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Practice each question, then open the answer or worked solution.
The enthalpy of atomisation for the reaction CH4(g)$\to $C(g) + 4H (g) is 1665kJ mol-1. What is the bond energy of C-H bond?
Ans: The reaction presented in the question is
CH4(g)$\to $C(g) + 4H (g)
Now, $\Delta $aH = 1665 kJ mol-1
The mean bond enthalpy of the C-H bond should be used here. For the atomisation of 4 moles of C-H bonds, the value is 1665 kJ/mol-1. So, per mole energy = 1665/4 = 416.2 kJ / mol-1.
Use the following data to calculate $\Delta $ latticeH$\ominus $for NaBr.
$\Delta $sub H$\ominus $for sodium metal = 108.4 kJ mol–1
Ionization enthalpy of sodium = 496 kJ mol-1
Electron gain enthalpy of bromine = – 325 kJ mol-1
Bond dissociation enthalpy of bromine = 192 kJ mol-1
$\Delta $fH$\ominus $for NaBr (s) = – 360.1 kJ mol-1
Ans: In order to calculate the lattice enthalpy of NaBr,
(i)Na(s) $\to $Na(g) ;$\Delta $ subH$\circ $=108.4 kJ mol-1
(ii) Na$\to $Na+ + e- ; $\Delta $iH$\circ $ = 496kJ mol-1
(iii) $\frac{~~~1~}{2}$Br2$\to $ Br, $\frac{~~1~~}{2}\Delta $ dissH$\circ $=96kJ mol-1
(iv) Br+e-$\to $Br-; $\Delta $ egH$\circ $= - 325 kJmol-1
$\Delta $fH$\circ $= $\Delta $ subH$\circ $+ $\frac{~~1~~}{2}\Delta $ dissH$\circ $+$\Delta $iH$\circ $+ $\Delta $ egH$\circ $+ $\Delta $ latticeH$\circ $
= -360.1 -108.4-96-496+325 = -735.5KJ mol-1
Given that ΔH= 0 for mixing of two gases. Explain whether the diffusion of these gases into each other in a closed container is a spontaneous process or not?
Ans: It is a spontaneous process. Although enthalpy change is zero, randomness or disorder (ΔS) increases and ΔS is positive. Therefore, in the equation, ΔG = ΔH – TΔS, the term TΔS will be negative. Hence ΔG will be negative.
Heat has a randomising influence on a system and temperature is the measure of average chaotic motion of particles in the system. Write the mathematical relation which relates these three parameters.
Ans: Heat has a randomising influence on a system and temperature is the measure of average chaotic motion of particles in the system. The mathematical relation which relates these three parameters is ΔS = qrev/ T
Here, ΔS = change in entropy ^
qrcv = heat of reversible reaction ‘
T = temperature
Increase in enthalpy of the surroundings is equal to decrease in enthalpy of the system. Will the temperature of the system and surroundings be the same when they are in thermal equilibrium?
Ans: Yes, when the system and the surroundings are in thermal equilibrium, their temperatures are the same.
At 298 K, Kp for the reaction N204(g)⇌ 2N02(g) is 0.98. Predict whether the reaction is spontaneous or not.
Ans: ΔrG° = -RT ln Kp
= -RT ln (0.98)
Since In (0.98) is negative
.’. ΔrG° is positive the reaction is non spontaneous
A sample of 1.0 mol of a monatomic ideal gas is taken through a cyclic process of expansion and compression as shown in the figure. What will be the value of ΔHfor the cycle as a whole?
Ans:
For a cyclic process, ΔH = 0
The standard molar entropy of H2O(l) is 70 J K-1 mol-1. Will the standard molar entropy H2O(s) be more, or less than 70 J K -1 mol-1?
Ans: The standard molar entropy of H20 (1) is 70 J K-1 mol-1. The solid form of H20 is ice. In ice, molecules of H20 are less random than in liquid water. Thus, molar entropy of H20 (s) < molar entropy of H20 (1). The standard molar entropy of H20 (s) is less than 70 J K 1 mol-1.
Identify the state functions and path functions out of the following: enthalpy, entropy, heat, temperature, work, free energy.
Ans: State functions: Enthalpy, entropy, temperature, free energy Path functions: Heat, work
The molar enthalpy of vaporization of acetone is less than that of water. Why?
Ans: Molar enthalpy of vaporization is more for water due to hydrogen bonding between water molecules.