Chemistry / Unit 4 Chem 0718 worked example, 60 to 90 seconds
Applying the Summation Form to a Multi-Reactant Reaction
The idea
Applying the summation form to a reaction with several reactants and products means multiplying each substance's known formation enthalpy by its coefficient in the balanced equation, summing those weighted values separately across all products and across all reactants, and subtracting the reactant sum from the product sum.
In practice
For nitrogen dioxide reacting with water to form nitric acid and nitric oxide, multiplying each formation enthalpy from a reference table by its coefficient gives four weighted values. Summing the two products' weighted contributions, separately summing the two reactants' weighted contributions, and subtracting the reactant total from the product total gives a standard enthalpy change near −138.4 kJ for the reaction as written.
Where people slip
Adding a reactant's weighted formation enthalpy into the product sum instead of subtracting it out produces an answer roughly twice the correct magnitude, with the wrong sign.
Your place on the trail
Every lesson rests on earlier ones and opens later ones. Tap any to jump.
Builds on
Unlocks
- Nothing yet depends on this.
Something wrong in this lesson? Flag it · Sources for Chemistry