enthalpy change

Question 1

reason standard enthalpy change of sth has no value quoted

Answer 1

• it is a mixture and thus has no fixed standard enthalpy change
• e.g. petrol, which is a mixture of hydrocarbons, does not have a fixed ∆Hcombustion

Question 2

why hydrogen is a good fuel

Answer 2

• only releases water as a by-product, which is a non-pollutant
• (if qn specifies “alternative”) must compare to other fuels, e.g as compared to methane, which releases CO2 as a by-product, which contributes to global warming

Question 3

what coating of black powder can imply
(during combustion)

Answer 3

• black powder is soot (fine solid of carbon)
• implies that incomplete combustion occurred
  
  • since complete combustion would only produce CO2 and H2O
• thus not all of the substrate has been burnt completely
  => amt of heat evolved will be less than expected
  => temp rise will be lower than expected

Question 4

how to prevent incomplete combustion / ensure only complete combustion occurs

Answer 4

need excess amt of O2 gas
- increase pressure of O2 (if in closed container)