Define Heat Capacity At Constant Volume And Constant

ENERGY BALANCE

Attempt ALL questions.

Question 1. (a) Define:
(i) specific heat capacity (ii) molar heat capacity.
(b) Distinguish between:
(i) heat capacity at constant volume and constant pressure
(ii) internal energy and enthalpy
(iii) sensible heat and latent heat.
(c) State the First Law of Thermodynamics as applicable to energybalance.

Question 2. Identify a process you are familiar with from your job (or everyday experience), which involves at least two different gases (you could for example use methane and air in a gas cooker if no other suitable alternative is available). Note the normal temperature (°C) of the gases in use.
(a) Estimate the molar heat capacity at constant pressure (Cp in J mol-1 K-1) for each gas at its normal operating temperature using:
(i) the correlation
Cp = a+ bT+ cT2 + dT3 where a, b, c and d are constants to be obtained from a relevant source and T is in K.
(ii) any other method (quote the source of data so it can be checked).
(c) Comment on the difference (if any) between the two answers foreach gas obtained in (a).
(d) Estimate the mean molar heat capacity at constant pressure over therange 273 K to T K and hence calculate the change in enthalpy over that range for each gas.

Question 3. (a) Write the balanced chemical equation for the complete combustion of 1 mole of liquid methanol (CH3OH) to carbon dioxide (CO2 (g)) and water (H2O(g)).
(b) Using the data given, calculate the standard heat of reaction(combustion) when methanol is completely burned in air.
Data:
ΔHf° of HO2 (g) = - 242 1. kJ mol-1
ΔHf° of CO2(g) = - 393 8. kJ mol-1
ΔHf° of CH OH3 (l) = - 238 7. kJ mol-1
(c) Using the answer obtained in (b) and the data given below, calculatethe standard heat of reaction (combustion) of methanol at 300°C.
Data:
Mean specific heat capacities over the range 25°C to 300°C for
CO2 (g) = 32.0 J mol-1 K-1 H2O(g) = 36.5 J mol-1 K-1
O2(g) = 30.0 J mol-1 K-1
CH3OH(l) = 50.0 J mol-1 K-1
(d) (i) Using the answer from (b) calculate a value for the heat of reaction (combustion) of methanol at constant volume.
Take R = 0.0083 kJ mol-1 K-1.
(ii) Give one instance where the heat of reaction at constant volume may be required rather than the one at constant pressure.

Question 4. Fuel oil is to be heated in a heat exchanger using steam under the following conditions:
Inlet temperature of oil = 40°C.
Outlet temperature of oil = 120°C.
Flow rate of oil = 420 kg h-1.
Mean specific heat capacity of oil = 2.05 kJ kg-1 K-1.
Temperature of steam = 165°C (dry saturated).
Temperature of steam condensate = 130°C
8% of the heat is lost to the atmosphere (i.e. process is 92% efficient).
(a) Using steam tables, find:
(i) the specific latent heat of vaporization of steam (hfg) at 165°C in kJ kg-1
(ii) the difference in enthalpy of liquid water (hf) between 165°C and 130°C in kJ kg-1.
(c) Calculate the quantity of this steam required per hour to perform this duty