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Question: compare dipole moment of ch3f,ch2f2,chf3,cf4...

compare dipole moment of ch3f,ch2f2,chf3,cf4

Answer

CF₄ < CHF₃ < CH₃F < CH₂F₂

Explanation

Solution

To compare the dipole moments of CH₃F, CH₂F₂, CHF₃, and CF₄, we need to consider their molecular geometry, bond polarities, and the vector sum of individual bond dipoles.

  1. Understanding Dipole Moment: A dipole moment arises from the unequal sharing of electrons in a molecule, creating a separation of positive and negative charges. It is a vector quantity, meaning it has both magnitude and direction. The net dipole moment of a molecule is the vector sum of all individual bond dipoles.

  2. Electronegativity: The electronegativity order is F > C > H.

    • C-F bond: Fluorine is much more electronegative than carbon, so the C-F bond is highly polar with the dipole pointing from C towards F (Cδ⁺-Fδ⁻).
    • C-H bond: Carbon is slightly more electronegative than hydrogen, so the C-H bond is weakly polar with the dipole pointing from H towards C (Hδ⁺-Cδ⁻).

  3. Molecular Geometry: All these molecules are derivatives of methane (CH₄) and adopt a tetrahedral geometry around the central carbon atom. The bond angles are approximately 109.5°.

Let's analyze each molecule:

  • CF₄ (Carbon Tetrafluoride):

    • Structure: Tetrahedral with four C-F bonds.
    • Bond Dipoles: All four C-F bonds are polar and identical in magnitude.
    • Net Dipole Moment: Due to the perfectly symmetrical tetrahedral arrangement, the four C-F bond dipoles cancel each other out vectorially.
    • μ(CF₄) = 0 D (Non-polar molecule).

    (Arrows indicate bond dipoles pointing towards F, cancelling out due to symmetry)

  • CH₃F (Methyl Fluoride):

    • Structure: Tetrahedral with one C-F bond and three C-H bonds.
    • Bond Dipoles: The strong C-F dipole points from C to F. The three weak C-H dipoles point from H to C. The resultant of the three C-H dipoles points along the C-F axis but in the opposite direction (towards the H atoms).
    • Net Dipole Moment: The C-F dipole is dominant. The small C-H resultant dipole partially opposes the C-F dipole. Thus, it has a significant net dipole moment.
    • μ(CH₃F) ≈ 1.85 D

  • CH₂F₂ (Difluoromethane):

    • Structure: Tetrahedral with two C-F bonds and two C-H bonds.
    • Bond Dipoles: The two strong C-F dipoles are at an angle to each other and their vector sum is significant. The two weak C-H dipoles are also at an angle and their vector sum is small. Both resultants generally align, leading to a large overall dipole moment. The molecule has C₂ symmetry.
    • Net Dipole Moment: This molecule generally has the largest dipole moment among the series due to the constructive addition of two strong C-F dipoles and two weak C-H dipoles.
    • μ(CH₂F₂) ≈ 1.96 D

  • CHF₃ (Trifluoromethane or Fluoroform):

    • Structure: Tetrahedral with three C-F bonds and one C-H bond.
    • Bond Dipoles: The three strong C-F dipoles point from C to F. Their vector sum points along the C-H bond axis, but towards the F atoms (away from H). The single weak C-H dipole points from H to C.
    • Net Dipole Moment: The resultant of the three C-F dipoles and the single C-H dipole are in opposite directions, causing partial cancellation. This cancellation makes its dipole moment smaller than CH₃F or CH₂F₂.
    • μ(CHF₃) ≈ 1.65 D

Comparison and Order: Based on experimental values and the analysis of vector addition:

  • CF₄: 0 D
  • CHF₃: ~1.65 D
  • CH₃F: ~1.85 D
  • CH₂F₂: ~1.96 D

Therefore, the order of increasing dipole moment is: CF₄ < CHF₃ < CH₃F < CH₂F₂

The reason CHF₃ has a smaller dipole moment than CH₃F is primarily due to the greater number of strong C-F dipoles in CHF₃ whose resultant is significantly opposed by the single C-H dipole. In CH₃F, the single C-F dipole is opposed by the resultant of three weak C-H dipoles, which is a smaller opposing effect.

The final answer is CF4<CHF3<CH3F<CH2F2\boxed{CF_4 < CHF_3 < CH_3F < CH_2F_2}

Explanation of the solution:

  1. CF₄: Perfectly symmetrical tetrahedral geometry. Four identical C-F bond dipoles cancel out, resulting in zero net dipole moment.
  2. CH₂F₂: Two C-F and two C-H bonds. The two strong C-F bond dipoles add up constructively. The two weak C-H bond dipoles also add up constructively. This leads to the largest net dipole moment among the series.
  3. CH₃F: One C-F and three C-H bonds. The strong C-F bond dipole is partially opposed by the resultant of the three weak C-H bond dipoles.
  4. CHF₃: Three C-F and one C-H bond. The resultant of the three strong C-F bond dipoles is significantly opposed by the single weak C-H bond dipole. This opposition is more pronounced than in CH₃F, leading to a smaller net dipole moment for CHF₃ compared to CH₃F.

Thus, the order of increasing dipole moment is CF₄ < CHF₃ < CH₃F < CH₂F₂.