IB DP Chemistry: SL复习笔记5.1.2 Standard Enthalpy Change

Standard conditions

• To compare the changes in enthalpy between reactions, all thermodynamic measurements are made under standard conditions
• These standard conditions are:
  • A pressure of 100 kPa
  • A concentration of 1 mol dm^-3 for all solutions
  • Each substance involved in the reaction is in its standard state (solid, gas or liquid)
• Temperature is not part of the definition of standard state, but a temperature of 298 K (25 ^oC) is usually given as the specified temperature
• To show that a reaction has been carried out under standard conditions, the symbol ⦵ is used
  • Eg. ΔH^Ꝋ = the standard enthalpy change

Standard Enthalpies

• There are a few Standard Enthalpy changes which are used commonly in energy calculations and they are summarised below:

• Practice your understanding of enthalpy changes on the following worked examples:

Answer:

• • Since two moles of water molecules are formed in the question above, the energy released is simply:

ΔH_r = 2 mol x (-286 kJ mol^-1)= - 572 kJ

Answer:

• • Since two moles of Fe₂O₃ (s) are formed the total change in enthalpy for the reaction above is:

ΔH_f =  2 mol x ( -824 kJ mol^-1)= - 1648 kJ

Answer:

Answer 1: ΔH_r^Ꝋ

Answer 2: ΔH_f^Ꝋ as one mole of CO₂ is formed from its elements in standard state and ΔH_c^Ꝋ as one mole of carbon is burnt in oxygen

Answer 3: ΔH_neut^Ꝋ as one mole of water is formed from the reaction of an acid and alkali