Work, heat and heat capacity, Chemistry tutorial

Introduction:

Thermodynamics is basically the study of systems comprising energy in the form of heat and work. A good illustration of a thermodynamic system is gas confined through a piston in a cylinder. Whenever the gas is heated, it will expand, doing work on the piston; this is one example of how a thermodynamic system can do work.

If the system undergoes change from one thermodynamic state to the final state due to change in properties such as pressure, temperature, volume and so on, the system is stated to have undergone the thermodynamic process.

The operation through which a system changes form one state to the other is termed as a process. If a system changes from one state to the other it is accompanied through change in energy. In case of open systems, there might be change of matter too.

Types of process:

There are a number of various thermodynamic methods which can change the pressure and/or the volume and/or the temperature of the system. To simplify matters, consider what occurs if something is kept constant. The various processes are then classified as follows:

1) Isothermal process:

If the system undergoes change from one state to the other, however its temperature remains constant; the system is stated to have undergone the isothermal process. For example: in hot water in thermos flask, if we take away certain quantity of water from the flask, however keep its temperature constant at 50 degree Celsius, the process is stated to be the isothermal process.

The ideal isothermal method should be infinitely slow and must comprise of infinitely small steps, in ideal thermal communication with the surroundings. The ideal equation for the Isothermal process is,

PV = Constant

2) Adiabatic process:

The method, throughout which the heat content of the system or certain quantity of the matter remains constant, is termed as the adiabatic process. Therefore in adiabatic process no transfer of heat among the system and its surroundings occurs. The wall of the system which doesn't allows the flow of heat through it, is termed as the adiabatic wall, whereas the wall which lets the flow of heat is termed as the diathermic wall.

For example:

Examining the stroke of a piston where heat transfer outside of the system can be least due to the short period of time analyzed.

Analysis of a combustion reaction utilizing the adiabatic assumption to provide an upper limit (that is, conservative) estimate of the flame temperature (termed to as the adiabatic flame temperature).

3) Isochoric process:

If the volume of the system is constant and its pressure and temperature changed, then the process is termed as an Isochoric process. The heating of gas in a closed cylinder is an illustration of the isochoric process.

In this, we are going to maintain constant volume in such a way that the work done will be zero.

ΔQ = ΔU + ΔW => ΔQ = ΔU + 0 => ΔQ = ΔU

The entire heat supplied is employed to raise the internal energy of the system.

4) Isobaric process:

The process throughout which the pressure of the system remains constant is termed as the isobaric process. Illustration: Assume that there is a fuel in piston and cylinder arrangement. If this fuel is burnt, the pressure of the gases is produced within the engine and as more fuel burns more pressure is formed. However, if the gases are allowed to expand via allowing the piston to move outside, the pressure of the system can be kept constant.

Work done by the gas:

W = P (V2 - V1) = μR (T2 - T1)

Reversible and Irreversible process:

Based on the value of the driving force applied, we can categorize the processes into two kinds: reversible and irreversible.

Irreversible Process:

The method is stated to be an irreversible process if it can't return the system and the surroundings to their original conditions whenever the process is reversed. The irreversible process is not at equilibrium all through the process.

For illustration: if we are driving the car uphill, it uses a lot of fuel and this fuel is not returned if we are driving down hill. Most of the factors contribute in making any process irreversible. The most general of these are:

  • Friction
  • Unrestrained expansion of a fluid
  • Mixing of two different substances
  • Heat transfer through a finite temperature difference

Reversible Process:

The main concept is that most of the thermodynamic processes encompass a preferred direction just as Heat always flows from hotter object to the colder object. Once a gas is discharged in a room, it expands in room and never contracts devoid of indulgence of any external force and so on.

However in several systems, the reverse takes place. Generally it happens whenever that system is close to the thermal equilibrium. This equilibrium has to be within the system itself and as well within the system and its surroundings. Whenever this phase is reached, even a small change can change the direction of the process and thus such a reversible process is as well termed as an equilibrium process.

For illustration: A very simple illustration can be of two metal jars 'A' and 'B' which are at the thermal equilibrium and are in contact with one other. Now, whenever we heat jar 'A' slightly, heat begins to flow from Jar 'A' to Jar 'B'. This is the direction of this process. Now this process can be reversed just via cooling Jar 'A' slightly. If Jar 'A' is cooled, heat flows from Jar 'B' to jar 'A' till the thermal equilibrium is reached.

Work, heat and heat capacity:

The Work, heat and energy encompass the similar unit, termed as the joule (J). Energy is the thermodynamic property of a system, while work and heat are not. Work and heat are meaningful only whenever a process occurs.

Heat:

By now, we all recognize heat as a form of energy. Heat is not the property of a system however it is exchanged between the system and its surroundings throughout a process, when there is a temperature difference between the two. 

Heat can be defined based on the ice-calorimeter. If we put ice in an ice-calorimeter and the stopper is pressed, there will be an increase in the level of Hp in the capillary and one can read   the calibrated capillary. If the reaction mixture is positioned in the reaction chamber of the ice-calorimeter and corked, two things might happen. The reaction may absorb heat from the surrounding jar, or the reaction mixture might produce heat. Now, what happens if the heat is absorbed? If your answer is that ice will be made, then you are correct.  If the reaction mixture generates heat, more ice will melt.

Work:

Let us now illustrate the word, work and describe its different types. The amount of work that attends a thermodynamic state is very important. The easiest concept of work (W) is stated as the product of the force applied (F) and the distance (x) moved all along the direction of the force.

W = Force x distance (x)

W = F.x

Forces encompass different physical origin, and work can be done in a variety of manners.

a) Gravitational work: If a body of mass 'm' is moved via a height 'h' against gravity, then force is equivalent to 'mg' and the gravitational work done is 'mgh'.

b) Electrical work: Whenever an electric potential 'E' is applied across the resistance 'R' in such a way that the current 'I' flows via it, then the work done per second is EI and in t seconds it is equivalent to EIt.

c) Pressure-volume work: This is a kind of mechanical work performed whenever a system changes its volume against the opposing pressure. This is as well termed as the work of expansion or compression.

The energy gained or lost throughout heat exchange between the system and its surroundings can be defined in terms of the heat capacity values.

Heat capacity:

Heat capacity is the heat needed to increase the temperature of a body by l K. If, throughout the process, the volume of the system remains constant, then it is termed as the heat capacity at constant volume (cv).  If the pressure remains unchanged, it is termed as heat capacity at constant pressure (cp).  For one mole of a pure substance, these are termed as molar heat capacity at constant pressure (CP) and molar heat capacity at constant volume (CV). The heat capacity per unit mass is termed as specific heat. The heat capacities change by temperature. This implies that the heat needed to change the temperature by 1K is different at different temperatures. Though, over small ranges of temperature, these are generally taken as constant. The molar heat capacity and specific heat are intensive properties, while heat capacity is the extensive property.

By changing the temperature of a specific system by dT, if the heat needed is dqv (at constant volume) or dq (at constant pressure), we encompass:

CV = ncv = dqv/dT

CP = ncp = dqp/dT

Here 'n' does the amount, that is, number of moles of the substance comprises the system.

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