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Q1. The enthalpy of combustion for H₂, C (graphite) and CH₄ are -285.8, -393.5, and -890.4 kJ/mol respectively. Calculate the standard enthalpy of formation ∆H_f for CH₄.

Solution

We can write equation as, C + 2H₂ → CH₄ Enthalpy of formation. (1) H₂(g) + 0.5 O₂(g) → H₂O(l)                    = -285.8 (kJ/mol) (2) C(graphite) + O₂(g) → CO₂(g)                 = -293.5  (kJ/mol)   Enthalpy of combustion methane (3) CH₄(g) + 2O₂(g) →CO₂(g) + 2H₂O(l)         = -890.4(kJ/mol) Step1. Multiply equation 1 by 2 (1) H₂(g) + 0.5 O₂(g) → H₂O(l)                    = -285.8 (kJ/mol) x 2   (4) 2H₂(g) + O₂(g) → 2H₂O(l)                      = -571.6 (kJ/mol) Step2. To 4 add 2 and subtract 3 = 2 x (-285.8) + (-393.5) - (-890.4) = -74.7 (kJ/mol)

Q2. Define lattice enthalpy.

Solution

Lattice enthalpy of an ionic compound is the enthalpy change which occurs when one mole of an ionic compound dissociates into its ions in gaseous state.

Q3. What is meant by reversible process? How does it differ from irreversible process?

Solution

A process which is carried infinitely slowly through a series of steps so that the system and surroundings always remain almost in equilibrium state is called reversible process. Differences between reversible and irreversible process Reversible process Irreversible process (i) It occurs through infinite number of steps under equilibrium conditions at each step. (i) Equilibrium exist only at the beginning and at the end. (ii) Work obtained is the maximum in this process. (ii) Work obtained is not the maximum. (iii) It is only an imaginary process and cannot be realized in actual practice. (iii) It is a natural process and occurs in a particular direction under given set of conditions.

Q4. Define standard enthalpy of fusion.

Solution

The enthalpy change that accompanies melting of one mole of a solid substance in standard state is called standard enthalpy of fusion.

Q5. How does the intermolecular interaction affect the magnitude of enthalpy change during phase transformation?

Solution

The strength of intermolecular interaction determines the magnitude of enthalpy change. For example in water there is strong intermolecular interaction in the form of hydrogen bond and in acetone the intermolecular interaction is weak dipole-dipole interaction. Hence enthalpy of vaporization is greater in case of water.

Q6. Compare lattice Enthalpy and Ionization Enthalpy.

Solution

1. There are two ways in which we can describe Lattice Energy: The lattice dissociation enthalpy is the enthalpy change needed to convert 1 mole of solid crystal into its scattered gaseous ions. Lattice dissociation enthalpies are always positive. The lattice formation enthalpy is the enthalpy change when 1 mole of solid crystal is formed from its scattered gaseous ions. Lattice formation enthalpies are always negative. 2. Ionisation Enthalpy is the energy required to remove the most loosely held electron from one mole of gaseous atoms to produce 1 mole of gaseous ions each with a charge of 1+.

Q7. Which process has the apparatus completely insulated?

Solution

An adiabatic process is carried out in a completely insulated apparatus.

Q8. State the effect of the following processes on the total energy content of the system (i) Work done by the system (ii) Heat transferred to the surroundings

Solution

i) Work done by the system is negative i.e. the total energy content of the system decreases. ii) When heat is transferred to the surroundings the total energy content of the system decreases.

Q9. At what temperature the entropy of a perfectly crystalline substance is zero?

Solution

The entropy of a perfectly crystalline substance is zero at absolute zero Kelvin.

Q10. Define molar enthalpy of fusion.

Solution

The enthalpy change that accompanies melting of one mole of a solid substance in standard state is called molar enthalpy of fusion or standard enthalpy of fusion.