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Which of these molecules have dipole-dipole forces? $(1) \;CO_2 \quad (2) \; CH_2Cl_2 \quad (3)\; CH_4$

$\begin{array}{1 1} Only\;(1) CO_2 \\All \;of\;them\;have\;dipole-dipole\;forces \\ Only\;(2) CH_2 Cl_2 \\ Both (2)\;Ch_2Cl_2\;and\;(3)\;CH_4 \mu T \end{array}$

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  • To determine whether a molecule has dipole–dipole forces see if is polar which means it must contain polar bonds that add together to form a net dipole moment
$(1) \;CO_2:$
Since the electronegativity of Carbon is 2.5 and that of Oxygen is 3.5, Carbon-di-oxide has polar bonds.
However, the geometry of $CO_2$ is linear and hence it is not polar as the dipoles cancel out.
$(2) \;CH_2Cl_2:$
Since the electronegativity of Carbon is 2.5 and that of Hydrogen is 2.1 and that of Chlorine is 3.0, it has 2 poplar bonds $(C-Cl)$ and two that are almost nonpolar $(C-H)$.
In this case, the geometry is tetrahedral. Since the $(C-Cl)$ and $(C-H)$ bonds are different, their dipoles don't cancel out. Therefore, this molecule has dipole-dipole forces.
$(3) \; CH_4:$
Since the electronegativity of Carbon is 2.5 and that of Hydrogen is 2.1, the $(C-H)$ bonds are almost nonpolar $(C-H)$.
While the geometry is tetrahedral, the polarities will cancel out and hence this is nonpolar.
Therefore the answer is (C): Only (2) $CH_2Cl_2$ has dipole-dipole forces.
answered Mar 22, 2014 by balaji.thirumalai
 

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