Question

Answer all parts as fully as you can, just as if it were a real exam

(a) State the First Law of Thermodynamics

(b) Explain the difference between heat and work

(c) Explain why the heat capacity of a gas is different at constant pressure than at constant volume

(d) Explain what is meant by the standard enthalpy of formation of a compound

(e) Explain what is meant by the standard enthalpy of combustion of a compound

(f) Use information in the Data section of the textbook to calculate the standard enthalpy at 298K of the reaction N2(g) + 3H2(g) → 2NH3(g)

(g) Use your result from (f) to determine the change in enthalpy

(a) when 1 tonne (103 kg) of N2 is consumed

(b) when 1 tonne of NH3 is formed

Answer 1

(a)

The first law of thermodynamics states that the energy of the universe is constant. Energy can be transferred from the system to its surroundings, or vice versa, but it can't be created or destroyed.

E_univ = E_sys + E_surr = 0

First law describes how energy is conserved. It says that the change in the internal energy of a system is equal to the sum of the heat gained or lost by the system and the work done by or on the system.

E_sys  = q + w.

(b)

Heat

Heat is the transfer of energy between two objects due to temperature differences.That is the energy transferred from a hot object to a cold object.

It is important to understand the difference between heat and temperature.

Temperature - Intensive property (are independent of mass or quantity). Temperature is proportional to the average energy per atom/molecule.

Heat - Extensive property (dependent upon the amount of a substance).Heat is proportional to the total energy of all atoms in an object.

Work:

The energy required to move an object against a force.

Work = Force x distance

or

The transfer of energy from one mechanical system to another. It is always completely convertible to the lifting of a weight.

In chemistry, the primary type of work discussed is called “PV work”. In fact, the concept of work is usually introduced in chemistry in order to discuss PV work's role in the definition of Enthalphy.

(c)

In general specific heat (C) gives us an idea of the amount of energy (heat) we need to provide to a system in order to bring about a unit rise in the temperature of the system. It's value may vary depending on the process you are providing this energy. Hence we have two values of C namely C_v and C_p .

C_v for a gas is the change in internal energy (U) of a system with respect to change in temperature at a fixed volume of the system i.e. C_v =(∂ U/∂ T)_v whereas C_p for a gas is the change in the enthalpy (H) of the system with respect to change in temperature at a fixed pressure of the system i.e C_p = (∂ H/∂ T)_p.

We know that, ΔH = ΔU + PΔV (+ VΔP, ΔP=0 for constant pressure) . So the enthalpy term is greater than the internal energy term because of the PΔV term i.e in case of a constant pressure process more energy is needed, to be provided to the system as compared to that of a constant volume process to achieve the same temperature rise, as some energy is utilized in the expansion work of the system. And the relation that correlates these two is C_p = C_v + R

But since liquids and solids can practically assumed to be incompressible, C_p and C_v for them have almost same values and hence only a single value of specific heat is used for them.

(d) Standard enthalpy of formation of a compound:

The standard enthalpy of formation of a compound is the change of enthalphy during the formation of 1 mole of the compound from its constituent elements, with all substances in their standard states at 1 atmosphere.

(e) Standard enthalpy of formation of a combustion:

The standard enthalpy change of combustion of a compound is the enthalphy change which occurs when one mole of the compound is burned completely in oxygen under standard conditions, and with everything in its standard state.