Write the definition of the standard state of a substance. Write the relationshi
ID: 555434 • Letter: W
Question
Write the definition of the standard state of a substance.
Write the relationship between Cv and Cp for a gas (e.g. monotatomic gas Cv = 3/2R)
Write (and make sure you can apply) Hess’s Law, using either a series of given reactions or with tabulated enthalpies of formation.
Write the definition of the enthalpy of formation (fH ) of a substance.
Write how to calculate H at T2 from H at T1 and heat capacities of a substance (Kirchoff’s Lab) Know the Carnot cycle and how to calculate U, S, q, w and efficiency of a heat engine and a refrigerator.
Explanation / Answer
The standard state of a chemical substance is its phase (solid, liquid, gas) at 25.0 °C and one atmosphere pressure.
for an ideal gas Cp- Cv = R (universal gas constant )
for mono atomic gas Cv = 3/2R . and Cp = 5/2R .
for diatomic gas Cv = 5/2R and Cp = 7/2R .
the ratio of Cp/Cv for a monoatomic gas = 1.66
and the ratio of Cp/Cv for a diatomic gas = 1.40
The standard enthalpy of formation(fH ) is defined as the change in enthalpy when one mole of a substance in the standard state (1 atm of pressure and 298.15 K) is formed from its pure elements under the same conditions.
delta H(T2) = delta H(T1) + (integral from T1 to T2) (Cp (products) - Cp (reactants) ) deltaT.
For a heat engine following carnot cycle U= 0 S= 0 the efficiency of the heat engine is = 1-{T2(low temperture sink) /T1(high temperature source )}
for a refrigerator the efficiency is expressed in terms of coefficient of performance
that is COP of refrigerator = 1/(TH /TL - 1 )
TH = Tempearture of the source (high temp)
TL = Temperature of the sink (low temp).
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