Definition of Enthalpy:

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Enthalpy is an interesting concept: it is defined by its change rather than a single entity. A state property, the word enthalpy comes from the Greek "heat inside". If you have a chemical system that undergoes some kind of change but has a fixed volume, the heat output is equal to the change in internal energy (q = ΔE). We will define the enthalpy change, ΔH, of a system as being equal to its heat output at constant pressure:

dH = q at constant pressure

Where:

ΔH = change in enthalpy

We define enthalpy itself as:

H = E + PV

Where:

H = enthalpy
E = energy of the system
PV = pressure in atm times volume in liters

You will not need to be able to calculate the enthalpy directly; in chemistry, we are only interested in the change in enthalpy, or ΔH.

ΔH = Hfinal - Hinitial or ΔH = H(products) - H(reactants)

Tables of enthalpies are generally given as ΔH values.

Example Problem:

Calculate the ΔH value of the reaction:

HCl + NH3 → NH4Cl

(ΔH values for HCl is -92.30; NH3 is -80.29; NH4Cl is -314.4)

Answer:

  ΔH = ΔHproducts - ΔHreactants
  ΔHproducts = -314.4
  ΔHreactants = -92.30 + (-80.29) = -172.59
  ΔH = -314.4 - 172.59 = 141.8

We can also represent enthalpy change with the equation:

ΔH = ΔE + P ΔV

Where:

ΔV is the change in volume, in liters
P is the constant pressure

If you recall, work is defined as P ΔV, so enthalpy changes are simply a reflection of the amount of energy change (energy going in or out, endothermic or exothermic), and the amount of work being done by the reaction. For example, if ΔE = -100 kJ in a certain combustion reaction, but 10 kJ of work needs to be done to make room for the products, the change in enthalpy is:

ΔH = -100 kJ + 10 kJ = -90 kJ

This is an exothermic reaction (which is expected with combustion), and 90 kJ of energy is released to the environment. Basically, you get warmer. Notice the convention used here -- a negative value represents energy coming out of the system.

You can also determine ΔH for a reaction based on bond dissociation energies. Breaking bonds requires energy while forming bonds releases energy. In a given equation, you must determine what kinds of bonds are broken and what kind of bonds are formed. Use this information to calculate the amount of energy used to break bonds and the amount used to form bonds. If you subtract the amount to break bonds from the amount to form bonds, you will have the ΔH for the reaction.

Example Problem:

Calculate ΔH for the reaction:

N2 + 3H2 → 2NH3

(The bond dissociation energy for N-N is 163 kJ/mol; H-H is 436 kJ/mol; N-H is 391 kJ/mol)

Answer:

  ΔH = ΔHproducts - ΔHreactants
  To use the bond dissociation energies, we must determine how many bonds
  are in the products and the reactants. In NH3 there are 3 N-H bonds so in 2 NH3
  there are 6 N-H bonds. In N2 there is 1 N-N bond and in 3H2 there are 3 H-H bonds.
  ΔHproducts = 6(391) = 2346
  ΔHreactants = 163 + 3(436) = 1471
  ΔH = 2346 - 1471 = 875
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