B. The heat of reaction for the process described in (a) can be determined by applying Hess's law. The heats of reaction shown in the table below can be
obtained experimentally or looked up in tables of enthalpy data. Which two of
these heats of reaction would be the easiest and safest to measure in the
laboratory, and which two are better obtained through reference sources?
Why? Hint: Consider whether a reaction takes place in aqueous solution or
instead involves noxious gases.

- lead + phosphoric acid = hydrogen + lead(ii) phosphate

hydrocarbons are organic compounds that contain only hydrogens and carbons. they can be saturated (those which contain only carbon-carbon single bonds) or unsaturated (those which contain carbon-carbon double or triple bonds) in nature. hydrocarbons can undergo several reactions like substitution, elimination etc.  

on substituting one or more hydrogen atoms in hydrocarbon it results in the formation of haloalkane. as the halogen atoms are large compared to the carbon and hydrogen atoms, due to which the molecular weight increases and the bond become polar because of presence electronegative halogen atom and results in the increase in boiling point of the haloalkane.

thus, the correct option is (b) that is the boiling point of the new compound increases on substituting a hydrogen atom with a halogen in a hydrocarbon.