During a chemical reaction between molecules D–E and F
CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
During a chemical reaction between molecules D–E and F, a transient species forms in which atoms D and F approach each other while the original D–E bond begins to break. Consider the potential energy along the D–F internuclear coordinate as D approaches F.
Time-resolved spectroscopy provides the following qualitative observations:
1. At long range, only weak dispersion forces exist between D and F.
2. As D–F distance decreases toward 230 pm, an attractive interaction develops that lowers the potential energy by roughly 20 kJ mol⁻¹ relative to the separated species.
3. At 190 pm, a transition state is reached where the potential energy is 45 kJ mol⁻¹ higher than that of the separated reactants.
4. A covalent D–F bond fully forms at 160 pm with a bond energy of 350 kJ mol⁻¹ relative to D+F.
Using only the verbal data above, analyze and represent the potential-energy changes that occur as D and F progress from infinite separation to formation of the D–F bond.
(a) On axes you construct, sketch a qualitative potential-energy curve for the approach of D toward F. Include the numerical anchor points specified in observations 2–4.
(b) Clearly label on your curve the regions corresponding to net attraction, the transition state, and net repulsion. Explain how each labeled region relates to changes in forces between the atoms.
(c) Indicate the equilibrium bond length and bond energy for the fully formed D–F bond on your diagram.
(d) Using your curve, explain why the transition state occurs at a shorter distance than the initial attractive minimum observed at 230 pm.
(e) Predict qualitatively how the curve would change if atom F were replaced by a smaller atom with similar electronegativity, and justify each predicted change in terms of atomic size and orbital overlap.