Question

The heats of combustion of yellow phosphorus and red phosphorous are -9.19 KJ and -8.78 KJ respectively, then heat of transition of yellow phosphorus to red phosphorous is

• A.
  • 1.13 KJ
• B.
  • 18.69 KJ
• C.
  • 18.69 KJ
• D.
  • 1.13 KJ

Answer 1

Answer: (A)

• 1.13 KJ

Answer 2

The transition reaction $P_4(yellow) \longrightarrow P_4(red)$ can be obtained by subtracting the combustion reaction of red phosphorus from the combustion reaction of yellow phosphorus.

$P_4(yellow) + 5O_2 \longrightarrow P_4O_{10}$ ($\Delta H_1$)

$P_4(red) + 5O_2 \longrightarrow P_4O_{10}$ ($\Delta H_2$)

Subtracting the second from the first:

$(P_4(yellow) + 5O_2) - (P_4(red) + 5O_2) \longrightarrow P_4O_{10} - P_4O_{10}$

$P_4(yellow) - P_4(red) \longrightarrow 0$

$P_4(yellow) \longrightarrow P_4(red)$

The enthalpy change for this reaction is $\Delta H_{transition} = \Delta H_1 - \Delta H_2$.

Using the likely intended values $\Delta H_1 = -9.91$ KJ and $\Delta H_2 = -8.78$ KJ:

$\Delta H_{transition} = -9.91 - (-8.78) = -9.91 + 8.78 = -1.13$ KJ.