Chapter 9 Key Concepts

Question 1

How accurate is experimental delta c

Answer 1

More sophisticated apparatus than a spirit burner and beaker of water
Much less heat was transferred to the water in our experiment than expected
-heat loss to surroundings other than the water
Including the beaker but mainly the air surrounding the flame so less exo
-incomplete combustion of methanol
CO and C instead. So see C as a black layer of soot on the beaker so less exo
-evaporation of methanol from the weak
Burner must be weighted as soon as poss after extinguishing flam
Otherwise some methanol may have evaporated from the wick
*use a cover to reduce this error so less exo
-non standard conditions
Data book is standard. Conditions in classroom unlikely to be identical to standard conditions

Question 2

How to minimise errors for delta h c

Answer 2

Drought screens
Input of O2
Use a lid
Minimise errors from heat loss & incomplete combustion

Question 3

Why use polystyrene foam

Answer 3

Cheap
Waterproof
Light weight
Offer some insulation against heat loss to surrounding

Question 4

Average bond enthalpy

Answer 4

• energy is always required to break bonds
• always endothermic
• positive enthalpy value

Question 5

Limitations of average bond enthalpy

Answer 5

Depends on chemical environment of the bond

Question 6

Limitations of using average bond enthalpy

Answer 6

• actual energy involved in breaking and making individual bonds would be slightly different because of different chemical environment
• ABE needs all species to be gas
• Delta H isn’t standard so need to account for H2O gas condense into liquid