What is the enthalpy change of reaction for this reaction?

#2"A" + B rightleftharpoons 2"C" + 2"D"#

Answer 1

#DeltaH_"rxn"^@ = - "237 kJ"#

The idea here is that we can use Hess' Law to calculate the standard enthalpy change of reaction, #DeltaH_"rxn"^@#, for this reaction using the standard enthalpy changes of formation, #DeltaH_f^@#, of the chemical species that take part in the reaction.

Hess' Law states that the enthalpy change of a reaction is independent of the path taken by the reaction.

In your case, you have

#color(blue)(2)"A" + "B" rightleftharpoons color(purple)(2)"C" + color(brown)(2)"D"#

Now, the standard enthalpy change of reaction can be calculated by taking the sum of the enthalpy changes of formation of the products multiplied by their respective stoichiometric coefficients and the sum of the enthalpy changes of formation of the reactants multiplied by their respective stoichiometric coefficients.

#color(blue)(ul(color(black)(DeltaH_"rxn"^@ = sum_i n xx DeltaH_"f i"^@ - sum_j m xx DeltaH_"f j"^@)))#

Here

So, you know that you have

You can say that for the products, you have

#sum_j m xx DeltaH_"f j"^@ = color(purple)(2)color(red)(cancel(color(black)("moles"))) * (+215 "kJ"/color(red)(cancel(color(black)("mol")))) + color(brown)(2)color(red)(cancel(color(black)("moles"))) * (-523 "kJ"/color(red)(cancel(color(black)("mol"))))#
#= "430 kJ" + (-"1046 kJ")#
# = -"616 kJ"#

Similarly, you can say that for the reactants', you have

#sum_i n xx DeltaH_"f i"^@ = color(blue)(2)color(red)(cancel(color(black)("moles"))) * (-225 "kJ"/color(red)(cancel(color(black)("mol")))) + 1color(red)(cancel(color(black)("mole"))) * (-403 "kJ"/color(red)(cancel(color(black)("mol"))))#
# = -"450 kJ" + (-"403 kJ")#
# = -"853 kJ"#

Therefore, you can say that the standard enthalpy change of reaction is equal to

#DeltaH_"rxn"^@ = -"853 kJ" - (-"616 kJ")#
#color(darkgreen)(ul(color(black)(DeltaH_"rxn"^@ = -"237 kJ")))#

The answer is rounded to three sig figs.

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Answer 2

To determine the enthalpy change of reaction, you need to calculate the difference between the enthalpies of the products and reactants. This can be done using Hess's law or by directly measuring the heat released or absorbed during the reaction under constant pressure conditions.

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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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