5 things you need to know about ENTHALPY

5 things you need to know about ENTHALPY

1. What does ENTHALPY (H) measure?

  • 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

2. What are the SYSTEM and SURROUNDINGS?

  • 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.

3. What does it mean if Enthalpy is POSITIVE or NEGATIVE?

  • 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

4. What are all the DIFFERENT TYPES of enthalpy?

  • ?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
  • Hf = 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, Hf = 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)

5. What FORMULAS do you need to know to find enthalpy?

  • 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 1C (J/gC or cal/gC)
  • ?T = final temperature ? initial temperature (doesn?t make a difference if you use C or K)
  • 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:

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  • ?Hrxn= ?Hf, products ? ?Hf, reactants
  • ?, or ?sigma?, means the sum
  • All the Hf 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 Hf 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

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