Different delta H

[Marnie Cavanaugh- 1A]

Could anyone explain the differences between delta Hr, delta Hf, and delta Hc? In what contexts would you use each one?

[Erin Nagel 1G]

Delta Hf is the standard enthalpy of formation for a species, delta Hc is the standard enthalpy of combustion for a species, and I think that delta Hr is the enthalpy change for a reaction. In general, I believe that we will just be given the ones we are supposed to be using for a specific problem, but the enthalpy of combustion can be used in a combustion reaction (some reactant + oxygen -> carbon dioxide and water, delta Hf for that reactant would give the overall enthalpy change for the reaction) and the enthalpy of formation can be used when you are forming something out of its basic components in their most stable forms (C (graphite, solid) + 2H2 (g) -> CH4 (g), delta Hf of CH4 would give overall enthalpy change) or when you are given delta Hf for all the products and reactants (delta H rxn = sum of delta Hf products - sum of delta Hf reactants).

[805939635]

The r in delta Hr stands for reaction, so it is the enthalpy change for a reaction, and will be often listed next to the chemical equation. Otherwise it can be derived. The f in delta Hf stands for formation, so that is the enthalpy change for the formation of a chemical species. The c in delta Hc stands for combustion, so this is the enthalpy change for the combustion of a certain species.

[samaagwani-disc2L]

Delta Hr: enthalpy change for a reaction

Delta Hf: standard enthalpy of formation for a chemical species

Delta Hc: standard enthalpy of combustion of a chemical species

[Valerie Jun 2D]

It's like the K constant, like Kc is the equilibrium constant for concentrations and Kp is the equilibrium constant for partial pressure.The different delta H's are all measuring enthalpy just in different situations. Hr is the enthalpy of a reaction, Hf is the enthalpy of a formation, and Hc is the enthalpy of combustion.

[Allyson_Khachaturov_3B]

Hello, Delta Hr represents the change in enthalpy in a reaction or whether the formation of reaction results in an absorption of energy or a release of energy. Delta Hf represents the enthalpy change for the creation of one mole of a compound in their standard states. Lastly, Hc is used to represent combustion reactions. Hope this helps!