5. ThermoChemistry

Standard Enthalpy Changes

Reaction energies depend on the conditions under which they were measured. In order to be able to record energies and tabulate them in a way that others can make sense of them, we define a standard state. This standard state is then used to define the state of both the reactants (before reaction) and products (after reaction). Recall that since ΔH and ΔU do not depend on the path, only the initial and final state. Hence this is a useful thing to do.

Thermodynamic Standard state should not be confused with the standard conditions (STP) used for Ideal Gas calculations and tabulations. They aren’t the same.

Standard State:

Pressure = 100 kPa (1 bar)*
Concentration = 1 mol/L, (solutions)
[Temperature = for tabulation purposes only, normally but not exclusively 25℃ ]

What are elements in their standard states? How do we decide which ones to use?

Take hydrogen:

Most hydrogen exists at standard conditions as H2(g) [On earth, minute amounts may exist as H(g). There is far more hydrogen in space, where the atomic form predominates].  We choose the most common form on earth at normal (standard) conditions, H2(g).

Now try Carbon:

Obviously, carbon is solid at Std Conditions but do we choose graphite, diamond, Bucky balls (Buckminster Fullerenes)? Graphite is the defined standard, it is more a common and more stable allotrope than diamond or Bucky balls.

So how do we specify standard state in general?  Normally, we just have to be careful to write the correct phase of the element or compound for the appropriate temperature.  The following is a list of elements and compounds and the proper standard state at two different temperatures

SubstanceT=25℃ (298.15K)T=100℃T=120℃
water*H2O()H2O() or H2O(g)H2O(g)
pentane (bp=36℃)C5H15()C5H15(g)C5H15(g)
methanol dissolved in water**CH3OH(aq)CH3OH(aq)CH3OH(g)

* We see that since water exists in two different standard, stable forms at 100℃ and P=1 bar, that there are two different possible states we can indicate, both of which are ‘standard’ states at 100℃.  The problem here would be that the difference between them would be the difference in energy (enthalpy) of vaporization.  Be careful of the states specified.

** The final entry in the table shows methanol, a molecule that can dissolve in water.  At or below 100℃, it is in standard state if the concentration is 1 mol/L.  Any other concentration means not standard state.  Note that above 100℃, water doesn’t exist in liquid form at standard conditions so methanol would only exist in gas form at p=1 bar along side the water, which would also have to be at 1 bar if it were to be claimed to be standard state.

We’ll see later that a simpler definition for standard state is “activity = 1″

Temperature is not part of the definition.  It is merely the experimental temperature for which most of the results are tabulated.  We indicate Thermodynamic quantities measured at standard conditions by using the superscript zero as in ΔH° (pronounced delta H not)

eg. CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)      ΔrH° = -890.4 kJ/mol

NOTE: When we see the superscript ‘not’ on the symbol ΔrH° it means that all the reactants and products must be in their defined standard state at a defined pressure of 1 bar (and/or concentration of 1 mol/L for solutes).

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