We use observations of the C I, C II, H I, and -->H2 column densities along lines of sight in the Galactic plane to determine the formation rate of -->H2 on grains and to determine chemical reaction rates with polycyclic aromatic hydrocarbons (PAHs). Photodissociation region models are used to find the best-fit parameters to the observed columns. We find the -->H2 formation rate on grains has a low rate ( -->R ~ 1 × 10−17 cm3 s−1) along lines of sight with low column density ( -->AV 0.25) and low molecular fraction ( -->fH2 10−4). At higher column densities ( -->0.25 ≤ AV ≤ 2.13), we find a rate of -->R ~ 3.5 × 10−17 cm3 s−1. The lower rate at low column densities could be the result of grain processing by interstellar shocks, which may deplete the grain surface area or process the sites of -->H + H formation, thereby inhibiting -->H2 production. Alternatively, the formation rate may be normal, and the low molecular fraction may be the result of lines of sight that graze larger clouds. Such lines of sight would have a reduced -->H2 self-shielding compared to the line-of-sight column. We find the reaction -->C+ + PAH−→ C + PAH0 is best fit with a rate -->2.4 × 10−7ΦPAHT−0.52 cm3 s−1 with -->T2 = T/100 K, and the reaction -->C+ + PAH0→ C + PAH+ is best fit with a rate -->8.8 × 10−9ΦPAH cm3 s−1. In high-column-density gas, we find -->ΦPAH ~ 0.4. In low-column-density gas, -->ΦPAH is less well constrained, with -->ΦPAH ~ 0.2–0.4.