Hess's Law

[Irene Ramos 1F]

When we are adding reactions in order to solve for the deltaH (using Hess's Law), are we supposed to flip the reactions in order to cross cancel?
If we are, is there a specific way to flip the reaction? or do we just flip them to our convenience?

[Chem_Mod]

Just flip them as needed, don't forget to change the sign accordingly.

[Emily_Vilshtein_3B]

Also regarding Hess's law, when you are given an equation for a reaction and the standard reaction enthalpies of each individual reactant and product of the given equation, how do you solve for the standard reaction enthalpy of the entire reaction?

[Hirmand_Sarafian_2N]

You multiply the standard enthalpies of formation of each product by the number of moles needed and all the values together; you then do the same for the reactants. Finally, subtract the reactants' value from the products' value to get the standard reaction enthalpy of the whole reaction in kJ.

[Alyssa_Hsu_2K]

When given standard reaction enthalpies of each individual reactants, you must add all the product enthalpies together and decrease that by the total reactant enthalpies.

[Selina_Han_3I]

As previously stated, the main formula is enthalpy of products – enthalpy of reactants. The goal is to have the given multiple reactions add up to equal the main reaction. To manipulate these reactions, you may sometimes need to either flip the reaction, or multiply it by a coefficient. For example, if you want all the reactions to equal the main reaction H2+Br2 --> 2HBr, and one of the multiple given reactions is NH3+HBr --> NH4Br, this reaction should be flipped because HBr should be a product, not a reactant. It should also be multiplied by 2 because you want HBr in the reaction to match 2HBr in the main reaction. Do not forget to switch the sign (because you flipped it) and multiply the enthalpy by 2.