1st Year NotesChemistry class 11th

1st year Chemistry Chapter 5 Energetics of Chemical Reaction

1st year Chemistry Chapter 5 Energetics of Chemical Reaction Question an Answers

Short and Simple Question an Answer

Q.1: How the lattice energy of the ionic compound can be measured by Born Haber cycle?

Ans: We first of all determine heat of formation of compound in laboratory. The heat of sublimation, heat of ionization, amount of electron affinity, bond dissociations energy of the substances are equalized with heat of formation.

Q.2: State the first law of Thermodynamic and give its mathematical form?

Ans: According to the first law of thermodynamic. energy can neither be created nor destroyed. If a gas is enclosed in a piston cylinder arrangement and it is heated by giving some calories of energy, then the piston can be pushed to the backward direction. So some of the heat is converted into internal energy and others used up to do the work.

Q.3: How do we determine the AH in the laboratory for food, fuel etc.?

Ans: For this purpose, bomb calorimeter is used. The substance under test is ignited electrically in the atmosphere of oxygen. The amount o at produced is calculated from the mass of the substance, specific heat of the calorimeter and change of temperature.

Q.4: The total energy of a system is the sum of translational, rotational and vibrational motions? Justify it?

Ans: The substances are consisted of atoms, ions and molecules. If a system is in liquid or gaseous state then its constituent particles have translational motion along three axis x, y and z space. They need energy for this. For rotational motion along three axis they also need energy. Various types of vibrational motions of the bonds make the bonds to compress and stretch. They also demand energy. So the total energy of a system is a sum of all these three types of energies.

Q.5: How the temperature of the system changes during exothermic and endothermic reactions?

Ans: In an exothermic reaction, heat is evolved with increases the temperature of the system. In an endothermic reaction, heat is absorbed so the temperature of the system falls down. These statement are true when the system is isolated.

Q.6: What is a spontaneous process?

Ans: That process which takes on its own is called spontaneous process. No external assistance is required. It moves from non-equilibrium state towards the equilibrium state. It is unidirectional and irreversible.

Q.7: What is state and state function?

Ans: The condition of a system is called state of a system. The initial state is a description before a change happens. Final state is a description after a change takes place. State function is the macroscopic property of a system, which has some definite values for initial and final states and is independent of the path which is adopted to bring out the change.

Q.8: Define the following terms:
Enthalpy
Endothermic reaction.
Ans: Enthalpy is the total heat content of the system. It cannot be measured. We can only measure the enthalpy changes of a system. Those chemical reactions whose enthalpy is positive and they absorb the heat during the process of the reaction are called endothermic reaction.

Q.9: What is the physical signifwance of equation AH = qp?

Ans: AH is the enthalpy change and qp is amount of heat supplied at constant pressure. This equation shows that the expansion of the gas takes place and work is done by the system.

Q.10: How will you differentiate between AE and AH? Is it true that AH and AE have the same value for the reactions taking place in the solution state?

Ans: AH is the heat change at constant pressure, while AE is the heat change at constant volume. In the solution state the pressure is constant, So, there is no difference in AE, and AH in such solution. There is no change in volume when the reaction happens in liquid or solid state.
Hence
PΔV = 0
ΔH = ΔE + PΔV
ΔΗ = ΔΕ + P Χ Ο
ΔH = ΔE

Q.11: Specific heat of a substance depends upon the nature of substance? Why?

Ans: Specific heat is the amount of heat required to raise the temperature of 1 gram of substance by 1 kelvin. The specific heat of H2O is 42 Jg¹k, while for ethanol it is 2.4 Jg¹k¹. Actually it is the capability depends upon the structure, bond angles and vibrational frequencies of the bonds etc.

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