What does a Standard Enthalpy with a subscript "c" mean?

[Yinhan_Liu_1D]

We've learned that enthalpy with subscript f is the standard enthalpy of formation.

What does the "c" mean?

I saw it on HW:8.57.

Appreciate your help; )

[Yinhan_Liu_1D]

Is it combustion?

Is it opposite to formation?

[Miya McLaughlin 2B]

The subscript "c" on the delta H denotes that it's the Standard Enthalpy value for a combustion reaction. A combustion reaction is when a compound burns completely with O2 gas to create CO2 and H2O. Here is an example.

Ex: 2 C2H2(g) + 5 O2(g) --> 4 CO2(g) + 2 H2O(g)

It's not the exact opposite of formation. Formation is reacting to create one mole of a compound from it's elements. Here is an example.

Ex: 2 C(s) + H2(g) --> C2H2(g)

The Standard Enthalpy of Formation(subscript f) and the Standard Enthalpy of Combustion(subscript c) are different values.