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Yes you add all of them up at the end. If you reverse a reaction, it is negative. You multiply/divide H by whatever you manipulate the reaction. Remember that you can do this because enthalpy is a state function!
ayushibanerjee06 wrote:Yes you add all of them up at the end. If you reverse a reaction, it is negative. You multiply/divide H by whatever you manipulate the reaction. Remember that you can do this because enthalpy is a state function!
This is correct thank you for the clarification.
You add up the values of the enthalpies and "cancel" out the reactants if those reactants are the same as those on the product side of another equation. If in case the coefficients are different, the equations can be combined algebraically. If one side of a reaction had say, 4H2 on the reactants side while another equation has 5H2 on the product side, the combined reaction would have H2 on the product side.
For the Hess Law, we basically have to manipulate the equation so that it matches the specific final reaction we want. The real first step is to write the chemical formula for what the question wants and to balance it. Then, take the other formula given and create a formula in which the stoichiometric coefficients and reactant match with the reactant in the problem and do the same for a product. Then, combine the change in enthalpy for both equations and write the equation so that all the products and reactants of the new manipulated equations. Whatever changes you do to these equations, do to the changes in enthalpy. Cross out any similar products or reactants. The third equation is usually used to get rid of some of the other leftover products or reactants not represented in the needed chemical forumla.
Hess's Law has to do with enthalpies being a state function - meaning they can be treated as a simple value that can be added. If a reaction is reversed (products become reactants and vice versa), then the sign of the reaction enthalpy switches (negative/positive). If a reaction is multiplied by a scalar, the reaction enthalpy is multiplied by the same number. If two reactions are added together, their enthalpies are also added together to get a net reaction enthalpy.
You would cancel out the reactants if those reactants are the same as the one's that are on the product side of another equation. The equations could be combined algebraically if the coefficients are different. Also, I think, if necessary you can flip the chemical equation, which you would also have to switch the sign of the delta H. Ultimately, you add up the values of the enthalpies.
As far as I know, if you're not multiplying by anything or reversing the reactions, just add the values together. If you reverse a reaction, add the opposite value of the reaction enthalpy. If you multiply by a coefficient, multiply the reaction enthalpy with the same coefficient.
When you flip a reaction, its delta H value will change signs. When you multiply a reaction by a number, multiply the delta H value by that same number. After doing all the necessary adjustments, add up all the delta H values together.
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