# Are the reaction rates of these equations equal? [closed]

$$\ce{CH3CH2CH2Br + OH- -> CH3CH2CH2OH + Br-}$$

$$\mathrm{rate} = k[\ce{CH3CH2CH2Br}][\ce{OH-}]$$

If I change the $$\ce{Br}$$ with any element from halogens (without changing concentration, volume, mass etc.), does the rate of reaction change? If so, how does it change? What factors change it?

• Why should the rates be the same ? Even rates for molecules with different isotopes of the same holegen are slightly different. – Poutnik Dec 14 '19 at 11:27
• @Poutnik Could you please explain why they must be different with examples? What makes them change? It seems like they will be the same. – Mathrix Dec 14 '19 at 11:39
• I will let you think about it for a while.... What major factors influence the kinetic rate constant ? One is kinematic, one is geometric, one is energetic. – Poutnik Dec 14 '19 at 13:38
• @Poutnik All I know is that it has an equation $k = Ae^{\frac{-E_a}{RT}}$. – Mathrix Dec 14 '19 at 13:46
• Start with that. And replace rather the rate by the k. Why should be A and E_a the same for all 4 reactions ? – Poutnik Dec 14 '19 at 13:50

For the reaction kinetic rate constants, there is the well known Arrhenius equation:

$$k=A \cdot \exp{\left(-\frac{E_\mathrm{a}}{RT}\right)}$$

$$A$$ is sometimes called frequency factor, interpreted as the rate of collisions with the proper orientation of molecules.

It has 2 terms:

1. The rate of general collisions, that is function of temperature and molecular masses, which determine speed of molecular motion (close relation to the kinetic theory of gases). Note that the temperature dependence is much smaller than for the exponential Boltzmann term.
2. The probability of the proper orientation of molecules, what depends on the molecular geometry. For molecules of otherwise the same geometry, it depends on covalent atom radii.

The exponential term follows the Boltzmann statistical distribution, determining the probability molecules would have enough kinetic energy to overcome the reaction activation energy barrier.

All 3 terms (2 for $$A$$ and the exponential term) Are different for different halogen atoms.

1. The mass molecules increases fluorine < iodine, so collision frequency as the rate constant term is the lowest for iodine.
2. As the reaction mechanism I suppose SN2. The geometrical aspects of $$\ce{-CH2X}$$ is tricky to determine from basic principles. Bigger halogen is more sterically blocking, but is also farther from the central carbon. More polar bond of the smaller halogen should cause stronger repelling of the other 3 bonds, so the other side is more open for sn2 reaction.
3. Activation energy would decrease ( and the Boltzmann term for the rate constant increase) in order F ... I

That about the principles. To compare the particular rates, it is matter of experimental data.

• Poutnik's explanation is perfectly correct. But it does not answer Mathrix's question., – Maurice Dec 14 '19 at 16:27
• Well, if you Taylor concentrations in such a way the rates are the same, the will be the same. Otherwise, they will be for the same concentrations different for reasons above. The same for different concentrations. All will add comments for oarciularveffects for the halogen serie, But I reserve the right to be partially wrong, as I am not an organic chemist. :-) – Poutnik Dec 14 '19 at 16:33