Lecture 3

Question 2

White phosphorus

Answer 2

• Elemental form of P₄
• Produces a very hot fire and lots of smoke from a very small amount of ignition.
• After the fire triangle has been interupted, once the element is added it can set on fire right away.
• The residual energy produced by the combustion process is enough to restart the combustion process.
• The ignition temperature of white phosphorous is really low so it is easy to achieve the activation energy to start the process again.
• There is more energy available the 2nd time which allows the process to restart.
• Pyrophoric species can be reignited easily

Question 3

Why does white phosphorus act this way?

Answer 3

Low ignition temperature
30 degrees celcius

Question 4

Pyrophoric

Answer 4

Ignites spontaneoulsy in the air

Question 5

Uses for white phosphorus in solid state?

Answer 5

• Effective smoke screens for military because of the size of the particles and the way that they scramble any light coming through it.
• That smoke is not going to be remotely good for you to breathe because it contains phosphoric acid and also some liberated P4 that has been evaporated

Question 6

White phosphorus powder

Answer 6

Those particles don’t stay solid. They tend to react with water in the air and that gives you phosphoric acid, which is a very strong polyprotic acid.

Question 7

Oxidation state of P in P₄?

Answer 7

0 as it is in its elemental state

Question 8

The oxidation state of P in P₄O₁₀

Answer 8

• The two different bonding modes of oxygen in this need to be taken into account
• Double bond provides P with -2
• Single bonds provide P with -1
• -1+-1+-1+-2 = -5 so P = +5

Question 9

SO4

Answer 9

Common anion with -2 oxidation state

Question 10

Military applications for white phosphorus

Answer 10

• Smokescreen grenade
• Tracer fire (signalling)
• Incendiary (to start fires)

Question 11

White phosphorus powder dangers on skin

Answer 11

• Is particularly nasty to use on people because it’ll stick to skin, and it’ll keep wanting to burn, which is really bad as we are 37 C and it self ignites at 32 C.
• It then hydrolyses on skin

Question 12

White phosphorus ban

Answer 12

• The use has been prohibited by the chemical weapons convention
• Clandestine use is still observed
• If you are exposed to white phosphorus and on fire, copper sulfate solution (aqueous) is the answer.

Question 13

How does aqueous copper sulfate quench white phosphorus?

Answer 13

White phosphorous (P₄) can be quenched by aqueous copper sulfate (CuSO₄) to give copper (Cu), phosphoric acid (H₃PO₄) and sulfuric acid (H₂SO₄).

Question 14

generic unbalanced equation for white phosphorus and aqeuous copper sulphate

Answer 14

P₄ + CuSO₄ → H₃PO₄ + Cu + H₂SO₄

Question 15

How does aqueous copper sulphate quench white phosphorus?

Final equation

Answer 15

P₄ + 10CuSO₄ → 16H₃PO₄ +10 Cu + 10H₂SO₄

Question 16

General graph of energy profile for combustion reaction

Answer 16

• Reactants start of at low energy
• Goes up to transition state
• Energy is lost when the products are formed

Question 17

ΔH

Answer 17

Change in enthalpy

Question 18

_o

Answer 18

Standard enthalpy change

Question 19

Hess’s law states

Answer 19

• “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.”
• Regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes.

Question 20

ΔH_f⁰

Answer 20

The change in heat in a system to form that material from its basic elements.