White Phosphorus
- Produces a very hot fire, loads of smoke
- P4 = oxidation state of 0
- When a dish is placed over the phosphorus, part of the fire triangle is lost (oxygen), and the flame is extinguished
- As soon as the dish is removed the oxygen returns and the species can set on fire again right away
- it is a pyrophoric species
What is a pyrophoric species?
A species which has a low ignition energy
* Ignites spontaneously in air
White phosphorus smoke grenades
- Can obscure the ability for targeting
- Because you get a burst of light, it can be used as a tracer fire to see where things are going
Why is it forbidden to use white phosphorus smoke grenades on people?
- It will stick to peoples skin
- Very hot
- It self-ignites at 32 C and our body temps are 37 C so it will continue to burn
What should you do if you are exposed to white phosphorus and on fire?
- Use copper sulphate solution (aq)
How does aqueous copper sulfate quench white phosphorus?
P4 + CuSO4 –> H3PO4 + Cu + H2SO4
- P4 –> PO4 (oxidised)
- CuSO4 –> Cu + SO4 (reduced)
- Balance the equations: P4 + 16 H2O –> 4 H3PO4 | 10 CUSO4 –> 10 Cu + 10 H2SO4
- Final equation: π4 + 10πΆπ’ππ4 + 16π»2π β 4π»3ππ4 + 10πΆπ’ + 10π»2ππ4
What is required to get to the transition state?
activation energy
What is the heat of combustion?
energy released during combustion
What does βπ» mean?
change in enthalpy
What does the f and c mean when it is next to βπ»?
- f = change in enthalpy of formation
- c = change in enthalpy of combustion
What does βπ»πβ mean?
- the enthalpy change of combustion measured under standard conditions
Why is βπ»πβ negative?
because heat is given out - energy is lost from the system
What are the products of combustion?
CO2 and H2O
How do you work out βπ»πβ?
from the values of standard enthalpies of formation using Hess’s law
What is enthalpy of formation?
the change of heat in a system to form that material from its basic elements
* if they are already in their basic form then they dont require any energy to stay in that form
What is Hess’s law?
the enthalpy change for a reaction is independent of the route by which the reaction is achieved but depends only on the initial and final stages
What is the equation for Hess’s law?
βπ»=[ββπ»πβ(πππππ’ππ‘π )] β [ββπ»πβ(πππππ‘πππ‘π )]
What are the enthalpy of formation of elements in their standard states?
zero
Is the enthalpy of formation of the products +ve or -ve?
+ve
Is the enthalpy of formation of the reactants +ve or -ve?
-ve
What are some basic elements?
- C(s)
- N2 (g)
- O2 (g)
- H2 (g)
What is the enthalpy of combustion for this:
πΆπ»4 (π) + 2O2 (π) β πΆπ2 (π) + 2π»2π (π)
βπ»πβ = -75 | 0 | -394 | -286 kJmol-1
βπ»cβ = [(-394) + 2(-286)] - [(-75) + 2(0)]
βπ»cβ = -891 kJmol-1
Why is βπ»cβ in a real world combustion slightly lower?
due to thr evaporation of water - energy needed to break the hydrogen bonding in water
What is the equation for the standard enthalpy of combustion?
(energy to break bonds) + (-energy to form bonds)
