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## AP®︎/College Chemistry

### Course: AP®︎/College Chemistry>Unit 6

Lesson 6: Hess's law

# Hess's law

Hess's law states that if a process can be expressed as the sum of two or more steps, the enthalpy change for the overall process is the sum of the ΔH values for each step. To use Hess's law, two principles must be understood: one, if an equation is reversed, the sign of the ΔH value is also reversed. Two, if an equation is multiplied by a coefficient, the ΔH value is multiplied by the same coefficient. Created by Jay.

## Want to join the conversation?

• At , how come the answer is not +226.8 kJ/molrxn? Isn't +1299.6 + (-787) + (-285.8)= 226.8 and not -226.8? • Why exactly is reversing equations necessary?
(1 vote) • Getting to a point where you can utilize Hess's Law just seems to convenient. How do chemists find equations that cancel out perfectly to get to that point?
(1 vote) • Chemists have recorded the enthalpy changes for most simple reactions using various calorimetry experimental techniques. Using Hess’ law we only need to experimentally determine the enthalpy of a relatively small number of the most simple reactions which then allows us to calculate the enthalpies of vast number of complex reactions.

For example here the actual reaction we want to know the enthalpy of is reacting solid carbon (graphite) with hydrogen gas to produce acetylene. So, we need a few simpler reactions which has graphite, hydrogen gas, and acetylene. And the three lower reactions involving those chemicals which we know the enthalpies of are about as simple as you can get in an experimental setting. All that is happening with those lower equations is that we’re reacting the desired chemicals with oxygen gas (essentially burning them) and observing the enthalpy change.

Hope that helps.
(1 vote)