CBSE Class 11 Chemistry Thermodynamics Worksheet Set A

Short Answer

Question. What are reversible processes? 
Answer. A process or change is said to be reversible, if a change is brought out in such a way that the process could at any moment, be reversed by an infinitesimal change. A reversible process precedes infinitely slowly by a series of the equilibrium states such that system and the surroundings are always in near equilibrium with each other.

Question. What do you mean by the term the state of the system? 
Answer. Thermodynamic state of a system is its condition at a specific time, which is fully identified by values of a suitable set of parameters known as state variables or state parameters or thermodynamic variables. A thermodynamic system is not simply a physical system.

Question. What is the first law of thermodynamics? 
Answer. The first law of thermodynamics is also known as Law of Conservation of Energy, it states that energy can neither be created nor destroyed, energy can only be transferred or changed from one form to another, for example: humans can convert the chemical energy in food, like this ice cream cone, into kinetic energy by riding a bicycle.

Question. How extensive properties are different form the intensive properties? 
Answer. 1. Intensive properties are physical properties that do not depend on the amount of matter whereas extensive properties are physical properties that depend on the amount of matter.
2. Intensive properties are independent of the amount of matter whereas extensive properties depend on the amount of matter.

Question. What is the difference between delta H and delta U?
Answer. Delta H is the change in enthalpy which is equal to change in U +P*change in V whereas the delta U is the change in internal energy of the system.
ΔH is the change in enthalpy .Since H=U+PV. SO, ΔH=ΔU+ PΔV + VΔP.
So, ΔH=dQ + VΔP. Also, ΔH= Cp Δ T.Cp = heat capacity at constant pressure.

Long Answer

Question. State the differences between reversible and irreversible process? 
Answer. 1. Reversible process is a slow process going through a series of smaller stages with each stage maintaining equilibrium between the system and surroundings whereas irreversible process is the process of the system attains final state from the initial state with a measurable speed. During the transformation there is no equilibrium maintained between the system and surroundings.
2. A reversible process can be brought back to the initial state without making an change in the adjacent surroundings whereas an irreversible process cannot be brought back to its initial state without making a change in the surroundings.
3. A reversible process can be made to proceed in forward or backward direction whereas an irreversible process can take place in only one direction.
4. A reversible process work done in a greater than the corresponding work done in irreversible process whereas in irreversible process work done is always lower than the same kind of work done in a reversible process.
5. A reversible process can be reserved whereas irreversible process cannot be reserved.
6. Reversible process infinite changes occur in the system whereas irreversible process finite changes occur in the system.

Question. Explain about system and types of system? 
Answer. A system in thermodynamics refers to that part of the universe in which observations are made and remaining universe constitutes the surroundings. System and surrounding together constitute the universe. The system according to the movements of the matter and energy in or out of the system. There are three types of the system:
1. Open system: An open system is a system that has external interactions. An open system is contrasted with the concept of an isolated system which exchanges neither energy, nor matter, nor information with its environment.
An open system is also known as a flow system. Organisations as open system have several characteristics or attributes such as boundary, inputs, throughputs or transformation process, outputs, feedback and environments. These attributes have to exist in any open organisations.
2. Closed system: A closed system can exchange energy as heat or work but not matter, with its surroundings. It allows heat to be transferred from the stove to the water. Heat is also transferred to the surroundings. Steam is not allowed to escape.
3. Isolated system: An isolated system is a thermodynamic system that cannot exchange either energy or matter outside the boundaries of the system. The system may be so distant from another system that it cannot interact with them.
The system may be enclosed such that neither energy nor mass may enter or exit.

Question. Write short note on standard enthalpy of reactions? 
Answer. Standard enthalpy of reaction is the change in the enthalpy of a chemical reaction that occurs at a constant pressure. It is a thermodynamic unit of measurement useful for calculating the amount of energy per mole either released or produced in a reaction. It is also known as heat of reaction. The standard enthalpy change of a reaction is the enthalpy change which occurs when equation quantities of materials react under standard conditions, and with everything in its standard state. The change in enthalpy does not depend upon the particular pathway of a reaction, but only upon the overall energy level of the products and reactants; enthalpy is a state function, and as such, it is additive. In order to calculate the standard enthalpy of a reaction, we can sum up the standard enthalpies of formation of the reactants and subtract this from the sum of the standard enthalpies of formation of the products.

Question. Explain about heat capacity? 
Answer. Heat capacity of a system is defined as the amount of heat required to raise the temperature of the system through 1°C. Molar heat capacity of a substance is defined as the amount of heat required to raise the temperature of 1 mole of a substance through 1° C. Heat capacity is an extensive property. The corresponding intensive property is the specific heat capacity. Dividing the heat capacity by the amount of substance in moles yields its molar heat capacity. The volumetric heat capacity measures the heat capacity per volume. Heat capacity is often referred to as thermal mass in architecture and civil engineering to refer to the heat capacity of a building. The unit of specific heat is J/kg.K. Cp = heat capacity at constant pressure. Cv = heat capacity at constant volume. Constant volume indicates that there is no change in volume. Therefore the only change in the system is internal energy of the system, due to the addition of heat. The heat capacity is the slope of the plot of internal energy U with temperature T. At very high temperatures many, many energy levels are populated and so the rate of increase becomes constant as the temperature increases and so the heat capacity becomes constant, i.e. it levels out to a maximum value.

Question. What are the enthalpy changes during phase transformations?
Answer. Enthalpy changes during phase transitions, is the enthalpy of fusion is the enthalpy change accompanying the transformation of one mole of a solid substance into its liquid state at its melting point. It is also known as molar enthalpy of fusion. The molar enthalpy of fusion (Δ fus H) of ice is +6 KJ mol-1. All phase changes are accompanied by changes in the energy of a system. The energy required to melt 1 mol of a substance is its enthalpy of fusion (ΔH fus). The energy change required to vaporize 1 mol of a substance is the enthalpy of vaporization (ΔH vap). The direct conversion of a solid to a gas is sublimation. It does not represent enthalpy change during phase transformation. While enthalpy of fusion, vaporization and sublimation represents enthalpy change during transformation from solid to liquid, liquid to gas and solid to gas respectively. The magnitude of the enthalpy change depends upon on the strength of the intermolecular interactions on the substances undergoing the phase transformation.