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^??= 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^?

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

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