5 things you need to know about ENTHALPY

1. What does ENTHALPY (H) measure?

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countdown.education

3 min readMar 26, 2017

enthalpy measures the potential energy contained in all the attractions and repulsions of protons, electrons, and atoms at the atomic level in a system
this potential energy is the internal heat contained in all these nuclear forces
enthalpy is usually measured in kJ/mol

System = the stuff that’s actually reacting → reactants and products
Surroundings = everything else in the universe, including the container, the water a solution is in, the air in the room, the planet Jupiter, Donald Trump’s hair, etc.

A positive ∆H means that a reaction is endothermic as heat is absorbed from the surroundings to the system and the surroundings feel cold as the temperature decreases
A negative ∆H means that a reaction is exothermic as heat is released from the system to the surroundings and the surroundings feel warm as the temperature increases

∆Hrxn = heat of reaction
the net change in enthalpy when a reaction occurs
the difference between the enthalpy of the products and the enthalpy of the reactants
∆Hsolution = heat of solution
A specific type of heat of reaction for when a solid compound dissociates into its ions in a solution
Ex: NaOH(s) → Na+(aq) + OH-(aq) has a ∆Hsolution of
Hºf = heat of formation
another specific type of heat of reaction when a compound is formed from its elements in a synthesis reaction
for standalone elements, Hºf = 0 because they are already elements and don’t need to be formed
∆Hfusion = heat of fusion
the enthalpy change when a substance goes from solid to liquid or vice-versa
∆Hfusion = positive if changing from solid to liquid (ice melts) (gains heat)
∆Hfusion = negative if changing from liquid to solid (water freezes) (loses heat)
∆Hvaporization = heat of vaporization
the enthalpy change when a substance goes from liquid to gas
∆Hvaporization = positive if changing from liquid to gas (water evaporates) (gains heat)
∆Hvaporization = negative if changing from gas to liquid (vapor condenses to water) (loses heat)

Q = ms∆T
Q = heat transferred to surroundings (Joules or calories)
m = mass of substance being heated, usually water or a solution, (grams)
s = specific heat, AKA how many joules it takes to raise one gram of a substance up 1ºC (J/gºC or cal/gºC)
∆T = final temperature — initial temperature (doesn’t make a difference if you use ºC or ºK)
Q = n∆Hfusion
you’ll often want to rearrange this to solve for Q when you’re looking for the heat when a substance is changing state from solid to liquid or liquid to solid
Use this in heat curves in the flat part where the temperature isn’t changing
Q = n∆Hvaporization
you’ll often want to rearrange this to solve for Q when you’re looking for the heat when a substance is changing state from gas to liquid or liquid to gas
Use this in heat curves in the flat part where the temperature isn’t changing
∆Hsolution = -Q / nsolute
Sometimes you’ll rearrange this to solve for Q or the moles of solute when you already know the molar heat of solution
∆Hrxn = Hproducts — Hreactants
basically, the change in enthalpy of a reaction is going to be the difference between the reactants and products
Hess’s Law: ∆Hrxn = ∆H1 + ∆H2 + ∆H3…etc.
This is like solving systems of equations in algebra, where you line up multiple reactions and add them together to get the desired reaction
ex:

∆Hrxn= ∑Hºf, products — ∑Hºf, reactants
∑, or ‘sigma’, means the sum
All the Hºf values will be given to you in a table
So basically just add up all values from your heat of formation table for the products, and subtract all the values for the reactantas
Remember, pure elements have a Hºf value of 0 because they are already formed
∆Hrxn= coefficient reactant x -Q / nsolute
This is useful in heat stoichiometry when you have a full reaction