Question

a. Define standard enthalpy change of combustion.

b. When red phosphorus burns in oxygen the enthalpy change is –2967 kJ mol–1. For white phosphorus the enthalpy change is –2984 kJ mol–1. For both forms of phosphorus the reaction taking place is:
P4(s) + 5O2(g) → P4O10(s)

i. Use this information to calculate the enthalpy change for the transformation: P4(white) → P4(red).
ii. Represent these changes on an enthalpy profile diagram.

Answer 1

a) Standard enthalpy change of combustion is defined as the enthalpy change when 1 mole of a compound undergoes combustion (i.e., reacts with oxygen completely) under standard condition (T = 298 K, P = 1 bar).

 

b) 

i)

1) P4(red, s) + 5O2(g) → P4O10(s).............................................      –2967 kJ mol–1.

 

2) P4(white, s) + 5O2(g) → P4O10(s).............................................     –2984 kJ mol–1.

 

So by subtracting 1 from 2 ( 2 - 1), we get the following transformation - P4(white) → P4(red).

 

Therefore the enthalpy change for the transformation: P4(white) → P4(red) is = {(–2984 kJ mol–1) – ( –2967 kJ mol–1)} = –17 kJ mol–1.

 

ii) Represention of  these changes on an enthalpy profile diagram is given below.

 

        

 Represention of  these changes on an enthalpy profile diagram is given below