State whether the following reaction is spontaneous or non spontaneous if ΔH = 50 kJ and ΔS = –130 JK–1 at 250 K.(b) Explain optical activity in 2-chlorobutane.(c) Write full name of DDT.

Answer:

(a) Non-spontaneous. \(\Delta H = 50\) kJ, \(\Delta S = -130\) J K^–1 \(= -0.130\) kJ K^–1, \(T = 250\) K.

\(\Delta G = \Delta H - T\Delta S = 50 - 250(-0.130) = 50 + 32.5 = +82.5\) kJ

Since \(\Delta G \gt 0\), the reaction is non-spontaneous. (With \(\Delta H\) positive and \(\Delta S\) negative, \(\Delta G\) is positive at every temperature.)

(b) Optical activity of 2-chlorobutane: In CH₃–CH₂–C*H(Cl)–CH₃, carbon-2 is attached to four different groups (–H, –Cl, –CH₃ and –C₂H₅), so it is a chiral (asymmetric) carbon. The molecule therefore has no plane of symmetry and is not superimposable on its mirror image. It exists as two enantiomers: one rotates plane-polarised light to the right, (+) or dextrorotatory, and the other to the left by the same angle, (–) or laevorotatory. An equimolar mixture of the two is a racemic mixture and is optically inactive.

(c) DDT: p,p′-Dichlorodiphenyltrichloroethane [IUPAC: 1,1,1-trichloro-2,2-bis(4-chlorophenyl)ethane].