reactants.
• What is the sign of ∆H in an exothermic reaction?

  ∆H is negative.
• In an endothermic change, where is energy transferred?

  Energy is transferred from the surroundings to the system (chemicals).
• What is required for an endothermic reaction to occur?

  An input of heat energy is required.
• How do the energy levels of products compare to reactants in an endothermic reaction?

  The products have more energy than the reactants.
• What is the sign of ∆H in an endothermic reaction?

  ∆H is positive.
• What are the standard conditions for measuring enthalpy changes?

  • 100 kPa pressure
  • 298 K (room temperature or 25°C)
  • Solutions at 1 mol dm^-3
  • All substances in their normal state at 298 K
• What symbol is used when an enthalpy change is measured at standard conditions?

  The symbol used is ΔH.
• What is the standard enthalpy change of formation?

  It is the enthalpy change when 1 mole of a compound is formed from its elements under standard conditions.
• What is the symbol for the standard enthalpy change of formation?

  The symbol is ΔfH.
• What is the enthalpy of formation of an element?

  The enthalpy of formation of an element is 0 kJ mol^-1.
• What is the standard enthalpy change of combustion?

  It is the enthalpy change that occurs when one mole of a substance is combusted completely in oxygen under standard conditions.
• What symbol is used for the standard enthalpy change of combustion?

  The symbol is ΔcH.
• What happens during incomplete combustion?

  It leads to soot (carbon), carbon monoxide, and water, and is less exothermic than complete combustion.
• What are common exothermic processes?

  • Combustion of fuels
  • Oxidation of carbohydrates such as glucose in respiration
• What is the definition of enthalpy change?

  Enthalpy change is the amount of heat energy taken in or given out during any change in a system provided the pressure is constant.
• What is the calorimetric method for measuring enthalpy change?

  • Measure temperature rise during a reaction.
  • Extrapolate temperature curve if reaction is slow.
  • Use the equation:

  $Q (J) =$$m (g) \times c_p (J g^{-1} K^{-1}) \times \Delta T (K)$
• What is the purpose of taking temperature readings before adding reactants in a calorimetric experiment?

  To get a better average temperature of the reactants.
• What is the equation used to calculate energy change for a reaction in solution?

  $Q (J) =$$m (g) \times c_p (J g^{-1} K^{-1}) \times \Delta T (K)$
• What does the calorimetric method assume about the specific heat capacity of solutions?

  It assumes all solutions have the heat capacity of water.
• What is a common error in the calorimetric method?

  Energy transfer from surroundings is usually lost.
• What is the general method for measuring enthalpy change in a calorimetric experiment?

  1. Wash and dry equipment.
  2. Insulate the cup in a beaker.
  3. Measure desired volumes of solutions.
  4. Measure initial temperatures.
  5. Add second reagent and stir.
  6. Record temperature changes.
• How do you calculate the enthalpy change of reaction, ΔH, from experimental data?

  ΔH = Q / number of moles of the reactant not in excess.
• What is the heat capacity of water?

  The heat capacity of water is 4.18 J g^-1 K^-1.
• How do you calculate the number of moles of a reactant?

  moles = concentration × volume.
• What is the state of H2O in the experiment mentioned?

  Gas
• What does Hess's Law state about total enthalpy change?

  Total enthalpy change is independent of the reaction route.
• How is Hess's Law related to the first law of thermodynamics?

  It is a version of the first law, which states that energy is always conserved.
• What is the chemical equation representing the reaction of hydrogen and chlorine?

  2H (g) + 2Cl(g) → 2HCl (g)
• In an energy level diagram, what do the directions of the arrows represent?

  They show the different routes a reaction can proceed by.
• Why can't the enthalpy change to form a hydrated salt from an anhydrous salt be done experimentally?


  It is impossible to add the exact amount of water.
• How are Hess's law cycles used in experiments?

  They measure enthalpy changes for reactions that cannot be measured directly.
• What is the formula to calculate the enthalpy change using Hess's law?

  ΔH reaction = Σ ΔfH products - Σ ΔfH reactants
• What is the standard enthalpy of formation for elements?

  ΔfH = 0
• What is the formula for calculating the enthalpy of combustion?

  ΔH reaction = Σ ΔcH reactants - Σ ΔcH products
• What is the significance of standard states in enthalpy calculations?

  They provide a reference point for measuring enthalpy changes.
• How does the enthalpy of combustion change in a homologous series?

  It increases as the number of carbon atoms increases.