CHANGE IN TEMPERATURE

• Based on the First law thermodynamic, quantity of heat is related to the temperature change in system.

• Temperature change can be calculated in two different system:

Closed system

Q = ΔU

I ) CLOSED SYSTEM

• The temperature change, ΔT result in the change of the specific internal energy, ΔȖ because specific internal energy depend on the temperature.

• The change of the specific internal energy is denoted as ΔȖ = Ȗ2 – Ȗ1 and related to the temperature change from the T1 to T2 at the constant volume and give an equation as shown below

dȖ = Cv (T) dT

ΔȖ = Cv (T) dT

• Example :Calculate the heat required to raise 250kg of nitrous oxide from 25OC to 150OC in a constant volume vessel. The constant volume heat capacity of N2O in the temperature range is given by the equation

Cv (kJ/kg. OC) = 0.855+9.42 x 10-4 T

Where T is in OC

Solution –ΔȖ (kJ/kg) = (kJ/kg. OC) dT=[0.855T]+ [9.42 x 10-4 T2/2] =106.88 + 10.30 =117.18kJ/kg

The energy balance for this closed system is

Q= ΔU = m (kg) x ΔȖ (kJ/kg) = (250kg) (117.18kJ/kg) = 29295kJ

II ) OPEN SYSTEM

• The temperature change, ΔT also result in the change of enthalpy, ΔḢ because enthalpy, ΔḢ also depend on the temperature.

• Change in specific enthalpy is denoted as ΔĤ and resulting in increasing of the temperature at the constant pressure.

• Change in the specific enthalpy is denoted as ΔĤ = Ĥ2-Ĥ1 for a temperature change from T1 to T2 at the constant pressure and give and equation:

dΔĤ = CP (T) dT

ΔĤ = CP (T) dT