Fires 3 Flashcards
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: 𝑃4 + 10𝐶𝑢𝑆𝑂4 + 16𝐻2𝑂 → 4𝐻3𝑃𝑂4 + 10𝐶𝑢 + 10𝐻2𝑆𝑂4
What is required to get to the transition state?
activation energy
What is the heat of combustion?
energy released during combustion
What does ∆𝐻 mean?
change in enthalpy
What does the f and c mean when it is next to ∆𝐻?
- f = change in enthalpy of formation
- c = change in enthalpy of combustion
What does ∆𝐻𝑐∘ mean?
- the enthalpy change of combustion measured under standard conditions
Why is ∆𝐻𝑐∘ 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 ∆𝐻𝑐∘?
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?
∆𝐻=[∑∆𝐻𝑓∘(𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠)] − [∑∆𝐻𝑓∘(𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠)]
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:
𝐶𝐻4 (𝑔) + 2O2 (𝑔) → 𝐶𝑂2 (𝑔) + 2𝐻2𝑂 (𝑙)
∆𝐻𝑓∘ = -75 | 0 | -394 | -286 kJmol-1
∆𝐻c∘ = [(-394) + 2(-286)] - [(-75) + 2(0)]
∆𝐻c∘ = -891 kJmol-1
Why is ∆𝐻c∘ 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)
What happens when a bond is broken and formed?
- broken: energy is required (+ve)
- formed: energy is released (-ve)
What does it mean if the energy of combustion is negative?
exothermic - releases heat
What does it mean if the energy of combustion is positive?
endothermic - absorbes heat
Which is more reliable and why? ethalpy of combustion from bond enthalpies or from heats of formation?
heats of combustion - bond enthalpies are not constant from compound to compound
Does oxidation reactions release energy when their ∆𝐻 is positive or negative?
negative
