How can I calculate the enthalpy of a reaction?

Answer 1

Enthalpy is a state function, so all we need to know to calculate the reaction's standard enthalpy are the reaction's beginning and end states.

Standard enthalpies of formation #DeltaH_f^@# are tabulated in Thermodynamics tables you should be able to find in your textbook appendix.

Using those, we can create the following equation, which essentially looks at the enthalpy of formation—the amount of energy needed to form each component of the reaction—and calculates the difference between the initial and final states:

#\mathbf(DeltaH_"rxn"^@ = sum_(P) n_PDeltaH_(f,P)^@ - sum_(R) n_RDeltaH_(f,R)^@)#

where

So, using the information from this source and using this reaction as an example:

#color(green)("CH"_4(g) + 2"O"_2(g) -> "CO"_2(g) + 2"H"_2"O"(g))#
#DeltaH_(f,"CH"_4(g))^@ = "-74.9 kJ/mol"#
#DeltaH_(f,"O"_2(g))^@ = "0 kJ/mol"#
#DeltaH_(f,"CO"_2(g))^@ = "-393.5 kJ/mol"#
#DeltaH_(f,"H"_2"O"(g))^@ = "-241.8 kJ/mol"#
We would be able to calculate the enthalpy for the reaction at standard conditions of #25^@ "C"# and #"1 bar"#:
#color(blue)(DeltaH_"rxn"^@) = sum_(P) n_PDeltaH_(f,P)^@ - sum_(R) n_RDeltaH_(f,R)^@#
#= [n_("CO"_2)DeltaH_(f,"CO"_2(g))^@ + n_("H"_2O)DeltaH_(f,"H"_2"O"(g))^@] - [n_("CH"_4)DeltaH_(f,"CH"_4(g))^@ + n_("O"_2)DeltaH_(f,"O"_2(g))^@]#
#= [(1xx"-393.5 kJ/mol") + (2xx"-241.8 kJ/mol")] - [(1xx"-74.9 kJ/mol") + (2xx"0 kJ/mol")]#
#=# #color(blue)("-802.2 kJ/mol")#
And then if you wanted to calculate #DeltaH_"rxn"# for the same reaction under the same conditions with a different amount of methane, since #DeltaH_"rxn"^@# is defined on a per-mol basis (you could also call it normalized to #"1 mol"#), simply multiply #DeltaH_"rxn"^@# by the quantity:
#"mass of Methane (Limiting Reagent) used"/"mass of 1 mol of Methane (Limiting Reagent)"#

And that reduces the standard reaction to your reaction (assuming, once more, that the standard reaction and your reaction are conducted under the same conditions and that the only variable is the amount of limiting reagent applied).

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Answer 2

The enthalpy change of a reaction can be calculated using the equation:

[ \Delta H = \sum \text{(products' enthalpies)} - \sum \text{(reactants' enthalpies)} ]

The enthalpies are typically given in kilojoules per mole (kJ/mol).

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Answer from HIX Tutor

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

When evaluating a one-sided limit, you need to be careful when a quantity is approaching zero since its sign is different depending on which way it is approaching zero from. Let us look at some examples.

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