Q4(b)(i). Thermal conductivity of the gas in an ageing foam
A certain closed cell foam used as an insulating material is initially filled with polyatomic gas of molecular weight ∼ 60. Later, the gas diffuses out of the foam and is replaced by dry air (mean molecular weight ∼ 30). Assuming that insulating property arises largely from the thermal conductivity of the gas, explain the factors which influence the thermal conductivity of the gas. For each factor, make an argument whether insulating ability increases or decreases. What is the overall effect upon the insulating ability?
Answer in brief
The gas in the cells conducts heat better (κ rises), so the insulating ability decreases: faster, smaller air molecules raise κ, while their lower heat capacity lowers it, but by less.
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Coversthermal conductivity of gaseskinetic theorymean free pathcollision cross-sectionheat capacity
All parts of Question 4
- 4(a) E.m.f. of a hydrogen–copper cell at 25 °C10 marks
- 4(b)(i) Thermal conductivity of the gas in an ageing foam10 marks
- 4(b)(ii) van der Waals constants of NO and CCl₄; nearness to ideal behaviour10 marks
- 4(c) Phenol–water system and the importance of tie lines20 marks