What is the basic difference between the two notations `Delta_(r)H` and `Delta_(f)H`?

What is the difference between two notations : nylon - 6 and nylon - 66 ?

The difference between Delta H and Delta U is equal to

Enthalpy is an extensive property. In general, if enthalpy of an overall reaction A rarr B along one route is Delta_(r) H " and Delta_(r) H_(1), Delta_(r) H_(2), Delta_(r) H_(3) ..... Represent enthalpies of intermediate reactions leading to product B. What will be the relation between Delta_(r)H overall reaction and Delta_(r)H_(1), Delta_(r)H_(2) ... etc for intermediate reaction.

Enthalpy is an extensive property.In general, if enthalpy of an overall reaction A to B along one route is Delta_(r)H and Delta_(r)H_(1) + Delta_(r)H_(2) + Delta_(r)H_(3) ... Represent enthalpies of intermediate reactions leading to product B.What will be the relation between Delta_(r)H for overall reaction and Delta_(r)H_(1), Delta_(r)H_(2) ....etc, for intermediate reactions.

If enthalpy of overall reaction XrarrY along one route is Delta_(r)H and Delta_(r)H_(1),Delta_(r)H_(2),Delta_(r)H_(3) …. Representing enthalpies of reactions leading to same product Y then Delta_(r)H is

The difference between between Delta H and Delta U for the formation of C_6H_(12)O_6( S) from its elements at 27^@ C is __ cal. (R=2 cal /K/mol)

Standard molar enthalpy of formation, Delta_(f) H^(Θ) is just a special case of enthalpy of reaction, Delta_(r) H^(Θ) . Is the Delta_(r) H^(Θ) ? Given reason for your answer. CaO (s) + CO_(2) (g) rarr CaCO_(3) (s) , Delta_(f) H^(Θ) = - 178.3 kJ mol^(-1)