Question

Predict all bond angles for these molecules. (a) \(\mathrm{CH}_{3} \mathrm{OH}\) (b) \(\mathrm{PF}_{3}\) (c) \(\mathrm{H}_{2} \mathrm{CO}_{3}\)

Short answer

Question: Predict the bond angles in the following molecules: CH3OH (methanol), PF3 (phosphorus trifluoride), and H2CO3 (carbonic acid). Answer: In CH3OH, the H-C-H bond angles are approximately 109.5°, while the H-O-C angle is approximately 104.5°. In PF3, the P-F-P bond angle is approximately 107°. In H2CO3, the O-C-O angle is approximately 120°, and the O-H-O bond angles are approximately 104.5°.

Step-by-step solution

Determine the central atom and electron pairs

For each molecule, first identify the central atom(s) - the least electronegative element (excluding hydrogen) and count the number of electron pairs (bonding and non-bonding) around them. (a) CH3OH Central atom: Carbon (C) Electron pairs around Carbon: 4 (3 single bonds with H and 1 single bond with O) Central atom: Oxygen (O) Electron pairs around Oxygen: 4 (2 lone pairs and 2 atoms bonded to it: C and H) (b) PF3 Central atom: Phosphorus (P) Electron pairs around Phosphorus: 4 (3 single bonds with F, and 1 lone pair) (c) H2CO3 Central atom: Carbon (C) Electron pairs around Carbon: 4 (2 single bonds with Oxygen, one double bond with the other Oxygen) Central atom: Both Oxygen atoms bonded to Carbon Electron pairs around Oxygen atoms: 4 (2 lone pairs and 2 atoms bonded to each Oxygen: H and C)

Molecular Geometry

Using the VSEPR theory, calculate the molecular geometry for each central atom. (a) CH3OH Carbon: 4 electron pairs, 4 atoms bonded - tetrahedral Oxygen: 4 electron pairs, 2 atoms bonded - bent (b) PF3 Phosphorus: 4 electron pairs, 3 atoms bonded - trigonal pyramidal (c) H2CO3 (considering both central carbonate and hydroxyl groups) Carbon: 4 electron pairs, 3 atoms bonded - trigonal planar Oxygen atoms: 4 electron pairs, 2 atoms bonded - bent

Predicting Bond Angles

Based on the determined molecular geometry, predict the bond angles for each molecule. (a) CH3OH Carbon (tetrahedral): H-C-H angles ≈ 109.5° Oxygen (bent): H-O-C angle ≈ 104.5° (b) PF3 (trigonal pyramidal) P-F-P bond angle ≈ 107° (c) H2CO3 Carbon (trigonal planar): O-C-O angle ≈ 120° Oxygen atoms (bent): O-H-O bond angles ≈ 104.5° These predictions should be a good estimation for most of the bond angles in given molecules. There might be slight deviations from the predicted angles due to factors such as resonance, electron delocalization or lone pair repulsions.