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.

Answer 1

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/(T_H /T_L - 1 )

T_H = Tempearture of the source (high temp)

T_L = Temperature of the sink (low temp).