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Course: AP®︎/College Chemistry>Unit 6

Lesson 5: Introduction to enthalpy of reaction

Enthalpy of reaction

The enthalpy change that accompanies a chemical reaction is referred to as the enthalpy of reaction and is abbreviated ΔH_rxn. The value of ΔH_rxn depends on how the balanced equation for the reaction is written and is typically given in units of kJ/mol-rxn. Created by Jay.

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• At why is 1/2 the conversion factor? The reactant has 2 moles and the product has 3 moles.
• When Jay mentions one mole of the reaction, he means the balanced chemical equation. And in the balanced chemical equation there are two moles of hydrogen peroxide. That's why the conversion factor is (1 mol of rxn/2 mol of H2O2).

Hope that helps.
• I always understood that to calculate the change in ▲H° for a rxn or if you wanted to calculate any change such as ▲S or ▲G or anything, you did products minus reactants. But I came across a formula for ▲H of reaction(not the standard one with the ° symbol) and it said that it was equal to Σ bond energy of bonds broken + Σbond energy of bonds formed. When do I know when to use the ▲H formula and when the ▲H° formula? Thanks!
(1 vote)
• 1. standard enthalpy (with the little circle) is the enthalpy, but always under one atmosphere of pressure and 25 degrees C.
2. I believe both equations can work for both types of enthalpy. To figure out which formula to use, you just look at the data you're given. Technically I think you can do either formula if you're given all the information. They should both give you the same delta h value.
3. For the bonds broken and formed formula, it tends to give slightly less accurate results because the bond energy is affected by surrounding atoms.
I'm not perfect with chemistry, so apologies if I got something wrong!