• Heat capacity (C): heat absorbed per degree (J/C or J/K)

           ○ Extensive property: depend on amount of substance                                                     

  • Specific heat capacity (cp): heat capacity per gram (J/C·g or J/K)

          ○ Amount of heat required to change one gram of a substance temperature by one degree C or K

  • Every substance has its own specific heat capacity

         ○ Cp of water is 4.18 J/C·g → requires 4.184 J (1 cal) of energy to heat a gram by one degree

  • Molar heat capacity: heat capacity per mol (K/C mol or K/K mol)
  • Specific and Molar heat capacity are intensive properties: independent of the amount (of substance)

Heat Transfer Equations

  • qA = -qB → heat lost = – heat gained         

          ○ qsystem = -qsurroundings; qsolution = -qreaction

  • q = the amount of heat absorbed or released

         ○ Note: “molar heat of solution” = q

  • Signage of Heat Transfer Equations:

         ○ Do q = mcpΔT and keep q always positive initially

         ○ Add a positive or negarive sign to the q value on the knowledge that…

              ■ If the temp of the water went down, then the reaction was endothermic and qsol = –

              ■ If the temp of the water went up, then the reaction was exothermic and qsol = +

  •  ΔHrxn =

        ○ Important: ΔH will be per 1 mole of substance → divide moles in sample by moles (coefficient) in the balanced equation

  • Questions involving specific heat → the amount of heat (J) gained/lost by a sample (q) can be determined by the formula: q = mcpΔT or ncpΔT

        ○ m = mass of sample

        ○ C = specific heat

        ○ Questions involving two substances: Do two mcats → mcΔT = – (mcΔT)

            ■ Ex:

        ○ Questions involving ΔT

           ■ If moles increases → more moles reacting so final temp will be higher → q increases b/c ΔT is greater

           ■ If heat is lost to the surroundings (ex: air) → q decreases b/c ΔT is smaller → (if exo) ΔH will be less negative

  • Example Question Proccess
  1. Water is heated by an external source (ex: combustion)
  • Since water is only thing undergoing temp change, only use the mass of water
  1. Chemicals are mixed in a solution and a temp change occurs in a solution (calorimetry)
  • Use temp change and cp of the solution, and the mass of the solution (mass of solute + solvent)