Fires 3

Question 1

White Phosphorus

Answer 1

• 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

Question 2

What is a pyrophoric species?

Answer 2

A species which has a low ignition energy
* Ignites spontaneously in air

Question 3

White phosphorus smoke grenades

Answer 3

• 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

Question 4

Why is it forbidden to use white phosphorus smoke grenades on people?

Answer 4

• 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

Question 5

What should you do if you are exposed to white phosphorus and on fire?

Answer 5

• Use copper sulphate solution (aq)

Question 6

How does aqueous copper sulfate quench white phosphorus?
P4 + CuSO4 –> H3PO4 + Cu + H2SO4

Answer 6

• 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

Question 7

What is required to get to the transition state?

Answer 7

activation energy

Question 8

What is the heat of combustion?

Answer 8

energy released during combustion

Question 9

What does ∆𝐻 mean?

Answer 9

change in enthalpy

Question 10

What does the f and c mean when it is next to ∆𝐻?

Answer 10

• f = change in enthalpy of formation
• c = change in enthalpy of combustion

Question 11

What does ∆𝐻𝑐∘ mean?

Answer 11

• the enthalpy change of combustion measured under standard conditions

Question 12

Why is ∆𝐻𝑐∘ negative?

Answer 12

because heat is given out - energy is lost from the system

Question 13

What are the products of combustion?

Answer 13

CO2 and H2O

Question 14

How do you work out ∆𝐻𝑐∘?

Answer 14

from the values of standard enthalpies of formation using Hess’s law

Question 15

What is enthalpy of formation?

Answer 15

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

Question 16

What is Hess’s law?

Answer 16

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

Question 17

What is the equation for Hess’s law?

Answer 17

∆𝐻=[∑∆𝐻𝑓∘(𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠)] − [∑∆𝐻𝑓∘(𝑟𝑒𝑎𝑐𝑡𝑎𝑛𝑡𝑠)]

Question 18

What are the enthalpy of formation of elements in their standard states?

Answer 18

zero

Question 19

Is the enthalpy of formation of the products +ve or -ve?

Answer 19

+ve

Question 20

Is the enthalpy of formation of the reactants +ve or -ve?

Answer 20

-ve

Question 21

What are some basic elements?

Answer 21

• C(s)
• N2 (g)
• O2 (g)
• H2 (g)

Question 22

What is the enthalpy of combustion for this:
𝐶𝐻4 (𝑔) + 2O2 (𝑔) → 𝐶𝑂2 (𝑔) + 2𝐻2𝑂 (𝑙)
∆𝐻𝑓∘ = -75 | 0 | -394 | -286 kJmol-1

Answer 22

∆𝐻c∘ = [(-394) + 2(-286)] - [(-75) + 2(0)]
∆𝐻c∘ = -891 kJmol-1

Question 23

Why is ∆𝐻c∘ in a real world combustion slightly lower?

Answer 23

due to thr evaporation of water - energy needed to break the hydrogen bonding in water

Question 24

What is the equation for the standard enthalpy of combustion?

Answer 24

(energy to break bonds) + (-energy to form bonds)

Question 25

What happens when a bond is broken and formed?

Answer 25

• broken: energy is required (+ve)
• formed: energy is released (-ve)

Question 26

What does it mean if the energy of combustion is negative?

Answer 26

exothermic - releases heat

Question 26

What does it mean if the energy of combustion is positive?

Answer 26

endothermic - absorbes heat

Question 27

Which is more reliable and why? ethalpy of combustion from bond enthalpies or from heats of formation?

Answer 27

heats of combustion - bond enthalpies are not constant from compound to compound

Question 28

Does oxidation reactions release energy when their ∆𝐻 is positive or negative?

Answer 28

negative